A Review on Fatigue Life Estimation of Welded Joints

IJSTE - International Journal of Science Technology & Engineering | Volume 4 | Issue 7 | January 2018 ISSN (online): 2349-784X

A Review on Fatigue Life Estimation of Welded Joints Himanshu Kumar Research Scholar Department of Automobile Engineering Rustamji Institute of Technology, BSF Academy, Tekanpur

Meenakshi Thakur Research Scholar Department of Automobile Engineering Rustamji Institute of Technology, BSF Academy, Tekanpur

Janmit Raj Assistant Professor Department of Automobile Engineering Rustamji Institute of Technology, BSF Academy, Tekanpur

Anand Baghel Assistant Professor Department of Automobile Engineering Rustamji Institute of Technology, BSF Academy, Tekanpur

Abstract Metals are the most broadly utilized materials in designing structures, and a standout amongst the most well-known failure methods of metal structures is Failure due to fatigue. Although metal exhaustion has been considered for over 160 years, numerous issues still stay unsolved. In this article, a best in class survey of metal failure due to fatigue is done, with specific accentuation on the most recent improvements in fatigue life forecast techniques. The literature is reviewed by considering primarily the papers of previous 10– 15 years. All factors which affect the fatigue life of metal structures are grouped into following categories: material selection, crack initiation/propagation and different type of approaches used.

Keywords: TIG, ARC, MIG, Process Parameters, TIG, Welded Joints. Fatigue Life ________________________________________________________________________________________________________ I.

INTRODUCTION

The actual strength of welded joints is affected by different parameters viz. weld quality, material, thickness, stress ratio, welding technique [14]. During the welding procedure, diverse kinds of material defects and blemishes are initiated into the subsequent welded joint. Material imperfections are for instance; softening in the heat affected zone (HAZ) and residual stresses, likewise greatly affect the diminishment of the fatigue life [27]. Fatigue strength is known to be firmly related with the exact geometrical attributes of the welded joint (i.e., weld toe radius, flank angle and weld size). The bead geometry relies upon the welding parameters, the working conditions and at some places, the skills of the welder. The bead shape (specifically the toe range) changes from joint to joint even in all around controlled manufacturing operations [7, 14]. The review is partitioned into following classifications:  Material Selection  Crack Initiation and Propagation  Nominal Stress Approach  Hot Spot Method II. MATERIAL SELECTION The remarkable characteristic of welded joints is that the fatigue strength is almost independent of the material strength, which results in the impossibility of using higher strength steel to meet the requirements of high dynamic load project as the material strength gets higher, the improvement becomes better [26]. Further studies have reported that material strength effects the UPT improvement to some degree. Accordingly, it will be protected to use the fatigue plan (S– N curve) acquired from UPT joint testing aftereffects of low strength steel to outline that of higher strength [29]. For different joint types and materials, incline benefits of comparing S– N curves of UPT joints change from 6.3 to 23.0 [7]. The high strength steel itself offer some critical good points of interest. In light of the high yield stress of the material the sheet-thickness might be diminished, keeping a comparative strength of the part. For whatever length of time that there are no, or just gentle notches, the fatigue strength increases with the materials strength [12]. Be that as it may, because of the high notch affectability of these materials, the fatigue strength diminishes altogether with expanding stress focus [10]. The hardness in the heat affected zone (HAZ) and in the combined zone is lessened, contrasted and the base metal, if there should be an occurrence of high strength steels specimens, and this can influence the fatigue strength [2, 28].

The welded joints are especially vulnerable to fatigue destruction, whereas fatigue processes are among the most often causes of failure [37]. The geometric profile configuration of weld and the heterogeneity of material structure in the proximity of welding zone both result from welding processes [35]. The reason of such susceptibility is their geometry and changeable structure of the material. The geometric profile configuration of weld and the heterogeneity of material structure in the proximity of welding zone All rights reserved by www.ijste.org

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A Review on Fatigue Life Estimation of Welded Joints (IJSTE/ Volume 4 / Issue 7 / 003)

both result from the welding process [38]. As a result, such basic element become point of stress concentration, often plays an essential role in safety of the entire structure. Theoretically, material behavior in the nearby areas such as weld roots and the weld toes depend on parameters such as load histories, residual stresses, heterogeneous metallurgical micro-structures,distortions,weld thermal cycles, heat source characteristics, etc. [39].

