Fatigue Failure Behavior Research Articles

Experiment and modeling on fatigue of the DP780GI spot welded joint

Fatigue life prediction of advanced high strength steels (AHSS) spot welded joints is of great importance for autobody design with AHSS being increasingly used. In the present work, the fatigue failure behaviors of the DP780GI spot welded joints under the tensile shear (TS) and coach peel (CP) loading conditions are experimentally investigated and numerically modelled. Two failure modes, eyebrow failure and weld button pull out occur in the tests of TS joints, while only interfacial fracture exists in CP joints. A fatigue life prediction method based the crack growth approach is proposed. The TS/CP specimens are simplified to the neural line models, by which the reaction forces and moments directly acting on the spot welds are calculated. Then the stress intensity factor is obtained by using the finite element method and analytical equations fitted to the FE results. Finally, the fatigue life of a spot weld is evaluated by applying the Paris law to the crack front and including the plastic zone in crack path update. The calculated fatigue lives show good agreement with experiment results for DP780GI, within five times error band.

Strain energy density based fatigue cracking assessment of load-carrying cruciform welded joints

This paper presents fatigue failure crack initial points and the fatigue failure mode transition relationship between the weld toe and root of load-carrying cruciform welded joints on the basis of LEFM and Williams’ notch stress intensity theory. The notch stress intensity factors (NSIFs) of different failure locations were calculated quantitatively. The averaged strain energy density (SED) in a control volume near the notches is used to unify the scalar quantity and rectify inconsistencies of NSIFs units for the weld toe and root. The failure mode transition relations of different geometrical parametric models (such as main plate thickness, attachment plate thickness, weld length, and penetration size) were compared and analyzed based on the SED values from weld toe and root locations, and the effect of unequal main plate thickness ratio in a cruciform joint on failure mode was also investigated. The results show that SED approach can be used to accurately predict the transition region of equal base plate thickness. The weld length and penetration size are related to significant changes in the fatigue failure location of cruciform joints. The SED approach effectively reflects failure characteristics and can be used to assess fatigue failure behaviors for load-carrying cruciform welded joints.

A new, direct approach toward modeling thermo-coupled fatigue failure behavior of metals and alloys

The objective of this study is two-fold. Firstly, new finite strain elastoplasticity models are proposed from a fresh standpoint to achieve a comprehensive representation of thermomechanical behavior of metals and alloys over the whole deformation range up to failure. As contrasted with the usual elastoplasticity models, such new models of much simpler structure are totally free, in the sense that both the yield condition and the loading–unloading conditions need not be introduced as extrinsic coercive conditions but are automatically incorporated as inherent constitutive features into the models. Furthermore, the new models are shown to be thermodynamically consistent, in a further sense that both the specific entropy function and the Helmholtz free energy function may be presented in explicit forms, such that the thermodynamic restriction stipulated by Clausius–Duhem inequality for the intrinsic dissipation may be identically satisfied. Secondly, it is then demonstrated that the thermo-coupled fatigue failure behavior under combined cyclic changes of stress and temperature may be derived as direct consequences from the new models. This novel result implies that the new model can directly characterize the thermo-coupled fatigue failure behavior of metals and alloys, without involving any usual damage-like variables as well as any ad hoc additional criteria for failure. In particular, numerical examples show that, under cyclic changes of temperature, the fatigue characteristic curve of fatigue life versus temperature amplitude may be obtained for the first time from model prediction both in the absence and in the presence of stress. Results are in agreement with the salient features of metal fatigue failure.

Effects of laser cladding on fatigue performance of AISI 4340 steel in the as-clad and machine treated conditions

Laser clad specimens were fabricated to characterize the axial fatigue failure behavior of laser clad 4340 steel in the as-clad condition and for specifically designed clad specimens with pre-clad machining and post-clad surface grinding processes. Three types of laser clad fatigue test specimens (Type I, II and III) were tested to study fatigue failure behavior and were compared to the unclad 4340 steel substrate specimen. Type I specimen investigates fatigue failure behavior from the clad toe region of as-clad specimens. Type II specimen evaluates fatigue strength enhancement effects by pre-clad machining of a groove for depositing a clad layer with post-clad surface grinding to process the added clad layer in the groove to be flush with the substrate material. Type III specimen studies clad specimens with extended clad area and surface grinding to promote fatigue failure within the clad region. The fatigue S-N curve for Type I, II and III clad specimens were compared to the S-N curve for the substrate specimens to show the reduction in fatigue strength for as-clad Type I specimens, and machining methods to improve fatigue strength for Type II and Type III specimens. The fatigue strength of all laser-clad specimens was observed to be lower as compared to that of the unclad substrate specimen.

