Joints in composite materials

REVIEW ARTICLE on

STRUCTURAL JOINTS IN POLYMERIC COMPOSITE MATERIALS

PREPARED FOR PERMANENT SCIENTIFIC COMMITTEE 56 FOR STRUCTURAL & CONSTRUCTION ENGINEERING

BY HOSSAM EL-DIN MOHAMMED SALLAM, Ph.D.

January, 2004

STRUCTURAL JOINTS IN POLYMERIC COMPOSITE MATERIALS

BY HOSSAM EL-DIN MOHAMMED SALLAM, Ph.D.

CONTENTS 1. INTRODUCTION 2. MECHANICALLY FASTENED JOINTS 2.1 Stress Concentration Factor in Composite Materials Containing an Open Circular Hole.

2.2 Failure Modes in Mechanically Fastened Joints 3. BONDED JOINTS 3. 1. Stresses in Adhesive Joints 3.2 Standard Mechanical Test Procedures 3.3. Fracture Mechanics Considerations 3.4. Fatigue behavior and Durability of Adhesive lap joints 3.5 Prevention of Peeling failure in Adhesive Joints 3.6 Adhesive Bonded Dissimilar Materials 3.7 Fusion Bonding for Joining Thermoplastic Matrix Composites 4. SNAP JOINT IN COMPOSITE STRUCTURES 5. HYBRID JOINTS 5.1. Hybrid Joints in Plated Beams 6. SUMMARY

6.1 Mechanical Joints (Bolts or Rivets) 6.2 Adhesive Bonding 6.3 Hybrid joints 7. REFERENCE

1 9 11 13 21 25 28 30 35 35 37 40 44 46 47 50 50 50 51 52

1. INTRODUCTION The permanent assembly of individual manufactured components is an important aspect of fabrication and construction. There are many ways for accomplishing this including the use of fastenings such as bolts and rivets, the use of adhesives, and by soldering, brazing and welding [1]. Joint design is dependent upon the nature of materials to be joined as well as the method of joining. The first cast iron bridge, at Ironbridge, in Shropshire, England, was erected in 1777-1779 by casting massive half-arches and assembling the mortise and tenon joints, with fit-on parts and tapered pegs, see Fig. 1-1.

Figure 1-1. The first cast iron bridge in Shropshire.

This mode of construction was the only one known to the designer, and is closely based on the design of timber structures. The only concession to the new structural material was in the use of large scale, single piece, curved members. Rolled bars of T- and L-sections did not become available until after Napoleonic wars and the first I-beams, for light loading only, were made in Paris in 1847. Heavy I-beams only become available in 1860 after the invention of mild steel. Fastening was originally by slotting a circular cylinder, cast as an integral of the beam, over a cylindrical column, cast with a retaining lip, but fish plates and bolting were introduced at early stage [2,3].

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Steel “nailing� was developed and primarily used for fastening sheet metal to structural steel for metal roof decks and claddings. Research undertaken at the University of Toronto has shown that the existing technology of steel nailing also can be used to connect structural steelwork, particularly hollow structural section steel members [4].

Cold-formed structural members may be joined with bolted connections that are designed with the aid of applicable national design standards, such as American Iron and Steel Institute (AISI). Winter [5] categorized the failure of bolted connections into four separate modes, described as end tear-out, bearing, net-section fracture, and bolt shear. Recently Rogers and Hancock [6] found AISI specification cannot be used to accurately predict the failure modes of thin cold-formed sheet-steel bolted connections that are loaded in shear.

It is sometimes necessary to use combination (hybrid) joints in steel construction. For example, high-strength bolts can be used in combination with welds, or rivets can be used in combination with bolts. The need for a hybrid joint can rise for a number different reasons. For example, the load demands on an existing bolted joint may change with time, necessitating renovation of that joint. If the geometry does not permit the addition of more bolts, welds can be added to the connection in order to increase its capacity [7]. Bonded structures can be of two types based on purely adhesive or an adhesive/mechanical connections. These types of connections include bonded-welded, bonded-riveted, and bonded-screwed connections [8]. The concept hybrid joint is also used in structural timber by injecting a resin into the gap between the connector and the wood to improve the performance of bolted and dowelled joints [9]. On the other hand, steel bolts/screws were used as shear connectors to joint steel/timber beam to concrete slab [10-12].

Elshafie [12] also used concrete shear key (dovetail) as shear connectors in timberconcrete composite joints.

It is convenient now to define an adhesive as a polymeric material which, when applied to surfaces, can join them together and resist separation. The structural members of the joint, which are joined together by the adhesive, are the adherends, a word first used by de Bruyne in 1939 [see Ref. 3]. Fracture mechanics has become a very popular tool for 2

characterization of adhesive metal joints in recent years [13]. Furthermore, Conrad et al.

[14] studied the effect of droplet diameter and droplet spacing on mode I fracture toughness of a discontinuous wood-adhesive bonds. The fracture mechanics concept is also used to understand the basic dentin adhesion mechanisms [15].

Advanced composite materials and structures have undergone rapid development over the past four decades [16]. The majority of advanced composite materials are filamentary with continuous fibers. As such, their behavior and the behavior of structures made from them are more complicated than that of monolithic materials and their structures.

Advanced composite materials can be divided into classes in various manners. One simple classification scheme is to separate them according to the reinforcement formsparticulate-reinforced, fiber-reinforced, or laminar composites. Fiber-reinforced composites, can be further divided into those containing discontinuous or continuous fibers. Polymers, ceramics, and metals are all used as matrix materials, depending on the particular requirements. The matrix holds the fibers together in structural unit and protects them from external damage, transfers and distributes the applied loads to the fibers, and in many cases contributes some needed property such as ductility, toughness, or electrical insulation.

The structure of polymers consists of long molecules with a backbone of carbon atoms linked by covalent bonds. In non-crystalline or amorphous polymers the molecular chains have an entirely random orientation and are cross-linked occasionally by a few strong covalent bonds and numerous but weaker van der Waals bonds. These weaker bonds break as the temperature reaches a value known as the glass transition temperature, Tg, characteristic for each polymer. Below Tg the polymer behaves as a linear elastic solid.

Creep becomes increasingly significant as the temperature increases and, above Tg, the polymer deforms in a viscous manner under load. In crystalline polymers the molecules are oriented along preferred directions, bringing with them optical and mechanical anisotropy. Polymers are described as being either thermosets (e.g., epoxy, polyester, 3

phenolic) or thermoplastics (e.g., polyimide, polysulfone, polyetheretherketone, polyphenylene sulfide). Ceramics, such as glasses, cement & concrete, and engineering ceramics ( Al2O3, SiC, Si3N4, ZrO2), have a wide range of engineering applications. The strong ionic and covalent nature of the bonding in most ceramics leads to a stable crystal structure with a high melting point and high stiffness. Many ceramic materials have very high elastic moduli and strengths, but the advantages of these properties bestow are often outweighed by their highly brittle nature, which leads to low and unpredictable failure stress resulting from the presence of flaws. Metal matrix composites typically comprise a light metallic alloy matrix, usually based on aluminum, magnesium or titanium alloys.

The major reinforcing elements used in composites are glass, carbon/graphite, organic, and ceramic. Both metal and ceramic materials have properties closer to those of likely reinforcements and this leads to a different choice of properties for which these composite systems are optimized. With polymer matrix composites, it is almost true to say that the properties of the composite are essentially those of the fibers, with little contribution from the properties of the matrix.

Fiber Reinforced Polymer (FRP), earlier limited to aerospace structures, are gaining acceptance in civil engineering applications also. FRP plates, sheets, rebars, and strands have been increasingly used in the construction industry due to their superior properties.

One of the potential structural applications of FRP composites with concrete or steel structures is strengthening of RC or steel beams with unidirectional fiber-composite sheets/plates bonded on their tension faces via the use of epoxy adhesives. Growing maintenance and durability problems in transportation infrastructure have led researchers to explore FRP box-girder, I-beam, or other shapes in bridges [17-19]. There are numerous examples of completed all-composite new bridges and several more are under construction, see Fig. 1-2. FRP materials could play an important role for the construction industry of tomorrow.

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Aberfeldy Footbridge in Scotland

Bonds Mill Lifting Bridge near Gloucester

Figure 1-2. Fully composite bridges [20].

Due to the relatively low weight and high strength of FRP girders and decks it has proven to be efficient to replace old bridges that no longer meet today’s requirements with FRP alternatives [21-26]. While design concepts vary, several systems employ a modular system to build up the bridge deck, meaning that composite profiles are transversely joined to form the required bridge span or occasionally width [22]. Kumar et al. [23] installed an all-composite bridge deck made up of FRP. The materials used for the fabrication of this 9.144 m long by 2.743 m wide deck were 76.2 mm pultruded square hollow glass and carbon FRP (GFRP and CFRP) tubes of varying lengths, Fig. 1-3.