It is possible that the higher notch sensitivity of high strength steels makes up for the higher fatigue strength of the base material, in this manner bringing about all evaluations of steel displaying essentially. The rule fundamentally proposes characteristics fatigue strength as a component of the material yield strength [18-20]. The S-N slope of the also changes from 3 to 5. This is because of weld details are limited by the S-N curve of the parent material, which may vary depending on the material strength. A purpose behind this may be the change in local strength because of the work-hardening of the material in the high frequency mechanical impact (HFMI) treated notch [37-40]. Cracks for the most part nucleate at welding, that is, in a zone described by both material heterogeneity and high stress gradients because of the welding geometry [15, 21]. For a material thickness of t >1mm, it is normal that the data values will rise consistently as the material thickness increments. So far, however, it is only possible to give recommendations. It is visible that the high residual stresses are obtained for the T-joint case with a uniaxial equivalent stress estimation of 357 MPa. Uniaxial equivalent stresses are reduced by 35% for 45° and 13% for 0° butt welds. Thus RS have inconvenient impact in the fatigue performance of the part. Therefore, base metal fatigue limit is decreased 44%, 59% and 68% respectively for 45°butt weld, 0° butt weld and T-joint respectively. Be that as it may, there is no a reasonable pattern of multiaxial impact in the fatigue performance. Subsequently, 45° butt weld case demonstrates 36% higher fatigue performance than consummately uniaxial 0° butt weld case, while T-joint case, likewise multiaxial, indicates 23% lower fatigue execution [30].

III. CRACK INITIATION AND PROPAGATION The fatigue life of a welded joint is calculated on basis of initiation of a crack followed by crack propagation. In the perspective of fracture mechanics, fatigue failure is a sequential process of fatigue crack initiation and propagation. Usually, initiation is a perplexing procedure that can lead to a crack as small as less than 1 mm (<1 lm) or one that pops in (starts and quickly develops) at up to 100 mm long. A fatigue break is framed as the aftereffect of localized plastic distortion amid cyclic stressing.

The size and the distribution of stresses in the potential crack plane controls both the fatigue crack initiation and propagation stages [25]. The fatigue appraisal of structures with numerous surface cracks requires displaying, their interaction, combination and crack shape development. The crack development period is relying upon the crack development resistance of the material [34].

Be that as it may, crack initiation is a surface wonder contingent upon surface conditions, and not on the crack development protection of the material [15]. The fatigue cracks proliferate from the underlying to the critical crack sizes before the last failure occurs [17].

The most well-known explanation behind failure of welded joint is the initiation of cracks at the weld toe because of sharp changes (between the weld and base metal), root cracks and cool laps. [19]. The fatigue cracks in the first welded joints frequently initiates from the weld toe slag, inclusions and small surface machining trace while the fatigue fractures in the ultrasonic peening treatment welded joints may have diverse kind of crack sources [20]. The nearness of weld defects, for example, undercuts, cold laps and little toe radii will expand the stress focus in the region of the weld toe and thus diminish the fatigue life. Fatigue cracks are probably going to start from these deformities because of the expanded stress fixation and material imperfection in the weld toe. In welded joints, cracks are by and large started in these imperfections at various areas along the weld crease and after that engender along the weld toe. The fatigue life stage and factors influencing the fatigue life is delineated in Fig.1

Fig. 1: Fatigue Life Phase and Factors [15]

