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Short Paper Proc. of Int. Colloquiums on Computer Electronics Electrical Mechanical and Civil 2011

Deformation Strength and Failure Mode Analysis of Laminated Composites Subjected to Impact Loading First A. H.K.Shivanand1, Second B. B.Anil Kumar 2, Third C. Sapthagiri.G3, Fourth D. Sadashiva.M4 1,2,3,4

University Visvesvaraya College of Engineering, Mechanical Engineering Department, Bangalore, INDIA Email: shivanand.uvce@gmail.com, sapthagiri.uvce@gmail.com

Abstract: During the last years, criteria of automobile and aircraft projects have been more and more rigorous for component developed in order to absorb impact energy. Research on the development of structural components with high crashworthiness has been carried out not only by the automobile and aeronautics industries, but also by naval, trains and elevators industries. The concept for structural components with high crashworthiness depends on the crash resistance. The crash resistance concept is based on the energy absorption capacity and structural integrity. For developing a project that reaches these requirements, it should change the material and/or architecture of the component. However, changes in the architecture can cause increase of costs and/or of weights, reducing the performance of the structure. The weight increase is not attractive for the aircraft development, because it reduces the aircraft performance. Nowadays, many kinds of components have been made using composite materials, because these materials can absorb a high amount of impact energy and can guarantee the survival of the passengers. However, the dynamic behavior of composite laminates is very complex, because there are many concurrent phenomena during composite laminate failure under impact load. Fiber breakage, delaminations, matrix cracking, plastic deformations due to the contact and large displacements are some effects which should be considered when a structure made from composite material is impacted by a foreign object. The dynamic behavior of composite laminates is very complex because there are many concurrent phenomena during composite laminate failure under impact load. Fiber breakage, delaminations, matrix cracking, plastic deformations due to contact and large displacements are some effects which should be considered when a structure made from composite material is impacted by a foreign object.

energy levels lower than those required to create visible damage. A similar behavior for graphite/epoxy honeycomb sandwich specimens was also shown. Since these studies, many have also confirmed, using different test methods and material systems, the marked susceptibility of sandwich structures to damage caused by the low-velocity impact of foreign objects. This type of barely visible external impact damage has been demonstrated to substantially reduce the tensile, compressive & bending strengths of the sandwich construction. In this project, qualitative and quantitative studies of the effects of low-velocity impacts on bi-woven GFRP honeycomb structures are performed. The test specimens are fabricated using traditional manufacturing methods. The specimens are then impacted using a FallingWeight impact test apparatus. A variety of impact-related quantities are related to the impact energy and conclusions are drawn. This segment of the report provides a brief account about composites, sandwich panels & how a sandwich beam actually works. II. LITERATURE SURVEY Over the years, the performance of composite materials in secondary aircraft/aerospace structures has shown superiority over metals. Currently, there is increasing interest to use composites for primary structures for higher weight savings and potential cost reduction. For this reason, a stringer-stiffened panel, with the potential to be used in aircraft fuselage structures, is considered in this research. A major concern in the use of fiber reinforced composites is their susceptibility to damage resulting from the effects of impact loads. Damage due to the low-velocity impact from accidents such as dropped tools or rough handling during maintenance may be undetectable by visual inspection but has the potential to alter the local composite stiffness and strength considerably. Typically, impact velocity of less than 5 m/s and mass less than 5 kg are considered under low-velocity impact. Tests conducted on coupons have shown that the damage can lead to reductions of up to 50% in the compressive properties [1-2]. Tests conducted on stringerstiffened panels show a reduction in compressive strength as high as 20% [3]. The aerospace industry has used laboratory drop tests on coupon size specimens to explore the nature of impact damage and to seek ways of minimizing it. However, for a given material the coupon test may be a poor guide to the performance of a real structure [4]. The ability of a large flexible structure to store more energy elastically plays a crucial role for a material whose fibers and resin are much more brittle than aluminum alloys. Hence, the

Index Terms: Composite Laminates, Failure Modes, Impact loading, E-glass Fiber, Epoxy Matrix.

I. INTRODUCTION Polymer composite sandwich panels are being utilized increasingly as primary load-carrying components in aircraft and aerospace structures. Serving in this capacity, these structures are subjected to impacts such as tool drops, hail, bird strikes, and runway debris. Unlike their solid metallic counterparts, predictions of the effects of low-velocity impact damage are difficult and are still relatively immature. For this reason, experimental studies have been performed to characterize the damage created by low-velocity impacts. These studies have been useful both for phenomenological classification and for analytical comparisons. Early work with graphite/epoxy and Kevlar/epoxy honeycomb sandwiches revealed that significant internal damage is achieved at impact ÂŠ 2011 AMAE DOI: 02.CEMC.2011.01. 512

