Fatigue Life Of Joints Research Articles

Assessment the Partial Welding Influences on Fatigue Life of S700MC Steel Fillet Welds

Fine-grained steels belonging to the HSLA group (High-Strength Low-Alloy steels) of steels are becoming increasingly popular and are used in both statically and dynamically stressed structures. Due to the method of their production, and thus also the method use to obtain the required mechanical properties, it is really necessary to limit the heat input values for these steels during welding. When applying temperature cycles, HSLA steels in highly heated heat-affected zones (HAZ) reveal intensive grain coarsening and also softening behaviour. This subsequently results in changes in both mechanical and brittle-fracture properties, and the fatigue life of welded joints. While grain coarsening and structure softening have a major effect on the change of strength properties and KCV (Charpy V-notch impact toughness) values of statically stressed welded joints, the effect of these changes on the fatigue life of cyclically stressed welded joints has not yet been quantified. The paper is therefore conceived so as to make it possible to assess and determine the percentage impact of individual aspects of the welding process on changes in their fatigue life. To be more specific, the partial effects of angular deformation, changes that occur in the HAZ of weld, and the notch effect due to weld geometry are assessed.

Residual stress effects on fatigue life prediction using hardness measurements for butt-welded joints made of high strength steels

The fatigue resistance of welded connections made of high strength steel (HSS) is one of the most important topics for the application of HSS in the construction sector. One of the most challenging issues is how to predict the fatigue life of welded structures with complex geometry based on the test results from relatively simple coupon specimens. However, there are generally pre-existing residual stresses in the welded coupon specimens during fatigue tests, and these residual stresses vary greatly in welded structures with complex geometry. This increases the difficulty in predicting the fatigue behaviour of welded structures based on results at coupon scale. Hence, it is important to establish a relationship between the residual stress independent material characteristics and fatigue life. The fatigue behaviour of complex welded structures can be predicted by this residual stress independent material characteristics calibrated at the coupon level and simulated local residual stress distribution. In this paper, the residual stress-free characteristics, hardness, is employed to predict the fatigue life of butt-welded joints. Besides, the residual stress of V-shaped butt welds on a plate made of high strength steels are analysed by modelling of the welding process based on subsequent thermal analysis and mechanical stress analysis by implementing kill/birth strategies. The results show that it contributes to a better prediction compared with experimental results after considering the residual stress effects.

Experimental study on fatigue performance of Q420qD high-performance steel cross joint in complex environment

The steel structure under the action of alternating load for a long time is prone to fatigue failure and affects the safety of the engineering structure. For steel structures in complex environments such as corrosive media and fires, the remaining fatigue life is more difficult to predict theoretically. To this end, the article carried out fatigue tests on Q420qD high-performance steel cross joints under three different working conditions, established a 95% survival rate S{ - }N curves, and analyzed the effects of corrosive media and high fire temperatures on its fatigue performance. And refer to the current specifications to evaluate its fatigue performance. The results show that the fatigue performance of the cross joint connection is reduced under the influence of corrosive medium, and the fatigue performance of the cross joint connection is improved under the high temperature of fire. When the number of cycles is more than 200,000 times, the design curves of EN code, GBJ code, and GB code can better predict the fatigue life of cross joints without treatment, only corrosion treatment, and corrosion and fire treatment, and all have sufficient safety reserve.

Reliability analysis for TO-247 multilayered power module packaging under mechanical oscillation based on finite element method

This paper presents the simulation and hardware test results for determining the fatigue life of the solder joints of a printed circuit board (PCB) including a DC to AC inverter circuit with six power metal oxide semiconductor field effect transistors (MOSFETs) by using the finite element method (FEM) under different vibration effects. This board is exposed under different angles by a vibration machine. The selected angles were performed on the power module to find the maximum stress points of the power module for different vibration frequencies. The results indicate that the maximum stress is observed at the corners of the solder layer. The stress of the solder joints significantly increases when the input frequency increases, and the failure, voids generation and crack formation and propagation are more observable under this condition. Furthermore, the loading direction changes show that the peeling stress of the solder layer is directly affected by lower angles. The shear stress occurs in lower angles of loading directions, and this situation results in the peeling stress. The simulation investigations are proved by the experimental results in the study. According to the observations, the fatigue effects result in the coalescence and the void growths in lower angles of vibration loadings. The paper also investigates the normal loading condition of the power inverter circuit, and the highest reliability is obtained compared to all other experimental tests.