Thermo-Mechanical and Low Cycle Fatigue Failure Behavior Relevant to Temperature Regime in a TBCed Superalloy Specimen

Steady state and non-steady state thermo-mechanical fatigue failure is great concern in this work. At first steady state thermo-mechanical fatigue failure behavior was investigated using the round-bar TBC specimens, after getting basic data of mechanical properties of the bond/top coats and the substrate alloy. The failure behavior was compared with that during isothermal low cycle fatigue (LCF). Next non-steady state TMF tests were carried out in which non-steady state thermal stress was significant in the TBC specimen, compared with the properties under the steady state TMF. The experimental work clearly demonstrated that the TMF failure lives were significantly changed depending on the temperature regime during TMF and LCF. Of particular importance was found in the non-steady state TMF tests. The non-steady state TMF cycling promoted the delamination of ceramic top coat, resulting in a significant reduction in fatigue life.

Fatigue behavior of resin-modified monolithic CAD–CAM RNC crowns and endocrowns

ObjectiveTo evaluate the influence of different types of modifications with resin on fatigue resistance and failure behavior of CAD–CAM resin nano ceramic (RNC) restorations for maxillary first premolars. MethodsSixty standardized resin composite root dies received CAD–CAM RNC endocrowns (n=30) and crowns (n=30) (Lava Ultimate, 3M Espe). Restorations were divided into six groups: full anatomic endocrowns (group A) and crowns (group D), buccal resin veneered endocrowns (group B) and crowns (group E) and buccal resin veneered endocrowns (group C) and crowns (group F) with a central groove resin filling. A nano-hybrid resin composite was used to veneer the restorations (Filtek Supreme, 3M Espe). All specimens were first submitted to thermo-mechanical cyclic loading (1.7Hz, 49N, 600000 cycles, 1500 thermo-cycles) and then submitted to cyclic isometric stepwise loading (5Hz) until completion of 105000 cycles or failure after 5000 cycles at 200N, followed by 20000 cycles at 400N, 600N, 800N, 1000N and 1200N. In case of fracture, fragments were analyzed using SEM and modes of failure were determined. Results were statistically analyzed by Kaplan–Meier life survival analysis and log rank test (p=0.05). ResultsThe differences in survival between groups were not statistically significant, except between groups D and F (p=0.039). Endocrowns fractured predominantly with a mesio-distal wedge-opening fracture (82%). Partial cusp fractures were observed above all in crowns (70%). Analysis of the fractured specimens revealed that the origin of the fracture was mainly at the occlusal contact points of the stepwise loading. SignificanceVeneering of CAD–CAM RNC restorations has no influence on their fatigue resistance except when monolithic crowns are modified on their occlusal central groove.

Prediction of progressive fatigue damage and failure behavior of IM7/977-3 composites using the reduced-order multiple space-time homogenization approach

This manuscript presents the blind prediction of fatigue life performance in three laminated carbon fiber reinforced polymer composite layups using a reduced-order space-time homogenization model. To bridge the spatial scales, the modeling approach relies on the Eigendeformation-based reduced order homogenization method. To bridge the time scales associated with a single load cycle and the overall life of the composite, a homogenization-based accelerated multiple-time-scale integrator with adaptive time stepping capability is employed. The proposed multiscale modeling approach was used to predict the evolution of composite stiffness and progressive damage accumulation as a function of loading cycles, as well as residual strength after fatigue in tension and compression, for three layups ([0,45,90,−45]2 s, [30,60,90,−60,−30]2 s, and [60,0,−60]3 s). Following blind prediction, the experimental data from the blind prediction specimens were employed to better understand the failure mechanisms and recalibrate the model. This study was performed as a part of the Air Force Research Laboratory's “Damage Tolerant Design Principles” Program.

