Cycle Fatigue Limit Research Articles

Low Cycle Fatigue Estimation Based on the Critical Distance Stress Theory

Generally the critical distance stress theory was applied for the fatigue limit estimation of general structures using fatigue limit of smooth specimen (σw0), and threshold stress intensity factor range (Kth). In this paper we extended this method for the estimation of low cycle fatigue life too. In this method we define the critical distance (rc) on static strength conditions, which is calculated using ultimate tensile strength (σB) and fracture toughness (KIC), in addition to the critical distance on fatigue limit condition (rc). Then the critical distances of any low cycle fatigue conditions can be calculated by interpolation of critical distance on fatigue limit (rc) with critical distance on static strength (rc). By unifying these low cycle fatigue life estimation method with high cycle fatigue limit estimation method we can estimate the full range fatigue life easily. And to confirm the availability of this estimation method we perform the fatigue test for any stress concentration specimens.s

Fatigue behavior of the aeronautical Al–Li (2198) aluminum alloy under constant amplitude loading

Tensile and fatigue mechanical behavior of wrought aluminum alloy 2198-T351 is examined and compared against 2024-T3 that is currently used in aerostructures. Experimental fatigue tests were carried out under constant amplitude stress ratio R=0.1 and respective stress–life (S–N) diagrams were constructed for both alloys. Fatigue behavior of both alloys is described with varying parameters being the percentage of fatigue life as well as the effect of maximum applied stress as a function of ultimate tensile strength. It was found that fatigue endurance limit of AA2024-T3 is approximately 40% below its yield stress, while only 9% below for the AA2198-T351. The latter was found to be superior in the high cycle fatigue and fatigue endurance limit regimes, especially when considering specific mechanical properties. Absorbed energies per fatigue cycle as well as dynamic stiffness of the fatigue hysteresis loop were calculated and plotted against the number of fatigue cycles and with varying maximum applied stress; both parameters are continuously decreasing due to the combination of hardening effect and micro-cracking in AA2024-T3, while this was the case only for the high applied stresses regime in AA2198-T351. Cyclic stress strain (CSS) curves were constructed and proved that work hardening exponent of AA2198-T351 is substantially decreasing with increasing fatigue life.

Influence of delamination on fatigue properties of a fibreglass composite

Residual fatigue life, stiffness and strength are directly related to presence of defects and damages in composite materials, among which delamination is one of the most diffused. This work aims to study the effect of delamination on fatigue behaviour of a glass fibre reinforced composite. Experimental tests are carried out to check fatigue life decrease of delaminated specimens, with a Teflon layer at half of its thickness, by a comparison with undamaged material. In order to evaluate defects presence and their progressive evolution, infrared thermography is taken into account, monitoring surface temperature of the damaged and undamaged samples.Static and fatigue tests are carried out, and a high cycle fatigue limit is identified for this composite material. Three approaches, based on thermal observations of static and dynamic tests at increased stress amplitude, revealed a relation between thermal response of the material and the fatigue limit. From this standpoint, thermography can be indicated as a valid tool to monitor damage initiation and its evolution during testing, and to estimate fatigue life.

Fatigue Strength and Life Estimation Methods Using Critical Distance Stress Theory

Generallythe critical distance stress theory was applied for the fatigue limitestimation of general structures. In thismethod, it needs only two parameters, fatiguelimit of smooth specimen (σw0), and threshold stress intensityfactor range (∆Kth). In this paper we extended this method for theestimation of low cycle fatigue lifetoo. In this improvement wedefine the critical distance (rc’) on static strength conditions,which is calculated using ultimate tensile strength (σB) andfracture toughness (KIC), in addition to the critical distance onfatigue limit condition (rc). Then the critical distances of any lowcycle fatigue conditions can be calculated by interpolation of criticaldistance on fatigue limit (rc)with critical distance on staticstrength (rc’). By unifying these low cycle fatigue life estimationmethod with high cycle fatigue limit estimation method we can estimate the fullrange fatigue life easily. And to confirm the availabilityof this estimation method we perform the fatigue test for circlehole specimens, sharp V notch specimens andfretting fatigue specimens.