Fatigue assessment of laser beam and friction stir welded joints made of aluminium

Available fatigue recommendations are typically developed for arc-welded joints. Some of them may need to be adjusted to fit fatigue life of joints manufactured with non-conventional welding techniques since they show different mechanical properties. Laser beam and friction stir welded butt and overlap joints made from aluminium are addressed in this paper focused on a comparison between different fatigue assessment methods. For an evaluation, S-N data from published papers was collected. Additionally, fatigue tests on laser beam and friction stir welded overlap joints were carried out. For a fatigue assessment, the nominal stress, notch stress and effective stress approach were applied. The comparison of the endurable nominal stresses showed a comparatively high fatigue strength of the tested friction stir welded overlap joints in comparison to literature data. The notches at the interface of the overlap joints were observed to have an asymmetric geometry compared to the common symmetric one, frequently found in literature, leading to a lower stress concentration. A comparison between endurable nominal stresses and the classes FAT 28 for butt welded and FAT 12 for overlap welded joints resulted in a conservative design for the majority of specimens. An evaluation based on notch stresses lead to partly non-conservative results that could be explained by the mild notches present at the laser-beam welded butt joints. For the effective stress approach, an evaluation of the micro-structural length was performed and a FAT-value is proposed.

Investigation on low cycle fatigue properties of CLAM steel electron beam welded joint at 550 °C

The low cycle fatigue (LCF) properties of structural materials at high temperatures are important for operational safety of fusion reactors. In this work, the investigation on LCF properties of CLAM steel joints welded by electron beam was conducted at strain amplitudes varying from 0.2 % to 0.7 % at 550 °C. It is found that the fatigue life and cyclic stress response of welded joints are lower than those of base metal. Continuous cyclic softening is observed in welded joints and base metal during LCF test. A majority of the fatigue cracks initiated at the surface of the the welded joint specimen with several initiation sites and propagated in a transgranular mode. The area of the crack propagation region decreased and the fatigue striation spacing increased with increasing strain amplitude. The cyclic softening is attributed to the coarsening of martensite laths and the reduction in dislocation density.

Thermal Fatigue Life Prediction of BGA Solder Joints Using a Creep Constitutive Equation Incorporating Microstructural Coarsening Effect

A thermal fatigue life prediction method is proposed for Sn–Ag–Cu solder joints that incorporates the effect of creep strength reduction due to microstructural coarsening of the solder during thermal cycling. The proposed method was used to predict the thermal fatigue life of solder joints in a BGA (ball grid array) semiconductor package. In the study, the thermal fatigue life was calculated through as follows. A creep constitutive equation for the solder that incorporates the strain-enhanced growth of the intermetallic compounds in the solder was used to update the constitutive equation at each given cycle and the relationship between inelastic strain energy density range and the number of cycles of the solder joint was obtained to calculate the fatigue life. As the number of thermal cycles increased, the intermetallic compounds grew, and in conjunction with this, the inelastic strain energy density range increased, causing the thermal fatigue life of the solder joint to decrease compared with that without the microstructural coarsening effect. Since the growth of intermetallic compounds under field conditions with a long dwell time is large compared with that during accelerated tests, the decrease in thermal fatigue life due to microstructural coarsening was found to be significant under field conditions with a long dwell time.