These tubes were bonded using an epoxy adhesive and mechanically fastened using screws in seven different layers to form the bridge deck with tubes running both longitudinal and transverse to the traffic direction. The cross-section of the deck was in the form of four identical I-beams running along the length of the bridge. They [23] concluded that, all-composite bridge decks made of pultruded glass and carbon tubes are judged to be a suitable replacement for short span bridges made of conventional materials.

In fact, the better fatigue performance of FRP was one of the main reasons suggested by design engineers to replace conventional materials in some applications [24].

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Figure 1-3. Experimental setup for the bridge deck test One promising structural form is frames, where replacement of steel members is by standard pultruded composite profiles of similar shapes [27,28]. The two most sensible production techniques, pultrusion and filament winding, yield products with considerable anisotropy in their properties. This results in considerable difficulties in making joints.

Drilling for bolts tends to sever the load-carrying fibers, and the lack of isotropy precludes other load paths. Welding is impossible[20]?!!(is it), and although adhesives can be used, the design of efficient and durable adhesive joints is not trivial. The most cost-effective way to manufacture constant-cross-section composite profiles is pultrusion, since this process is highly automated and uses low-cost forms of raw material. The nature of the fiber reinforcement lends itself to novel forms. Pultrusions are difficult to bend into conventional reinforcement shapes without seriously weakening the element at the bend. So why not make use of the fibers’ flexibility to form the links before adding resin to stiffen the material? There is research in Germany into the use of a variety of textile processes, including the double-layer and Raschel knitting techniques (Figure 1-4) which naturally give three dimensional structures [29]. The performance advantages of three-dimensionally (3D) reinforced textile composites (as compared to traditional 2D ones) have been well documented [30]. These advantages include full suppression of delamination, lower notch sensitivity, and better fatigue and impact properties. 3D woven 6

composites can be easily machined into parts, very similar to metals, without considerable effect on their properties.

Figure 1-4. Double-knit textile reinforcement [31] .

Just because FRP’s have good potential, they cannot be used in every structure [32-37].

One thing that needs to be considered is the beam-to-column connections. Mosalam [32] concluded from an evaluation of available information that a majority of connections research and manufacturers’ information was mimicking ‘steel-like’ details, i.e. they copied the connection practice of steel frames, and that this wrongly ignored the effect of difference in mechanical properties. The development of his Universal Connector (UC) [33] was therefore presented as the first positive step in correcting this deficiency of current connection design details. It has the conventional leg-angle shape of steel cleats, but with side ribs to substantially increase its stiffness and strength. Appropriate placement of the E-glass fiber reinforcement has been achieved by manufacturing the UC with the resin transfer method. Cleated connections are the most favoured method for pultruded frames due to the unsuitability of composite materials for conventional steel endplate or welded joints [34,35]. Mottram and zheng [34,35] have tested both connections consisting of FRP materials and those of steel. The conclusions from this test were that the steel option worked better and was recommended in the article.

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Although many FRP structures can be molded in single pieces to simplify the manufacturing process and to reduce assembly coast, they cannot be completely exempted from the joining process. Both mechanical fastening and adhesive bonding are commonly used in structure assemblies. Each technique has its own advantages and disadvantages in terms of function and cost. In terms of strength, different techniques also lead to different damage modes and failure strengths. Based on the high in-plane strengths of FRP materials, mechanical fastening is commonly used in assembling composite laminates.

However, mechanical fastening can cause high stress concentration due to structural discontinuity. Adhesive bonding, although free from structural discontinuity, is primarily based on the shearing strength of the adherend, the adhesive and the bonding between them. These shearing strengths are not greater than the in-plane strengths and are limited to the bonding surface, instead of through the thickness. As the joining components become thicker, a much larger bonding area is required, not only to achieve a higher joining strength but also to reduce the high stress concentration around the bonding ends.

Since mechanical fastening is actually a three-dimensional technique, it is superior to two-dimensional adhesive bonding in joining thick composite laminates [36].

The main objective of this review article is to present a sound background regarding the development of the understanding of the mechanical behavior and the failure modes of the bolted, adhesive, and hybrid joints in polymeric composite materials. Section 2 presents a review of mechanically fastened joints in FRP including various modes of failure. In Section 3, the review article discusses the bonded joints in FRP and dissimilar materials. Section 4 presents a review of snap joints in FRP. Hybrid joints in composite materials are introduced in section 5.

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2. MECHANICALLY FASTENED JOINTS Mechanical joints are used when repeated disassembly and reassembly is required or when surface preparation is not practical. Mechanical joints require that bolt or rivet holes are drilled into the composite, that reduced the net cross sectional area of the structure and introduce localized stress concentration.

These stress concentrations can cause ply delamination since they will include through thickness tensile and shear stresses. Mechanical joints add weight to the structure from the added weight of the bolt or rivet. They also pose a risk for corrosion since the laminate and fastener may comprise dissimilar materials and moisture can be trapped in the crevices inherent in such joints. However mechanical joints can be readily inspected before assembly and while in service. Examples of two typical bolted joints are the single lap joint and double strap joints as shown in Fig.2-1.

e P

D

W

P t

Bolted Single Lap Axial Pitch

P

P

Bolted Double Strap Figure 2-1. Basic types of mechanical joints.

The single lap joint is the simplest and most weight efficient but the load results in a moment due to off-set load. The double lap joint eliminate the moment but adds additional weight from the straps and additional bolt.

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A circular hole in tensioned FRP plates may be classified into three types as follows [38]: (1) open-hole tension: the FRP plates were subjected to uniaxial tension with no constraint imposed on the hole, (2) filled-hole tension: a bolt, φ = dB, was inserted inside the hole, φ = dH, with/without a clamp-up load. A washer, φ = dW, was inserted between a bolt head and tail and the FRP plate to distribute the clamp-up load, as shown in Fig. 2-2. The FRP plates were subjected to uniaxial tension, and (3) bolted joint, bolt-loaded hole: double-lap bolted joints were subjected to a uniaxial load with/without clamp-up load.

Tab

Tab Rigid Washer

Load Cell Washer

D

Dw

Nut

Strain Gauge Load Cell

Washer

Bolt

W

Specime

W

(a)

(b)

(c)

Figure 2-2. Geometries of the (a, b) specimens for open- and filled-hole tension tests, and (c) bolted joint test setup.

In the case of open-hole tension, the mode of failure mainly depends on the fiber/matrix interface. A weak interface results in longitudinal crack propagation along the interface, while a strong interface results in transverse crack propagation across fibers leading to premature composite failure.

However, an interface of intermediate strength leads to optimum composite performance between these extreme conditions. Based on the specimen geometry and the interfacial strength of unidirectional FRP, the crack emanating from notches, such as circular holes, may grow parallel to the loading and fiber direction, i.e. notch insensitive [39]. A schematic description of the damage pattern for a composite laminate is shown in Fig. 2-3. In the case of Fiber Breakage & Matrix Cracking mode of failure, damage is localized to the 10

stress concentration areas before final failure. Edge Delamination mode of failure may occur in both open- and filled-hole specimens in an early loading stage. The edge delamination grew throughout the width of the open-hole specimens. In addition, a Fiber-Matrix Splitting mode occurred along the zero degree plies (in the direction of the applied load) emanating from the edge of the hole [38].

Fiber Breakage Matrix Cracking

Delamination

Fiber-Matrix Splitting

Figure 2-3. Typical tensile failure modes in composite laminates containing a circular hole.

2.1 Stress Concentration Factor in Composite Materials Containing an Open Circular Hole The susceptibility of composite materials due to effects of stress concentrations such as those caused by notches, holes, etc., is much less than for metals.