Fatigue cracks in functional structures generally have a place with blend mode cracks because of the complexity of stacking and local geometry. In this manner, the blend mode crack propagation investigation is of extraordinary significance to uncover the more real fatigue conduct of basic subtle elements for the situation of straightforward geometry, load designs like the edge or semicircular surface crack in a plate are subjected to unadulterated twisting or strain stack. The semi-circular crack supposition gives widely longer fatigue lives than the test fatigue lives and in this manner to be unseemly [36]. The trial contemplates uncovered that tests on small size welded examples that the fatigue qualities of low yield strength (Sy) steels match with those of welded joints

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A Review on Fatigue Life Estimation of Welded Joints (IJSTE/ Volume 4 / Issue 7 / 003)

containing the high tractable lingering stresses that exist in genuine welded structures. Sy tests with little size butt welded examples likewise demonstrated that the fatigue quality diminished with expanding yield strength, particularly in the long-life area. Further, it was accounted for in the investigation that the fatigue quality of little size welded joints diminishes with increment in the stress proportion [2]. The proportion of the fatigue crack initiation life to the length of the fatigue crack development period differs relying upon the load and weld geometrical components and its relative commitment to the aggregate fatigue life of a weldment increments in with the lessening of the connected cyclic load and the expansion of the aggregate life [25]. On account of steady amplitude loading the fatigue crack development depends just on the segment geometry, applied loading and material properties and if there should be an occurrence of variable amplitude loading the fatigue crack development depends likewise on the previous cyclic loading [17].

The exploratory perceptions of fatigue crack conduct under variable amplitude loading uncovered that the lingering stress power factor for the present cycle isn't just a component of the remaining stress field in front of the crack tip actuated by the last cycle, however can likewise rely upon the leftover stress fields created by the past cycles of the loading history. Be that as it may, comprehend that the real fatigue crack development augment because of load inversion is short of what it could be under the connected scope of stress power factor because of the presence of the compressive (residual) stresses left behind the crack tip. In the entire consistently loaded specimen's failure occurred in the weld zone with the crack starting in closeness of the weld toe, where there are high stress fixations [14]. For the fatigue crack development under variable amplitude loading the conclusion display depends on the plastic disfigurement and crack face interaction in the wake of the crack [17].

Residual or compressive stresses affect the fatigue strength of welded structures, and it is notable that high pliable residual stresses detrimentally affect fatigue life and compressive residual stresses could favorably affect fatigue life [16]. The presence of residual stresses decreases the rigidity and the dimensional soundness of parts. Residual stresses influence both initiation and development phases of fatigue cracks [9].Compressive residual stress at the weld root tends to close the crack under little elastic load. It generally lessens the SFF (stress force factor), and subsequently increment the break strength and reduction the crack development rate in cyclic loading [1]. Residual or compressive stresses in front of the crack tip can either defer or quicken the ensuing fatigue crack development relying upon the present crack length and past loading cycles. Notwithstanding, when the crack proliferates through the highest piece of the residual stress field actuated by the overload the residual stress force factor begins rotting and ought to in the long run achieve an indistinguishable level from that for the steady amplitude loading. This impact originates from the way that the piece of the stress field near the crack tip is considerably more essential than the staying one [17].

Further, the fatigue crack development conduct is overwhelmed by residual stress outside the weld zone. Lack of penetration, absence of fusion or configuration root crack will all go about as cracks and if the connected stress power variety is sufficiently high and the residual stress level is in strain at that point fatigue life will be short in correlation if the residual stress is in compression [13].

The bonded patch repair of a cracked panel gives an impressive increment in the residual strength and also fatigue life [5, 11].

The Reinforcement utilization of steel structure is another system which can adequately lessen the crack development rate and fundamentally expand the fatigue life, by decreasing the stress extend around the crack tip, diminishing the crack opening relocation and by advancing crack closure [27, 32]. Notwithstanding, Reinforcement deboning is the overwhelming failure method of strengthened steel part. The presence of a deboned territory in the crack tip district essentially decreases the viability of the reinforced technique and ought to be considered in the assessment of the fatigue lifetime [14-16]. For long introductory crack size, the underlying crack development represents a significant bit of the entire fatigue life. The repair of long starting crack size is substantially less proficient then the instance of short beginning crack size [31]. The strategies for fatigue life estimation is appeared underneath in Table 1.

