Microstructure-based fatigue life prediction for cast A356-T6 aluminum-silicon alloys

Fatigue life prediction and optimization is becoming a critical issue affecting the structural applications of cast aluminum-silicon alloys in the aerospace and automobile industries. In this study, a range of microstructure and porosity populations in A356 alloy was created by controlling the casting conditions and by applying a subsequent hot isostatic pressing (“hipping”) treatment. The microstructure and defects introduced during the processing were then quantitatively characterized, and their effects on the fatigue performance were examined through both experiment and modeling. The results indicated that whenever a pore is present at or near the surface, it initiates fatigue failure. In the absence of large pores, a microcell consisting of α-Al dendrites and associated Si particles was found to be responsible for crack initiation. Crack initiation life was quantitatively assessed using a local plastic strain accumulation model. Moreover, the subsequent crack growth from either a pore or a microcell was found to follow a small-crack propagation law. Based on experimental observation and finite-element analysis, a unified model incorporating both the initiation and small crack growth stages was developed to quantitatively predict the dependency of fatigue life on the microstructure and porosity. Good agreement was obtained between the model and experiment.     

轴承钢的滚动接触疲劳与寿命预测模型

轴承是现代工业中最重要的钢铁部件之一,而次表面滚动接触疲劳(RCF)是轴承主要的失效模式。为了深入理解轴承钢的RCF失效过程,首先从力学和材料学两个角度对轴承钢的RCF过程进行了描述,提出RCF过程的实质是由位错和碳原子的交互作用造成的次表面组织演变过程;然后介绍了预测轴承RCF寿命的工程模型和理论模型,指出将工程模型与理论模型相结合、将在循环载荷作用下微观组织的演变与RCF寿命相结合是未来工作的两个重要方向;最后对全流程、多尺度的轴承钢设计问题进行了展望,提出耦合物理冶金学算法与人工智能算法以及多学科交叉的重要工作思路。     

随机载荷作用下的疲劳寿命计算

In two-dimensional fatigue load spectrum,the range and mean of random load are regarded as two random variables which could be obtained by rainflow method.Joint probability density function of the two variables is built up on the statistical analysis of load history.Applying constant amplitude median Sa-Sm-N surface function,a fatigue life prediction method of two-dimensional load spectrum is presented based on Miner’s rule.In the last part,an example is presented to demonstrate the application of the prediction method,and it shows that the two-dimensional fatigue life prediction method is practicable in fatigue life analysis for engineering structures.     

Scatter in fatigue life due to effects of porosity in cast A356-T6 aluminum-silicon alloys

Porosity is well known to be a potent initiator of fatigue cracks in cast aluminum alloys. This article addresses the observed scatter in fatigue life of a cast A356-T6 aluminum-silicon alloy due to the presence of porosity. Specimens containing a controlled amount of porosity were prepared by employing a wedge-shaped casting mold and adjusting the degassing process during casting. High-cycle fatigue tests were conducted under fixed stress conditions on a series of specimens with controlled microstructures (especially, the secondary dendrite-arm spacing), and the degree of scatter in the results was assessed. Stochastically, such scatter was found to be adequately characterized by a three-parameter Weibull distribution function. Large pores at or close to the specimen surface were found to be responsible for crack initiation in all fatigue-test specimens, and the resultant fatigue life was related to the initiating pore size through a relationship based on the rate of small-fatigue-crack propagation. With respect to the probabilities for the pores of various sizes and locations to initiate a fatigue crack, a statistical model was developed to establish the relationship between the porosity population and the resultant scatter in fatigue life. The modeling predictions are in agreement with the experimental results. Moreover, Monte-Carlo simulation based on this model demonstrated that the average pore size, pore density, and standard deviation of the pore sizes, together with the specimen size and geometry, are all of consequence regarding scatter in fatigue life.     

连铸用钢包的耐火内衬优化及疲劳寿命预测

摘要：连铸技术对钢包的性能要求越来越高,而传统钢包炉衬构件普遍存在使用寿命短、消耗高等问题。通过研究耐火材料特性,优化内衬结构布置,设计出一种长寿命、超保温的新型钢包,并基于数值模拟技术,对新型钢包与传统钢包在典型工况下的温度与应力进行对比分析。温度场模拟结果表明,新型钢包在保温性能上有较大的提升,钢包壳最高温度较传统钢包降低54℃。同时,应力场结果表明,新型钢包壳的最大应力减小了66.7MPa且整体应力分布更加均匀。最后将温度场和应力场的分析结果反馈到钢包的生产、制造、维护上,并进行实验验证。实验结果表明,新型钢包在保温与长寿等性能方面表现更好,内衬寿命提高了119炉次,达到了钢包设计、制造、维护一体化的效果。     

Temperature evolution and life prediction in fatigue of superalloys

Low-cycle fatigue behavior of two superalloys, ULTIMET® alloy, Co-26 pct Cr-9 pct Ni (wt pct), and HAYNES® HR-120® alloy, Ni-33 pct Fe-25 pct Cr, was studied at room temperature. An infrared thermography system was employed to monitor the temperature evolution of fatigue processes for both superalloys. Temperature changes during fatigue were related to the hysteresis effect, and were successfully predicted, based on the consideration of the hysteresis effect and heat conduction. The temperature increase of a specimen from the initial to the equilibrium stages was used as an index to predict the fatigue life of the two superalloys. It was found that the fatigue-life predictions using the present model were in good agreement with the experimental results.     