Failure in long fiber-reinforced laminated composite structures containing stress concentration areas, such as circular holes or bolt-loaded holes, has been one of the technological issues by many researchers during the last decades [38-42]. The mechanisms of such failures are significantly affected by fiber orientation, relative strength of the matrix and fiber and the bond strength between them. Open holes, notches, scratches, inclusions, and so on, all produce concentrations of strain and stress. Stress concentrations can seriously weaken brittle materials and can shorten the fatigue life of both ductile and brittle materials. But a ductile material under static load can redistribute stress by yielding without fracture. Thus, although strain concentration factor persists,

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stress concentration factor (SCF) decreases markedly. The theoretical SCF for an infinite orthotropic plate containing an open circular hole, K T∞ , can be calculated from the following relation [43]:

K ∞T = 1 +

 E  E 11 2  − υ12  + 11  G 12  E 22

(2-1)

Where E11, E22, ν12, and G12 are the elastic constants for an orthotropic plate, i.e. E11 & E22, are longitudinal and transverse elastic modulus respectively, and ν12, and G12 are the principal in plane Poisson’s ratio and shear modulus respectively. It is worth to note that, in the case of isotropic plate, i.e. E = E11 = E22, and G = G12 = E/2(1+ν), K T∞ will be equal to the value obtained from the will known relation K T∞ = 1+2(D/ρ)0.5 = 3, where D and ρ are the depth and the radius of the notch respectively. The inverse of isotropic finite width correction (FWC) factors for open circular hole can be written, according to [44], as K∞  3(1 − α ) FWC Iso−1. =  T  = 3  K T  Iso. 2 +(1 − α )

(2-2)

Where α = dH/w, and KT is the finite SCF. The inverse of orthotropic FWC factor equals [45]:  K T∞  = FWC Iso−1. + 0.5 ( M α ) 6 ( K T∞ − 3) 1 − ( M α ) 2    K T  Orth

(

)

(2-3)

& M=

(

)

1 − 8 FWC Iso−1. − 1 − 1 2(α )

(2-4)

2

Finally, SCF based on net section instead of the whole width, KTn, can be calculated as follows:

K Tn = K T (1 − α )

(2-5)

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2.2 Failure Modes in Mechanically Fastened Joints There are six basic failure modes in mechanically fastened joints in FRP [46,38]: net-tension, shear-out, transverse splitting, cleavage, bearing, and pullthrough as shown in Fig. 2-4. The bearing stress, σb is the load, P divided by the projected transverse cross sectional area of the hole σb = P /(dHt). The shearout stress is determined by the longitudinal shear surfaces, and is given as σSO = P /(2et). The net section stress is σN = P /[(W-dH)t]. The transverse splitting stress is a localized stress normal to the applied load. The gross stress defined as σ = P /[Wt] is used to rate the effectiveness of the joint. Joint efficiency is the ratio of the gross section stress at failure to the strength of the laminate in the gross section. For metals single fastener joints can have efficiencies as high as 80%. Polymer matrix-fiber composite laminates have efficiencies generally less than 50% due to their strength anisotropy and inability to redistribute stress. Net-tension failure is associated with matrix and fiber tension failures due to stress concentrations. Bearing failure leads to an elongation of the hole. Shear-out failure can be regarded as a special case of bearing failure. Shear-out and bearing failures result primarily from the shear and compression failures of fiber and matrix. Cleave failures are associated with both an inadequate end distance (e) and too few transverse plies. Pull-through failure occurs mainly with countersunk fasteners or when the plate thickness (t) to hole diameter (dH) ratio, t/dH, is sufficiently high to precipitate failure. Li et al [47] found two others failure modes, i.e., bending induced cross-section failure and rivet cap penetration failure. Further, they [47] found that, failure modes may change with increasing loading rate and rivet rotation decreases with increasing loading rate. Generally, the total energy absorption of composite riveted joints increases with increasing loading rate, except for joints used countersunk rivets. It was also shown that rivet rotation is significant in some joint designs, which could damage the laminate and thus reduce the joint strength.

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(a)

(b)

(c)

(d)

(e)

(f)

Pull-out P

P

P

P

P

Figure 2-4. Failure Modes (a) Net-Tension, (b) Shear-Out, (c) Transverse splitting, (d) Cleavage, (e) Bearing, and (f) Pull-out.

Net section failures can be prevented by increasing the ratio of the plate width to hole diameter, W/dH. Generally, W/dH > 6 is sufficient to prevent net section failures. Shearout failures can be eliminated the ratio, e/dH to 3 or greater.

Transverse splitting is rare but will occur if there is a high fraction of the fibers in the load direction such as would be the case in a unidirectional composite.

Bearing failure is the preferred failure mode since the joined members are not catastrophically separated. Bearing failures are associated with localized hole damage such as local delamination and matrix crazing. The experimental observations of the effects of joint geometry, ply-orientation, lay-up, t/dH, dH to plate width (w) ratio dH/w, e/dH, and clump-up load, i.e. through-thickness pressure, on the joint behavior were reviewed by Camanho and Matthews [46].

The effect of e/dH on the SCFs of unidirectional CFRP single-bolt tension joints with Îą = 0.2, i.e. w/dH = 5, was studied numerically by Sallam [48].

SCFs were calculated based on equivalent, shear, and longitudinal stresses. The calculated SCFs based on equivalent stress are very high due to the high compressive stress ahead of the loaded-bolt. SCFs decrease with increasing e/dH up to e/dH equals three, as shown in Fig. 2-5. After that, the effect of e/dH is insignificant.

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Stress concentration factor

41

α = 0.2

31

E11 = E22 = E33 = G12 = G13 = G23 =

Based on

σEq. τ XY

21

σYY 11 1 0

2

4

e/dH

6

8

GPa 165 11 11 5.3 5.3 3.9

ν12 = 0.26 ν13 = 0.5 = 0.5 10 ν23

Figure 2-5. SCFs of unidirectional CFRP single-bolt tension joints.

Sallam [48] found through an experimental investigation that, the failure mode of all tested bolt-loaded unidirectional CFRP plates is shear-out, regardless the values of dH, dB, e, and w or their ratios. This is due to the weakness in the bond between the fiber and matrix or in the shear strength of the matrix compared with the tensile strength of the fiber. Load-bolt displacement responses of all tested bolt-loaded unidirectional CFRP plates are shown in Fig. 2-6. The static failure is defined as the maximum load, Pult, achieved during the test to failure [49]. As shown in the Fig. 2-6, the stiffness of the joints is not a function of the values of dH, dB, e, and w, whereas, the ultimate load is dependent on the contact area alone, i.e. the value of dB. This observation is acceptable only in the case of shear-out failure in unidirectional composite plate, i.e. lower stiffness and strength in transverse direction.

Load, kN

4

Test # 1 2 3 4 5 6 7 8 9

2

0 0

2 4 Bolt displacement, mm

dH d B 7 6 10 6 10 6 10 6 13 6 13 6 10 8 10 8 13 8

e 10 11 25 25 17 25 30 30 25

w 20 30 30 50 50 50 30 50 50

6

Figure 2-6. Load-bolt displacement curves of unidirectional CFRP plates.

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Aktas and Karakuzu [50] found experimentally and numerically that, the failure mode of the unidirectional composite pinned joint is shear-out except for the cases of e/dH and w/dH ratios of 4 and 5 respectively. For those values the mode is bearing mode. When failure caused by the pin continues up to 2–3 mm from the free end of the plate in the direction of loading, the plate tears immediately. Net-tension mode occurs in the following situations: fiber orientation ranged between 30o and 60o, w/dH = 2 –3, and e/dH = 4. Although the failure mode is the same, the propagation of failure is a bit different for each criterion, because the angle of first failure area is different in each case, as shown in Fig. 2-7 .

(a) (b) Figure 2-7. Ultimate failure of specimen for (a) E/D, and (b) W/D series.

Furthermore, Karakuzu and his co-workers [51,52] studied the effect of joint geometry and ply orientation on failure strength and failure mode of composite laminates with a pin-loaded hole. They concluded that, there is a definite dependence of bearing strength on stacking sequence and joint geometry. The lay-up [0/±45]s laminate was found to have higher bearing strength than the lay-up [90/±45]s laminate. Ultimate strengths are sensitive to w/dH and e/dH values in a larger range compared to the laminate tensile strength. Increasing the end distance increased the bearing strength of the joint until a critical end 16

distance was reached; any increase of the end distance beyond that value did not result in a corresponding increase in the strength of the joint. Pin bearing strength decreases with decreasing w/dH ratio. As the width of the specimen decreases, there is a value where the failure changes from the bearing to nettension failure mode. Maximum bearing strength is reached when the edge distance ratio, e/dH, is equal to or greater than 3, and the side distance ratio, w/dH, is equal to or greater than 4 for the [90/±45]s laminate. For the [0/±45]s laminate, the critical e/dH and w/dH ratios are 3. In net-tension and shear-out failure, the reduction in load is both greater and more sudden. But in bearing failure load doesn’t drop suddenly. Therefore, joints that fail in the bearing mode are stronger and safer joints although this depends on selected design rule. The net-tension strength of a single-hole joint is dependent on ply orientation and specimen width. The effect of a change in the ratio of specimen width to hole diameter is least for [90/±45]s laminate and most marked for [0/±45]s laminate. The shear strength of single-hole joints has been shown to be strongly dependent on the ply orientations within the laminate and the edge distance. For the [0/±45]s laminate, the maximum shear stress at failure is reduced to 50% with increasing edge distance to diameter ratio. Whereas for [90/±45]s the effect of a change in the ratio of edge distance to diameter is of about 66% loss in strengths. It is evident from the load–displacement curves that the [(±45)3]S laminate with small w/dH fails in a more sudden fashion than the [(0/90)3]S laminate. For this reason, the use of mechanically fastened joints in [(±45)3]S laminates with small w/dH is not recommended. Turvey [53] found that, both the initial stiffness and the ultimate load depend on the w/dH and e/dH for pultruded GRP plate. It is worth to mention that, the reinforcement of pultruded GRP plate is in two forms, viz. E-glass roving (unidirectional fiber bundles) and Continuous Filament Mat (CFM). The CFM provides the transverse stiffness and strength. Further, Turvey found that the failure mode changed from bearing to cleavage to shear-out with decreasing e/dH with constant w/dH. However, the failure mode changed from bearing to tension with decreasing w/dH with constant e/dH. In drilling operations, high axial

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forces are induced locally in the material at the tip of the drill because of the stationary tool center. In composite materials, the high axial contact force between the drill and the composite may cause separation of piles at the exit.