S.NO 1 2 3

Table â&#x20AC;&#x201C; 1 Inputs Required For the Three Main Fatigue Life Estimation Methods FATIGUE LIFE METHOD ASSESSMENT INFORMATION The nominal stress method S-N curve of weldment Structural detail The local strain life method Peak notch stress â&#x2C6;&#x2C6;-N curve of material The fracture mechanics method Stress intensity factor at the crack tip Through thickness stress distribution

IV. NOMINAL STRESS APPROACH The nominal stress strategy utilizes the standard stress life of the welded structure. This technique depends on the on the worldwide geometry and does not represent the local impacts, neither at full scale level (weld shape and size) nor at smaller scale level, (for example, weld toe or root) [8-10]. Utilization of this approach requires fatigue S-N bends, which are produced through fatigue testing of either little examples or close full scale structures [1-2]. All large scale geometrical factors, for example, the brokenness impacts prompted by different connections and all miniaturized scale geometrical factors, for example, the local notch impacts from the weld geometry are incorporated into the fatigue strength acquired tentatively [16]. The S-N approach is extremely straightforward and simple to apply on a fundamental level. Practically speaking the technique is extremely hard to apply to complex joints and loading histories in light of the fact that the bends utilized as a part of this strategy were produced from fatigue testing of agent tests of particular geometries [8, 25, and 30]. The framework of the fatigue marvel utilizing the stress amplitude versus life (S-N) graphs was finished by amid the time of 1850 and 1860 [12]. The typical S-N bend is appeared beneath in Fig. 2

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A Review on Fatigue Life Estimation of Welded Joints (IJSTE/ Volume 4 / Issue 7 / 003)

Fig. 2: Typical S-N Curve [8]

Wohler conducted fatigue test under repeated bending stress. The result of the experiment showed that fatigue life decreased with the increase of the stress amplitude and that below certain stress amplitude, the test specimens did not fail [8]. Wohler suggested that for determining the fatigue life the amplitude of stress was more important than the maximum stress. The next major contribution was done by Goodmanâ&#x20AC;&#x2122;s, by developing the model for accounting the effect of mean stress on fatigue life of metal [6], in 1910 presented an empirical law to mathematically describe the S-N curve for fatigue life determination [6] depicted that in the finite life region the S-N curve could be represented as a linear log-log relationship Miner proposed the linear damageaccumulation hypothesis [8].

In the application of S-N approach for determining the fatigue life a well-defined nominal stress is used. This nominal stress gives the fatigue strength of the same welded joints. In order to properly apply this method, the nominal stress range â&#x2C6;&#x2020;đ?&#x153;&#x17D; should be clearly defined and the structural discontinuity should be comparable with one of the classified details used in design rules for generating the fatigue S-N curves several standards and guidelines are developed based on the statistical evaluation of relevant fatigue tests [8].

The International Institute of Welding issued comprehensive set of S-N curves along with catalogue of details for steel and aluminum alloys. As per the recommendation by the International Institute of Welding, the S-N curves are characterized by their fatigue strength at N = 2x106 cycles with the survival probability, Ps = 97.7%, the FAT class N/mm2, a slope exponent k = 3 above and k = 5 below the knee point at 10 million cycles for variable amplitude loading [22]. The curves for welded joints are limited by the FAT 160 curve for parent material (steel) with k = 5. For this purpose, results of existing fatigue tests are gathered and reevaluated with regard to the fatigue strength of comparable details [33]. Furthermore, by assuming the slope of the S-N curve to be independent from the structural detail and obtains a specific value, the endurance limit of welded joints and can also be used to explain the damage process from the crack nucleation up to the final failure [15].

