Fatigue Performance Of Joints Research Articles

Fatigue strength of misaligned non-load-carrying cruciform joints made of ultra-high-strength steel

Misalignments and distortions in welded plate components act as stress raisers and can significantly decrease the fatigue strength of welded connections. This paper investigates the effect of the plate misalignment of transverse attachments on the fatigue behavior of axially-loaded non-load-carrying cruciform (NLCX) joints. Experimental fatigue tests with and without the plate misalignment are carried out for fillet-welded NLCX joints. The test specimens were fabricated of S1100 ultra-high-strength steel grade, and the fatigue tests were conducted using an applied stress ratio of R = 0.1. Numerical finite element analyses were conducted to obtain the stress concentrations induced by the misalignment. In addition, varied geometry parameters were applied to investigate their effect on the magnitude of stress concentrations. Stress concentrations were obtained using the structural hot spot stress method applying linear surface extrapolation and 1 mm below depth methods, and the effective notch stress concept with the reference radius of 1 mm. Experimental fatigue tests showed a decrease of up to 12% in fatigue strength depending on the degree of misalignment. The highest stress concentration was induced when the misalignment to the joint width ratio was e/L = 0.2–0.4. Structural stresses cannot be estimated using the linear surface extrapolation. Instead, structural stress at the 1 mm depth and effective notch stress concept accurately evaluated the misalignment effect on the fatigue performance of NLCX joints, and provided a good correspondence between the theoretical and experimental fatigue strength estimations.

Influence of Rivet Diameter and Pitch on the Fatigue Performance of Riveted Lap Joints Based on Stress Distribution Analysis.

Interference-fit riveting is one of the most widely used mechanical joining ways in aircraft assembly. The fatigue performance of riveted joints has a significant impact on the service life and reliability of aircraft. In this paper, the fatigue performance of the riveted lap joints with various rivet diameters and pitches are studied based on stress distribution analysis under tensile load. First, a theoretical model of the riveted lap joint under tensile load is developed by using the spring-mass model. The rivet-load stress, bypass stress, and interference stress around the riveted hole are analyzed. Then, the finite element (FE) model of riveted lap joints are established. The influence of rivet diameter and pitch on stress distribution around the riveted hole are discussed. Finally, the fatigue tests are conducted with riveted lap joint specimens to verify the theoretical model and FE results, and a good agreement is observed. Based on the simulation and experimental results, a good combination of structural parameters of the riveted lap joint is found which can optimize the stress distribution around the riveted hole and improve the fatigue life of the riveted lap joint.

Fatigue Behavior of Bolted Joints: Comparative Analysis for the Effect of Fretting and Stress Concentration

Abstract In this paper, a methodology is developed to investigate the combined effect of some main factors (bolt pretension load, shear load, coefficient of friction, and bolted joints structure) on the fatigue performance of bolted joints. A parameter k, which reflects the ratio of bolt pretension load to shear load, is used to evaluate the competition effect of fretting and stress concentration on fatigue damage of bolted joints. The analysis result deduces that as the increase of k, the effect of stress concentration is decreased, while the influence of fretting is increased. At the increase of the bolt pretension load, the fatigue crack formation position will be far away from the hole center. This parameter k can be used as a design or assembly parameter to improve the strength performance of the bolted joints.

Fatigue behavior improvements of laser-induction hybrid welded S690QL steel plates

In this paper, the improvements of fatigue performances of S690QL steel welded by laser-induction hybrid welding (LIHW) method were performed, mainly through the infrared imaging device to obtain the weld thermal cycle, fatigue machine to test the fatigue strength and scanning electron microscope to observe the fracture morphology. In fatigue tests, tension-tension fatigue loading and stress ratio R = 0.1 was selected. The LIHW fitted S-N curves were derived. It was found that the cracks with symmetrical grooves were initiated at the weld center (WC), while the single-laser welding (SLW) fatigue samples were fractured at the WC. However, the LIHW samples were finally fractured near heat-affected zone (HAZ). The fracture morphology of fatigue samples with stress amplitude of 108 MPa and 144 MPa were further selected to analysis. Due to different degree of defects or stress concentration, and different welding heat input absorption ability and cooling rate, in sudden fracture region, SLW fatigue samples mainly contained the brittle transient failure mode, LIHW fatigue samples mainly showed a ductile failure mode. It can thus be concluded that the LIHW method could improve the fatigue performance of S690QL steel joints.

