High‐temperature low cycle fatigue damage assessment in near alpha IMI‐834 titanium alloy

In the present work, evolution of damage under high‐temperature (823 K) low cycle fatigue loading condition in near α IMI‐834 titanium alloy has been studied. The in situ damage has been experimentally measured during cyclic deformation using the alternating current potential drop (ACPD) technique. The measured damage curve has been compared with the damage curves calculated through mechanical variables such as cyclic modulus and stress amplitude. The ACPD damage curve has been found most sensitive towards high‐temperature low cycle fatigue damage evolution.     

Simulated flight fatigue of an alpha/beta titanium alloy

Simulated flight (FALSTAFF) fatigue tests have been carried out on precracked single edgenotch test-pieces of (Ti4Al4Mo2Sn0.5Si) IMI 550 titanium alloy. Predictions of simulated flight fatigue behaviour have been made from constant amplitude fatigue data, using a damage accumulation approach, with no allowance for load history. The predicted lives were conservative compared with the measured lives, and accurate within a factor of approximately two. Retardation of fatigue crack growth increased with increasing load amplitude. The microstructure produced by β-solution heat treatment at 1010°C, followed by ageing, was found to improve simulated flight fatigue lives by up to approximately 100% compared with standard solution treatment at 900°C, followed by ageing.     

An anisotropic damage model for tensile fatigue

Fatigue damage in materials is considered to be the effect of material degradation, and the dispersion in fatigue life is attributed to variability in microstructure. This paper presents a numerical model to simulate fatigue damage evolution using continuum damage mechanics to characterize material degradation. An explicit microstructure topology representation is achieved using Voronoi tessellations. Unlike conventional models which use a scalar approximation for damage, this model treats the damage variable as an anisotropic tensor. The model is used to simulate tensile fatigue failure in thin steel specimen. The fatigue life estimations from the model compares well with published experimental results. The results predict a high variability in fatigue life that is characteristic of metals and alloys, as compared with the existing isotropic damage models available in the literature. The model was also used to study the influence of material inhomogeneity on fatigue life dispersion.     

The life evaluation by linear cumulative damage rule for cold dwell fatigue of Ti-6Al-4V alloy

Low cycle fatigue (LCF) life at ambient temperature of Ti alloys is well known to decrease with stress dwell. This phenomenon, called cold dwell fatigue (CDF), is influenced by the peak stress, dwell time, and microstructure. For this study, the CDF life was evaluated by the linear cumulative damage rule. The influence of test conditions and microstructure on the linear cumulative damage rule was also verified. By the linear cumulative damage rule, when creep damage is calculated using the time exhaustion rule, theCDF damage was evaluated by the inequality of D_Total = (D_F, D_C) ≤ (0.01, 10^?6). However, the CDF damage can be evaluated in the range of D_Total = 0.6–1.2 when creep damage was calculated using the ductile exhaustion rule. Results indicate that the evaluation was almost independent of the dwell time, peak stress, and microstructure, so it is also a versatile method for evaluating CDF responses.     

Ti60钛合金室温保载疲劳性能及断裂行为

研究高峰值应力条件下Ti60钛合金双态组织和片层组织的低周疲劳与保载疲劳性能,利用金相显微镜(OM)、扫描电子显微镜(SEM)和电子背散射衍射(EBSD)等观察和分析Ti60钛合金的显微组织与疲劳断裂行为。结果表明:显微组织对低周疲劳性能影响不大,但显著影响保载疲劳性能,双态组织保载疲劳敏感性大于片层组织;保载情况下,疲劳寿命显著下降;随峰值应力的提高,疲劳寿命下降,保载疲劳敏感性增加;相同循环周次内,保载疲劳塑性应变累积大于低周疲劳,双态组织的塑性应变累积大于片层组织;低周疲劳裂纹萌生于试样表面,为单裂纹源,而保载疲劳裂纹为内部多源萌生;断口表面均存在准解理小平面,双态组织断口准解理小平面密度大于片层组织。     

Low cycle fatigue of a titanium 829 alloy

The fatigue behaviour of titanium 829 in its oil quenched (‘basketweave’) and air cooled (‘aligned’) microstructural forms has been examined at 600°C and room temperature under fully reversed, total strain controlled conditions. Identical endurances are observed for each microstructure together with a low transition life. Similarly, almost perfect cyclic stability is exhibited irrespective of microstructure, temperature, strain range and rate. This is tentatively attributed to the ability of the aligned colonies present in both microstructures to accomodate plasticity. It is argued that a reduction in strain rate shortens life due to environmental effects. Multiple crack initiation is generally associated with facet-like features, with later growth surfaces bearing striations only after fatigue at 600°C.     