Comparison of Mechanical Properties of a Biomedical β Titanium Alloy Added with Pure Rare Earth and Rare Earth Oxides

The microstructures and fatigue properties of the TNTZ added with Y or Y2O3 have been investigated. The results indicate that TNTZ added with Y or Y2O3 are all found to be composed of β phase and the small amount of Y2O3. The grain size of TNTZ added with Y or Y2O3 is smaller than that of TNTZ. The Young’s modulus of TNTZ added with Y or Y2O3 are maintained at a low level, and Young’s modulus of TNTZ added with Y is smaller than that of TNTZ added with Y2O3. The mechanical properties are both improved by adding Y or Y2O3, while the tensile strength of TNTZ added with Y2O3 is slightly higher than that added with Y. The high cycle fatigue limit of the alloys added with Y or Y2O3 are similar, while the low cycle fatigue strength of TNTZ added with Y is higher than that added with Y2O3. The improvement in fatigue properties ascribes to the microstructure refinement and the pining effect of Y2O3 particles. On the other hand, Y elements form Y2O3 with the Oxygen elements in the matrix, thus lead to the weakening of the Oxygen solution effect.

Rapid Determination of the High Cycle Fatigue Limit Curve of Carbon Fiber Epoxy Matrix Composite Laminates by Thermography Methodology: Tests and Finite Element Simulations

Self-heating measurements under cyclic loading allow a fast estimation of fatigue properties of composite materials. The tensile fatigue behavior of a high stress carbon fiber epoxy-matrix composite laminates is examined at room temperature. Tension- tension cyclic fatigue tests are also conducted under load control at a sinusoidal frequency of 5Hz to obtain high-cycle fatigue stress curves (S-N). The fatigue limits of the different composite lay-ups tested were successfully compared with data resulting from the self-heating test method on the same laminates. This comparison reveals a good agreement between the two methods dedicated to stress fatigue limit determination. In addition, a tomographic analysis is used to perform comparisons at the microscale between both fatigue methods. The nonlinear heat transfer laminate theory is used for self-heating tests simulation. Self-heating simulations involving conduction, convection, and boundary radiation are performed with the Finite Element code Cast3M.

Low cycle fatigue of welded joints with aging influence

The article is a continuation of welded joints research, which is intended to improve the fatigue life prediction of the exhaust system's welded parts. It is focused on the low cycle fatigue (LCF) range, where the influence of high loads on the material durability was analyzed. Sometimes the practice shows an unprofitable production of some intensively loaded parts evaluated in the range of high cycle fatigue or fatigue endurance limit. Numerous LCF tests of samples without weld, with weld and with aging influences were performed. Testing was performed on a servohydraulic test machine where the strain was controlled. The tables and graphs show the results of the monotonic tension tests, LCF tests and the results of the artificial aging influences on the welded joints. The article also includes the fatigue degradation regarding the weld influence and minor aging effect on fatigue properties at higher strains. The results present a contribution to the progression of knowledge and the research of new technologies, especially in the area of the fatigue of welded parts under high loads.

Optimal test planning for High Cycle Fatigue limit testing

In High Cycle Fatigue (HCF) limit testing the fatigue limit of a mechanical component is determined by applying cyclical stress of a certain amplitude and noting whether the component breaks or not. Since testing is time consuming and expensive, the number of test samples should be kept to a minimum. A common protocol for finding the fatigue limit distribution is the staircase method, in which the testing amplitude is decreased or increased with a fixed step depending on whether the component in the previous test did break or not. We have developed and implemented an alternative protocol, based on Bayesian experimental design, in which the amplitude of each test is selected to maximize the expected information gain of the test. Simulations show that with the proposed method the number of required test samples is significantly decreased as compared to with the staircase method.

Effect of Hydrogen Absorption on the Fatigue Strength Reduction Caused by Multiple Overloads in Notched Component

Effects of multiple overloads and hydrogen on high cycle fatigue limit was examined to establish a criterion which assesses whether hydrogen utilization machines can be used after large earthquakes. Test materials were SUS304 and SUS316L. The test environments were 0.6MPa hydrogen gas and air. Hydrogen pre-charged specimen was used for in-hydrogen gas test. The reduction of fatigue strength was caused by overloads in both materials. The cause of the reduction was small cracks formed by overloads. In SUS304, the reduction of fatigue limit was enhanced by hydrogen since propagation of small cracks during overloads was accelerated due to hydrogen. In SUS316L, there was no reduction of fatigue limit due to hydrogen. Maximum overload amplitude which caused no reduction of fatigue limit was 0.5σ0.2 for SUS304 and 0.75σ0.2 for SUS316L. These values were regarded as the upper limits of overload amplitude below which the continuous use of components are allowed after earthquakes.