This effect can also be observed when sawing with a core drill. At the entrance of the hole, delamination may occur if the cutting angle is too small and the cutting edge is wedged between two laminates [54], Fig. 2-8. Zachrisson et al.

[55] developed a new method (KTH-method) which gives defect free holes.

Failure analysis of glass woven roving composites of bolted connections with drilled and moulded-in holes were examined by Lin & Tsai [56]. They found that, laminates with moulded-in holes are stronger when the (e/dH =1).

Lamina

(a)

Interface

(b)

Figure 2-8. Delamination at (a) the exit and (b) the entrance sides of the hole.

The influence of bolt spacing (i.e. pitch distance, row spacing, end distance, and bolt diameter) and the degree of laminate anisotropy on the bolt load distribution and failure prediction in multi-fastened composite joint was investigated numerically by Sergeeva et al.[57] and Chutima and Blackie [58].

Failure loads for two laminate lay-ups [10/60/30] and [25/50/25], with fastener diameter specified as D= 8 mm and varying spacing, S, are shown in Fig. 2-9.

There is no notable interaction among the stress concentrators for the case of distantly placed fasteners, and the relationship between bolt spacing and failure load is approximately linear. As the fastener positions are closer to each other and the high stress areas near the fastener holes merge, the joint strength starts to decrease. The results indicate that the optimal bolt spacing may be different

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for different laminate lay-ups: in the case of the quasi-isotropic lay-up [25/50/25], the maximum joint strength is achieved at S/D = 3, while for the [10/60/30] laminate the optimal fastener spacing corresponds to S/D=3.5.

Figure 2-9. Variation of the failure load for different fastener spacing [57].

The effect of edge geometry (i.e. round or rectangular, see Fig. 2-10), stacking sequence, and clamping force on failure modes in mechanically fastened joints in FRP was studied by several researchers [59,60] The lateral clamping pressure suppresses the onset of delamination and continuously suppresses the propagation of interlaminar cracks. The failure mode changes from a catastrophic mode to a progressive one. Consequently, the lateral clamping pressure increases both the delamination and ultimate failure strengths of bolted joints in composite materials [59].

(a)

(b)

(c)

Figure 2-10. Types of the failure modes: (A) Net-tension failure mode (B) Shear-out failure mode (C) Bearing failure mode [60].

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It was found that, either bolt-to-washer or bolt-to-hole clearance can affect the initial failure loads but not the ultimate failure loads [61, 62].

Starikov and

Schon [63] found that the amount of load carried by the part with ordinary holes compared to the part with countersunk holes increased during fatigue loading. The contact stresses in the cylindrical part of the holes are probably larger in the plate with countersunk holes than in the part with ordinary holes since the cylindrical part of the hole is less for countersunk holes than for ordinary holes. This probably results in more fatigue damage in countersunk holes than in ordinary holes and the applied load is redistributed to the plate with ordinary holes. Figure 2-11 shows joints with protruding head bolts and countersunk head bolts that failed by bolt failure [64]. In all cases of bolt failure, the bolt failed at the threads. It can be seen that extensive bearing damage existed at the hole before the bolt failed. The bearing damage at the shear plane appears more extensive in the countersunk case, which is most likely due to the bearing load being taken almost entirely by the cylindrical portion of the hole in the countersunk laminate.

(a) protruding head bolt

(b) countersunk head bolt Figure 2-11. Final failure of protruding head and countersunk head bolts.

20

3. BONDED JOINTS The key advantage of adhesive bonded joints over other joint approaches, e.g. mechanical fasteners (bolting or riveting), is that it enables the development of large, cost-effective, and highly integrated structures [37]. Adhesively bonded joints have high structural efficiency and are used extensively to join composites in advanced aerospace structures. Bonded joints can be between two composite laminates or between a composite laminate and a metal/concrete structure. Adhesively bonded joints can distribute load over a much wider area than mechanical joints. Since no holes are required, the risk of local delamination is practically eliminated. Compared to bolted joints with weight of the joint is significantly reduced. On the other hand adhesively bonded joints cannot be disassembled without destroying the substrate. Some adhesives are susceptible to degradation by temperature and humidity. The most critical drawback for safety critical structure such as airframes is their inspectability.

Critical joints may require ultrasonic inspection over their entire area.

Corrosion can be problem in carbon fiber composite to steel/aluminum joints due to galvanic action. In such cases an intermediate insulating layer can be used. Some of the most common types of adhesively bonded joints are illustrated in Fig. 3-1. In addition to the lap and strap joints that are used in bolted joints additional types are possible with adhesive bonding such as the stepped joints and scarf joints [3,65,66].

Single-lap joints are efficient at transferring in-plane shear, but they should not be used for compression loads unless the joint is stabilized because the eccentricity increases as the load increases in compression. Adhesively bonded doublers transfer load through the adhesive in the same manner as other structural joints. The adhesive between the flanges of a stiffener and a skin is loaded similarly to a bonded doubler case. The single-strap butt joint has two disadvantages caused by the eccentricity in which the members butt together.

These disadvantages are (1) high bending moment in the splice plate and (2) high adhesive peel stress, which may lead to either premature failure of the

21

Single lap Double lap Double strap Stepped lap

Double stepped lap

Single scarf

Double scarf

Figure 3-1. Common adhesive joint configurations. joint or delamination of the composite members. Thick adherends in single-lap joints should have the overlap ends tapered. This reduces adhesive peel stress so that the adhesive shear strength can be fully developed. The taper also reduces the magnitude of the peak in the shear stress distribution, and when both overlap ends are tapered, the strength of the joint increases.

Double-lap and double-strap joints are balanced stiffness designs: that is, the stiffness of a strap or splice sheet should be equal or slightly higher than onehalf of the central sheet stiffness, and the two straps have equal stiffness.

Again, these straps and splice plates should be tapered at the ends to reduce the adhesive peel stress. A maximum taper tip thickness of 0.5 mm for the splice 22

sheets. Another detail that reduces the adhesive peel stress and the peak shear stress is locally thickening of the adhesive at the tips of the splice plates and doublers. The stress condition at the end of the central sheet remains constant with these modifications, but it is possible to increase the adhesive thickness at this point by slightly tapering the end of the central sheet. Tension loads on the joints do not generate adhesive peel stresses at the end of the central sheet because these are compression stresses.

The stepped-lap bonded composite joint strength is improved when the adherend stiffness is balanced, that is, a constant stiffness joint. The tip and end thicknesses should be limited. The length and number of steps are additional design variables, and the number of steps has the most effect on joint strength.

Step-lap joints have been used to bond composite to metal adhesively where high magnitude concentrated loads are transferred, and the composite cannot withstand the high local stresses associated with mechanical fasteners.

Adhesively bonded stepped-lap joints of composite to metal can be found in the tails of the F-14, F-15, F-16, and F-18 and in the wings of F-18 aircraft. Scarf joints are usually considered uniformly strained. In practice, however, the tips of the adherends have a finite thickness and they must be analyzed as an approximation to a stepped-lap joint with a fine grid over the length of the overlap. In scarf joints that have adherends with different stiffnesses. there is a tendency for the thin tip of the stiffer adherend to fail in fatigue [16].

Adhesively bonded composite joints have the following three basic failure modes: 1. The failure is in the adherend outside of the joint, which fails at 100% of the adherend strength. This is the strongest joint. There is no adhesive failure.

2. The failure is caused by the shear strength of either the adhesive or the laminate/adhesive interface. This is the next strongest joint.