Fatigue response of friction stir welded joints of Al 6061 in the absence and presence of inserted copper foils in the butt weld

The present study examines the impact of inserted copper foil on n mechanical characteristics and fatigue strength of aluminum 6061 butt joints welded through the Friction Stir Welding (FSW) process. Thin copper foils were inserted between the faying surfaces of the sheet joints alloying Al 6061 locally with copper foil over FSW operation. Test specimens were then cut off from the welded plates to examine the quasi-static and fatigue behavior of the joints. The load-controlled condition tests were conducted on the specimens with a stress ratio of R = 0.1. Insertion of copper foils of 100 μm and 200 μm in the stirred zones, improved tensile strengths respectively 48 % and 31 %. The fatigue life of alloyed joints was improved as compared to as-welded joints in the absence of foils. A three-dimensional finite element (FE) analysis was employed to simulate the maximum stress and strain values in the weld zones. Weld regions due to their different mechanical properties experienced different stress magnitudes. The simulated stress and strain components were then used to predict uniaxial fatigue lives of FSW joint samples through the Smith-Watson-Topper (SWT) approach.

Fatigue behavior of low alloy structural steel single-lap bolt-welded joint

The fatigue behavior of Q345B low alloy structural steel single-lap bolt-welded joint is studied compared to bolted and welded joints. A series of static and fatigue experiments of bolt-welded, bolted and welded joints were carried out, and its static finite element analysis were performed. The experiments showed that the fatigue failure modes of bolt-welded and welded joints are fatigue fracture at weld toe. With the increase of stress levels, the location of crack initiation trends to move from weld toe to weld root. Compared to welded and bolted joints, the usage of welds and bolts in bolt-welded joint can enhance fracture resistance of the joint. The welds and bolts of bolt-welded joint can improve the bearing capacity. The nominal stress, hot spot stress (HSS) and local strain energy density (SED) approaches were used to estimate fatigue life of bolt-welded and welded joints. The local SED of bolt-welded joint shows a smaller scatter band in fatigue life estimation compared to the nominal stress and HSS method. The results showed the bolt-welded joint has a higher fatigue strength, and the fatigue assessment of bolt-welded joint under local SED can better estimate the fatigue life. This study provides theoretical and experimental references for the fatigue design of single-lap bolt-welded joint.

An Experimental Investigation on the Repairing Performance and Fatigue Life of Asphalt Pavement Potholes With an Inclined Interface Joint

A pothole is a typical structural damage of asphalt pavements that significantly influence the life of asphalt pavements and driving safety. The durability of the existing pit repair methods is generally low. The existing studies in the context of pothole repair mainly focus on the selection and the amount of tack coat materials, nonetheless, very limited studies emphasize the effect of the joint interface shape. This study aims to investigate the influence of the interface joint shape on the service life of pothole repair by experimental testing. The strength and fatigue behavior of the joints were studied and the effectiveness of pothole repairs was evaluated under various conditions, including four temperature levels (5, 10, 15 and 25°C), four strain levels (750 με, 1,000 με, 1,250 με, and 1,500 με) and three loading frequencies (2, 5, and 10 Hz). The optimal interface joint shape was obtained by orthogonal tests. The results indicated that the bond strength and fatigue life of the high viscoelastic emulsified asphalt with an area density of 0.6 kg/m2 in the form of a 30° inclination joint was 473 and 80 times higher than those of traditional pothole repair (i.e., vertical joint form and no tack coat), respectively. Finally, a prediction model was proposed for the interface joint fatigue life considering external parameters through multiple regression analyses. This prediction model can provide a reference for the further study of asphalt pavement pothole repair.

Effect of Water Environment on Fatigue Behavior in X80 High Strength Steel CO2 Arc Welding Welded Joint

Fatigue life tests and fatigue crack growth rate (FCGR) tests in the air and water environment were conducted on X80 pipeline steel welded joints (welded by CO2 arc welding). Scanning electron microscope (SEM) and electron backscatter diffraction (EBSD) were utilized to investigate the internal influential mechanisms of the water environment during fatigue crack initiation and propagation stages, respectively. Results show that a great many oxide particles induced by the water environment gradually formed the fatigue crack initiation site and decreased fatigue life of welded joints. Meanwhile, the preferred grain orientation of <001>//ND and CSL boundaries of Σ3, Σ11, Σ13c, Σ17b, Σ25a, and Σ25b are both prone to fatigue propagation when loading in the water environment. In addition, a coalescence of the stress intensity factor (SIF) range and water environment accelerated FCGR by motivating secondary slip systems of {112}<111> and {123}<111> in bcc crystalline structures.