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Short Paper Proc. of Int. Colloquiums on Computer Electronics Electrical Mechanical and Civil 2011 dynamic response of an impacting mass will be quite different if a panel is supported by stiffeners, spars, ribs, frames and so on. One way of establishing the response or damage of composite structures due to impact loads is through an intense structural testing program. This option is very time consuming and costly for fully representative panels. A numerical model of a realistic structure would be beneficial in order to limit the impact tests at all structural levels and support design and maintenance of composite structures. Impact events involve contact between the impactor and the target. The response of the composite structure under impact is dynamic in nature and depends upon the progressive failure of the laminate. Numerical modeling of such a phenomenon is therefore complex and requires progressive degradation to simulate loss in stiffness. Explicit finite element codes with contact and failure definitions for composites offer the means of analyzing damage in composite structures due to impact load. Based upon experimental results low-energy impact results in four major damage modes, viz. contact damage delamination, matrix failure and fiber failure. The exact sequence of events is difficult to ascertain because of the large number of parameters involved and small duration of the phenomenon. The general description of the sequence of events is illustrated in Fig. 1 showing the occurrence of damage in four stages with each stage absorbing different amounts of impact energy [4-5]:

fibers, along with resins and epoxies are common materials laminated together with a vacuum bag operation. A polyurethane or vinyl materials are used to make the bag, which is open at both ends. This gives access to the piece or pieces to be glued. A plastic rod is laid onto the bag, which is then folded over the rod. A plastic sleeve with an opening in it , is then snapped over the rod. This procedure forms a seal at both ends of the bag, when the vacuum is ready to be drawn. A “platen” is used inside the bag for the piece being glued to lay on. The platen has a series of small slots cut into it, to allow the air under it to be evacuated. The platen must have rounded edges and corners to prevent the vacuum from tearing the bag. All bags have access to the vacuum via a nipple which penetrates the bag. When a curved part is to be glued in a vacuum bag, it is important that the pieces being glued be placed over a solidly built form or have an Air Bladder placed under the form. This air bladder has access to “free air” outside the bag. It is used to create an equal pressure under the form, preventing it from being crushed.

III. MANUFACTURE OF COMPOSITE PANELS VACUUM BAG PROCESSING: Used for complex form panels. Vacuum bag molding, a refinement of hand lay-up, uses a vacuum to eliminate entrapped air & excess resin. After the lay-up is fabricated on either a male or female mold from precut plies of glass mat or fabric and resin, a non-adhering film of polyvinyl alcohol or nylon is placed over the lay-up and sealed at the mold flange.A vacuum is drawn on the bag formed by the film while the composite is cured at room or elevated temperatures. Compared to hand lay-up, the vacuum method provides higher reinforcement concentrations, better adhesion between layers, and more control over resin/glass ratios.

Figure 1. Final Composites Specimens

IV. EXPERIMENTAL INVESTIGATION Test apparatus details This test method is designed to provide load versus deformation response of plastics under essentially multiaxial deformation conditions at impact velocities. This test method further provides a measure of the rate sensitivity of the material to impact. Multiaxial impact response, while partly dependent on thickness, does not necessarily have a linear correlation with specimen thickness. Therefore, results should be compared only for specimens of essentially the same thickness, unless specific responses versus thickness formulae have been established for the material. For many materials, there may be a specification that requires the use of this test method, but with some procedural modifications that take precedence when adhering to the specification. Therefore, it is advisable to refer to that material specification before using this test method. Table 1 of Classification System D 4000 lists the ASTM materials standards that currently exist. The equipment /Impact Tester are designed as per ASTM D 3763. SPECIFICATIONS Height of Fall : 1.5 m Impactor mass : 2.5 to 12.5 kgs

Steps in vacuum bag operation A vacuum bag is a bag made of strong rubber-coated fabric or polymer film, open at one end & used to bond or laminate materials. In some applications the bag encloses the entire material or in other a mold is used to form one face of the laminate with the bag being single sided to seal the outer face of the laminate to the mold. The open end is sealed & air is drawn out with a vacuum pump. As a result, uniform pressure approaching one atmosphere is applied to the surfaces of the object inside the bag, holding parts together while the adhesive cures. The entire bag may be placed in a temperature-controlled oven, oil bath or water bath and gently heated to accelerate curing. Laminating of flat objects can be performed more efficiently in a heated laminating press, but when the objects are curved or have irregular shapes, a vacuum bag is normally used. Vacuum bagging is widely used in the composites industry- Carbon fiber fabric and glass © 2011 AMAE DOI: 02.CEMC.2011.01. 512

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Short Paper Proc. of Int. Colloquiums on Computer Electronics Electrical Mechanical and Civil 2011 Impactor tip : 12.5 to 25mm dia Impact velocity: 2 -6 m/s Impact Energy : 4.9- 180J Load Cell : 5000N Specimen Size: 150 mm x 150 mm Fully Computer Controlled An impact velocity measurement is through with the help of Optical Diodes. High Speed Data Acquisition System is built in to capture and analyze peak load, velocity, penetration and energy absorbed.