3. The failure is caused by peel loads either as failure of the adhesive or as delamination of the adherend.

23

For the first failure mode, the strength of the joint is proportional to the adherend thickness and no bending of the adherend is assumed. The loading can be tension, compression, or in-plane shear. For the second failure mode, the shear strength of the bonded joint is proportional to the square root of the laminate thickness. For the last failure mode, the peel strength is proportional to the quarter-power of the laminate thickness.

New joint designs were proposed for adhesive bonding of thick multi-layered composite adherends by Bahei-El-Din and Dvorak [67] to reduce or eliminate the failure modes associated with delamination and tensile and/or shear failure of the surface plies that are often observed in lap joints, and provide for a better stress distribution in the adhesive. In contrast to lap-joint designs, which transfer in-plane tensile stresses and other loads from the adherends to doubler plates by out-of-plane shearing of the surface plies, the new joint configurations transfer most of the load by in-plane shear and normal stresses, through bonded inserts or interlocking interfaces which have the same thickness as the laminate adherends, as shown in Fig. 3-2. Doublers will

δ=15 b=15

transfer a calculated percentage of the load.

b

A

d,h

c,g

b,f

3td δ=0.2

l1

l=70

tA=30

A

l2 δ 3td=9

Dimensions in mm

X3 td=3

X1

a

b

e

f

X1

td=3

Section A-A

Figure 3-2. New design with diamond-shaped inserts [67]

24

Avila and Bueno [37] addressed the advantages and disadvantages of the wavylap joint proposed by Zeng and Sun [68], Fig. 3-3, and they modified it. The results showed an average increase on loading to failure close to 41%. This fact could be due to the compressive stress field developed inside the wavy-lap joint. In addition, this stress field distribution can also be the reason for the adherent delamination observed on the wavy-lap joints. So far, the modiffed wavy-lap joint seems to lead to stronger joints.

101.6

Tab

12o

6.15

Adhesive

2.80

12o

8.40

24.6

25.40

6.15

227.8

Figure 3-3. Wavy-lap joint main dimensions.

3. 1. Stresses in Adhesive Joints The single lap joint, in which two sheets are joined together with an overlay, is one of the most common joints encountered in practice. The joint is easy to make and the results are sensitive to both adhesive quality and adherend surface preparation. The simplest analysis considers the adherends to be rigid and the adhesive to deform only in shear. If the width of the joint is b, the length λ, and the load P, then the shear stress

τ

is given by:

τ

= P/(bλ).

However, the adherend tensile stress will decrease linearly to zero over the joint length from A to B, as shown in Fig. 3-4(a). In Fig. 3-4(b) is shown a similar joint but in which the adherends are now elastic (i.e. deformable). For the upper adherend، the tensile stress is a maximum at A and falls to zero at B. Thus, the tensile strain at A is larger than that at B and this strain must progressively reduce over the length λ. The converse is true for the lower adherend. Thus, assuming continuity of the adhesive/adherend interface, the uniformly sheared parallelograms of adhesive shown in Fig. 3-4(a) become

25

distorted to the shapes given in Fig. 3-4(b). This phenomenon is called differential shear. Essentially, this is what Volkersen analyzed in 1938 (the first one proposed a simple shear lag model based on the assumption of onedimensional bar-like adherends, i.e. the adhesive deforms only in shear, while the adherend deforms only in tension) [see Refs. 3 and 69]. The linear elastic analysis by others and the elasto-plastic analysis (Hart-Smith’s solution) including the end effects of lap joints are reviewed in Ref. 3. There are many different forms of structural joint, but most transmit essentially collinear loads such as the lap joints. More complex configurations exist, such as corner or Tjoints, but these are very difficult to analyze.

A

B

P

P (a)

L

Ď„ A

P

x

B P L

(b)

Ď„ x Figure 3-4. Exaggerated deformations in loaded single-lap joint: (a) with rigid adherends; (b) with elastic adherends.

In general, failure takes place in the adhesive (called cohesive failure) rather than between the adhesive and the adherend or substrate (called adhesive failure). Adhesive failure is often due to environmental degradation and cannot be generally analyzed, although it is possible to treat specific cases, providing simplifying assumptions are made. Peel loads are the greatest enemy of the

26

designer of bonded joints. Wherever possible, the adhesive should be loaded in shear so that peel and cleavage stresses are avoided. Increasing the width of lap or peel joints increases the strength pro rata, whereas increasing the length is beneficial only for very short overlaps. However, the benefits to be gained from having a large area of lightly stressed material in the middle of the joint, especially when creep and fatigue and faulty manufacture need to be taken into account. Figure 3-5 shows various ways of joining sheet loaded in tension. The “loading'” rating is indicative of how well the joint will withstand the applied load and the “cost” is an estimate of the assembly, cutting and purchasing costs. Great accuracy is not claimed for the numbers quoted as these will vary with adherend material and thickness and the choice of adhesive.

Rating Load Cost 1

1

10

2

15

1

15

7

7

3

10

2

12

4

2

2

10

4

10

8

Figure 3-5. Joining of sheet materials in tension; the higher the load rating, the stronger is the joint.

27

3.2 Standard Mechanical Test Procedures Since adhesives were first used, there has always been a need to define a series of tests which can be carried out in order to quantify (or at least qualify) their suitability in technology. Various tests have been proposed and some are still in use today for structural adhesives which have been developed from the fabric and wood-working industries. The wide variety of standard test procedures as listed by the International Standards Organization (ISO), European Standards (EN), American Society for Testing and Materials (ASTM), British Standards Institution (BSI) and other organizations are essentially for testing adhesives and surface treatments rather than joints. Unfortunately, most if not all of these standard tests consist of joints in which the adhesive stresses are far from uniform [3]. Let us therefore examine the reasons why we might need to carry out any form of test on an adhesive, other than the not unworthy cause of curiosity. The most commonly used test is the single-lap joint illustrated in Fig. 3-6(a). These dimensions are as specified by ASTM D 1002 (very similar to ISO 4587) which also specifies the adherend materials. It is recommended that the specimens be cut from a 177mm wide bonded plate since this gives the most representative results. The outer strips should be discarded. Note that this joint is automatically misaligned before it is placed in the testing machine.

Some laboratories bond tabs at the ends to improve alignment, as shown in Fig. 3-6(b). Even so, under load the joint will bend as shown in Fig. 3-6(c), giving rise to large transverse peel stresses in the adhesive layer. As known, the adhesive shear stress is nonuniform owing to differential straining in the adherend and other factors: this is illustrated schematically in Fig. 3-6(d) together with the associated adhesive peel (transverse) stresses in Fig. 3-6(e).

Even though it is still recommended in ASTM D 1002 and similar standards that the results be given as the average shear stress at failure (i.e. load divided by bond area), it has long been recognized that this average shear stress bears little relationship to what is actually happening in a joint, especially when geometric, adhesive and adherend nonlinearities become significant.

28

1.62Âą0.125 F

F

25.4 63.5 101.6

12.7Âą0.25

Grib area

(a) Single-lap joint F

F Alignment tabs

(b) F

F

(c) F F

Average shear Stress = F / Area

Adhesive Shear stress

(d) Transverse adhesive stress (acting across the bond-line thickness)

(e) Figure 3-6. Single-lap joint test piece to ASTM D 1002 (dimensions in mm).

29

The advantages of the single-lap test are that it is simple, cheap, uses a standard tensile testing machine, and there are a lot of data available for comparison. Its main disadvantage is that the reported average shear stress is not an intrinsic adhesive property. Curiously, this very disadvantage is also an advantage since no-one really believes that the average shear stress means anything fundamental. In other tests with apparently more precisely controlled conditions, it is common for misleadingly definite values to be quoted. Also, the complex stress situation in the single-lap test makes it quite representative of many structural applications and loading situations. This test is widely used, often abused, but remains one of the most trusted standards. Nevertheless, there have been many attempts to improve the single-lap test. These include the laminated assembly (ASTM D 3165) and sometimes wrongly referred to as a double-lap joint.