A new creep damage assessment method for metallic material under variable load conditions at elevated temperature

It is significant for structural design and maintenance to assess behaviour and life under varying load conditions. For structures operating in high‐temperature environments, creep is one mechanism responsible for material failures. In this paper, different damage accumulation rules were reviewed, and a new creep damage assessment method was proposed based on the creep damage tolerance parameter λ and load factor Φ. By introducing the creep damage tolerance parameter λ and the minimum creep rate, the loading process and creep behaviour of the material are taken into account in a damage assessment. The parameters in the model can be obtained by the simple variable load creep test, and the remaining life and strain can be predicted using uniaxial creep test data. To analyse the applicability and accuracy of this new model, the strain histories, the life of step load, and the constant load creep from experiments on a titanium alloy at 500°C were obtained, and the prediction results of the novel and previous methods were carefully investigated.     

The influence of dynamic strain aging on the low cycle fatigue behavior of near alpha titanium alloy IMI 834

Total strain controlled low cycle fatigue tests on IMI 834 have been conducted in air in the temperature range between 375 and 500 °C at a temperature interval of 25 °C at the nominal strain rate of 6.67 × 10⁻⁴ s⁻¹. The observed maximum peak stress ratio, minimum half-life plastic strain range and lower fatigue life at 425 °C indicates the occurrence of dynamic strain aging (DSA). Pronounced deformation bands, increased dislocation density and non-uniform dispersion of dislocations inside primary α grains observed by the study of transmission electron microscopy supports the occurrence of dynamic strain aging. Initial cyclic softening was attributed to shearing of Ti₃Al precipitates as revealed by TEM evidences.     

Dwell sensitive fatigue in a near alpha titanium alloy at ambient temperature

The ambient temperature fatigue performance of the near alpha titanium alloy IMI834 was evaluated using laboratory specimens manufactured from two material sources: rolled bar stock and an isothermally forged compressor disc. The effect of dwell periods imposed at peak stress and R value were assessed. Significant differences were defined between the two variants with respect to their sensitivity to dwell loading. Variations in microstructural form together with a localised texture within the disc material are considered responsible for controlling the dwell performance in each case. The findings are consistent with a previously proposed model for facet development in this class of material.     

Physics‐based multiscale damage criterion for fatigue crack prediction in aluminium alloy

In this paper, a physics‐based multiscale approach is introduced to predict the fatigue life of crystalline metallic materials. An energy‐based and slip‐based damage criterion is developed to model two important stages of fatigue crack initiation: the nucleation and the coalescence of microcracks. At the microscale, a damage index is developed on the basis of plastic strain energy to represent the growing rate of a nucleated microcrack. A statistical volume element model with high computational efficiency is developed at the mesoscale to represent the microstructure of the material. Also, the formation of a major crack is captured by a coalescence criterion at mesoscale. At the macroscale, a finite element analysis of selected test articles including lug joint and cruciform is conducted with the statistical volume element model bridging two scale meshes. A comparison between experimental and simulation results shows that the multiscale damage criterion is capable of capturing crack initiation and predicting fatigue life.     

