LIFE PREDICTION BY SIMULATION OF CRACK GROWTH IN NOTCHED COMPONENTS WITH DIFFERENT MICROSTRUCTURES AND UNDER MULTIAXIAL FATIGUE

A modelling procedure was developed which is applicable to crack growth in notched components subjected to multiaxial fatigue for materials with different microstructures. An algorithm for crack growth, in a microstructure that was modelled as hexagons, was established as a competition between growth by crack linkages during the crack initiation and propagation stages and the propagation of a dominant crack as a single crack. Analytical results simulated by using the developed model were compared with experimental results from fatigue tests which had been conducted using notched specimens of pure copper, carbon steel and two kinds of titanium alloy. Cracking morphology, which was experimentally observed to depend on the microstructure and the loading mode, was well simulated using the present model. The fatigue failure life of a notched specimen was statistically estimated by a Monte Carlo procedure based on the model. The simulated life with a statistical scatter-band almost coincided with the experimental data.     

复合材料疲劳寿命预测

在疲劳载荷作用下,复合材料的弹性模量会随着载荷循环数的增加而不断下降,而材料中的内部损伤则不断增大。为此,本文提出复合材料的疲劳模量和累积应变的概念,并由此定义出三种预测复合材料疲劳寿命的疲劳损伤模型。文中应用这三种模型对单应力水平和多应力水平下的玻璃纤维增强环氧树脂复合材料的疲劳寿命进行了估算,并同实验结果进行了比较。     

MODELING THE INFLUENCE OF INITIAL MATERIAL INHOMOGENEITIES ON THE FATIGUE LIFE OF NOTCHED COMPONENTS

This paper summarizes a fracture mechanics analysis developed to study the effect of initial size and spatial distribution of inherent material inhomogeneities on fatigue life. Calculations were performed for a set of experiments conducted previously to determine the influence of microporosity size and frequency on the fatigue life of notched 7050-T7451 aluminum specimens. The favorable comparison between the numerical and experimental results indicates the analysis method could be used to predict the benefits to fatigue life obtained by reducing the size and distribution of initial inhomogeneities such as those due to microporosity.     

LOW CYCLE FATIGUE LIVES OF NOTCHED COMPONENTS

Abstract— In low cycle fatigue situations, the plastic behaviour of the material at the root of stress concentrators is of prime importance in determining the cyclic life. However, simple procedures such as Neuber's rule do not adequately describe the development of plastic behaviour at a notch root, while the expense of a finite element analysis is not justified in many instances. This paper describes a simple, approximate numerical method of calculating plastic notch stresses and strains that would be of use in such situations. The usefulness of the technique is demonstrated by comparing low cycle fatigue lives predicted from notch plastic strains with those determined by fatigue testing of smooth specimens subjected to similar plastic strain ranges.     

PREDICTION OF FATIGUE LIFETIME OF NOTCHED MEMBERS

Abstract— A simple model for the estimation of the total fatigue life of notched members is presented. The number of cycles to failure is estimated as the summation of the cyclic life spent in: (1) the initiation of a dominant fatigue crack at the notch root; (2) the very early growth of this crack within the notch plastic zone; (3) the subsequent fatigue crack growth in the elastic stress-strain field of the notch; and (4) the elastic stress field of the bulk material. Theoretical and experimental results are compared.     

FRACTURE MECHANICS APPROACH FOR PREDICTION OF FATIGUE LIFE OF HIGH STRENGTH STEEL COMPONENTS

Abstract— A fatigue life estimation procedure based on a fracture mechanics approach is analytically developed for components made of a 4140 grade high strength steel. The maximum allowable flaw sizes for both static and cyclic loads are computed. For different stress levels and shapes of the defects, initially-existing, allowable defect size versus number of operating cycles, as well as the thickness versus tolerable defect sizes curves, are presented. The maximum allowable operating cycles of components for non-destructively detectable minimum defects are also estimated. From these estimations, a more realistic prediction of the safe service life of components is possible.     

非比例载荷下缺口件疲劳寿命预测

针对Mod.9Cr-1Mo铁素体钢缺口件进行了一系列非比例载荷低周疲劳试验,采用直流电位差法测量裂纹萌生寿命,比较了缺口半径和应变路径对疲劳裂纹萌生寿命的影响。结果表明,缺口件裂纹萌生寿命占总寿命的比例与材料类型、应变路径相关,更与缺口半径尺寸直接相关。同一路径下,随着缺口半径增加,裂纹萌生寿命所占比例增大。采用Neuber律进行缺口局部应力-应变损伤的计算,结合Smith-Watson-Topper (SWT)模型和Kandil-Brown-Miller (KBM)模型进行疲劳寿命预测。结果表明,除单轴路径和比例路径外,SWT模型得到的预测结果偏于不安全;KBM模型除对单轴预测偏于保守外,其他预测值较好,总体预测结果位于2倍分散带内。     