Competing fatigue failure behaviors of Ni-based superalloy FGH96 at elevated temperature

Fatigue experiments were performed on a polycrystalline P/M processed nickel-based superalloy, FGH96 at 600°C to investigate competing fatigue failure behaviors of the alloy. The experiments were performed at four levels of stress (from high cycle fatigue to low cycle fatigue) at stress ratio of 0.05. There was large variability in fatigue life at both high and low stresses. Scanning electron microscopy (SEM) was used to analyze the failure surfaces. Three types of competing failure modes were observed (surface, sub-surface and internal initiated failures). Crack initiation sites were gradually changed from the surface to the interior with the decreasing of stress level. Roles of microstructures in competing failure mechanism were analyzed. There were six kinds of fatigue crack initiation modes: (1) surface inclusion initiated; (2) surface facet initiated; (3) sub-surface inclusion initiated; (4) sub-surface facet initiated; (5) internal inclusion initiated; (6) internal facet initiated. Inclusions at surface were the life-limiting microstructures at higher stress levels. The probability of occurrence of inclusions initiated is gradually reduced with decreasing of stress level, simultaneously the probability of occurrence of facets initiated is increasing. The existence of the inclusions resulted in large life variability at higher stress levels, while heterogeneity of material caused by random combinations of grains was the main cause of fatigue variability at lower stress levels.

Experimental and numerical investigations on extremely-low-cycle fatigue fracture behavior of steel welded joints

Beam-to-column welded joints in moment-resisting steel frames may undergo extremely low cycle fatigue in earthquake action. The aim of this paper is to investigate the fatigue failure behavior by means of experimental study and finite element analysis. Experiments were conducted on the six full scale beam-to-column welded joints. The seismic performance of welded joints with different weld access hole geometries was compared among them and the effect of crack initiation and growth on the load carrying capacity of welded joints was discussed. In addition, a continuum damage mechanics model used previously for monotonic loading was reformulated in order to account for the extremely low cycle loading condition. The model was implemented in a commercial nonlinear finite element software of ABAQUS, which was adopted to predict the fracture of beam-to-column welded joints subjected to monotonic loading and extremely low cycle loading. The predicted load displacement response agrees well with the test results. The distribution and evolution of damage in the welded joints were calculated during crack initiation and propagation. It is found that the methodology based on continuum damage mechanics proposed in the paper can successfully predict fracture behavior of beam-to-column welded joint with reasonable accuracy.

Experimental and analytical verification of the characteristics of shear fatigue failure in the adhesive interface of porous foam materials

In the past, many studies have been conducted to examine the effect of static load and fatigue load on the adhesive interface between two different materials or the same materials, but little research has been done on porous materials. Thus, this study was carried out to examine the effect of fatigue load on the adhesive interface formed by aluminum foam, which exhibits porous characteristics. For the experiment, five specimens were fabricated with the thicknesses varied in increments of 10 mm from 25 mm to 65 mm. The aluminum foam was bonded using the single-lap method, and MTS landmark was used to conduct the fatigue experiment. Based on the initial static experiment, the maximum reaction force at which total failure occurred in the adhesive interface was obtained, and fatigue load was applied on the lower load cell in the 10 Hz sine graph form. The results of the experiment showed that for all five of the specimens, the adhesive strength of the adhesive agent was maintained in the adhesive interface during the 5000 cycle of the fatigue load. Also, based on the correlation between displacement and repeated load cycles, it was discovered that the adhesive interface underwent total failure after a sharp displacement in the interface in all five cases when the load was repeated for more than 5000 cycles In addition, a numerical analysis was performed based on the experimental results, and the stress distribution was visualized. The numerical analysis results showed similar tendencies as the experimental results, which confirmed the reliability of the analysis results. Thus, it was deemed that it would be possible to analyze the fatigue failure behavior of actual, bonded structures made of a porous material based on the experimental and numerical analysis results obtained through this study.

Investigation on the effect of Sn grain number and orientation on the low cycle fatigue deformation behavior of SnAgCu/Cu solder joints

Generally, lead-free solder joints in electronic device are near single or multi crystalline rather than poly crystalline, and the fatigue failure behavior of solder joints is greatly influenced by the solder joint thickness. In this study, the grain number and orientation of solder joints with four solder thicknesses (0.1, 0.2, 0.3, 0.6 mm) were analyzed and the effect of that on the low cycle fatigue deformation behavior of solder joint was also investigated. The orientation analysis was conducted by means of electron backscatter diffraction and the fatigue test was performed under room temperature with a fixed frequency of 1 Hz. Moreover, the deformation evolution on side surface of solder joint was observed by utilizing scanning electron microscope. It is shown that there is no preferred solidification orientation for lead-free solder joints. However, grains in one joint seem to have a similar orientation when the solder thickness is <0.3 mm. Solder joint grain number increases with solder thickness. Besides, the slip direction and slip band density are different for grains with different orientations. Consequently, the deformation on both sides of the grain boundary is inhomogeneous and the fatigue crack is prone to initiate and propagate in this area.