Fig. 3: Typical Cyclic Loading Parameters [8]

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A Review on Fatigue Life Estimation of Welded Joints (IJSTE/ Volume 4 / Issue 7 / 003)

V. HOT SPOT METHOD The hot-spot stress method is an augmentation of the Sâ&#x20AC;&#x201C; N Curve approach in that it makes utilization of S-N curves got from tests on real welded joints [5].The hot-spot method was originally developed to assess the fatigue behavior of offshore structures, with its use being subsequently extended to other structural applications [32]. Hot spot stress method estimates the fatigue life of welded joints through calculation of maximum principal surface stress at critical regions adjacent to the weld toe, and then comparing this quantity with the S-N curves of geometric stress category [35].

The nominal stress concept is easier to use in the case of a simple weld joint but it is not practical to determine nominal stress on real structures.[25] The presence of gross structural discontinuities, non-uniform stress distributions and through-thickness stress gradients and after that select one of the plan S-N curves for the investigated complex welded joint.[8] The auxiliary hot spot stresses, likewise called geometric stresses, incorporate nominal stresses and stresses from basic discontinuities and the presence of attachments, yet do exclude stresses because of the presence of welds [23].

The hot spot stress decides the fatigue life expectation of any basic detail by extrapolation of stress esteems at specific purposes of interests on the surface of the specimen and considering a satisfactory Sâ&#x20AC;&#x201C; N curve [30] Extensive Detail of Hot Spot is accessible on [35] , [23] Hot spot stress technique in Eurocode3 estimates the fatigue life of welded joints through estimation of maximum principal surface stress at critical areas neighboring the weld toe, and afterward contrasting this amount and the S-N curves [6] In this strategy, no thought is taken for the impacts of base metal plate thickness and the proportion of secondary bending stress to membrane stress. Utilizing break mechanics, in view of Parisâ&#x20AC;&#x201C; Erdogan condition, [3] demonstrated the last two elements can extraordinarily influence fatigue life of a welded T-joint and played out a parametric report on the impact of these two factors on fatigue life of breathing networks. A later technique in view of break mechanics created [4], proposed that the weld classification in light of S-N curves can be significantly lessened into a solitary 'ace' S-N curve [35]. The plate thickness, material, welding composes, joints and welding parameter taken by the past specialists is condensed in Table 2 S. No

Plate Thickness

Material

Table â&#x20AC;&#x201C; 2 Summary of Past Research Work.

Type of Welding Welding Types Joints

Welding Parameters

References

[10]

[14]

1.

12 mm Thick Plates.

Steel Plates

Metal Arc Gas Welding (MAG)

(1) Fillet (2) T-joint (3) Cruciform Joint

Flank Angle - 45° Throat Thickness - 5 mm Leg length of 5/16 inch (7.9 mm) Nominal Throat Thickness of the weld is 4.5 mm.

2.

50 mm Wide, 5 mm Thick and 300 mm Long

AH 36 Steel

Thermo Graphic Method

Butt Joints

-

3.

-

Steel SS2172

Metal Arc Gas

Fillet Welded TubeTo-Plates and SingleU Weld Groove

4.

190 x 60 mm

Low Carbon Steel Q235B

Ultrasonic Peening Treatment

Butt and Longitudinal Fillet Joint

5.

10 mm Thick Steel Plates

Stainless Steel SS2333

Hybrid Laser Metal Inert Gas (L/MIG)

Fillet Welded joints

Welding Current-210 A Voltage-26 V Welding Speed-5.1 m/s Heat Input-1.07 KJ/mm Ultrasonic Vibration is 20 kHz and The Amplitude Range is 0â&#x20AC;&#x201C;25 đ?&#x153;&#x2021;m MIG current-328A MIG voltage (resulting)- 27V Pulse Time - 2.4 ms Frequency â&#x20AC;&#x201C; 90 Hz Wire Stick Out Length 16 mm Wire Feed Rate 4.2 m/min Shielding gas - Ar Shielding Gas Flow 20 l/min Laser beam angle 108° Welding Speed1.05m/min Laser Power -3.25Kw

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[16]

[18]

[19]

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A Review on Fatigue Life Estimation of Welded Joints (IJSTE/ Volume 4 / Issue 7 / 003)

6.

