Fatigue Life Behavior Research Articles

Modeling fatigue behavior of quasi-isotropic laminates

Fatigue behavior of quasi-isotropic carbon/epoxy laminates were studied, using six stacking sequences. Experimental results show that there is a significant stacking sequence effect on monotonic and cyclic performance of laminates. Though by changing the stacking sequence the difference between failure strength of the strongest and weakest lay-ups reaches only to ∼ 12%, the difference between fatigue lives (cycles to failure) increased ∼ 1000% in some loading regions. In addition, both linear and sigmoidal functions were examined to model the fatigue life behavior.

Understanding the influence of particle size on strain versus fatigue life, and fracture behavior of aluminum alloy composites produced by spray deposition

The strain versus fatigue life and fracture behavior of spray-formed Al–Si composites reinforced with SiC particles of two different sizes were studied under total strain amplitudes. Both composites exhibit short low-cycle fatigue (LCF) which follows a Coffin-Manson relationship, and display cyclic hardening at all strain amplitudes. The LCF endurance of the composite with large particles is higher than that of composite containing small particles in the high strain amplitudes, however, at low strains the difference in fatigue endurance between the two composites decreased. Moreover, the decrease in particle size results in a higher degree of hardening at low and middle strains, but reduces the magnitude of hardening at highest strain. Fractographic analysis reveals that particle/matrix debonding is the main mechanism of failure in composite with small particles, while fracture and debonding of SiC particle are predominant in the large particle reinforced composite.

Low‐cycle fatigue life behaviour of BS 460B and BS B500B steel reinforcing bars

ABSTRACTThis paper investigates the low‐cycle fatigue resistance of BS 460B and BS B500B steel reinforcing bars and proposes models for predicting their fatigue life based on plastic‐strain (ɛap) and total‐strain (ɛa) amplitudes. Constant‐amplitude, strain‐controlled low‐cycle fatigue tests were carried out on these bars under cyclic load with a frequency of 0.05 Hz. The maximum applied axial strain amplitude (ɛs,max) ranges from 3 to 10% with zero and non‐zero mean strains. The strain ratios (R = ɛs,min/ɛs,max) used are R =−1, −0.5 and 0. Hysteresis loops were recorded and plastic and total strain amplitudes were related to the number of reversals (2Nf) to fatigue failure and models for predicting the number of reversals to fatigue failure were proposed. It is concluded that the predicted fatigue life of these bars is very accurate when compared with the measured experimental fatigue life results for wide range of values of strain ratios. It is also observed that based on plastic‐strain amplitude, BS B500B consistently has a longer life (higher number of cycles to failure) than those of BS 460B for all R values; however, at low plastic‐strain amplitudes they tend to behave similarly, irrespective of R value. Other observations and conclusions were also drawn.

Anisotropic fatigue behavior of rolled Mg–3Al–1Zn alloy

The anisotropy in the fatigue behavior of rolled Mg–3Al–1Zn alloy between the rolling direction and normal direction to the rolling plane was investigated. The {10-12} twinning–detwinning characteristics were found to play key roles in the anisotropic fatigue deformation behavior by inducing a change in the predominant plastic deformation mechanism, which controlled the flow stress and finally influenced the fatigue resistance by generating mean stress. Energy-based approach was successfully used to describe anisotropic fatigue life behavior.

Low-cycle fatigue behavior of AA2618-T61 forged disk

The cyclic stress–strain response and the low-cycle fatigue life behavior of an aluminum alloy AA2618-T61 forged disk were studied. Fully reversed strain-controlled tests were performed at 200 °C in air at a constant total strain rate and under the total strain ranges of 0.5–0.9%. Specimens cut from longitudinal direction of disk displayed cyclic hardening or softening which was dependent on the total strain range. The variation of low-cycle fatigue life with plastic strain amplitude followed a single-slope Coffin–Manson power-law relationship. Fracture of the samples was predominantly ductile fracture of high density microdimples.

