An Energy-Based Torsional-Shear Fatigue Lifing Method

An energy-based fatigue-life prediction framework for the determination of full-life, remaining-life, and critical-life of in-service structures subjected to torsional-shear loading has been developed. This framework is developed upon the existing foundation of energy-based fatigue models crafted for the axial, uniaxial bending, and transverse-shear loading cases, which state: the total strain energy density accumulated during both a monotonic event and a cumulative cyclic process is the same material property. The modified energy-based torsional-shear fatigue-life prediction framework is composed of the following entities: (1) the development of a torsional-shear fatigue testing procedure capable of assessing strain energy density per cycle in a pure shear stress state and (2) the determination of the remaining-life and critical-life of in-service aluminum (Al) 6061-T6 structures subjected to shear fatigue through the application of the energy-based prediction method. Experimental data was shown to be affected by load-frame misalignment which was estimated and successfully incorporated into the validation results. Close correlation between adjusted experimental results and the full-life and critical-life predictions stemmed from a 3-to-2 shear-to-axial biaxial loading assumption, which was supported by crack path comparisons. Results of the study effectively demonstrated the versatility of the energy-based lifing method.     

A Method of Modeling the Interaction of Creep and High-Cycle Fatigue

The interaction of creep and fatigue in structural materials under high-cycle loading is modeled using isochronic limit stress diagrams. The hypothesis of a unified limit diagram invariant to the time to failure is used. The unified diagram is given by a cosine power function with the exponent describing creep-fatigue interaction and encompasses convex, concave, and S-like curves. The models build are tested for aluminum alloys, heat-resistant steels, creep-resistant steels and alloys, and laminates__________Translated from Prikladnaya Mekhanika, Vol. 41, No. 1, pp. 25–36, January 2005.     

Prediction of fatigue crack growth life under variable-amplitude loading using finite element analysis

This article presents an elastic-plastic study aiming at predicting the fatigue crack growth (FCG) of 2024-T3 aluminum alloys under variable-amplitude loading. The proposed analysis needs the estimation of the residual stress distribution ahead of the crack tip during propagation. An elastic-plastic FE analysis has been implemented for modeling FCG using Chaboche's model. The FE study has been carried out through consideration of the loading history effect using the memory rules. Three different loading spectra have been applied in this work. The obtained results have been compared to the experimental ones and it has been proved that the suggested model has a better prediction of the FCG lives of cracked 2024-T3 aluminum alloy structures subjected to variable-amplitude loading.     

Random-load fatigue tests on a fail-safe structural model

Experiments performed on a ten-member redundant “fail-safe” structural model subjected to randomized load sequences confirm predictions of fatigue life and reliability based on a probabilistic approach. The statistical variation in ultimate strength of 2024-T4 aluminum alloy combined with an exponentially distributed, Markovian, bending load-amplitude sequence with a constant-amplitude S-N relation for single specimens, is utilized in the analysis. Experimental results are presented for the statistical distribution of ultimate bending strength of 2024-T4 aluminum alloy. Constant load-amplitude flat-bending fatigue tests on single specimens and on multimember structures, and variable-amplitude flat-bending tests on fail-safe structures are reported. Life to failure of the weakest member, as well as the remaining “fail-safe” life in the parallel structure, are examined.     

Fatigue-life prediction by an order statistics treatment of acoustic-emission signals

In fatigue, both the crack-propagation rates and the cumulative acoustic-emission activity are known to be related to the applied stress-intensity range. By considering the energy balance during crack propagation and the relation of strain energy release to the acoustic-emission characteristics, a formal relation between acoustic emission amplitudes and initial fatigue-crack-propagation rates has been derived. Continuous monitoring of acoustic emission during low cycle (tension-tension) fatigue tests has been conducted on aluminum 2024-T3 and 7075-T6 alloys, until fracture. Initial crack sizes and orientations in the fatigue specimens were randomly distributed. Every few hundred cycles, the acoustic signal having the highest peak amplitude was recorded as the extreme acoustic-emission event for the elapsed period. The extreme peak amplitudes, related to extreme crack-propagation rates, were shown, by an order statistics treatment, to be extremally distributed. Statistical, nondeterministic, approach to fatigue considers that only extreme crack-propagation rates are vital to fatigue lives. Knowledge of the distribution function of propagation rates is therefore essential in design for fatigue. Such knowledge can now be obtained in a nondestructive manner, during service in real time, by analyzing the distribution of amplitudes of acoustic-emission signals emitted during cyclic stressing. The statistical treatment enables the prediction of the number of cycles left until failure. Predictions performeda posteriori, based on results gained early in each fatigue test, were in good agreement with actual fatigue lives. The amplitude distribution analysis of the acoustic signals emitted during fatigue tests has been proven to be a feasible nondestructive method for predicting fatigue life.     

Fatigue and static strength of notched and unnotched aluminum-alloy and steel specimens

This paper describes a method of presentation of fatigue data on three commonly used aircraft materials, 2024-T3 and 7075-T6 aluminum alloys and normalized SAE 4130 steel, such that variations in fatigue strength with stress-concentration factor can be shown. Comparisons of the fatigue strengths of 2024-T3 and 7075-T6 aluminum are made for the most useful range of stress-concentration factors. Static-strength results of notched and unnotched specimens of the three materials are presented to show how the strength varies with some parameters of the stress concentration. Comparison of the data with one theory for the strength of cracked specimens was made.     