不同应力比交替作用下单参数疲劳寿命预测方法

为了准确预测管道在不同应力比交替作用下的剩余寿命,在疲劳裂纹扩展速率实验的基础上,建立了一种新的含缺陷管道剩余寿命预测方法,即不同应力比交替作用下单参数疲劳寿命预测方法.该方法全面考虑了管线运行过程中不同应力比交替作用引起的破坏.以X52管线钢为例,通过模拟天然气实际运行情况(R=0.1和R=0.6交替作用)预测了其剩余寿命.     

Finite element prediction of high cycle fatigue life of aluminum alloys

A new method is described for calculating the long fatigue life (>10⁵ cycles) portion of the stress-life (S-N) fatigue curve for precipitation-hardened aluminum alloys. It is based upon a finite element model of the deformation of a persistent slipband (PSB), and the only material parameter required is the ultimate tensile strength (UTS) of the alloy. The stress dependence of the plastic strain at the tip of a PSB is shown to be very pronounced and to closely match that of anS-N fatigue curve. Very good agreement is obtained for 6061-T6, 2014-T6, 2024-T4, and 7075-T6 aluminum, and the fatigue strength (at 10⁸ cycles) is calculated to be 26 pct of the tensile strength of each alloy, in agreement with experimental data. By contrast, the plastic strain at a crack tip has a much weaker stress dependence. Thus, these calculations also confirm that the elongation of a PSB, and not crack growth, is the rate-controlling process in high cycle fatigue.     

Finite element prediction of high cycle fatigue life of aluminum alloys

A new method is described for calculating the long fatigue life (>10⁵ cycles) portion of the stress-life (S-N) fatigue curve for precipitation-hardened aluminum alloys. It is based upon a finite element model of the deformation of a persistent slipband (PSB), and the only material parameter required is the ultimate tensile strength (UTS) of the alloy. The stress dependence of the plastic strain at the tip of a PSB is shown to be very pronounced and to closely match that of anS-N fatigue curve. Very good agreement is obtained for 6061-T6, 2014-T6, 2024-T4, and 7075-T6 aluminum, and the fatigue strength (at 10⁸ cycles) is calculated to be 26 pct of the tensile strength of each alloy, in agreement with experimental data. By contrast, the plastic strain at a crack tip has a much weaker stress dependence. Thus, these calculations also confirm that the elongation of a PSB, and not crack growth, is the rate-controlling process in high cycle fatigue.     

基于EM算法的不完全疲劳寿命数据可靠性分析

针对随机截尾寿命数据模型的参数估计,提出基于EM算法的通用算法,并以疲劳寿命计算中所常用的二参数威布尔分布模型为例,给出基于EM算法的寿命数据拟合通用方法.另外对于服从复杂分布模型的数据,则采用与蒙特卡洛法相结合的EM算法.最后通过计算模拟的不完全疲劳寿命数据,验证了EM算法对于随机截尾寿命数据模型参数估计的有效性.     

A model for roughness-induced fatigue crack closure

A model for predicting the crack closing stress intensity factor for roughness-induced closure of fatigue cracks is developed based on a two-dimensional approach considering crack opening and closure of an idealized crack path. The model highlights the contribution of irreversible cyclic planar slip at the crack tip, and is extended to real cases describing roughness-induced crack closure as a function of fracture surface roughness parameters at low ΔK levels where planar slip prevails. The resulting equation indicates that roughness-induced crack closure depends on the maximum stress intensity factor, the standard deviation of heights as well as the standard deviation of angles of the crack profile elements, and the yield stress of the material. Comparison between the prediction of the model and experimental data of K _cl for lamellar microstructures of Ti-2.5 Cu as well as TIMETAL 1100 shows good agreement.     