3.3. Fracture Mechanics Considerations Adhesive joint studies proposed in the literature usually fall into two classes: those based on stress analysis and those based on fracture mechanics. Fracture mechanics has become a very popular tool for the characterization of adhesive joints in recent years. Fracture mechanics tests are routinely conducted by industry during materials development and have also found extensive application in fatigue and durability studies over the past 20 years. More recently, fracture mechanics data has been used to predict the impact failure response of, for example, the impact wedge peel test and currently fracture mechanics data are finding application in structural impact studies via the use of cohesive zone models. The use of Linear Elastic Fracture Mechanics (LEFM) tests to measure the mode I adhesive fracture energy, GIC; of adhesive joints dates back to the work of Ripling and co-workers in the 1960s who developed a mode I test method to measure the toughness of structural bonds between metallic substrates. This work led to the publication of an ASTM standard in 1973 [see Ref 70]. Since then, the many developments in the application of fracture mechanics to, for example, FRP composites has created

30

great potential for the development of a new test protocol for structural adhesive joints. It was against this background that a technical committee of the European Structural Integrity Society (ESIS) commenced work on structural adhesives test methods in 1997. The new protocol accommodated the use of both metallic and FRP composite substrates. During the course of the wide ranging program involving ten test laboratories, modifications were made to both the experimental and analytical procedures compared to the original ASTM standard of 1973. For example, a new corrected beam theory analysis for the tapered double cantilever beam was developed and a correction for system compliance and additional validity checks were built into the experimental procedure. Following these modifications, the revised protocol was submitted to the British Standards Institution for consideration as a British Standard (under the direction of the ‘‘Adhesives Standards Policy Committee PRI/52’’) and was accepted and subsequently published in 2001. It is intended that this document should also be published as a European standard. Blackman et al. [70] described the stages in the development of this new protocol and the modifications made in the light of the results from the round-robin tests. The results from the inter-laboratory round-robin highlighted the importance of correcting for system compliance effects if accurate and reproducible results are to be obtained. The values of GIC deduced were shown to be independent of test geometry but dependent upon the substrate material used to make the joints. Additional studies have shown that the substrate dependence was due to the cured adhesive in the different joints possessing different values of glass transition temperature. The existence of pre-bond moisture in the CFRP substrates and variations in heat-up rate during cure were both shown to affect the Tg of the cured epoxy-paste adhesive employed in the present work. The pre-bond moisture effect was however, much more important than the heat-up rate effect but both would need to be considered when optimizing joints for toughness with the present adhesive.

31

There are three types of delamination test to obtain pure mode I loading, pure mode II, and mixed mode I/II loading [71]. For the mode I Double Cantilever Beam (DCB), Fig. 3-7(a), if it is assumed that the compliance CI is a function of

(a + |∆|)3 where “a” is the crack length and ∆ is a correction parameter, a

first strain energy release rate expression is

GI =

3Pδ 2b(a + ∆ )

(3-1)

where P is the applied load, δ the crack opening displacement, b the specimen width. ∆ is determined from the plot of C1/3 versus a. Another compliance expression is the following: C = k an

(3-2)

where n is the slope of the ln(C) versus ln(a) curve. Therefore, the energy release rate expression becomes GI =

nPδ 2ab

(3-3)

For the mode II End Notched Flexure (ENF) test, Fig. 3-7(b), a first energy release rate formulation comes from beam theory:

G II =

9a 2 P 16b 2 h 3 E f

(3-4)

where P is the applied load, Ef is the flexural modulus such that L3 Ef = 4bh 3C o

(3-5)

32

in which C0 is the compliance obtained experimentally from a three-point bending test in the elastic range on the uncracked specimen. Another expression involving

GII =

may be used:

9a 2 Pδ 2b(2 L3 + 3a 3 )

(3-6)

If we assume that the compliance determined experimentally can be written as C = C0 + ma3

(3-7)

the energy release rate is expressed in the form

G II =

3ma 2 P 2 2b

(3-8)

The Mixed Mode Bending (MMB) test, Fig. 3-7(c), is the superposition of the two previous tests, and it allows any combination of mode I and mode II loadings to be obtained. It consists of a three point bending test, in which the load P is applied by means of a lever with a central fulcrum, at a distance c from this fulcrum. The load bends the MMB specimen at the fulcrum and at the same time pulls the delamination open, which gives a combination of mode II (bending) and mode I (crack opening) loadings. The mixed mode ratio I/II can be modified by changing the lever length c. An MMB test analysis is based on beam theory. The expressions for mode I and mode II energy release rate components are 12a 2 PI2 , b 2 h 3E 9a 2 PII2 G II = 16b 2 h 3 E

GI =

(3-9)

33

PI and PII are the pure mode loading components:  3c − L  PI =  P,  4L 

c+L PII =  P,  L 

(3-10)

where c is the lever length, and ¸ is half of the distance between lower loading points. The mixed mode ratio is defined by 2

G I 4  3c − L  , = G II 3  c + L 

c ≥ L/3

(3-11)

It is independent of the crack length a and it only depends on the lever length c.

P P

δ a

L

P

(a) DCB test configuration.

P

L

a

δ

(b) ENF test configuration.

c

L

2h a L

L

(c) MMB test configuration Figure 3-7. Test configuration of three different types of delamination test.

34

3.4. Fatigue behavior and Durability of Adhesive lap joints The response of bonded joints to fatigue loading has been extensively researched in the last years concluding that the failure generally initiates at the adhesive [72,73] for moderate stress levels. The fatigue life is strongly influenced by the profile of the edges of the joint. The initiation of small cracks at the edges points represent the major part of the fatigue life. The static and fatigue properties of polymeric adhesives can be influenced by centered defects [73,74] and environmental conditions such as: temperature, moisture and chemical agents [72]. It has been shown that thermal effects, whether due to mismatch of the adherends or to adhesive contraction by temperature of cure, lead to significant changes in the stress state of lap joints. The temperature Tg of the adhesive should be above the maximum temperature expected in service.

It has been also verified that the strength of joints usually decreases in presence of humidity and time of exposure. Also the debonding of adhesives caused by cleavage of cracks increases with the water presence in form of liquid or vapor.

This is the result of the hydrophilic nature of adhesives, which is caused by the polar groups, needed to confer adhesive properties on polymeric materials.

Therefore chemical degradation of the adhesive, substrate and chemical bonds across the interface is possible as a result of interaction with water. The water can enter in the adhesive and then attack it by diffusion through the adhesive and adherent, and finally transport along the interface and move by capillary action through cracks in the adhesive. The moisture can not only affect the adhesive but also the mechanical behavior of the adherend composite. However no significant effect of moisture was observed on the static strength and fatigue crack growth of laminate composites.

3.5 Prevention of Peeling failure in Adhesive Joints Where peel is encountered with lap joints, or even to counter the peel loads inherent in loading even in the double-lap joint, various techniques can be used; these are illustrated in Fig. 3-8. Of these, the best is probably the positive constraint of the joint end by riveting, bolting or spot welding [3]. However,

35

the arrangement shown in Fig. 3-8(d) is also common and is used in the construction of nearly all motor car doors, the joint being called a clinch or hem-flange join. Where lap joints are subjected to bending moments a rivet, bolt or spot weld near the end A, where the joint tends to open in peel, should be used if possible, as shown in Fig. 3-9(a). At the other end B, the adhesive is in compression and failure will not occur there. Alternatively, an overhanging reinforcement plate may be used, as shown in Fig. 3-9(b). This has the advantages of reducing the bending strain in the upper adherend, thus strengthening the joint, and transferring the load further into the joint [3].

Peel action

Increase area Rivet, bolt or spot weld

(b)

(a)

Increase stiffness

Bead end (if possible)

(c)

(d)

Figure 3-8. Techniques for combating peel.

36

B (a)

A Reinforcement B

(b)

A

Figure 3-9. Combating bending loads in lap joints using (a) a rivet, and (b) overhanging reinforcement.

3.6 Adhesive Bonded Dissimilar Materials The use of adhesive bonded structure is significantly increasing recent years. It is well known that mechanical properties of the bonded structure depend on the surface conditions of the substrates. Accordingly, many studies have been made on the surface treatment of polymer, ceramic, and metallic substrates to improve the mechanical properties of FRP/metal, FRP/concrete, steel/concrete, metal/metal and metal/ceramic bonded structures [75-84]. One appealing way to bond composite parts to steel structures is to mold a steel edge into the composite, and then weld this assembly to the steel structure via this edge. This requires that the steel can successfully be molded into the composite during the manufacturing. The strength of these joints can be increased [76] by using perforated steel, as patented by Unden and Ridder [77]. The perforations will decrease the elastic mismatch between the stiff steel and the relatively compliant FRP, and as an added benefit provide mechanical interlocking.

37

Co-cured joining method [78], which is regarded as an adhesively bonded joining method, is an efficient joining technique because both the curing and joining processes for the composite structures can be achieved simultaneously.

The co-cured joining method requires neither an adhesive nor a surface treatment of the composite adherend because the excess resin, which is extracted from composite materials during consolidation, accomplishes the cocured joining process. Since the adhesive of a co-cured joint is the same material as the resin of the composite adherend, the analysis and design of the co-cured joint for composite structures are simpler than those of an adhesively bonded joint, which uses an additional adhesive.