Crack growth in bi-material laminates

Growth rates of fatigue cracks have been measured in laminates fabricated by adhesively bonding layers of 2024-T351 and 7075-T6 aluminum alloys. The laminates had either two or four layers, with equal thicknesses and numbers of layers of each alloy. Fatigue-crack-propagation tests were performed with through-cracks, giving a crack-divider geometry, the results being compared to those for the two alloys tested in monolithic form. Crack-propagation rates in the bi-material laminates were intermediate between those of the monolithic alloys, with the slower growth in 2024-T351 tending to dominate over a portion of the growth-rate range. Fracture toughnesses of the laminates are also discussed.     

Evaluation and prediction of fatigue resistance of metals from the kinetics of variation in surface properties

The paper studies the fatigue resistance of metallic samples subjected to high-cycle loading and microhardness measurement. The fatigue damage of materials during loading is identified as decrease in the thickness of the barrier surface layer, which prevents fatigue failure. It is shown that the thickness of this layer is independent of the plastic characteristics of the material and the level of stress. A method to evaluate accumulated fatigue damage is developed. Kinetic curves of damage accumulation are analyzed. Methods to predict fatigue characteristics are proposed __________ Translated from Prikladnaya Mekhanika, Vol. 44, No. 3, pp. 86–95, March 2008.     

Fatigue strength of metallic and composite materials under repeated high-cycle bending

The problem of fatigue strength analysis of metallic and composite materials subjected to combined static and cyclic bending is resolved. The analysis is based on limit-state models, which allow describing limiting-stress curves of all known shapes, including convex, rectilinear, S-shaped, and concave. The fatigue strength of the following materials is evaluated: carbon and alloyed steels, aluminum alloys, unidirectional organic plastics, laminated plastics, and glassfiber-reinforced plastics. The calculated and experimental data are in satisfactory agreement __________ Translated from Prikladnaya Mekhanika, Vol. 42, No. 5, pp. 26–36, May 2006.     

J resistance curves in aluminum SEN specimens using Moiré interferometry

TheU _y -displacement field obtained by white-light moiré interferometry were used to estimate the approximate far- and near-fieldJ-integral values associated with the subcritical crack growths in fatigue precracked 7075-T6 and blunt notched and fatigue precracked 2024-0 and 5052-H32 aluminum, single-edged notch (SEN) specimens. The initial phases of theJ-resistance curves for the somewhat brittle 7075-T6 and the two ductile 2024-0 and 5052-H32 aluminum SEN specimens are presented. Paper was presented at the 1987 SEM Spring Conference on Experimental Mechanics held in Houston, TX on June 14–19.     

A study of cyclic plastic stresses at a notch root

An experimental study is presented for cyclic plastic stresses at notch roots in specimens under constant-amplitude repeated tension and reversed loading. Edge-notched,K _T=2, 2024-T3 aluminum-alloy sheet specimens were cycled until local stress conditions stabilized. Local stress histories were determined by recording local strain histories during cycling and reproducting these histories in simple, unnotched specimens. The fatigue lives for these notched specimens were estimated using stabilized local stresses and an alternating vs. mean stress diagram for unnotched specimens of the same material. These predictions compared favorably with lives from S-N data for the notch configuration tested. In addition, an expression is presented for calculating local first-cycle plastic stresses. An acceptable correlation is shown between predicted stresses and experimental data within the scope of the investigation.     

In Situ Observation of Adiabatic Shear Band Formation in Aluminum Alloys

We used high-speed X-ray phase contrast imaging and infrared thermal imaging techniques to study the formation processes of adiabatic shear bands in aluminum 7075-T6 and 6061-T6 alloys. A modified compression Kolsky bar setup was used to apply the dynamic loading. A flat hat-shaped specimen design was adopted for generating the shear bands at the designated locations. Experimental results show that 7075-T6 exhibits less ductility and a narrower shear band than 6061-T6. Maximum temperatures of 720 K and 770 K were locally determined within the shear band zones for 7075-T6 and 6061-T6 respectively. This local high temperature zone and the resulting thermal instability were found to relate to the shear band formation in these aluminum alloys.

     

随机载荷下疲劳裂纹的闭合和扩展估算模型

本文分析了裂尖前,后方塑性区对闭合性能的影响,提出从裂尖塑性钝化量和尾迹区残余塑性变形两个方面来确定裂纹面的残余变形,并讨论了压缩载荷对闭合应力的影响,由此建立了一个疲劳裂纹闭合模型,然后通过模型“有限记忆”性质的假定将它应用到随机加载情况。用此模型对铝合金2219——T851受飞机谱载荷的CCT平面应力试件进行了疲劳寿命估算,估算值与实验结果接近。     

Approche multi-échelle en fatigue polycyclique anisotherme

An extension of the macro-meso approach developed by Dang Van in isothermal high-cycle fatigue is established in the case of structures subjected to anisothermal cyclic loadings. The fatigue limits of such structures are assumed to be temperature-dependent. This proposal is based upon the assumption of elastic shakedown at macroscopic and mesoscopic scales. The use of a local shakedown condition established for structures with temperature-dependent yield limits leads to a thermomechanical formulation of the high-cycle fatigue criteria.     

