13SiMnNiCrMoV钢切口疲劳裂纹萌生门槛值

本文研究在13SiMnNiCrMoV 结构钢中用尖裂纹的应力强度因子来反映三点弯曲缺口试样的疲劳裂纹萌生规律。当 R=0.1,f=100Hz,试样尺寸 B×W×L=12.5×25×117mm 时,缺口名义应力幅的门槛值为Δσ_(th)=3315ρ~(0.352) MPa,0.5mm≤ρ≤5mmΔK_(th)=250ρ~(0.352) MPam~(1/2)门槛值与ρ有关。同时得到缺口裂纹萌生的循环次数 N_i 与名义应力幅Δσ、缺口曲率半径ρ的定量关系是N_i=3.98×10~(22)ρ~(1.62)/Δσ~(4.85),N_i≤10~(5.5)并讨论了(ΔK_1/ρ~(1/2))_(th)与ρ无关的结论。     

Micronotches for studying growth of small cracks

Well‐defined starter cracks at preselected sites in the microstructures are needed for systematic investigation of the characteristic features of microstructure controlled growth of small cracks. A kinked ellipsoidal micronotch with very high notch factor at the trailing kink is proposed, which triggers controlled crack initiation along the notch contour. These micronotches can be machined by femtosecond laser ablation with virtually no heat‐affected zone at the edges. Crack growth results obtained for an intermetallic γ‐TiAl alloy are presented as an illustrative example.     

On the estimation of fatigue life in notches differentiating the phases of crack initiation and propagation

Over the last three decades, a variety of models have been developed in order to predict the life of components under fatigue. Some of the models are based on the definition of the fatigue process as a combination of the phases of crack initiation and crack propagation, considering component life as the sum of the duration of each phase. Other models consider only one of the phases; some consider only initiation while others only propagation, though in this case, from cracks with lengths in the order of the microstructural dimensions. This article will carry out a comparative analysis of the methods that consider life as the sum of the duration of both phases. In this same line, it proposes yet another method, which simulates crack growth according to damage theories. In analysing the behaviour of each model, this paper will describe various elements: the prediction that each of them produces regarding notched specimens submitted to testing, the advantages and inconveniences of each, and lastly, the possibilities of applying each of the models to more realistic geometries.     

The use of multiaxial fatigue models to predict fretting fatigue life of components subjected to different contact stress fields

This work describes the application of multiaxial fatigue criteria based on critical plane and mesoscopic (Dang Van, 1973, Sciences et Techniques de lÁrmement, 47 , 647—722) approaches to predict the fatigue initiation life of fretted components. To validate the analysis, several tests under closely controlled laboratory conditions are carried out in a Ti‐6Al‐4V alloy. These classical Hertzian tests reveal a size effect where fretting fatigue lives vary with contact size. Experimentally available data for fretting fatigue of an Al‐4Cu alloy are also used to assess the models. Neither the critical plane models nor the mesoscopic criterion considered can account for the effects of different contact stress fields on the initiation life, if the calculation is based only on highly stressed points on the surface. It is shown, however, that satisfactory results can be achieved if high values of the fatigue parameters are sustained over a critical volume.     

Investigation of rolling contact crack initiation in bainitic and pearlitic rail steels

The stress–strain history and the crack initiation lives of bainitic and head‐hardened pearlitic rail steels were determined under rolling contact loading by implementing the semi‐analytical Jiang–Sehitoglu rolling contact model that incorporates both ratchetting and multiaxial fatigue damage. The calculations revealed that the bainitic steel withstands higher loads than the pearlitic steel at low shear tractions, however; both materials behave in an increasingly similar manner as the shear tractions increase. Furthermore, maximum damage occurs in both steels when ratchetting and fatigue damage coincide on the surface. In addition to shedding light on the rolling contact fatigue (RCF) performance of bainitic and pearlitic rail steels, the current work also establishes a methodology for the realistic prediction of crack initiation under RCF.     