基于夹杂-细晶粒区-鱼眼疲劳失效的超长寿命预测模型

本文旨在研究夹杂-细晶粒区-鱼眼诱发疲劳失效的超长寿命预测模型.基于Cr-Ni-W合金钢疲劳试验结果,结合局部应力-寿命法和位错-能量法,分别构建了局部裂纹萌生寿命模型(LCIL)和考虑夹杂及细晶粒区影响的裂纹萌生寿命模型(IFCIL),并与Tanaka-Mura模型(T-M)进行了对比分析.其次,分别对细晶粒区内的小裂纹扩展行为和细晶粒区外鱼眼内的长裂纹扩展行为进行建模,最终形成了包含裂纹萌生和扩展在内的全寿命预测模型.结果表明,考虑夹杂及细晶粒区影响的裂纹萌生寿命模型(IFCIL)有较高的预测精度;对应细晶粒区的裂纹萌生寿命几乎等同于全寿命;裂纹扩展寿命仅占据全寿命很小的一部分;预测结果全部处于2倍偏差以内,即全寿命模型可有效地用于夹杂-细晶粒区-鱼眼诱发失效的超长寿命预测.     

A study of the mechanisms of fatigue life improvement in an ion implanted nickel-chromium alloy

Wires of the alloy Ni20Cr with and without carbon ion implantation have been tested in tension-tension fatigue. A 17% increase in endurance limit was found with implantation. The fatigued surface was examined by SEM, and the wire then nickel plated so that transverse sections could be made for TEM study. It was found that bulk slip was unaffected by implantation but slip in surface grains was unable to penetrate the implanted layer to significant degree. Slip band cracking which was found in unimplanted specimens was replaced by grain boundary cracking in the implanted specimens.     

高温多轴比例与非比例循环加载下疲劳寿命预测

利用薄壁圆管和缺口试样,在MTS 809电液伺服材料试验系统上对GH4169合金的高温多轴疲劳特性进行了实验研究．实验采用对称轴向和扭转应变控制、比例与非比例循环加载,轴向与扭转应变的相位差分别为0°,45°,90°．通过对薄壁圆管和缺口试样的高温多轴疲劳寿命特性分析,基于临界面方法提出了一个的多轴疲劳寿命预测模型,在考虑临界面上最大剪应变和正应变对多轴疲劳损伤贡献的同时,还引入了应力状态对多轴疲劳寿命的影响因素．应用新模型对GH4169合金高温多轴疲劳寿命进行预测结果表明,该模型对于缺口试样和薄壁圆管的高温多轴疲劳寿命估算具有较高的准确性．     

Creep-fatigue life prediction in terms of nucleation and growth of fatigue crack and creep cavities

In low cycle fatigue at elevated temperature, the interaction between fatigue crack and creep damages is known to be responsible for the significant reduction of the fatigue life. In this investigation, a model for the life prediction for low cycle fatigue with hold time at tensile peak strain is suggested for the temperature range of 0.5T _m. This model is formulated on the basis of the assumptions that the creep cavities are formed due to the vacancies generated during fatigue, and are grown during the hold period. The fatigue crack nucleated at the surface due to fatigue loading is affected by the creep damages for its propagation. The model is checked by experimental results with various hold time periods. The predicted creep-fatigue lives are in good agreement with experimentally observed ones for 304 stainless steel and 13CrMo44 steel. Formerly Graduat3e student, Department of Materials Science and Engeneering ,KAIST, Seoul, Korea.     

A new approach to the prediction of low-cycle fatigue data

Low-cycle fatigue data for annealed AISI Type 304, 316, and 348 stainless steel to 816°C have been analyzed and a new relationship has been identified associating fatigue data with shortterm tensile behavior. A logarithmic plot of plastic strain range vs time to fracture was shown to be linear with a slope of minus unity. This line was also found to originate at the point corresponding to the tensile ductility. A similar analysis for elastic strain range data revealed a relationship with the strain-hardening exponent measured in a short-term tensile test at the same temperature and strain rate. These correlations have been combined to provide an equation for predicting the relationship between total strain range and cycles to fracture. This approach has been termed the Method of Characteristic Slopes.     

A computational model for the prediction of steel hardenability

A computational model is presented in this article for the prediction of microstructural development during heat treating of steels and resultant room-temperature hardness. This model was applied in this study to predict the hardness distribution in end-quench bars (Jominy hardness) of heat treatable steels. It consists of a thermodynamics model for the computation of equilibria in multicomponent Fe-C-M systems, a finite element model to simulate the heat transfer induced by end quenching of Jominy bars, and a reaction kinetics model for austenite decomposition. The overall methodology used in this study was similar to the one in the original work of Kirkaldy. Significant efforts were made to reconstitute the reaction kinetics model for austenite decomposition in order to better correlate the phase transformation theory with empiricism and to allow correct phase transformation predictions under continuous cooling conditions. The present model also expanded the applicable chemical composition range. The predictions given by the present model were found to be in good agreement with experimental measurements and showed considerable improvement over the original model developed by Kirkaldy et al.     

转炉炼钢过程静态控制模型的改进

简要介绍了采用统计回归方法建立增量多元回归方程,从而建立达到对冶炼终点进行控制目的的转炉炼钢静态控制模型,并通过采集到的某钢厂120 t转炉的实际生产数据,对模型进行了验证.