The manufacturing process of the specimens of co-cured single and double lap joints of the plate type with steel and composite adherends was introduced by Shin et al. [78]. They found that, the initial failure mechanism of the co-cured single lap joint was interfacial failure between steel and composite adherends and the failure mechanism of the co-cured double lap joint was cohesive failure by delamination at the first ply of the composite adherend. Out-of-plane tensile and shear stresses played an important role of the interfacial failure of the cocured single lap joint and only an out-of-plane shear stress played a most important role of the cohesive failure of the co-cured double lap joint.

Dvorak et al. [79] explored a new approach for the joining of thick, woven Eglass/vinyl-ester composite laminated plates to steel or other composite plates, with applications in naval ship structures. Adhesive is applied along throughthickness contoured interfaces, employing tongue-and-groove geometry. They found that adhesively bonded tongue-and-groove joints between steel and composite plates loaded in monotonically increasing longitudinal tension are stronger than conventional strap joints even in relatively thin plates.

Melogranaa et al. [80] studied tongue-and-groove geometries, see Fig. 3-10, in thin composite laminated plates. Adherends were comprised of 2.7 mm thick

38

stainless steel and 1.6 mm thick T700/SE84HT carbon fiber/epoxy composites.

The adhesive used was the two-component epoxy paste Hysol 9430. They compared their thin tongue-and-groove joints to conventional single lap joints manufactured with the same materials and similar dimensions. The tongue-andgroove specimens with large aspect ratio tongues were stronger than the conventional single lap joints.

w=25.4 [1.0]

tg=0.1 [0.039]

Rtip

Wgroove

Rmax

T700 carbon/epoxy

AL–6XN steel L=25.4[1.0]

Figure 3-10. The general geometric parameters for a tongue-and-groove joint.

So far, FRP has mostly been used for strengthening concrete structures [21].

Perhaps the single most critical parameter in strengthening reinforced concrete with externally bonded FRP/steel plate is the bond achieved between the reinforcing FRP/steel and concrete, which is responsible for the mechanism of stress transfer and therefore the composite behavior. Debonding of the FRP/steel plate from the concrete surface can significantly limit the potential flexural and shear strength enhancement provided by the FRP/steel to the repaired structure[81-84]. A brief review of this topic was presented by Nehdi et al. [81] and De Lorenzis et al. [84]. Figure 3-11 summarizes some of the experimental techniques used in the literature for testing the bond of FRP plates to concrete. These tests often involve direct tension or bending and therefore require special handling.

39

Figure 3-11. Various techniques for testing bond of FRP plates to concrete.

3.7 Fusion Bonding for Joining Thermoplastic Matrix Composites An ideal structure would be designed without joints, since joints are potentially sources of weakness and additional weight. In practice however, upper limit to component size is generally determined by the manufacturing processes.

Further requirements for inspection, accessibility, repair and transportation or assembly mean that load-bearing joints will be part of an engineering structure.

This is particularly so in the manufacturing of thermoplastic composites (TPC) for which high melt resin viscosity and constraints imposed by the continuous reinforcement limit the production to relatively simple geometry components which must be joined together to produce large, complex structures. The extensive experience available from the thermoplastic polymer (TP) industry showed that to make large or complex parts, the most cost-effective method often involves molding two or more parts and joining them together.

Thermosetting (TS) adhesive bonding is inherently preferable to mechanical 40

fastening because of the continuous connection avoiding large stress concentrations induced at each discrete fastener hole. However, there are a wide range of contaminants present on substrate surfaces [85]. These need to be eliminated prior to bonding by using a surface preparation treatment and increase the surface roughness (improving mechanical interlocking and increasing bonding surface area). Extensive surface preparation and long curing times make adhesive bonding labor intensive. Typical surface treatments used for adhesive bonding are generally hard to control and affect directly the strength and durability of bonded joints.

Fusion bonding, or welding, is a long established technology in the thermoplastic industry where the efficiency of the welded joint can approach the bulk properties of the adherends [85]. Although welding may induce residual stresses if performed without adequate control, it eliminates the stress concentrations created by holes required for mechanical fasteners and so does TS adhesive bonding. In addition, welding reduces processing times and surface preparation requirements. However, the high content in carbon fiber (CF) reinforcement in TPCs, resulting in high thermal and electrical conductivity, imposes difficulties such as uneven heating, delamination and distortion of the laminates. These problems become more difficult when bonding large components. In addition, as fiber volume fraction increases, the amount of resin available to melt and reconsolidate into a fused joint is reduced and this can affect the welding quality.

Fusion bonding techniques have often been classified according to the technology used for introducing heat. In this review article, fusion bonding techniques were classified into four classes, as shown in Fig. 3-12, namely bulk heating (co-consolidation, hot melt adhesives, dual resin bonding), frictional heating (spin welding, vibration welding, ultrasonic welding), electromagnetic heating (induction welding, microwave heating, dielectric heating, resistance welding) and two-stage techniques (hot plate welding, hot gas welding, radiant

41

welding). Bulk heating techniques such as autoclaving, compression molding or diaphragm forming are available for performing co-consolidation.

Co-consolidation is an ideal joining method as no weight is added to the final structure, no foreign material is introduced at the bondline, essentially no surface preparation is required and the bond strength is potentially equal to that of the parent laminate. However the entire part is brought to the melt temperature, and this generally implies the need for complex tooling to maintain pressure on the entire part and to prevent de-consolidation. Hot melt thermoplastic adhesive films may be inserted at the bondline to improve filling of parts mismatch. Inserting of an amorphous polymer interlayer proved to reduce the scatter of strength, which widens the processing window.

The dual resin bonding, or amorphous bonding, involves co-molding an amorphous TP film to a semi-crystalline TP matrix laminate prior to bonding as in the Thermabond速 process, in which a PEI film is coated onto APC-2 laminates. During the joining step, the amorphous PEI film can be fused at a temperature above its glass transition temperature below the melting temperature of the semi-crystalline PEEK polymer avoiding any deterioration of the bonded structure.

In two-stage techniques the heating device needs to be removed from between the substrate surfaces between the stages of heating and forging. This aspect involves limitations on size of the component since the whole welding surface must be heated in a single step. Heating times are normally long as they rely on the low thermal conduction of heat through the polymer. Between the heating and forging steps, surface temperature drops and the region experiencing the maximum temperature is located below the skin of the laminate. The high pressure required to consolidate the bondline may cause warpage/flow in the higher temperature inner region.

42

Spin welding and vibration welding have been extensively used in the plastics industry but are less appropriate to joining TPCs as the motion of the substrates relative to one another may cause deterioration of the microstructure, such as fiber breaking. The process was however investigated for joining APC-2 and GF/PP systems.

Microwave and dielectric welding are available for joining thermoplastics but the fact that heating occurs volumetrically and that multi-layer composites are excellent shields in the microwave range make these techniques poorly suitable to welding of TPCs particularly when they are reinforced by CFs. The three most promising fusion bonding techniques are ultrasonic welding, induction welding and resistance welding. In these techniques, only the welding interface is brought to the melt temperature, minimizing the impact on the rest of the structure. Welding times are very short. Large-scale welding may be performed through sequential or scanning approaches, and on-line monitoring of the consolidation is possible.

Fusion Bonding Bulk Heating

Frictional Heating

Electromagnetic Heating

Co-consolidation Hot-melt Adhesives Dual Resin Bonding

Spin Welding Vibration Welding Ultrasonic Welding

Induction Welding Microwave Heating Dielectric Heating Resistance Welding

Two-stage Techniques Hot Plate Welding Hot Gas Welding Radiant Welding

Infrared Welding Focused Infrared Welding Laser Welding Solar Energy

Figure 3-12. Fusion bonding techniques.

43

4. SNAP JOINT IN COMPOSITE STRUCTURES In the majority of the composite structural components, both bolted and/or adhesive bonded joint was used. Most of the details are similar to those for metal joints. It was shown from extensive testing on bolted composite joints that failure always occurs in a catastrophic manner due to high stress concentration developed at the bolt locations. Due to the inherent low bearing and interlaminar shear strengths of composites, these stress concentrations threaten the downfall of every piece of the composite structure [86-87]. The optimum composite joint design is the one capable of distributing stresses over a wide area rather than to concentrate them at a point. Adhesively bonded joints can satisfy these requirements, however, most of the adhesives are brittle, and brittle failure is unavoidable. This was the motivation of developing what is called the SNAP joint, Fig. 4-1.

A

B

Snap Joint Concept

Hardware for fasternerless snap joint

Figure 4-1. Snap joint.

