Crack growth from naturally occurring material discontinuities under constant amplitude and operational loads

This paper discusses how cracks that grow from small naturally occurring material discontinuities under operational load spectra behave. The growth of small cracks under a representative maritime aircraft flight load spectrum is discussed first. The results of this study, when taken in conjunction with the authors previous studies into cracks growing under combat aircraft load spectra, illustrate how for cracks that grow from small naturally occurring material discontinuities under operational load spectra crack growth can often be easily and accurately computed. It is also shown that the Hartman–Schijve variant of the NASGRO crack growth equation is able to accurately represent the growth of small cracks in two different rail steels. It is further shown that the growth of both small and long cracks can be described by a family of da/dN versus ΔK curves and that, for 7050-T7451, the experimental procedures commonly used to determine a closure free da/dN versus ΔK curve produce curves that are consistent with those obtained using the Hartman–Schijve equation and allowing for small variations in the term ΔK_thr.     

考虑多裂纹的焊趾裂纹扩展行为的数值仿真分析

针对焊趾处存在多裂纹的情况,对多裂纹扩展过程进行了数值仿真.考虑短裂纹扩展阶段,采用M积分计算整个裂纹前沿的应力强度因子,模拟并分析了不同初始表面裂纹间距条件下,该因素对裂纹前沿融合前后的扩展行为的具体影响.结果表明,裂纹间距的不同将直接导致其对整个裂纹前沿前后的应力强度因子的影响的差异;受初始裂纹间距的影响,裂纹前沿断口形貌呈现不同形状;可将相对裂纹间距L/a=1分界拐点作为后续工程研究的有效参考.后续进行了十字焊接接头疲劳验证试验,对仿真结果取得了较好的验证.     

Fatigue life predictions using fracture mechanics methods

In the present work, a simple engineering approach which is based on a relatively solid background and which is checked against fatigue test data for various test conditions was developed: it may provide a practical and reliable basis for the analysis of structures under in-service loading conditions, in the presence of previous corrosion attack, or in the presence of a residual stress field, by using widespread fracture mechanics software. In particular, the approach was checked against an experimental program which consists of the following fatigue tests: base and friction stir welded (FSW) material under constant amplitude loading at different loading ratios (R = 0.1, 0.5, −1); pre-corroded base and FSW material under constant amplitude loading at load ratio R = 0.1; centre hole FSW specimens under the standardised variable amplitude loading spectrum FALSTAFF. Moreover, from the literature fatigue experiments under FALSTAFF of cold expanded as well as not cold expended holes were also used to validate the approach. The predictions were performed with the last version of AFGROW and NASGRO 3.0 software.     

考虑形状比的焊趾裂纹扩展行为的数值仿真分析

针对焊趾处初始裂纹的不同形状比,基于NASGRO裂纹扩展速率模型对裂纹扩展过程进行了数值仿真. 以具有不同形状比的半椭圆表征焊趾表面裂纹,考虑短裂纹扩展阶段,采用M积分计算整个裂纹前沿的应力强度因子,并将裂纹前沿应力强度因子中值点作为裂纹扩展的主要控制参量,模拟并分析了不同形状比条件下裂纹前沿的形状演变和应力强度因子的分布及变化特点. 结果表明,随着裂纹的扩展,短裂纹阶段的不同形状比变化将趋于一致;短裂纹的形状比对早期裂纹扩展有较大影响,并最终导致整体疲劳寿命的显著差异性;综合初始形状比对裂纹前沿应力强度因子、扩展形状以及疲劳寿命的影响,可将其作用效果的强弱拐点作为界定短裂纹与长裂纹的有效参考. 开展了十字焊接接头疲劳验证试验,试验结果与仿真结果具有较好的一致性.     