Effect of Stress Gradient on Heat Cycle Fatigue Life Prediction of Solder Joints by Repetitive Bending Test

Strength of solder joints is usually evaluated by a shear test and a pull test. The reliability of the solder joint is evaluated by the repetitive pull tests of solder bulk specimens. However, the stress and strain field that caused by thermal load on the solder joint of the product model for estimating the reliability is different from these tests. Therefore, we proposed a repetitive bending test as a reliability test of solder joints producing the stress and strain field caused at the solder joint of product model. We proposed a repetitive multi-point bending test as a method to predict the fatigue life of the solder joint in the thermal cycle test in a short period of time. The influence of strain gradient on the inelastic strain amplitude used for lifetime evaluation is estimated. The controllability of the strain gradient by the three-point bending test parameters is investigated. The effect of residual stress on inelastic strain amplitude during sample preparation for thermal cycle test is also evaluated.

Improvement of fatigue life of riveted joints in helicopter airframes

Using original cold-formed rivets in repairs of airframes of helicopters is difficult due to no access to inside parts of the airframe. Thus, the main aim of the study was to investigate the possibility to use the blind rivets or hybrid joints by verification the fatigue performance of such joints that must be better than with original rivets. Riveted and hybrid joints have been experimentally tested under static and fatigue loads. Furthermore, numerical calculations of stress distribution for strapped joint have been conducted. The test results covered fatigue life of lap joints and models of repaired airframe sheets using ordinary mushroom head rivets ref. 3558A-4-10, titanium driven blind bolts with pin, ref. MBF2110AB-05-150 and modified hybrid joints. Using titanium driven blind bolts with pin instead of ordinary hammer-bucked rivets, can improve the fatigue life of element made of aluminum alloy AW 2024T3. There are advantages of replacing riveted joints with modified hybrid (rivet & adhesive) joints in threefold increase in fatigue life of repaired airframe structures.

Forming Quality and Fatigue Behavior of Self‐Piercing Riveted Joints of DP590 and AA6061 Plates

Two kinds of self‐piercing riveted (SPR) joints were prepared with DP590 and AA6061 plates. The forming qualities of the joints were studied using the finite element method. The relationships between the fatigue life and failure forms of the joints with different upper plates were discussed. Finally, the failure mechanisms of the joints were analyzed. The results show that the maximum static tension of DA32 joints (with an upper plate of DP590) is significantly greater than that of the other kind, and the fatigue life of DA32 joints is always longer than that of AA40 joints when with a 2.0 mm thick AA6061 aluminum alloy upper plate under the same fatigue load. The failure mode of SPR joints changes obviously due to different upper plates, and the fatigue life of the joints can be effectively improved by reducing the microvibration wear.

ИССЛЕДОВАНИЕ ВОЗДЕЙСТВИЯ АГРЕССИВНЫХ ФАКТОРОВ ВНЕШНЕЙ СРЕДЫ НА РАЗВИТИЕ КОРРОЗИОННЫХ ПОРАЖЕНИЙ НА ОБРАЗЦАХ СЛОИСТОГО МЕТАЛЛОСТЕКЛОПЛАСТИКА КЛАССА СИАЛ

The article presents the results of examination of the influence of aggressive environmental factors during tests in a salt fog chamber and moisture saturation on the strength characteristics and low-cycle fatigue of riveted joints of laminated glass-reinforced plastic with and without an integrated protection system. It is shown that the multilayer structure of SIAL prevents the development of corrosion damage in layers along the depth of the material. The mechanical properties of samples made of SIAL material with three-layer and five-layer structure after moisture saturation practically do not change. It was found that the lack of corrosion protection of the laminated material leads to a decrease in the fatigue life of riveted joints.