Analysis of fatigue-related aspects of FCAW and GMAW butt-welded joints in a structural steel

The classificatory method is considered a straightforward and robust procedure for the assessment of welded joints in metallic structures. The methodology adopts a set of S–N diagrams with a conservative survival rate, which expresses the fatigue performance of the most employed joint configurations. In the design phase, the adoption of a structural detail correlated with a higher curve in the set conducts to high structural integrity. Nevertheless, fatigue phenomenon is essentially multifactorial and, consequently, several optimizing alternatives may be applied for life increasing. Studies focused on continuous (weldless) parts correlate fatigue performance with the material ultimate strength. Transferring this approach to welded parts, adoption of a high-strength filler metal, or any procedure intended to mitigate metallurgical inhomogeneity in the weld-affected region may propitiate fatigue life improvement. In other sense, issues related to bead geometry, notably the notch effect associated with weld toe, are also reported to have an essential influence on fatigue life of structural details. This work aims to counterbalance material and bead geometry aspects related to fatigue performance of butt joints subjected to repeated transverse loading. Two welding processes, FCAW and GMAW, as well as different filler metals and parameters, were adopted for specimens manufacturing. The ASTM-A572-Gr.50 steel was employed as a parent metal. After welding, FCAW joints presented better mechanical and metallurgical characteristics. Additionally, in order to characterize the relevance of the notch effect, half of the specimens had the original bead geometry maintained, while the others underwent a manual removal of the reinforcement. The obtained results indicate that FCAW joints present better fatigue performance in the as-welded state. However, after removing the reinforcements, both welding processes attained similar results, suggesting that the notch effect has more relevance for fatigue life than mechanical and metallurgical characteristics of the joint. Consequently, the superior performance presented by the FCAW joints in the as-welded state is assigned to the final shape of the reinforcements, which is smoother than in the GMAW case. The higher capacity of penetration and better filler metal wettability propitiated by the FCAW process is responsible for the lower notch effect.

Study on Low-Cycle Fatigue Performance of Aluminum Alloy Temcor Joints

Aluminum alloy Temcor joints have been extensively applied to aluminum alloy latticed shells, but easily experience low-cycle fatigue under the effects of strong winds. In this paper, the low-cycle fatigue performance of the Temcor joints was studied by experimental researches, finite element simulations and theoretical analyses. Six kinds of displacement loads were selected and the low-cycle fatigue failure mechanism and fatigue life of the Temcor joints were studied through experimental research. Under the effect of the low-cycle reciprocating load, the bolt went through continuous brittle failure and the failure position was located at the junction between the screw root and the nut. Consistent with the experimental situations, the finite element model was established and the simulation results were compared well with the experimental results. Then, a prediction method for low-cycle fatigue life based on the local stress-strain method was attempted, and the finite element simulation method was employed to calculate the key parameters. The low-cycle fatigue life prediction results based on the local stress-strain method accorded well with the test results.

Numerical modeling of residual stresses and fatigue damage assessment of ultrasonic impact treated 304L stainless steel welded joints

Welding technology has been widely adopted to fabricate structures and components due to its low cost and easily accessible procedure. However, the welding methods usually impose tensile residual stresses on the fabricated part, that tend to be detrimental to fatigue properties of welded joints. Ultrasonic impact treatment (UIT) has been introduced as an increasingly popular post-weld treatment technique to improve fatigue performance of welded joints due to its high efficiency and reliability. In this paper, the simulations of welding and UIT processes were performed by means of finite element modeling to predict the residual stress distribution of the 304L stainless steel welded joints. Effects of the UIT on the residual stress distribution and fatigue performance were individually investigated. Finite element results indicated that the UIT introduced beneficial compressive stress up to depths of 2–3 mm in the butt and T-joint welds. The stress distribution at the potential crack plane and the respective stress intensity factors were calculated by taking into account residual stresses, to evaluate the fatigue crack propagation. The fatigue crack growth model was subsequently employed to predict fatigue lives of the welded joints. Results of the fatigue analysis showed that the UIT remarkably extended fatigue lives of the 304L welded joints.