动车组铸钢制动盘裂纹扩展寿命预测

为研究动车组铸钢制动盘出现裂纹后裂纹扩展速率和扩展寿命,根据制动盘材料参数,使用ANSYS软件建立制动盘的循环对称三维瞬态计算模型,采用间接耦合方法计算制动盘的温度场和应力场,得到在动车组速度为300 km/h的工况下,裂纹处的温度为355.33℃。以温度计算结果作为初始载荷计算制动盘热应力,制动盘最大热应力为899 MPa,盘面裂纹处的应力为501 MPa。并将计算结果作为计算制动盘的载荷输入到NASGRO中,对裂纹扩展速率和扩展寿命进行计算和分析。计算和分析结果表明,此材料制动盘径向裂纹长度尖端处的应力强度因子和扩展速率均高于深度尖端处;计算得出制动盘裂纹扩展寿命为制动48 831次,为该制动盘的使用提供参考。     

The influence of cyclic stress intensity threshold on fatigue life scatter

Understanding factors that contribute to scatter in fatigue lives of metallic structures (particularly airframes) subjected to identical spectrum is critical to maintaining safety and optimising designs. This paper first briefly discusses the sources of scatter, and then concentrates on the effect of variations in the “cyclic stress intensity threshold” (ΔK_thr) on fatigue crack growth. It shows that a version of the NASGRO equation can be used to account for the crack growth scatter seen in a number of classical fatigue experiments by accounting for variations in ΔK_thr. This is an important outcome for safety and is particularly useful when considering lead cracks for which ΔK_thr is small (approaching zero) as these cracks appear to commence growing soon after introduction into service.     

Estimations of fatigue life and variability under random loading in aluminum Al-2024T351 using strip yield models from NASGRO

This paper is concerned with the application of the strip yield models implemented in NASGRO to estimate fatigue crack growth under random loading. The two different strip yield model options (constant constraint-loss (CCL) and variable constraint-loss (VCL)) implemented in the software are considered and compared. In addition, three crack grow rate relations, obtained from constant amplitude data, the NASGRO material database and the literature, were also considered and compared. The capacity of the models to estimate the fatigue life and variability is analyzed by comparing simulated results with experimental data of fatigue crack growth under different stationary Gaussian random load processes on compact, C(T), specimens of aluminum alloy 2024-T351. The analysis performed showed that both model options provide very similar good fatigue life predictions. The estimated fatigue lives are within ±13% of the test lives with the best option. The variability of the results due to the randomness of the load is also analyzed. In this case, the variable constraint-loss option provides better estimation than the constant constraint-loss option.     

The development of a damage tolerance concept for railway components and its demonstration for a railway axle

A draft procedure for damage tolerance analysis of railway components is presented and illustrated by a case study on a railway axle. The scheme is based on the recently developed European flaw assessment procedure SINTAP, the NASGRO/ESACRACK procedure for fatigue crack extension and other documents. As the result of the worked example the crack size was quantified which has to be detected in non-destructive testing if the inspection interval is fixed by an existing maintenance plan. The resulting numbers are aimed at illustrating the method and cannot be used for industrial implementation without appropriate modification.     

Sigmoidal crack growth rate curve: statistical modelling and applications

The present paper proposes a statistical model for describing sigmoidal crack growth rate curves. Major novelties are: a) exploitation of the maximum likelihood principle for obtaining material estimates by pooling together experimental data belonging to the different crack propagation regions; b) a general formulation which allows to adopt different sigmoidal models and any kind of statistical distribution for the model variables; c) fatigue life predictions through numerical integration of analytical functions with no need of Monte Carlo simulations. Experimental data taken from NASGRO database are used to check the validity of the statistical model in estimating material parameters included in the crack growth NASGRO algorithm. Illustrative plots of number of cycles to failure and crack length after a given number of cycles are presented, showing good agreement between the proposed statistical model and NASGRO results.     

Experiments and stochastic model for propagation lifetime of railway axles

Inspection intervals of critical safety components should be calculated on the basis of sophisticated crack growth algorithms able to describe crack propagation under service loads. The present paper deals with a probabilistic application of the NASGRO crack growth algorithm to estimate the propagation lifetime of railway axles. The analysis of experimental crack growth and threshold data of an A1N steel has enabled us to propose two random variable models for the estimation of propagation lifetimes, discussing their statistical properties and their implications for lifetime estimates.