The effect of seismic loading on the fatigue strength of welded joints

Earthquakes sometimes damage steel structures. Structures which are not seriously damaged are still used after earthquakes but their fatigue strength may have been reduced by the large cyclic loadings imposed by the earthquakes. In order to clarify the effect of seismic loading on the fatigue strength of welded joints, high cycle fatigue and variable amplitude fatigue tests after a number of large initial strain cycles were performed. The large strain cycles formed a short crack at the toe of the weld in a low cycle fatigue that triggered a high cycle fatigue strength reduction. The high cycle fatigue limit of welded joints after initial strain cycles is governed by the threshold stress intensity factor of the short crack. The formation of short cracks also enhanced the damage accumulation for subsequent variable amplitude loading. It is important to keep all of the stress variations after earthquake below the fatigue limit of the cracked welded joints to avoid fatigue damage accumulation after an earthquake.

Effect of Sc and Zr additions on the microstructure and fatigue properties of AA6106 produced by equal-channel-angular-pressing

The effect of Zr and Sc addition on the grain refinement and mechanical properties of 6106 aluminium alloy produced by equal-channel-angular-pressing (ECAP) have been analyzed in the present study and compared to the polycrystalline conditions. The concomitant effect of both Zr, Zr + Sc, and precipitation hardening secondary phases, and the severe plastic deformation of the commercial alloy was found to increase the tensile strength and high cycle fatigue limit. The fatigue crack growth tests revealed a reduction of crack initiation but a faster crack growth after ECAP.

Research on low cycle fatigue properties of TA15 titanium alloy based on reliability theory

This paper introduces a method to determine the symmetric cycle fatigue limit of TA15 alloy at given confidence γ and survival probability P. It gives a general method to calculate the true survival probability of this material fatigue limit. The median and data of the LCF at γ = 95% and P = 99.9% are acquired after studying the LCF properties of aircraft construction material TA15 at the temperature of 25 and 250 °C. The strain–life curve, cyclic stress–strain curve and parameters of LCF are also achieved. These provide reference to analyze the reliability of aircraft construction and estimate the life.

Fatigue strength of ultrafine-grained pure Ti after severe plastic deformation

Abstract Fatigue life and notch sensitivity of the ultrafine-grained pure Ti produced by equal channel angular pressing (ECAP) was investigated. The ECAPed Ti showed a significant improvement in its high cycle fatigue limit (by a factor of 1.67) but its notch sensitivity was increased.

ECAP가공에 의한 초미세립 순수 티타늄의 피로 특성 향상

Fatigue life and notch sensitivity of the ultrafine grained pure Ti produced by ECAP was investigated. The ECAPed sample with the true strain of 460% showed near equiaxed grains with an average size of about 0.3㎛. After ECAP, the ultimate tensile strength was increased by 60%, while the tensile ductility was decreased by 31%. The ECAPed ultrafine grained pure Ti samples showed high notch sensitivity and significant improvement of high cycle fatigue limit by a factor of 1.67. The ECAPed samples also show high notch sensitivity (Kf / Kt = 0.96). It can be concluded that ECAP is the effective process for achieving high fatigue strength in Ti by increasing its tensile strength through grain refinement.

The effect of notch geometry on critical distance high cycle fatigue predictions

A critical distance method for predicting the fatigue limit stresses of notched specimens was implemented for notched specimens with a wide range of notch dimensions. Circumferentially notched cylindrical specimens ( k t=1.97–4.07) taken from Ti–6Al–4V forged plate were cycled to failure ( R=0.1 and 0.5) using a step loading method for estimating the 10 6 cycle fatigue limit stresses. These experimental data were used in combination with finite element solutions for all specimen geometries to determine a ‘critical distance’, a quantity or parameter determined from the stress distribution surrounding the notch in combination with fatigue limit stress data from unnotched specimens. A unique parameter was not found for all of the specimen geometries. However, predictions for the fatigue limit stresses of the larger notch geometries may be made with some amount of accuracy using a single value of the critical distance parameter, while reasonable predictions for the specimens with the smallest notch dimensions may be made upon the recognition of an apparent size effect.