Effect of the surface morphology over the fatigue performance of metallic single lap-shear joints

The effect provided by the state of the surface on the quality of the adhesion, as well as the sensitivity of the position of the locus of failure to the surface morphology, is known to be one of the most crucial issue to be addressed when evaluating the capability of the bonded joint to withstand any mechanical stress. Therefore, the need for the substrates to undergo a pre-treatment before being bonded is to be considered. In this work, different pre-treatments were selected to be applied over aluminum and stainless steel adherents’ surfaces with the goal to produce single lap joints to undergo cyclic loading until complete failure. In particular, the experimental campaign aimed to correlate the morphology generated by the different surface pre-treatment (laser ablation, grit blasting and simple degreasing) with the quality of the fatigue performance, measured as the number of cycles to failure. Result of this research shows that the surface morphology generated by the laser ablation was able to reduce or avoid interfacial failures, leading to an increase of the fatigue performances if compared with grit blasted and degreased joints.

Influence of process parameters on the microstructural evolution and mechanical characterisations of friction stir welded Al-Mg-Si alloy

Heat generation and plastic deformation during Friction Stir Welding (FSW) produce profound changes in the microstructure and structural properties of welded joints. Strengthening precipitate, grain size and crystallographic texture evolution are the most important microstructural changes in the case of welding aluminium alloys. An interaction relationship has been developed in this study to understand the evolution of microstructure during FSW of Al-Mg-Si alloy for a wide range of welding temperatures and plastic deformations by controlling two important process parameters (tool rotation and welding speeds). Electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM) have been used for microstructural characterisation. The mechanical properties were evaluated using microhardness, tensile and low cycle fatigue tests. In all cases, fine recrystallised equiaxed grains with a partial <111> fibre texture was evolved in the nugget zone. Microstructural development was found to be significantly influenced by the weld pitch (welding speed/rotation speed), as it is controlled the heat input, cooling rate, exposure time and plastic deformations. The strength of the FSW joints is improved by increasing the welding speed because of grain refinement, incomplete dissolution or re-precipitate of strengthening precipitates Mg5Si6(β)´´ and the introduction of edge dislocations. The fatigue performance of FSW joints is dependent on grain size with a remarkable improvement in the fatigue life observed for smaller grain sizes.

Fatigue performance of single lap joints with CFRP and aluminium substrates prior and after hygrothermal aging

AbstractAdhesive bonding has been extensively used to join composites and aluminium alloys in the automotive industry, but a deeper understanding of its fatigue behaviour is still required. This work presents a novel evaluation of the combined performance of joints manufactured with carbon fibre reinforced plastic (CFRP) and aluminium substrates subjected to fatigue loads in the unaged, hygrothermally aged, and dried after hygrothermal aging states. The experimental results allowed to conclude that the fatigue performance of joints can be affected by changes induced by the drying process or losses in the interface strength and that dissimilar combination of substrates sustains higher number of cycles to failure. The fatigue performance of joints with dissimilar substrates was found to be better than that of joints with similar substrates, which can be attributed to the lower stresses acting on the adhesive layer. The presence of water has also noticeably changed the fatigue performance of joints with dissimilar substrates.

Effect of autoclave cure time and bonded surface roughness on the static and fatigue performance of polyurethane film Adhesive Single lap joints

This study investigates the effect of autoclave cure time and bonded surface roughness on the static and fatigue performance of film-adhesive single lap joints. Joint static performance is assessed in terms of its load transfer capacity in a quasi-static tensile-shear test to failure. Effect on fatigue life under a mean and cyclic tensile-shear amplitude is also investigated. Two levels autoclave cure (soak) time and two levels of bond surface roughness are investigated. All other autoclaving process variables are kept constant; namely, the ramp rate of temperature rise/cooling, pressurization/depressurization, as well as the cure temperature and cure pressure levels. Test joints are made of aluminium-aluminium or aluminium-magnesium adherends, joined with a polyurethane film adhesive. The results suggest that: an increase of the surface roughness is beneficial to static strength and detrimental to fatigue strength; an increase of the autoclave soak (cure) time is beneficial both to static and fatigue strength. Test data, failure mode analysis, discussion, observations and conclusions are provided.

Fatigue Performance of Friction Stir Weld-Bonded Al-Mg joints

The need for weight reduction and leaner manufacturing and assembly processes in aircraft construction has led to the pursuit of alternative joining processes to conventional riveting. One such technology that has been considered for this application is friction stir welding (FSW). Since it is a solid-state joining method, it results in high performing joints in a wide range of materials while avoiding overlap lengths and added weight from fasteners, crack stoppers, doublers, etc. However, the adoption of this technology to the assembly of large fuselage shell components is challenging, due to geometric tolerance management requirements. A hybrid friction stir weld-bonding method combining overlap friction stir welding and adhesive bonding (AB) has been proposed as an alternative, aims to incorporate properties and characteristics of both joining technologies, as well as improving damage tolerance. Fatigue performance of single lap joints of AA6082-T6 Al-Mg alloy was assessed and benchmarked against FSW overlap and hybrid friction stir weld-bonding joints. Significant strength and ductility increase were achieved through the hybridization of the overlap FSW joints. Fatigue strength of the hybrid joints was also higher than FSW overlap joints.