Low-cycle fatigue failure behavior and life evaluation of lead-free solder joint under high temperature

In this study, low-cycle fatigue test was conducted for a lead-free solder joint at two test temperatures (348K, 398K) and three strain amplitudes (3%, 4%, and 8%). Fatigue failure behavior was analyzed and the fatigue life was evaluated using the Coffin–Manson relationship and Morrow energy-based model. The results show that the maximum load gradually drops with increasing the number of loading cycles. When the strain range or temperature is low, the maximum load drop curve can be divided into three stages. Then, it degrades into a linear stage with increasing the strain range or temperature. Both the softening of solder and the reduction of effective load-bearing area are responsible for the maximum load drop depending on the test condition. Fatigue crack initiates at the corner of the solder joint and propagates along the strain concentrated zone. Spacing distance between fatigue striations is enlarged with increasing the temperature in accordance with the degradation of fatigue resistance. In addition, both the Coffin–Manson model and Morrow energy-based model can be used to evaluate the fatigue life of solder joint under high temperature. The fatigue ductility exponent α in Coffin–Manson model and the fatigue ductility coefficient C in Morrow model are dependent on temperature, whereas other parameters in these two models keep stable under different temperature.

Frequency-dependent failure mechanisms of nanocrystalline gold interconnect lines under general alternating current

Thermal fatigue failure of metallization interconnect lines subjected to alternating currents (AC) is becoming a severe threat to the long-term reliability of micro/nanodevices with increasing electrical current density/power. Here, thermal fatigue failure behaviors and damage mechanisms of nanocrystalline Au interconnect lines on the silicon glass substrate have been investigated by applying general alternating currents (the pure alternating current coupled with a direct current (DC) component) with different frequencies ranging from 0.05 Hz to 5 kHz. We observed both thermal fatigue damages caused by Joule heating-induced cyclic strain/stress and electromigration (EM) damages caused by the DC component. Besides, the damage formation showed a strong electrically-thermally-mechanically coupled effect and frequency dependence. At lower frequencies, thermal fatigue damages were dominant and the main damage forms were grain coarsening with grain boundary (GB) cracking/voiding and grain thinning. At higher frequencies, EM damages took over and the main damage forms were GB cracking/voiding of smaller grains and hillocks. Furthermore, the healing effect of the reversing current was considered to elucidate damage mechanisms of the nanocrystalline Au lines generated by the general AC. Lastly, a modified model was proposed to predict the lifetime of the nanocrystalline metal interconnect lines, i.e., that was a competing drift velocity-based approach based on the threshold time required for reverse diffusion/healing to occur.

TESTING THE FLEXURAL FATIGUE BEHAVIOR OF E-GLASS EPOXY LAMINATES

With the advent of fiber reinforced composite materials the application of polymer composite materials is increasing in aerospace, marine and automobile fields, and non-conventional energy harvesting sectors in view of deriving advantages of polymer composites towards enhancing the equipment performance. Composites are replacing the conventional materials almost in every application. Due to their high specific strength, stiffness, better fatigue life, wear resistance and corrosion resistance. So polymer composites play a vital role in weight reduction with better functional characteristics of critical components like leaf spring, wheel drum, crash bumper etc. The tailor-designing capability with composite materials provides large scope of mechanical properties by adopting advanced design techniques. The tailored properties of laminated composite throws a challenge on design engineers to arrive at optimum design solutions. The flexural fatigue failure is common in rotating components such as wind turbine rotors and aero plane wing structures. The present work is focusing on flexural fatigue failure behavior with the help of flexural fatigue test rig established in the mechanical engineering department of JNTU. The explicit work publishes the experimental test results to understand the influence of frequency on the fatigue failure behavior due to cyclic flexing loads on the glass epoxy laminates

Effect of hold time on the mechanical fatigue failure behavior of lead-free solder joint under high temperature

In this paper, effect of hold time on the low cycle mechanical fatigue failure behavior of Sn3.0Ag0.5Cu solder joint was investigated. Fatigue tests were conducted utilizing trapezoid waveform with four hold times (0, 2, 5, 10 s) under 348 K. The results show that samples tested under waveform with hold time has the faster maximum stress drop rate comparing to that without hold time. Accordingly, the fatigue life of solder joint is declined with increasing hold time. However, there exists a threshold value, and when the hold time exceeds this value, the fatigue life changes little. Stress relaxation occurs during the hold time, which is completed in a short time under high temperature. Deformation of the solder joint is not uniform, which is concentrated in the solder matrix near the substrate. Micro crack initiates at the corner of the solder joint and propagates in the strain concentrated zone with a direction parallel to the applied load. In addition, Creep voids are observed from the fracture morphology of solder joints tested with hold time, which nucleate at the slip bands and the dendrite boundary tensile ledges. Moreover, the fracture morphology appears an inter-granular rupture mode. However, for solder joints tested without hold time, the rupture mode is trans-granular with some dimples dispersed on the morphology.