Optimization of fatigue life and fractography analysis of knuckle joint

Steering knuckle is an important component of a vehicle suspension system, which acts as a connection point between the wheel hub and the suspension system. Since, the forces coming from the tires are transferred to knuckle with high varying loads. The design of a knuckle becomes very critical in terms of its life with high factor of safety. At the same time, the design also should be robust with the required fatigue life, right material selection and weight optimization, without affecting the desired performance. There has been a considerable effort placed in this direction to reduce the overall weight of the knuckle system by employing high strength alloys instead of conventional materials or optimizing the geometry without altering the desired functionality and its fatigue life. This project focuses on optimization and fatigue life estimation of an optimized Knuckle using Finite Element Analysis approach and fatigue tester. This is done by studying and comparing the results of a rigid baseline model with that of an optimized model through simulation and experimentation. The optimized model is arrived based on the stresses obtained from the baseline study. The fatigue life of the optimized model is evaluated under fatigue tester and fractography analysis is carried out.

Comparison of Findley and MWCM multiaxial fatigue methods using the notch stress method on welded joints

Fatigue assessment of welded joints is vital for accurately designing and predicting lifetime of many engineering structures. Assessing weld fatigue is challenging due to the high variability in weld quality and geometry because of the volatile nature of the joining method and further challenges arise when assessing multiaxial fatigue. In this paper, two approaches, the Findley criterion and Modified Wöhler Curve Method, on several multiaxial experimental data sets of welded joints are compared to predict the fatigue life of the welded joints. Six experimental sets from literature are tested under a proportional torsion/bending multiaxial load. The notch stress method is used for calculating stresses at the toes of a weld.

Paper Review of “Effect of heat affected zone microstructure behavior under cyclic loading on fatigue life of weld joint”

Paper Review of “Effect of heat affected zone microstructure behavior under cyclic loading on fatigue life of weld joint”

Finite element shape optimization of weld orientation in simple plate structure considering different fatigue estimation methods

Dynamically loaded structures are generally in risk of fatigue damage. The fatigue lives of such structures can be improved using traditional methods like producing a better weld or using post-weld treatment. Another method is to improve the overall design of the structure by using the finite element method in combination with shape optimization. In this paper, a framework for finite element shape optimization has been developed, considering the fatigue life of welded joints. Simplicity and the possibility of including different fatigue estimation methods have been in focus for the framework. The framework has been used on a simple plate structure, where the weld orientation has been optimized based on four different fatigue criteria. The results show that the criteria from governing guidelines predict the same optimized weld orientation. However, the critical plane approaches cannot produce optimized weld orientations, when using the nominal stress approach as it does not account for increased stresses from the weld.

Mechanical behaviors of electron beam welded titanium alloy up to very high cycle fatigue under different process conditions

The mechanical behavior of electron beam welded joints for titanium alloy has been investigated in static and cyclic loading up to very high cycle fatigue. It can be concluded that a higher welding voltage parameter and optimal heat treatment condition in combination will produce higher static and cyclic properties of titanium alloy welded joints. The metastable martensite α′ structures in the β grains of the fusion and heat-affected zones are wholly decomposed into small lamellar α phases, forming α + β phases through the post-weld heat treatment (PWHT). Hence, the welded joints by PWHT could transform the “concave” shape microhardness distribution into “convex” shape distribution. The fatigue cracks are initiated from the internal pores of the material in very high cycle fatigue regime. The higher voltage parameter applied to the welding process could reduce the density of pores in the material, which is in favour of the fatigue life of the welded joints. • A higher voltage and optimal heat treatment in combination produce higher mechanical properties of welded joints. • Microstructure and microhardness of welded joints were analyzed before and after heat treatment. • The higher voltage reduced the pores density in the welded joints, which improved fatigue life.