Fatigue properties of Ti-6Al-4V alloy friction stir welding joint obtained under rapid cooling condition

Friction stir welding joints of Ti-6Al-4V titanium alloy were obtained under air and rapid cooling conditions. After post welding heat treatment, the low-cycle fatigue properties of the joints were manly investigated. The results show that the application of liquid nitrogen on the joint top surface decreases the peak welding temperature and the temperature gradient along the joint thickness, which results in a finer and more homogeneous microstructure. Consequently, the improved tensile properties and microhardness are obtained. Fatigue tests show that the joints obtained under rapid cooling condition possess an increased fatigue life. The W-Re alloy contaminations resulting from the tool wear play a significant role in the joint fatigue performance. The fatigue specimens fracture in the SZ due to the existence of contaminations and these contaminations with different sizes have different influences on the fatigue crack initiation and propagation.

Failure analysis of adhesively bonded GFRP/ aluminum matrix single composite lap joint with cold worked penetrative reinforcements

A new cold worked, penetrative mechanical reinforcement method was evaluated for its potential to enhance joining between 2024-T3 aluminum matrix and glass fiber reinforced polymer (GFRP) for aerospace and automotive applications. In this study, the aluminum substrate surface was modified using a new cold working process which formed hooks directly from the substrate material. This surface was then treated with grit blast and Sol-Gel surface treatment before it was co-cured to both side of a GFRP, forming a single lap joint for testing. This type of joints was compared to a baseline bonded joint without mechanical reinforcement using the same surface treatment. The joint strength and fatigue performance of these sets of samples were experimentally evaluated under quasi-static and fatigue loading conditions. The bonded joints with penetrative mechanical reinforcements withstood greater peak load and higher strain prior to failure, as compared to non-reinforced bonded joints. Fatigue testing on the other hand showed better performance of non-reinforced joints over the reinforced ones. The failure surfaces of both quasi-static samples and fatigue samples were evaluated using a scanning electron microscope to determine failure mode(s). It was determined that the stress concentration at the base of reinforcements could be the main underlying cause for earlier onset of fatigue failure of reinforced samples.

Fatigue performance and evaluation of welded joints in steel truss bridges

While welded joints are extensively used in the connections of steel truss bridges, service life of the bridges is largely dependent on the fatigue resistance of the welded joints. Stress concentration and weld residual stress are two primary causes of fatigue damage in the welded joints. In this study, corner-fillet profile (CFP) and ultrasonic impact treatment (UIT) are used to improve the fatigue performance of the welded joints through relieving stress concentration and weld residual stresses, respectively. The fatigue resistance of welded joints is evaluated through experimentation. The results indicate that the use of CFP and/or UIT can alter the initiation location of fatigue crack. The fatigue resistance of welded joints was increased by 24% and 36% by using the CFP and UIT, respectively. The fatigue performance of welded joints was evaluated using three different methods, including the nominal stress method, effective notch stress method, and peak stress method. The peak stress method with a single fatigue resistance curve demonstrated the highest applicability and accuracy.

Load transfer mechanism and fatigue performance evaluation of suspender-girder composite anchorage joints at serviceability stage

To evaluate the mechanical and fatigue behavior of composite suspender-girder anchorage joints in arch bridges, fatigue tests of two composite joints including pure-shear and shear-compression types were conducted and the load transfer mechanisms were evaluated based on the static loading. The experimental results indicated that both proposed composite anchorage joints presented great combined behavior. The load ratio of PBLs dropped sharply with the increase of embedded depth. Approximately 1/3 of the suspender tensile load was carried by the PBLs in the first row. Due to the redundant PBLs and large embedded depth, few loads were resisted by the bearing plate. Before and after the fatigue loading process, the shear amplitude of PBLs and the vertical stress distribution of concrete were almost consistent, indicating the favorable fatigue performance of joints. Parameterized solid nonlinear finite element models were established and verified by the test results to investigate the effect of total row number of PBLs and presence of bearing plate on the load transfer mechanics. The numerical results showed that the increase of total row number of PBLs enlarged the degree of irregularity of load distribution and reduced the load ratio of bearing plate. Finally, the load ratio formula of PBLs and bearing plate under serviceability stage was derived. The calculated load ratios were in good agreement with the test and numerical results. It was proposed that the total row numbers of PBLs were no >4 or 6 for the joints with or without bearing plate.