Effects of cyclic torsional prestraining and overstrain on fatigue life and damage behavior of brass alloy

Many practical structural members and parts may be subjected to fluctuating plastic deformation by prestraining due to manufacturing and machining process (forming operation, straightening, etc.) and unintentional overstrains (misuse, accidents, under design, etc.). For this reason, the effect of the prestrain and periodic overstrains on fatigue life and damage behavior was being necessary considered for reasonable fatigue design. In this context, an experimental program was conducted to study the effects of overstrain and prestraining on fatigue life and damage behavior of brass alloy subjected to cyclic torsional loading. To establish baseline fatigue behavior, several virgin specimens were tested under fully-reversed strain control and constant amplitude fatigue torsional loading up to failure. The obtained experimental results showed that the fatigue life depends strongly on the strain amplitude and prestraining type (monotonic or cyclic). In addition, a beneficial effect in the fatigue life was observed for all tests with periodic overstrain. Cyclic fatigue fracture on a macroscopic scale revealed features reminiscent of locally ductile and brittle mechanisms. At the same time, microscopic analysis indicated a difference on fatigue fracture surface morphology between the conducted tests and those performed under constant amplitude loading.

The effects of ultrasonic peening treatment on the ultra-long life fatigue behavior of welded joints

The conventional fatigue design codes was formulated based on the data from test stress cycles less than 1 × 10 7. Most of them could not be applied for welded structures serviced in the ultra-long life region so the new recommendations were discussed. The method of ultrasonic peening treatment (UPT) was introduced in this thesis as a post-weld treatment to improve the fatigue strength of the welded structures. The fatigue behavior of welded structures in the ultra-long life region was investigated using an ultrasonic fatigue test machine under two post-weld treatment states: the as-welded and the UPT. Discussion had also been made about the fracture mechanism on the UPT joints in ultra-long life fatigue test.

Structural Appraisal-based Different Approach to Estimate the Remaining Fatigue Life of Railway Bridges

The current fatigue life estimation methods of railway bridges are generally based on combinations of measured strain histories, Miner’s rule, and railway code provided fatigue curve. Even though the past measured strain histories are available for major bridges, most of the old bridges do not have past strain measurements. Furthermore, in case of existing railway bridges where the detailed loading history is known, Miner’s rule might provide incorrect results because of its omission of load sequence effect. These reasons hinder the usage of current methods to estimate the realistic remaining fatigue life of most of the existing railway bridges. Therefore, a new method is presented in this article to estimate remaining fatigue life of riveted railway bridges. The method mainly consists of predicted stress histories, recently developed sequential law and fully known Wöhler curve. Here, it is essential to use the fully known Wöhler curve as the related fatigue curve. Hence, the technique, which utilizes transfer of the partially known Wöhler curve to fully known curve, is also discussed in this article. Further, this method describes a reasonably accurate procedure to obtain the past stress histories from present day measured strains. Initially, the article describes the proposed method for remaining fatigue life estimation. Then the sequential law and associated fully known S-N curve determination procedure are verified by comparing theoretical fatigue life, damage behavior with real fatigue life and damage behavior of few materials. Finally, the remaining fatigue life of an existing railway bridge is estimated. Hence, validity and merits of the proposed method is confirmed by comparing the results with previous method-based fatigue lives.

Fatigue life and plastic deformation behavior of electrodeposited copper thin films

Using a commercial, electrodeposited copper film of 12 μm thickness, tensile tests, fatigue tests and static loading tests were performed. The elastic modulus, two types of yield strengths, tensile strength and strain-hardening exponent were obtained. Fatigue tests were performed at four levels of mean stresses and cyclic plastic strain and ratcheting strain (monotonic plastic strain) were measured continuously during the tests. Irrespective of mean stress, fatigue life can be well represented by the maximum stress. The monotonic plastic strain at fracture is nearly constant, irrespective of fatigue life or mean stress. Based on the results obtained, the reason why fatigue life can be well represented by the maximum stress is discussed.