On the use of critical distance theories for the prediction of the high cycle fatigue limit stress in notched Ti–6Al–4V

Methods are investigated for predicting the high cycle fatigue (HCF) lives of notched cylindrical Ti–6Al–4V specimens using critical distance concepts that employ the stress distribution in the vicinity of the notch. Cylindrical fatigue specimens had circumferential V-notches with a range of elastic stress concentration factors ( k t=1.97–4.07). Notched and unnotched specimens were cycled to failure using a step-loading technique to generate points on a Haigh (Goodman) diagram for a constant fatigue life of 10 6 cycles. Finite element solutions were generated to provide stress distributions for the notched gage sections. The stress distributions were used in the search for a critical distance over which the quantities of mean stress, stress range, or elastic strain energy may contribute to the fatigue process and can be correlated to similar quantities from smooth, unnotched specimens. If the decrease in the local stress ratio at the notch root for high applied stress ratio is accounted for in the analysis, trends independent of applied stress ratio were found in the calculated critical distances. Predictions based upon the results gave accuracy to within 12% of the experimental fatigue limit stresses and illustrate the method has promise for use in fatigue design of Ti–6Al–4V components.

HCF notch predictions based on weakest-link failure models

Weakest-link models for the prediction of the high cycle fatigue (HCF) limit stress of notched components were investigated. The models employed surface area elements in the high-stress region surrounding the root of a notch. Statistical descriptions of unnotched specimen experimental data were developed using Weibull distributions and median ranking, and incorporated into some of the weakest-link notch formulations. The predictions from each of the failure models were compared to the experimental 10 6 cycle fatigue limit stresses. The fatigue limit stresses were estimated using a step-loading technique at stress ratios from R=−1 to 0.8, for five geometries (elastic stress concentration factors of K t =2.0, 2.8 and 4.1) of circumferentially notched Ti–6Al–4V specimens. The methods were shown to be modestly successful, although data obtained at R=−1 and R>0.65 could not be correlated well with data at intermediate values of R.

Effect of predamage from low cycle fatigue on high cycle fatigue strength of Ti-6Al-4V

Effects of prior low cycle fatigue (LCF) cycling on the subsequent high cycle fatigue (HCF) limit stress corresponding to a life of 10 7 cycles are investigated for Ti-6Al-4V at room temperature. Tests are conducted at 420 Hz on an electrodynamic shaker-based system at several different LCF maximum loads and under subsequent HCF at R=0.1, 0.5 and 0.8 using a step loading procedure. Under these load combinations, which include the possibility of overload or underload effects if cracks form, there is no statistically significant effect of the prior LCF on the subsequent HCF limit stress. While LCF loading at a high stress of 900 MPa is seen to result in strain ratcheting, no distinct features on the fracture surface and different mechanisms of crack propagation from those obtained at lower maximum loads were observed. LCF loading up to 50% of expected life did not produce any indications of crack formation from either the stress limit data or the fracture surfaces.

High cycle fatigue limit stresses for airfoils subjected to foreign object damage

The foreign object damage (FOD) on leading edge geometries from particles ingested into gas turbine engines found to be comprised of geometric discontinuities like a notch, the residual stresses in regions adjacent to the notch, and material degradation in the form of micro-cracks, plastic deformation, voids, etc. The extent of the damage is related to the geometry and characteristics of the impacting object as well as those of the leading edge in terms of the resultant high cycle fatigue (HCF) strength of a simulated airfoil geometry. Steel spheres having diameters ranging from 0.5 to 2.0 mm are used as the impacting objects at velocities over a range from 60 to 500 m/s. The role of residual stresses is identified through the use of samples subjected to stress relief annealing. Simple notch analysis is used to estimate the effect of the geometry on the fatigue strength. Step-loading fatigue tests in tension-tension at 350 Hz are used to establish the HCF strength corresponding to 10 7 cycles. Impacts that result in the formation of craters are found to absorb more energy than those that produce chipped or fractured notches of the same geometry. Chipping, on the other hand, is found to be more detrimental to the HCF strength. Simple notch analysis is used to predict crater dimensions, but is found to be applicable only under limited conditions when residual stresses and material damage are not significant.

Notch size effects on high cycle fatigue limit stress of Udimet 720

Notch size effects on the high cycle fatigue (HCF) limit stress of Ni-base superalloy Udimet 720 were investigated on cylindrical specimens with three notch sizes of the same stress concentration factor K t=2.74. The HCF limit stress corresponding to a life of 10 6 cycles was experimentally determined at a stress ratio of 0.1 and a frequency of 25 Hz at room temperature. The stresses were calculated using finite element analysis (FEA) and the specimens analyzed using scanning electron microscopy (SEM). Test results show that at the same K t value, notch size can slightly affect the HCF limit stress of U720 when notch root plasticity occurs. FEA and SEM results reveal that the notch size effects are influenced by a complicated combination of the stress and plastic strain fields at the notch tip, the nominal stress, and the effects of prior plastic deformation on fatigue crack initiation.