7.

8.

76 mm Thickness (Thickness of 24 mm, a Width of 50 mm) Specimen Thickness Varies From 5 to 25 mm

Carbon Steel Q235B Steel Q345

Ultrasonic Peening Treatment

316L Forged Stainless Steel Pipes

Automatic Gas– Tungsten Arc Welding (GTAW)

Butt Welded joints

The Steel Grades vary from S235 to S1100

Metal Arc Gas Welding (MAG)

Butt Joint

Cruciform Welded Joints Welding Speed of about 5.1 cm/min and The Heat Input Rate of About 20 KJ/cm

-

9.

12 mm Thick

Aluminium Alloys 2219-T62

Friction Stir Welding (FSW)

Butt Joint

10.

4 x 4 x 23.625 in and 2 x 6 x 14.313 in

A22-H steel

Flux Cord Arc Welding

T-Joints with Single Fillet Weld

11.

3 and 4 mm

Steel plates

Conventional Arc (CV), Laser Hybrid (HY) and Pure Laser (LA) Welding

Butt and Fillet Welded T-Joints

10 mm Thick

S275JR Structural Steel.

Gas Metal Arc Welding (GMAW)

Tilt Angle= 2.5° Tool Rotary Speed = 300 rpm, 400 rpm, 500 rpm Traverse Speed = 60 mm/min, 80 mm/min and 100 mm/min Weld Radius-0.0312in Weld Angle-45° Weld Height-0.312in -

(i) 0° Butt 12.

[20]

(ii) 45° Butt (iii) T-Joint

13.

16 mm Thick

Steel Plate SM490YA

Gas Shielded Arc Welding (GSAW)

14.

4 mm Thick

AISI 304 austenitic stainless steel

Gas Tungsten Arc Welding(GTAW)

Square Butt Joint

15.

-

AI 6061 aluminium alloy

Metal Inert Gas Welding (MIG)

Butt Welded Joints

T-Joint

(i) 0° Butt Weld Welding power-7090.9 W Welding speed-550 mm/min (ii) 45° Butt Weld Welding power-7090.9 W Welding speed-550 mm/min (iii) T-Joint welding Welding Power- 8258.9 W Welding Speed-500 mm/min Inter-Pass Temperature is to be Below 200°C and Heat Input is About 40 kJ/cm Welding Current-226A Welding Speed75mm/min Welding Voltage-29.5V Gas flow rate-161/min Welding Current-180A Welding Speed194mm/min Welding Voltage-20V

[21]

[22]

[24]

[25]

[28]

[30]

[33]

[41]

[42]

VI. OUTCOMES OF THE LITERATURE SURVEY 1) 2) 3) 4)

A fatigue crack is formed as the result of localized plastic deformation during cyclic straining.

Depending upon surface conditions the crack growth period is depending on the growth resistance of the material.

Fatigue cracks propagate from the initial to the critical crack sizes before the final failure occurs.

The presence of weld imperfections such as undercuts, cold laps and small toe radii will increase the stress concentration in the vicinity of the weld toe and hence decrease the fatigue life.

5) Butt welded specimens also showed that the fatigue strength decreased with increasing yield strength.

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A Review on Fatigue Life Estimation of Welded Joints (IJSTE/ Volume 4 / Issue 7 / 003)

6) Compressive residual stress at the weld root tends to close the crack under small tensile load. It always reduces the SIF (stress intensity factor), and hence increase the fracture strength and decrease the crack growth rate in cyclic loading.

The results showed that fatigue life decreased with the increase of the stress amplitude and that below certain stress amplitude, the test specimens did not fail.

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