Investigation on high temperature mechanical fatigue failure behavior of SnAgCu/Cu solder joint

In this paper, high temperature mechanical fatigue tests on SnAgCu/Cu solder joints were carried out under three test temperatures (100, 125, 150 °C). Failure mechanism was analyzed through observation of micro-crack evolution and fracture morphology. The results show that the deformation curve of solder joint under high temperature mechanical fatigue tests can be divided into three stages: strain hardening stage, stable deformation stage and accelerated failure stage, which is similar to the curve under creep test condition. In addition, the cyclic life decreases rapidly with increasing temperature. Deformation field in the solder joint is non-uniform and shear strain concentration occurs in solder close to the intermetallic compound (IMC) layer. Micro-crack initiates at the corner of the solder joint and then tend to propagate along interface between Cu substrate and solder. The fracture morphology under three temperatures all exhibits ductile fracture mode and the failure path transforms from cutting through the top of Cu6Sn5 to propagation in solder matrix close to IMC layer with increasing temperature.

Fatigue failure and electrical resistance behaviors of carbon nanotube-based polymer composites under uniaxial tension–tension loading in a cryogenic environment

This paper investigates the fatigue failure and electrical resistance behaviors of carbon nanotube-based polymer composites at cryogenic temperatures. Tension–tension fatigue tests were performed on carbon nanotube /polycarbonate composites at room temperature and liquid nitrogen temperature (77 K), and the electrical resistance of the specimens was monitored during the tests. Based on the obtained results, the dependence of the mechanical and electrical responses of the nanocomposites on the temperature and the nanotube content was studied. Microscopic examinations were also carried out on the specimen fracture surfaces, and the failure mechanisms of the nanocomposites were discussed.

Effect of mean shear stress on torsion fatigue failure behavior of 2A12-T4 aluminum alloy

Fatigue experiments with solid cylindrical bar specimens were carried out to investigate the effects of mean shear stress on the torsion fatigue failure behavior of 2A12-T4 aluminum alloy. The torsion S–N curves under different values of mean shear stress were obtained. Experiment results showed that fatigue life under the same shear stress amplitude gradually reduces with increasing the mean shear stress, when mean shear stress is lower than a threshold value. Macro-analysis and micro-analysis of specimen fracture appearance were conducted in order to obtain the fracture characteristics for different mean shear stress values under torsion fatigue loading.

Cyclic deformation and fatigue behaviors of Hadfield manganese steel

The cyclic deformation characteristics and fatigue behaviors of Hadfield manganese steel have been investigated by means of its ability to memorize strain and stress history. Detailed studies were performed on the strain-controlled low cycle fatigue (LCF) and stress-controlled high cycle fatigue (HCF). Initial cyclic hardening to saturation or peak stress followed by softening to fracture occurred in LCF. Internal stress made the dominant contribution to the fatigue crack propagation until failure. Effective stress evolution revealed the existence of C–Mn clusters with short-range ordering in Hadfield manganese steel and demonstrated that the interaction between C atoms in the C–Mn cluster and dislocation was essential for its cyclic hardening. The developing/developed dislocation cells and stacking faults were the main cyclic deformation microstructures on the fractured sample surface in LCF and HCF, which manifested that fatigue failure behavior of Hadfield manganese steel was induced by plastic deformation during strain-controlled or stress-controlled testing.

Ultrasound Signals Response Associated to Fatigue Failure Behaviour using Statistical Analysis

In this paper, ultrasound signals had been analysed using a statistical-based approach to evaluate and predict fatigue failure of carbon steel AISI 1045. Fatigue tests were performed according to the ASTM E466-96 standard with the attachment of an ultrasound sensor to the tested specimen. The material used in this test was the AISI 1045 carbon steel due to its extensive application in automotive and machinery industry. Fatigue test was carried out at a constant loading stress at the sampling frequency of 8 Hz. A set of data acquisition system was used to collect those fatigue ultrasound signals. All obtained data were analysed using specific software. Ultrasound signals were collected during fatigue test in order to detect any structural changes occurs during the test. Fatigue damage characteristics were observed based on the ultrasound signals characteristics and a further analysis was performed using statistical approach. The results of signals distribution, r.m.s value and energy content of the signals were discussed to correlate fatigue failure behaviour and ultrasound signals.