Fibre waviness in pultruded bridge deck profiles: Geometric characterisation and consequences on ultimate behaviour

Abstract Conventional tests cannot be used to establish the important influence of fibre waviness, a manufacturing legacy at the flange-web joints (FWJs) of pultruded GFRP bridge decks, on the local ultimate behaviour of such decks. Hence a novel, simple and reliable three-step experimental scheme for that purpose is presented herein, using one pultruded deck profile as an exemplar. First, the joints to be targeted for testing - namely those within the single deck unit and the hybrid joints formed by bonding deck units together - are identified. Second, an effective manual method is put forward to map this waviness at the FWJs. Third, a test is introduced which enables statically determinate loading (via an adjoining flange) of one joint at a time, while also ensuring continuity between this joint and the remaining deck so that the real load paths within the deck are preserved. During the tests failure always occurred by fracture of the wavy fibre-resin interfaces within the FWJs. In the hybrid joints, this failure occurred on the same side of the adhesive layer as the load. The strongest joint had the least waviness. For the flange-single web (but not the flange-double web) joint, bonding transformed the wavy fracture pattern and quadrupled the failure load. It is concluded that this simple test, which also reveals the influence of fibre waviness on the flange's local flexural stiffness, can be further used to characterise joint fatigue performance.

Experimental Investigation on High-Cycle Fatigue of Inconel 625 Superalloy Brazed Joints

The high-cycle fatigue performance and crack growth pattern of transient liquid phase-brazed joints in a nickel-based superalloy Inconel 625 were studied. Assemblies with different geometries and types of overlaps were vacuum-brazed using the brazing paste Palnicro-36M in conditions such as to generate eutectic-free joints. This optimal microstructure provides the brazed assemblies with static mechanical strength corresponding to that of the base metal. However, eutectic micro-constituents were observed in the fillet region of the brazed assembly due to an incomplete isothermal solidification within this large volume of filler metal. The fatigue performance increased significantly with the overlap distance for single-lap joints, and the best performance was found for double-lap joints. It was demonstrated that these apparent changes in fatigue properties according to the specimen geometry can be rationalized when looking at the fatigue data as a function of the local stress state at the fillet radii. Fatigue cracks were nucleated from brittle eutectic phases located at the surface of the fillet region. Their propagation occurred through the bimodal microstructure of fillet and the diffusion region to reach the base metal. High levels of crack path tortuosity were observed, suggesting that the ductile phases found in the microstructure may act as a potential crack stopper. The fillet region must be considered as the critical region of a brazed assembly for fatigue applications.

Characteristics of Residual Stresses Generated by Induction Heating on Steel Plates

This paper presents experimental and numerical investigations on the characteristics of residual stresses generated by induction heating (IH) on 12 mm thick steel plates. IH at 250 °C and 350 °C provided high tensile residual stresses in the heating field but high compressive stresses away from the heating field. The double heating case generated higher compressive residual stresses—around 200 MPa—than the single heating case because of the superposition of the compressive residual stresses. It will be expected to improve the fatigue performance of welded joints when IH is applied for repair work on existing steel structural members susceptible to fatigue damage. Numerical simulation models for predicting residual stresses by IH were proposed by adopting the body heat flux input and the surface heat flux input. They will be beneficial for identifying the optimum heating conditions, such as the target temperature and the heating field, for applying IH to the actual repair work of the steel structural members.

Surface modification methods for fatigue properties improvement of laser-beam-welded Ti-6Al-4V butt joints

Surface and internal defects formed upon laser beam welding (LBW) have been recognized as a serious problem because they cause stress concentration leading to premature failure of a welded component. This paper seeks to remedy these weld imperfections by applying various post-weld treatments and analyzing their effect on the high cycle fatigue (HCF) performance of welded joints. High efficiency of laser-based post-processing techniques after welding such as laser surface remelting (LSR) and laser shock peening (LSP) was demonstrated and compared with conventional approaches. The study reveals that welding porosity determines the internal crack initiation of the surface-treated weldments. Influence of process parameters on porosity level and the HCF properties is presented in detail. Based on an extensive experimental study, practical guidelines needed to mitigate the notch effect from defects and to maximize the fatigue performance of the laser-welded Ti-6Al-4V butt joints are given.