Static and Fatigue Behavior of Epoxy/Fiberglass Composites Hybridized with Alumina Nanoparticles

Advanced composites are hybridized by the integration of alumina nanoparticles into the matrix and onto the fabric surface. Alumina was pre-dispersed by sonication. Pre-processing of alumina was carried out by resin modification or fiber modification, prior to the consolidation into composite laminates. Alumina nano-particles were also functionalized by silane coupling agent tris-2-metoxyethoxy vinyl silane (T2MEVS). Vacuum assisted resin transfer molding (VARTM) was used to fabricate the composite panels for mechanical performance evaluation and characterization. Material property characterization for tensile, fatigue life and inter-laminar fracture toughness were determined for these hybrid composites and compared with the traditional fiber—matrix composite system as a baseline. Experimental characterization indicated that mode-I fracture toughness was significantly improved with the inclusion of alumina nanoparticles as well as by the functionalization of alumina nano-particles. However, the influences on the tensile behavior and tension/tension un-notched fatigue behavior in [0/90] configuration were not significant. In applications that involve only tension/tension fatigue loading, hybrid composites with nano-particulate inclusions can provide improved delamination failure characteristics without impacting the fatigue life and tensile behavior significantly.

The effect of load ratio on fatigue life and crack propagation behavior of an extruded magnesium alloy

Fatigue experiments were carried out in laboratory air using an extruded magnesium alloy, AZ31, to investigate the effect of load ratio on the fatigue life and crack propagation behavior. The crack propagation behavior was analyzed using a modified linear elastic fracture mechanics parameter, M. The relation crack propagation rate vs. M parameter was found to be useful in predicting fatigue lives at different R ratios. Good agreement between the estimated and the experimental results at each stress ratio was obtained.

Environmental effects on fatigue behavior of adhesively-bonded pultruded structural joints

The fatigue response of adhesively-bonded pultruded GFRP double-lap joints has been investigated under different environmental conditions. Tests were performed at −35 °C, 23 °C and 40 °C. A fourth set of fatigue data was collected from tests on preconditioned specimens in warm (40 °C) water. The tests were performed at 40 °C and at 90% relative humidity. Specimens were instrumented with strain and crack gages to record fatigue data. In addition to the S– N curves, stiffness fluctuations and crack initiation and propagation during fatigue were monitored. The dominant failure mode was a fiber-tear failure that occurred in the mat layers of the GFRP laminates. In the presence of high humidity, the failure shifted to the adhesive/composite interface. Although the testing temperature was lower than the glass transition temperature of the adhesive, its influence on the fatigue life and fracture behavior of the examined joints was apparent and was aggravated by the presence of humidity.

FATIGUE LIFE PREDICTION OF ROLLED AZ31 MAGNESIUM ALLOY USING AN ENERGY-BASED MODEL

Fatigue behavior of rolled AZ31 magnesium alloy, which shows an anisotropic deformation behavior due to the direction dependent formation of deformation twins, was investigated by carrying out stress and strain controlled fatigue tests. The anisotropy in deformation behavior introduced asymmetric stress-strain hysteresis hoops, which make it difficult to use common fatigue life prediction models, such as stress and strain-based models, and induced mean stress and/or strain even under fully-reversed conditions; the tensile mean stress and strain were found to have a harmful effect on the fatigue resistance. An energy-based model was used to describe the fatigue life behavior as strain energy density was stabilized at the early stage of fatigue life and nearly invariant through entire life. To account for the mean stress and strain effects, an elastic energy related to the mean stress and a plastic strain energy consumed by the mean strain were appropriately considered in the model. The results showed that there is good agreement between the prediction and the experimental data.

Tensile-hold low-cycle-fatigue properties of solid-solution-strengthened superalloys at elevated temperatures

Total strain-controlled low-cycle-fatigue (LCF) tests on three solid-solution-strengthened superalloys, HAYNES 188, HAYNES 230, and HASTELLOY X, were carried out at temperatures ranging from 816 °C to 982 °C in laboratory air. Various hold times were introduced at the maximum strain to investigate the high-temperature fatigue and creep–fatigue interactions. The cyclic-stress response and fatigue life of three superalloys were compared. It was found that the fatigue life of three superalloys was dependent on the imposed total strain range, the duration of hold time, and the test temperature. The different fatigue-life behavior indicated by three superalloys appeared to be related to the difference in the damages from creep and oxidation. Several material properties for three superalloys were obtained using the Holloman and Coffin–Manson relations. The fracture surfaces and metallographic sections of the fatigued specimens were observed to determine the crack initiation and propagation modes.