多轴载荷下缺口件的疲劳寿命估算方法

提出了一种多轴载荷下缺口件的疲劳寿命估算方法。该方法基于临界平面理论,计算出缺口件各部位的多轴疲劳损伤参数,以损伤参数最大的部位为缺口件的多轴疲劳危险点。根据临界距离思想,提出了热点法和线法的临界距离的计算方法,采用热点法和线法考虑缺口件疲劳危险点附近损伤梯度的影响,以临界距离修正的损伤参数计算多轴载荷下缺口件的疲劳寿命。采用SAE1045钢缺口件的多轴疲劳试验对该文提出的寿命估算方法进行评估和验证,结果表明：该文所建立的寿命预测方法具有较好的预测能力,预测结果大部分分布在试验结果的3倍分散带之内。     

ELASTIC-PLASTIC BEHAVIOUR AND UNIAXIAL LOW CYCLE FATIGUE LIFE OF NOTCHED SPECIMENS

Experimental and numerical analyses were carried out to examine the elastic-plastic behaviour and low cycle fatigue (LCF) life of EN15R steel notched specimens. Notch root strains were measured and compared with estimates obtained from three methods: Neuber, Glinka and hite element (FE) analyses. All methods provided fairly accurate estimates of cyclic strain up to net section yield, from which point the Neuber and Glinka predictions were greater than measured. The finite element results compared well with measured results. The estimated notch root strains were used to predict the life of the notched specimens based on LCF results from unnotched specimens. Uniaxial Coffom-Manson and multiaxial Lohr-Ellison approaches were used. Improved fatigue life predictions were obtained when the FE predictions of the multiaxial strains were combined with a multiaxial strain parameter. The possible influence of strain gradient is inferred by comparing LCF lives for hollow thin-walled and solid bar specimens.     

PREDICTION OF FATIGUE LIFE BY X-RAY DIFFRACTION METHODS

Abstract— X-ray double crystal diffractometry and reflection topography were employed to examine the defect structure induced by fatigue of A1 2024 specimens. Analysis after various amounts of tension compression cycling revealed that the excess dislocation density in the surface layer increased rapidly early in the fatigue life, and maintained virtually a plateau value from 20 to 90% of the life. Beyond 90%, the excess dislocation density increased rapidly again to a critical value at failure. Investigation of the defect distribution in depth showed that the excess dislocation density in the bulk material, by contrast to the surface layer, increased more gradually during the life. Using deeply penetrating molybdenum radiation, the grains in the bulk could be analyzed, and thus the fatigue life and onset of failure could be predicted nondestructively. It was also shown that, after removal of the surface layer, the defect structure induced in the bulk by prior cycling was unstable. The resultant extension of fatigue life by surface layer removal was explained on the basis of this structural instability of the bulk material.     

线图法预测疲劳寿命的改进

线图法预测工程的疲劳寿命简单，方便。在已有预测公式的基础上，提出了预测低应力高周疲劳寿命的新公式，并进行了严密的公式推导，对一实例进行了计算并进行了验证，给出了新旧公式的预测结果，并进行了比较。     

转子裂纹检测与寿命预测技术研究

现代的抗疲劳设计法——损伤容限设计使零件在出现疲劳裂纹后通过严格的检修、剩余寿命计算和断裂控制仍能安全使用 ,使充分利用零件的剩余强度成为可能。对损伤容限设计的关键技术——初始缺陷或服役中出现的疲劳裂纹检测和疲劳剩余寿命预测技术进行了研究。成功地研制了一套性能优良的交流电位表面裂纹监测系统 ,分辨率为 0 .1 mm,测量精度达到± 2 % ,测量范围为 0～ 1 0 0 mm;开发出一套微机平台上的基于断裂力学的 (含表面裂纹 )转子剩余寿命计算程序 ,为分析裂纹扩展形态和概率断裂力学方法估算剩余寿命提供了快速计算途径。有较强的工程实用性和良好的市场开发前景。     

考虑棘轮效应的疲劳寿命预测方法

多轴棘轮加载时轴向加载的恒定应力、剪切应变幅对轴向棘轮应变和疲劳寿命有很大的影响.考虑棘轮效应影响的Coffin模型将棘轮效应与循环部分相结合来计算疲劳寿命,预测结果较好,绝大部分预测结果分布在2倍分散带内.     

FATIGUE LIFE PREDICTION OF WELDED JOINTS—A RE-ASSESSMENT

Abstract— Fatigue life prediction of welded joints needs an accurate and exhaustive theoretical Fracture Mechanics characterization of weld toe crack propagation. The method proposed by Albrecht et al. leads rapidly to accurate solutions of the LEFM δK-parameter. However, non-LEFM short crack behavior within the notch (weld toe) plastic zone must be taken into account. Available information on notch fatigue is surveyed, and practical cases where short crack growth is likely to occur are identified. Based on an elastoplastic finite element analysis, the LEFM validity limits and errors resulting from the misuse of LEFM in fatigue life prediction are quantified.     