The snap joint technology developed by W. Brandt Goldworthy & Associates, Inc. [86]. The concept is based on similar joining technology used for connecting wooden parts. Also, this technique is very similar to techniques which were used a decade or so again for plastic. Figure 4-1 shows a pultruded structural composite member (A) with one end shaped as a fir-tree, and therefore has a large load bearing area. In this figure, part (A) has been snapped into another structural shape (B). From Fig. 4-1 one can see that, the later shape has been designed to combine its structural shape with functionality that allows

44

for the engagement of the load-bearing surface of member (A). It is possible to "snap" joint both parts together since part (A) has been cut for a short distance along it length to provide enough lateral flexibility to move out of the way when entering part (B). In order to make this joining concept successful, the fiber architecture of part (A) must be designed in such a way that the load bearing surfaces have higher interlaminar shear strength capacity. Also, it can be noticed from the figure, that a circular hole was introduced at the end of the horizontal slot of part (A) to inhibit the crack propagation along the length of the pultruded member. The applications of this technology in composite structures will have benefits as follow: • The structures are easy to assembly.

• Installation of structure members become faster.

• Installation needs smaller number of labor and equipment.

• Since it use composite materials, its weight is less than traditional structures.

The first prototype or "Demonstration project" using this joining method was in designing and construction of three Transmission Tower Structures, Fig.4-2(a), near Los Angeles by W. Brandt Goldworthy & Associates, Inc. and Ebert Composites

Corporation

[86].

Through

California

Department

of

Transportation, they have proposed the design and the construction of a truss structure to carry highway singes, Fig. 4-2(b).

The snap joining technique is considered to be one of the optimum techniques to join composite structural members. However, it has a major limitation, and can only be used in specific applications. That is, this method can only be used to transmit axial loads, which make it ideal for truss-type structures. However, in my opinion, this method should NOT be recommended when out-of-plane loads or any shear loads are introduced since the connection is not design to carry any major bending moments. Under flexural loads, it is expected that the joint will be very flexible, and the artificial cracks introduced to members will

45

propagate and a complete failure will occur even under moderate service flexural and/or shear loading.

(a) Composite Transmission Tower

(b) Truss Joint

Figure 4-1. Snap joint projects.

5. HYBRID JOINTS In an attempt to improve the joint strength of composite materials, a hybrid of adhesive and bolted joints has also been explored [88], Fig. 5-1. Hybrid joints failed at a higher load than the bolted joints and with the proper clamping torque reached the same failure load as the adhesive joints. Furthermore, unlike the adhesive joints, hybrid joints failed in two steps, first by initiation of fiber tear (akin to delamination in laminated continuous fiber composites) at one of the lap ends and then by tensile failure across the bolt hole. This led to a slightly higher overall elongation at failure for specimens with the hybrid joints. Failure in fatigue also started by fiber tear and when the fiber tear progressed to the bolted area, a combination of half-net-tension failure and splitting (cleavage failure) occurred. In both static as well fatigue, failure was initiated by fiber tear and the round washers with their edges located slightly away from the lap ends were not effective in preventing fiber tear. Fu and Mallick [88] found that, hybrid joints give better static as well as fatigue

46

performance than adhesive joints in structural reaction injection molded composites when fiber tear, the primary failure mode in adhesive joints, is either prevented or delayed by the presence of clamping. Their finite element analyses proved that the presence of the lateral clamping pressure can significantly decrease the maximum peel stress at the interface, which helps in achieving improved joint performance.

Figure 5-1. Sketch of an adhesive/bolted (hybrid) joint.

5.1. Hybrid Joints in Plated Beams The main drawback of the RC member strengthened by externally bonded plates is the peeling failure [89,90], Fig. 5-2. Several researchers suggested analytical models to predict the critical tensile and shear stresses at the ends of glued plate at the bottom face of the RC beams. This means that, the peeling crack may grow under mode I, opening mode, or mixed mode, opening and sliding modes. The anchorage applied at the ends of externally bonded plate may take several forms, including bolted anchorage systems, bonded angle sections to provide anchorage of the reinforcing plate to both sides of the beam, and trapping the plate under the beam supports, Fig. 5-3.

Figure 5-2. Cover separation failure mode.

47

Adhesive-filled bore CFRP plate

Bearing pad Support roller

a: Plate ends under supports CFRP plate

Anchorage block

Mild steel bolt

b: Plate ends bolted to beam

Figure 5-3. Plate end anchorage systems adopted by Garden and Hollaway [see Ref. 90] Saba [89] suggested two different techniques to preclude the peeling failure at the ends of steel plates glued to the soffits of reinforced concrete, RC, beams.

In the first technique, concrete cover was replaced by grout to enhance the resistance of substrate to crack initiation and propagation. In the second one, permanent compressive forces at the ends of bonded plate were inserted using different plate end anchorage systems, i.e. end anchorage by bolts or by clamps, Fig. 5-4.

P/2

P/2

Grout

35 cm 5 cm 25 cm 2m

P/2 80 cm

P/2

70 cm

80 cm

(a) 15 mm St.

P/2

P/2

6 Studs 14 mm

25 cm P/2

P/2

2m 80 cm

70 cm

80 cm

(b) Figure 5-4. Plated beam with (a) partial replacement of concrete cover, and (b) plate end anchorage by steel clamps 48

Schnerch et al. [91] used a steel angle to clamp the fiber to the steel base plate before the pot life of the epoxy had been reached as shown in Fig. 5-5.

Figure 5-5. Steel Angles used to mechanically anchor CFRP plys.

Figure 5-6 shows the upgrading techniques, which used hybrid joint concepet, for reinforced concrete beam窶田olumn joints proposed by El-Amoury and Ghobarah [92].

Steel plates

U-shaped steel plates

Steel plate FRP sheets

FRP sheets Steel rods

Figure 5-6. Retrofitting schemes

49

Steel angle

6. SUMMARY 6.1 Mechanical Joints (Bolts or Rivets) Mechanical joints are used when repeated disassembly and reassembly is required or when surface preparation is not practical. Unfortunately, well-established joining technologies for metallic structures are not directly applicable to composites for several reasons, e.g.: • Stress concentration created by the presence of holes and cut-outs, which is worsened by the lack of plasticity limiting stress redistribution.

• Delamination originating from the localized wear occurring during drilling.

• Differential thermal expansion of fasteners relative to composite.

• Water intrusion between fasteners and composite.

•Possible galvanic corrosion at fastened joints.

•Additional weight of fastening system.

Evolution of Mechanical Joints In mechanical joining, there seems to be a distinct shift toward integral attachment using features either designed into or formed into the parts to be joined: Snap-fit integral features are beginning to appear in composites:

The use of so-called “hook-and-loop” attachments (likeDuPont VelcroTMand 3M Dual-LocksTM) will find greater and morediverse application:

Tog-L-Loc®

Die side "Button"

Cross Section Punch side

A generic hook-and-loop concept representing DuPon's Velcro

TM

6.2 Adhesive Bonding This joining technique is inherently preferable to mechanical fastening as the continuous connection avoids large stress concentration induced at each discrete fastener hole. The 50

presence of a wide range of contaminants on substrate surfaces (release agents and bagging materials, fluorocarbon release sprays and films, machining oils, fingerprints, etc.) makes however necessary a preliminary surface preparation treatment, which may also serve to: • Improve wetting of low energy surfaces.

• Chemically modify the surfaces (introducing polar groups or coupling agents).

• Increase the surface roughness.

Adhesive Bonding Benefits • It is still possible to make joints not only between two composite laminates, but also between a composite laminate and a metal, or a concrete, or a wood structure.

• Loads are distributed over a much wider area than in the case of mechanical joints.

• Since no holes are required, the risk of local delamination is practically eliminated.

• Significantly reduced weight of the joint.

• Possibility to make a wide range of configurations (e.g. stepped or scarfed joints).

Adhesive Bonding Drawbacks • Adhesively bonded joints cannot be disassembled without destroying the substrate.

• Some adhesives are susceptible to degradation by temperature and humidity.

• Poor inspectability(critical joints may require ultrasonic inspection over their entire area).

• Corrosion can be a problem due to galvanic action.

• Typical surface treatments are generally hard to control in an industrial environment and directly affect the strength and durability of bonded joints.

6.3 Hybrid joints Hybrid joints give better static as well as fatigue performance than adhesive joints in composites when fiber tear, the primary failure mode in adhesive joints, is either prevented or delayed by the presence of clamping. The presence of the lateral clamping pressure can significantly decrease the maximum peel stress at the interface, which helps in achieving improved joint performance. The square washer representing full clamping to the edges of the overlap area, gives a better performance compared to round washer, representing partial clamping.

51

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