A new scatter band assessment procedure for multi-heat fatigue life data

Two methods for fatigue life data assessment were introduced and verified for low cycle fatigue (LCF) tests on a 10%Cr-steel grade. With these methods mean fatigue life curves were calculated withregression analysis for single temperaturestemperature dependent approach on the basis of a cycle-temperature-parameterThe first method shows an acceptable fit of the experimental data. However, an unfavourable cross-over of mean curves could only be avoided with a graphical optimization, which affects the repeatability of the assessment procedure adversely. Further the description of the plastic strain amplitude is unsatisfactory.The second method represents a new procedure. This temperature dependent description of the fatigue life behaviour is based on a cycle-temperature-parameter which is developed from the hysteresis loop at mid fatigue life time. The necessary data can be derived from the standard assessment of LCF tests. This second method delivers an acceptable description of multi-heat fatigue life in comparison to the first method.Summarizing it can be stated that an analytic procedure was developed which considers the influence of temperature in a suitable way and which allows a determination of mean curves and scatter band parameters in a reproducible manner. Future work will focus on the influence of hold times and of strain rates. An extension of the assessment method for different materials and temperature ranges is also planned.

Fatigue life behaviour of the dual-phase low carbon steel sheets

This paper presents the effect of heat treatment on the fatigue life of low carbon steel sheet with dual-phase microstructure. The steel sheets were heat treated with two different procedures; intermediate quenching, intermediate quenching and tempering. The properties of tensile strength, fatigue life, hardness, micro hardness and microstructure were evaluated by the mechanical tests and metallographic analysis, respectively. The results showed that dual-phase steel (DPS) microstructures, composed by ferrite and martensite had higher fully reversed plane bending fatigue strength when compared with as-received steel and tempered martensite (TM) steel. The experimental results showed that fatigue life of the heat-treated steel sheets enhanced with increasing amount of martensite in the microstructure. The highest fatigue strength was observed on the intermediately annealed steel sheets at 870 °C. Internal microstructures, fatigue crack initiation and propagation of the heat-treated steel were analyzed by scanning electron microscopy (SEM).

High temperature strength and failure of the Ni-base superalloy PM 3030

The strength, fatigue life and fracture behavior of the oxide dispersion strengthened (ODS) nickel-base superalloy PM 3030 are investigated. The high Al content in PM 3030 leads to the formation of coherent γ′ particles and, thus, to additional precipitation strengthening. A coarse and elongated grain structure (R34) and two isotropic batches with mean grain sizes of 1 μm (R90) and 17 μm (R901315) are considered. Compressive constant strain rate tests and high cycle fatigue (HCF) tests are performed. Optical, scanning and transmission electron microscopy (OM, SEM and TEM) are carried out. The properties are compared with those of the solely oxide dispersion strengthened Ni-base alloy PM 1000 [Estrin, Y., Heilmaier, M., Drew, G., 1999. Creep properties of an oxide dispersion strengthened nickel-base alloy: the effect of grain orientation and grain aspect ratio. Mater. Sci. Eng. A 272(1), 163–173]. It is found that additional γ′ hardening provides an increase in quasi-static strength by about a factor 2 and in HCF life by about a factor 10 2–10 3 at temperatures up to 850 °C. When fatigue life is compared at a fixed ratio of stress amplitude-to-yield or ultimate compressive strength, R34 shows a fatigue life similar to that of PM 1000 at lower temperature (e.g. 600 °C) indicating that the quasi-static strength advantage is proportionally translated into improved fatigue performance; for higher temperatures (850 °C) however, R34 shows a shorter fatigue life as compared to PM 1000. Grain size reduction, as exemplified with the fine grain R90 batch, also provides an increase in strength up to the equicohesion temperature ( T E) [Dieter, G.E., 1986. Mechanical Metallurgy. SI Metric ed. McGraw-Hill Book Company, London]. Above T E, faster diffusion and grain boundary sliding [Raj, R., Ashby, M.F., 1971. On the grain boundary sliding and diffusional creep. Metall. Trans. 2, 1113–1127; Spingarn, J.R., Nix, W.D., 1978. Diffusional creep and diffusionally accommodated grain rearrangement. Acta Metall. 26, 1389–1398] lead to a drastic drop in strength for the R90 material. In contrast, the batch with intermediate grain size (R901315) shows strength comparable to that of R34 up to 850 °C. Furthermore, R901315 shows improved crack tolerance compared to its coarse grain counterpart R34. Due to premature crack initiating coarse oxide particles however, R901315 does not show any improvement in elongation to failure during tensile tests. Eliminating those coarse particles is expected to improve the ductility and toughness of this isotropic batch.