疲劳缺口系数Kf与理论应力集中系数Kt之间的关系

疲劳缺口系数Kf是表征缺口部位疲劳强度降低程度的参量,这一系数在结构抗疲劳设计中经常被使用.一般认为Kf与理论应力集中系数Kt呈线性关系,线性的斜率与不同材料类别(铝合金、钛合金和钢)有关.通过对大量不同材料、不同应力比和不同缺口试样形式的高周疲劳极限的系统分析,发现大多数情况下K f与Kt的呈线性关系,但有些情况下二者不服从线性关系.Kf与Kt的关系除与材料类别有关外还与实验的应力比有关.     

Fatigue life prediction of welded box structures

The objective of this study is to predict fatigue life of metal welded boxes. Experimental results of fatigue life are satisfactory predicted using an analytical scheme based on the volumetric approach.Translated from Problemy Prochnosti, No. 6, pp. 17–25, November–December, 2004.     

High-cycle fatigue mechanisms in 1045 steel under non-proportional axial-torsional loading

Detailed microscopic analyses have been made on the high-cycle mechanisms in 1045 steel under various stress-controlled axial-torsional loadings. A special attention has been paid to a critical example of non-proportional loading, i.e., 90° out-of-phase loading with different stress ratios. The replica technique has been used to monitor crack initiation and propagation from the microstructure scale. The orientations of persistent slip bands and Stage I cracks are in good agreement with the critical plane concept. The evolutions of crack length with cycle life as well as the crack aspect ratios depend on the loading condition. However at a given life, the data are consolidated in terms of crack depth versus cycle life. The McDiarmid parameter correlates stress-life data under proportional loadings. However, it underestimates fatigue lives under out-of-phase loading at high stress ratio and it overestimates them in the case where all planes experience the same shear stress amplitude (stress ratio = 0.5). More damaging mechanisms are involved in crack initiation and crack propagation. It is recommended to test the fatigue performance of materials in this last condition that involves the worst damage mechanisms.     

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。     

A method for determining the fatigue critical plane for biaxial random vibration in the frequency domain

This paper provides a direct method to determine the critical plane orientation for biaxial random vibration. The critical plane is obtained by finding the direction that maximizes the normal stress variance. It is found that the shear stress is uncorrelated with the normal stress in this orientation. Furthermore, the direction of maximal normal stress is shown to coincide with the principal direction in the case of proportional stress components. Spectral fatigue damage methods proposed in recent literature involve a Monte Carlo enumeration step to find the critical plane orientation. By using the proposed technique, computationally expensive enumeration methods are avoided and greater accuracy in the fatigue damage estimate may result.     

On the corrosion fatigue crack initiation model and expression of metallic notched elements

In the present study, attempts are made to extend the application of the mechanical model for the fatigue crack initiation (FCI) and the FCI life formula of metallic notched elements in laboratory air to those in the corrosive environment. The test results and analysis of the corrosion FCI (CFCI) life of aluminum alloys and Ti---6A1---4V show that the expression of the CFCI life obtained by modifying the FCI life formula in laboratory air can give a good fit to the test results of the CFCI life. The salt water (3.5% NaCl) environment has no effects on the CFCI resistant coefficient compared with the FCI resistant coefficient in laboratory air. However, 3.5% NaCl environment greatly decreases the CFCI threshold of aluminum alloy, but has little effect on the CFCI threshold of Ti---6A1---4V. The loading frequency ranging from 1 Hz to 10 Hz has no appreciable effect on the CFCI life, and thus, the CFCI threshold of aluminum alloys investigated. Hence, the expression for the CFCI life of metallic notched elements proposed in this study is a better one, which reveals a correlation between the CFCI life and the governing parameters, such as, the geometry of the notched elements, the nominal stress range, the stress ratio, the tensile properties and the CFCI threshold. However, this new expression of the CFCI life needs to be verified by more test results.