Verification of a neuber-based notch analysis by the companion-specimen method

Measurements of actual notch-root stress-strain behavior by means of the companion-specimen method are compared with predictions of the notch analysis using both a constant (long-life) fatigue-notch factor and a variable fatigue-notch factor determined from fatigue-life data. The results, using cyclicly stabilized specimens, show thatK _f is dependent upon the stress level of the degree of local straining. The amount of error incurred in the use of either the variable or constantK _f was about 7 percent in the prediction of the local strains. In addition, test results using specimens that had not been cyclicly hardened, and specimens subjected to a random-load history, indicated the effects of material-deformation phenomena such as cyclic hardening, stress-strain hysteresis-loop shape, and material memory, on local stress-strain predictions.     

一个高周疲劳损伤演化修正模型

将连续损伤力学应用到疲劳问题中, 得到合理的疲劳损伤演化方程,被认为是预测疲劳寿命最有效的方法之一. 在研究Lemaitre最新疲劳损伤演化方程基础上, 根据试验数据提出了一个修正的高周疲劳损伤演化方程, 该方程可以考虑应力幅、平均应力等影响因素. 以2A12-T4铝合金为例, 得到了修正模型的材料常数. 将该修正模型以UMAT子程序形式嵌入ABAQUS主程序, 计算了两种构件14种不同受载情况下的疲劳寿命, 所得计算寿命与试验结果误差均值约15%, 说明修正的疲劳损伤演化方程可以很好地计算金属构件的高周疲劳寿命.      

Application of ultrasonics to detection of fatigue cracks

An ultrasonic system was developed and used to observe the formation of fatigue cracks in centernotched sheet specimens of unalloyed aluminum, two aluminum alloys, a mild steel, and a nickel-base alloy tested in axial tensile fatigue. S-N curves of life-to-initial detectable cracks as well as life-to-fracture were obtained. With the reflection technique, fatigue cracks that ranged in length from 0.0005 to 0.005 in. were detected while the test was in progress. Cracks were detected within approximately 1 to 3 percent of total specimen life for all of the materials considered over the range of stresses considered. The through-transmission technique was utilized to measure relatively long cracks.     

Development of a Closed-Loop High-Cycle Resonant Fatigue Testing System

In this paper, a vibration-based testing methodology to assess fatigue behavior of a metallic structure is presented. To minimize the testing duration, the test setup is designed for a base-excited multiple-specimen arrangement driven in a high-frequency resonant mode, which allows completion of fatigue testing in an accelerated period. The shaker operates in closed-loop control with dynamic specimen response feedback provided by a scanning laser vibrometer. A test coordinator function is developed to synchronize the shaker controller and the laser vibrometer and complete the closed-loop scheme: the test coordinator monitors structural health of the test specimens throughout the test period, recognizes change in specimen dynamic behavior due to fatigue crack initiation, terminates test progression, and acquires test data in an orderly manner. The test methodology is demonstrated with cantilever specimens that are clasped on the shaker armature with specially-designed clamp fixtures. Experimental stress evaluation is carried out to verify the specimen stress predictions. A successful application of the experimental methodology is demonstrated by validation tests with Al 6061-T6 aluminum specimens subjected to fully-reversed bending stress.     

Prediction models of fatigue crack closure and growth under random loading

The effects of plastic zones both in front of and behind crack tip on crack closure have been analysed. The total residual deformations of crack surfaces involve two parts, that is, the amount of plastic blunting of crack tip and the residual deformation in the wake of the tip. This paper presents a fatigue crack closure model in which the influences of compressive load on closure stress are discussed. The model is applied to random loading conditions by the assumption of limited memory properties. The fatigue lives are predicted using the proposed crack growth model for CCT plane stress specimen cut from 2219-T851 aluminum alloy under flight spectrum loadings, and the prediction values agree with the test results.The project was supported by the natural science foundation of China.     

Fatigue life predictions by integrating EVICD fatigue damage model and an advanced cyclic plasticity theory

An event independent cumulative damage (EVICD) fatigue prediction model was previously developed for the fatigue damage prediction under general multiaxial stress state and loading conditions. The model takes the plastic strain energy as the major contributor to the fatigue damage. The application of the EVICD model does not require a cycle counting method for general random loading. In the current effort, derivations were made to explicitly and directly relate the material constants in the fatigue model to the parameters in the Manson–Coffin equations and the cyclic stress–strain curve of the material. In addition, an advanced cyclic plasticity theory was implemented for the determination of the detailed stress–strain response that was required as the input for the EVICD fatigue model. Three metallic materials were used to demonstrate the capability of the modified fatigue model for the predictions of fatigue lives under different loading conditions. The results show that the fatigue model can provide fatigue life predictions in close agreement with the experimental observations.