High temperature deformation behavior of a near alpha Ti600 titanium alloy

The high temperature deformation behavior of a near alpha Ti600 titanium alloy was investigated with isothermal compression tests at temperatures ranging from 800 to 1000 °C and strain rates ranging from 0.001 to 10.0 s⁻¹. The apparent activation energy of deformation was calculated to be 620.0 kJ mol⁻¹, and constitutive equation that described the flow stress as a function of the strain rate and deformation temperature was proposed for high temperature deformation of Ti600 titanium alloy in the α + β phase region. The processing map was calculated to evaluate the efficiency of the forging process in the temperatures and strain rates investigated and to recognize the instability regimes. High efficiency values of power dissipation over 55% obtained under the conditions of strain rate lower than 0.01 s⁻¹ and temperature about 920 °C was identified to represent superplastic deformation in this region. Plasticity instability was expected in the regime of strain rate higher than 1 s⁻¹ and the entire temperature range investigated.     

考虑位错密度和损伤的NiCoCrFe高熵合金晶体塑性有限元分析EI北大核心CSCD

结合实验和晶体塑性有限元方法研究准静态加载NiCoCrFe高熵合金有限变形过程中的宏观和微观力学响应、损伤行为以及微观结构演化。使用电子背散射衍射技术(EBSD)对拉伸实验变形前后NiCoCrFe的微观结构进行表征。通过修改强化模型和流动准则分别在CPFEM模型中引入位错密度内部状态变量和连续介质损伤因子,并结合拉伸实验应力-应变曲线确定NiCoCrFe相关的模型参数。结果表明:考虑位错密度和损伤的CPFEM模型可以有效地描述NiCoCrFe宏观和微观力学响应。CPFEM模型合理预测NiCoCrFe颈缩区域的变形形状和尺寸,其中实验获得的颈缩区域长度比预测结果小7%,CPFEM预测的颈缩区域宽度比实验结果大23%。CPFEM模型预测NiCoCrFe拉伸变形后的织构演化同EBSD表征结果大致相同,均表现为弱的(100)∥RD以及强的(111)∥RD纤维织构。在三维微观结构损伤分析中,CPFEM模型预测的损伤在应力集中以及位错密度集中的晶界处萌生,表现为晶间损伤机制,并且随着变形的增加损伤逐渐向晶粒内部扩展。     

An unfitted finite element method using discontinuous Galerkin

In this paper we present a new approach to simulations on complex‐shaped domains. The method is based on a discontinuous Galerkin (DG) method, using trial and test functions defined on a structured grid. Essential boundary conditions are imposed weakly via the DG formulation. This method offers a discretization where the number of unknowns is independent of the complexity of the domain. We will show numerical computations for an elliptic scalar model problem in ?² and ?³. Convergence rates for different polynomial degrees are studied. Copyright © 2009 John Wiley & Sons, Ltd.     

A method of assessing the strength of metal surface using film samples on titanium alloy Ti‐1023

The fatigue life of aerospace components depends greatly on the mechanical properties of the finished surface layer. However, no independent strength test of this layer has been reported because of the lack of suitable samples. Therefore, a direct method of assessing the surface tensile strength using film samples with thicknesses of approximately 40 μm is proposed in this paper. The immediate objective of this research is to demonstrate the fundamentals of surface strength testing and prove the feasibility of preparing films by tracking the evolution of the surface integrity. The test results show that layer‐by‐layer grinding and polishing is a feasible method for preparing film samples with sufficient area, controllable thickness, and well‐maintained surface integrity. During the preparation of the film samples, the roughness and micro‐hardness of the test side (the side kept unprepared for testing) are protected, and those of the processed side (the side that is ground and polished) are controlled. The residual stress on both sides is released to zero. The film specimens exhibit regular fracture behaviour in the tensile tests, and their stress–strain curves can be explained as weighted averages of the stress–strain functions of multiple layers.     

Small fatigue crack growth behavior of titanium alloy TC4 at different stress ratios

In this paper, the small fatigue crack behavior of titanium alloy TC4 at different stress ratios was investigated. Single‐edge‐notch tension specimens were fatigued axially under a nominal maximum stress of 370 MPa at room temperature. Results indicate that fatigue cracks in TC4 initiate from the interface between α and β phases or within α phase. More than 90% of the total fatigue life is consumed in the small crack initiation and growth stages. The crack growth process of TC4 can be divided into three typical stages, ie, microstructurally small crack stage, physically small crack stage, and long crack stage. Although the stress ratio has a significant effect on the total fatigue life and crack initiation life at constant σ_max, its effect on crack growth rate is indistinguishable at R = ?0.1, 0.1, and 0.3 when crack growth rate is plotted as a function of ?K.     