(SIZE – 150

150MM, THICKNESS - 3 MM) G LASS FIBER

V. EXPERIMENTATION Material testing The experimental process involves impacting a 150mm wide x 150 mm tall x 3mm thick for both glass fiber and graphite fiber specimens plate with an impactor made of mild steel mounted on a Falling Weight Impact Tester operated with the help of a compressor. The specimens were impacted at low velocities (<6m/s).This process was an effective way in determining the degree of fragmentation, & observing the amount of damage initiation, crack progression & propagation. A detailed description & documentation regarding the experimental results has been carried out & explained in the subsequent sections.

GRAPH 1. B1 GLASS FIBER 3mm

VI. RESULTS OF IMPACT SPECIMENS

TABLE I. RESULTS

3MM E-GLASS FIBER

T ABLE II PANEL A (SIZE – 150

150MM, THICKNESS - 2 MM) G LASS FIBER

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Short Paper Proc. of Int. Colloquiums on Computer Electronics Electrical Mechanical and Civil 2011

GRAPH 5. B3GLASS FIBER 3mm

VII. DISCUSSIONS OF RESULTS Damage analysis on impact specimens The composite panels on testing were prone to damage from impact events resulting in delamination and cracking of the exposed facesheet, debonding of the facesheet from layer by layer. Typically, low velocity impact results in little surface damage; however, the residual strength of the composite panel is severely degraded. The damage was initiated by the facesheet failure in all the tests irrespective of the impact energy. While some of the specimens showed partial failure (no damage caused) and few specimens showed total failure. The damage occurred to the composite structure can be divided into three main areas: damage to the top layer that results in an in-plane stiffness and strength degradation, damage to the facesheet results in an out-of-plane stiffness and strength degradation, and damage that results in a localized geometric perturbation. In the first region of all the graphs, all the samples showed steep increase until maximum load (Fmax) and the first major break on the load窶電isplacement curves occurred at the onset of the failure process, i.e., impactor side face sheet cracking. Some curves show a zigzag (several humps) pattern suggesting that there exists a multiple step failure mode during the impact. The interface of the core and the skin resist further crushing, which is manifested as humps. Further valleys are due to the interface failure caused by debonding. This behaviour, which continues throughout the second region, probably means that the local rigidity, rather than the overall structural rigidity, is involved in the impact phenomenon. Damage of the backsheet under the impactor is either visible or hidden. The impact force can produce high throughﾂｩ 2011 AMAE DOI: 02.CEMC.2011.01. 512

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Short Paper Proc. of Int. Colloquiums on Computer Electronics Electrical Mechanical and Civil 2011 thickness shear in the skin, which causes local delamination of the skin. These delaminations can grow during the impact process, and if spring-back occurs, part of the skin below the delamination may remain attached to the core as the remainder of the skin recovers, opening the delamination further.

REFERENCES [1] Sjoblom, P.O. et.al, 1988, “On low velocity impact testing of composite materials”, Journal of composite materials, 22:30-52. [2] Sun C.T and S.Chattopadhyay, 1975. “Dynamic Response of Anisotropic Laminated Plates under Initial Stress to Impact of a Mass”, Journal of Applied Mechanics, 52:693-698. [3] Kwon, Y.S and B.V.Sankar, 1993, “Indentation-Flexure and Low velocity Impact Damage in Graphite/Epoxy Laminates,” ASTM Journal of Composites Technology & Research, 15(2): 101-111. [4] Prasad, C.B, D.R.Ambur and J.H.Starnes, 1993, “Response of Laminated Composite Plates at low speed impact by Air gun propelled and Drop weight Impactors,” 34th AIAA/SDM Technical Conference, AIAA-93-1402-CP, pp 887-900. [5] Rhodes MD. Impact fracture of composite sandwich structures. AIAA/ASME/SAE 16th Structures, Structural Dynamics, and Materials Conf. New York: AIAA, 1975. p. 75-748. [6] Oplinger DW, Slepetz JM. Impact damage tolerance of graphite/ epoxy sandwich panels. In: Greszczuk LB, editor. Foreign Object Impact Damage to Composites: A Symposium (ASTM STP 568).p. 30-48. [7] Bernard ML, Lagace PA. Impact resistance of composite plates. In: Proceedings of the American Society for Composites, Second Technical Conference. Lancaster, PA: Technomic, 1987. p. 16776.

VIII. CONCLUSIONS This study consisted of fabrication, low-velocity impact testing, inspection, & damage analysis of composite sandwich panels. Examination of the damaged samples at the conclusion of testing revealed that many of the low-density samples experienced significant shear failure near the periphery of the sample. The specimens experienced cracking (or tearing) from the center of the impact site to the edge of the sample. A drop in both the absorbed load & absorbed energy of the composite panel was clearly observed with the increase in thickness of the different composite panel specimens under test.

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