A NEW METHOD FOR PREDICTING FATIGUE LIFE IN NOTCHED GEOMETRIES

The objective of this paper is to develop a notch crack closure model, called NCCM, based on plasticity-induced effects and short fatigue crack growth in the vicinity of the notch, and to predict the fatigue failure life of notched geometries. By using this model the regime for non-propagating cracks (n.p.c.) and the relationship between the fatigue strength reduction factor, Kf , and the elastic stress concentration factor, Kt , under mean stress conditions, can be determined quantitatively. A crack closure model is assumed to apply in the notch regime based on an approach developed to explain the crack growth retardation behavior observed in smooth specimen geometries after an overload. Notch plasticity effects are also applied in the NCCM model. Fatigue failure life is calculated from both short fatigue crack growth in the notch region where elastic–plastic fracture mechanics (EPFM) is applied and from long fatigue crack growth remote from the notch where linear elastic fracture mechanics (LEFM) occurs. This prediction is obtained using a quantity called the effective plasticity-corrected pseudo-stress. The NCCM can be used to account quantitatively for various observed notch phenomena, including both the relationship between Kf and Kt and n.p.c. The effects of the tensile mean stress on the Kf versus Kt relationship is investigated and leads to the little recognized but technologically important observation that mean stress conditions exist where Kf can be greater than Kt . The role of notch radius and tensile mean stress on n.p.c. behavior is also explored. The model is verified using experimental data for notch geometries of aluminum alloy 2024-T3, alloy steel SAE 4130 and mild steel specimens tested at zero and tensile mean stress.     

光滑与含孔复合材料压-压疲劳实验研究

本文新设计了光滑与含孔两种复合材料试样的抗失稳装置, 简述了这两种试样的压-压疲劳实验概况, 分析和讨论了其疲劳破坏机理, 提出复合材料疲劳损伤的“转捩点”概念。     

FATIGUE LIFE PREDICTION IN BENDING FROM AXIAL FATIGUE INFORMATION

Bending fatigue in the low cyclic life range differs from axial fatigue due to the plastic flow which alters the linear stress-strain relation normally used to determine the nominal stresses. An approach is presented in this report to take into account the plastic flow in calculating nominal bending stress S_bending based on true surface stress. For a given surface strain ε_S, hence the surface life (N_S), an equation is derived to express S_bending in terms of axial fatigue stress (S_axial), involving material constants c, b, N_T which are obtained from axial fatigue information and f₁ and f₂ which are functions of strain hardening exponent and depend on the geometry of cross-section. These functions are derived in closed form for rectangular and circular cross-sections. The nominal bending stress and the axial fatigue stress are plotted as a function of life (N_S) and these curves are shown for some materials of engineering interest.     

NOTCHED MEMBER FATIGUE LIFE PREDICTIONS COMBINING CRACK INITIATION AND PROPAGATION

Abstract Fatigue lives of notched members are considered to be divided into crack initiation and propagation phases. Apparent size effects caused by crack propagation through the strain gradient of the notch are accounted for if initiation is defined as a crack size within the local notch field. The extent of this field may be estimated from fracture mechanics analysis, with its size being of the order of one tenth of the notch radius. Plasticity effects must be properly handled in predicting crack initiation, but linear elastic analysis is generally satisfactory for handling the propagation phase.     

PREDICTION OF THE FATIGUE LIFE OF LASER WELDED STAINLESS STEEL JOINTS

Abstract— A fatigue scatter band has been computed for laser welded austenitic stainless steel joints. These laser welded steels have a very small heat affected zone. The unified scatter band provided by standards for welded structural steels does not adequately describe the trend of the experimental data of laser welded steels and this makes their design parameters scarcely realistic. The scatter band proposed in this paper has been computed by re-sorting experimental data relative to joints with high stress concentration factors and has subsequently been assessed with data relevant to butt welded joints.     

BIMODAL CONCEPT OF HIGH-CYCLE FATIGUE LIFE PREDICTION FOR CONSTANT AND VARIABLE AMPLITUDE FATIGUE

A bimodal concept for the prediction of the high-cycle fatigue life of structural details subjected to constant- or variable-amplitude loading is considered in this paper. The total fatigue life was separated into two phases: crack initiation and crack propagation. The portion of life spent in crack initiation was estimated by using S–N data obtained on smooth specimens. A fracture mechanics concept was used to calculate the portion of life spent in crack propagation, and the S–N curve, including the fatigue limit of a structural detail, was determined by using material properties and the geometry of the detail. The bimodal concept was applied to a welded stiffener and the results are compared with experimental data reported in the literature.