Effect of stress ratio on long life fatigue behavior of Ti-Al alloy under flexural loading

A new ultrasonic three-point bending fatigue test device was introduced to investigate fatigue life ranging up to 10 10 cycles and associated fracture behavior of Ti-Al alloy. Tests were performed at a frequency of 20 kHz with stress ratio R=0.5 and R=0.7 at ambient temperature in air. Three groups of specimens with different surface roughness were applied to investigate the effect of surface roughness on fatigue life. Furthermore, optical microscopy(OM) and scanning electron microscopy(SEM) were used for microstructure characteristic and fracture surface analysis. The S— N curves obtained show that fatigue failure occurs in the range of 10 5 -10 10 cycles, and the asymptote of S— N curve inclines slightly in very high cycle regime, but is not horizontal for R=0.5. Fatigue limit appears after 10 8 cycles for R=0.7. Surface roughness (the maximum roughness is no more than 3 μm) has no influence on the fatigue properties in the high cycle regime. A detailed investigation on fatigue fracture surface shows that the Ti-Al alloy studied here is a binary alloy in the microstructure composed of α 2-Ti 3Al and γ-Ti-Al with fully lamellar microstructure. Fractography shows that fatigue failures are mostly initiated on the surface of specimens, also, in very high cycle regime, subsurface fatigue crack initiation can be found. Interlamellar fatigue crack initiation is predominant in the Ti-Al alloy with fully lamellar structure. Fatigue crack growth is mainly in transgranular mode.

A Study on the Thermal Fatigue Behavior of Solder Joints Under Power Cycling Conditions

Failure mechanisms exposed by environmental accelerating testing methods such as thermal cycling or thermal shock test, may differ from those at service operating conditions. While the device is heated up or cooled down evenly on its external surface during environmental testing, real operating powered devices experience temperature gradients caused by internal local heating, components' different heat dissipation capability, and ambient temperature variation, etc. In this study, a power cycling technique is introduced to better approximate the field operating conditions so as to activate the field failure modes. Power cycling thermal fatigue test is performed with different ball grid array solder joints, that is, lead contained [Sn/37 Pb (SP)] and lead free [Sn/4.0Ag/0.5 Cu (SAC)], and the result is compared. In order to account for the thermal fatigue life behavior discrepancy for different solder joint composition, real time Moire interferometry is applied to measure the global/local thermo-mechanical behavior during power cycling excursion. Effective damage parameter, the total average shear strain, is extracted from the experiment and applied to account for the difference in fatigue life result of two different solders. In addition, amount of experimentally measured total average shear strain is mutually verified with finite element method analysis. It is clear that total average shear strain of a solder joint can be an effective damage parameter to predict thermo-mechanical fatigue life. A physical mechanism in terms of thermal material property of solder joints' is proposed to offer some thoughts to abnormal shear strain behavior that leads to discrepancies in fatigue life of two solders. An importance of power cycling testing method is emphasized for certain package designs.

Fatigue Life and Short Crack Behavior in Ti-6Al-4V Alloy; Interactions of Foreign Object Damage, Stress, and Temperature

High-cycle fatigue (HCF) failures associated with foreign object damage (FOD) in turbine engines of military aircrafts have been of major concern for the aeronautic industry in recent years. The present work is focused on characterizing the effects of FOD on crack initiation and small crack growth of a Ti-6Al-4V alloy at ambient and also elevated temperatures. Results show that the preferred crack initiation site depends on applied stress and temperature as maximum fractions of cracks emanating from the simulated damage site, and naturally initiated cracks are observed at 25 °C under the maximum stress of 700 MPa and at 300 °C under the maximum stress of 300 MPa. The fatigue crack growth rate is influenced by increasing temperature, and the FCG rate at 300 °C is higher than that at room temperature under the same ΔK, whereas this effect for FOD-site initiated cracks is not so remarkable. This observation seems to be due to the effect of stress relaxation at 300 °C. Results also indicate that fatigue crack initiation life (Ni) and fatigue life (Nf) are expressed by three-parameter Weibull distribution function.