On the mechanisms of fatigue facet nucleation in titanium alloys

A crystal plasticity model for near‐alpha hcp titanium alloys embodying a quasi‐cleavage failure mechanism is presented and employed to investigate the conditions necessary in order for facet nucleation to occur in cold‐dwell fatigue. A model polycrystal is used to investigate the effects of combinations of crystallographic orientations (and in particular, a rogue grain combination), the essential role of (cold) creep during hold periods in the loading cycle and the more damaging effect of a load hold rather than a strain hold in facet nucleation. Direct comparisons of model predictions are made with dwell fatigue test results. More generally, the crystal model for faceting is found to be consistent with a range of experimental observations.     

钛合金超声扭振铣削强度优化设计仿真

钛合金具有比强度大、热传导率低等特点,对其进行强度优化设计仿真,可以提高其切削加工性能。运用Abaqus软件建立了二维超声扭振铣削模型,并采用John-cook分离准则、任意的拉格朗日-欧拉(ArbitraryLagrange-Euler,ALE)网格划分方法和施加相应边界等条件,根据刀-屑的周期分离原理,确立了接触率与角速度、振动频率和超声振幅之间的关系。此外,采用单因素法依次改变角速度、频率和振幅,以此进行强度优化设计仿真。结果表明,切削力和温度均随角速度的增加而增加;振幅的增大使得切削力有所降低,而温度却有所上升;频率的变化对两者影响不大。并得出结论:给定参数范围内,影响切削力的权重依次为振幅、角速度和频率;影响温度的权重依次为角速度、振幅和频率。     

Evaluation of the probability distribution of crack propagation life in metal fatigue by means of probabilistic finite element method and B-models

In this paper, a new model for the prediction of the cumulative distribution function of fatigue life of structural elements during the crack propagation stage is established. This problem is considered as a cumulative damage process following the probabilistic approach of Bogdanoff and Kozin (B-models). The initial and final crack lengths, the crack propagation angle, the material fracture and elastic parameters and the external loads have been the random variables considered here. The theoretical bases of the model and the procedure to construct it are described in the forthcoming paragraphs such as several examples for mode I problems including the comparison with experimental results.     

Quantitative relationship between microstructure characteristics and fatigue parameters of A319 casting alloy

This paper describes a microstructure‐based uniaxial strain‐controlled fatigue life prediction model applied to A319 aluminum alloy which is widely used in automobile industry. The materials made with different casting conditions are characterized and quantified in terms of secondary dendrite arm spacing (SDAS), size, and aspect ratio of eutectic Si particles. Uniaxial low cycle fatigue tests have been performed on four groups of A319 alloy under different casting conditions in which cooling rate and Sr addition are variables. It is shown that the effect of various degrees of microstructure on the fatigue life and fatigue behavior is obvious. The first part of the paper is quantitatively characterizing the microstructure of samples to identify the influence of different casting conditions. With regard to mechanic properties, the tensile properties and fatigue behavior of samples are analyzed combining with microstructure. Finally, a microstructure‐based Manson‐Coffin‐Basquin model is proposed to predict fatigue life of Al‐Si alloy.     

Three-dimensional finite element simulations of microstructurally small fatigue crack growth in 7075 aluminium alloy

Three‐dimensional finite element simulations were performed to study the growth of microstructurally small fatigue cracks in aluminium alloy 7075‐T651. Fatigue crack propagation through five different crystallographic orientations was simulated using crystal plasticity theory, and plasticity‐induced crack opening stresses were calculated. The computed crack opening stresses were used to construct small crack da/dN‐ΔK diagrams. The generated da/dN‐ΔK curves compared well with experimental small crack data from the literature. The variance observed among the da/dN‐ΔK results, which occurred as a consequence of the different crystallographic orientations employed, was found to be of the same order of magnitude as commonly observed variability in small fatigue crack growth data. This suggests that grain orientation is a major contributor to observed small fatigue crack data scatter.