Modeling of cavity coalescence during tensile deformation

The effect of material properties such as the cavity growth rate and initial cavity population on cavity coalescence during uniaxial tensile deformation was determined. To this end, a two-dimensional model that treats the growth of a random array of spherical cavities inside a deforming tension specimen was developed. Simulation results included predictions of the conditions under which cavity coalescence occurs, the variation of average cavity radius and fraction of cavities which have coalesced as a function of strain, and the evolution of cavity size distribution as a function of strain, cavity growth rate, and cavity population. For a given cavity density, it was found that the fraction of cavities which has coalesced is independent of the cavity growth rate and varies linearly with the cavity volume fraction; a relationship between the fraction of coalesced cavities and the cavity volume fraction was established from the simulation results. In addition, simulation predictions of average cavity radius as a function of strain were compared to previous relations for cavitation under conditions involving growth and coalescence. Predictions of average cavity size as a function of strain from the present work gave good agreement with previous experimental and theoretical work of Stowell and Pilling.     

Characterization by X-ray computed tomography of decohesion,porosity growth and coalescence in model metal matrix composites

Damage mechanisms of model materials have been studied using in situ tensile tests coupled with high resolution X-ray tomography. This non destructive technique revealed that 50% of the particles were pre-damaged by the extrusion. The initiation and growth phases of the damage process were quantified using the three dimensional images. The growth phase, measured both locally (on isolated particles) and globally (in the entire block) was compared with the Rice and Tracey prediction which was shown to overestimate the global prediction and to give a reasonable agreement of the local growth rate. Discrepancies between prediction and experiments could be partly quantified by introducing the effect of the growth threshold in the Rice and Tracey analysis. The scatter in the measured thresholds and growth rates were attributed to local crystallography and to local spatial arrangement effects.     

An orthotropic ductile damage model for visco-plastic materials

Numerical modelling of bulk forming processes has to identify the conditions that may result in unsatisfactory products. In ductile materials, damage may occur with the micro void nucleation at the site of second phase particles and inclusions in the plastic or visco-plastic matrix and then with the micro void growth. In this work an isotropic ductile damage model is extended to load cases with successive tensile and compressive steps and with pure deviatoric stress state. The constitutive equations satisfy the Clausius-Duhem inequality for negative or positive voids volume fraction rate. The constitutive parameters are identified with the Rice and Tracey model modified for a sphere initially filled with a soft or a hard inclusion. Axisymmetric geometries are considered for remote strain fields without distortion. A plane cell and a 3D unit cell are analysed numerically for the deviatoric strain state and various combined deviatoric and volumetric deformations.     

Visualization by X-ray tomography of void growth and coalescence leading to fracture in model materials

The literature contains many models for the process of void nucleation, growth and coalescence leading to ductile fracture. However, these models lack in-depth experimental validation, in part because void coalescence is difficult to capture experimentally. In this paper, an embedded array of holes is obtained by diffusion bonding a sheet filled with laser-drilled holes between two intact sheets. The experiments have been performed with both pure copper and Glidcop. Using X-ray computed tomography, we show that void growth and coalescence (or linkage) are well captured in both materials. The Brown and Embury model for void coalescence underestimates coalescence strains due to constraining effects. However, both the Rice and Tracey model for void growth and the Thomason model for void coalescence give good predictions for copper samples when stress triaxiality is considered. The Thomason model, however, fails to predict coalescence for the Glidcop samples; this is primarily due to secondary void nucleation.     

静水压力对Fe-20Cr合金点蚀行为的影响

基于统计学方法和随机理论,利用动电位极化曲线和恒电位测试技术研究静水压力对Fe-20Cr合金点蚀行为的影响。随着静水压力的增加, Fe-20Cr合金的击破电位降低,维钝电流密度减小,耐蚀性能变差。静水压力对Fe-20Cr合金点蚀产生和点蚀生长的研究表明：高的静水压力下,亚稳态点蚀发生的频率加快且向稳态点蚀发展的倾向增大,从而导致点蚀的产生速度加快,点蚀的孕育期缩短,但点蚀的产生机制并没有发生改变;静水压力的增加增大了点蚀的生长概率,高压下产生的点蚀更容易成长为大的腐蚀坑。     

Initiation and growth of damage in a dual-phase steel observed by X-ray microtomography

In situ tensile tests have been carried out on a high-strength, dual-phase steel. These experiments show that it is possible to follow and quantify the evolution of damage nondestructively and in three dimensions in this type of material. The measurements were analyzed in terms of density, size, aspect ratio of the cavities but also of the local deformation and the stress triaxiality in the sample. It was found that damage initiation is progressive with the applied tensile strain and that the initiation of new small cavities reduces the average diameter while the growth of the previously created ones increases the average diameter. This information was used to develop a new model for void growth based on the classical Rice and Tracey approach. This simple approach was modified to account for progressive damage initiation. The results of the proposed model are in good agreement with the measurements.     

Mechanical behavior of LC_4 alloy in semisolid state at high volume fractions of solid

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Deformation mechanism of tension of 2024Al alloy at semi-solid state

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Ductile shear failure damage modelling and predicting built-up edge in steel machining

This paper reports an improved ductile shear failure model for steels and its application, through finite element simulations, to predicting the conditions for built-up edge formation in steel machining. The model has two parts, a standard damage accumulation law and (the improved part) how damage affects the steel's flow stress after failure. The accumulation law includes a strain to failure with inverse exponential dependence on hydrostatic pressure and reducing in a blue-brittle temperature range. The flow stress after failure remains finite in compressive hydrostatic conditions, to create a friction resistance to shear across the failure surface. Predictions of built-up edge formation depend strongly on strain hardening behaviour. This affects the hydrostatic stress field in the chip formation region. Simulations show the general features of built-up edge formation (a finite cutting speed range with an upper limit determined by increased ductility with temperature and a lower limit determined, depending on conditions, by insufficient heating for blue-brittleness, lower chip/tool friction or a change to unsteady chip formation). The simulations are tested against previously published observations of built-up edge formation in orthogonal cutting of a Russian steel equivalent to AISI 5130. To extend the work to a wider range of steels requires more data to be gathered on individual steels’ damage accumulation law coefficients. Also, at this stage, the simulations only predict the conditions (cutting speed, uncut chip thickness) in which built-up edge forms. They are not able to follow the growth of the built-up edge to its final shape.     

Long fatigue crack growth behaviour of ferritic steel weld metal

Abstract

Single edge notched weld joint specimens were tested at different stress levels to study the long fatigue crack growth behaviour of a ferritic steel (nuclear grade SA333 Grade 6 steel) weld metal at two stress ratios R of 0·1 and – 1. A two slope behaviour was noticed in the crack growth rate versus stress intensity factor range plots at both stress ratios. Different parameters were employed to generalise the load ratio effect on fatigue crack growth rate.     

冲裁成形与断裂的数值模拟

本文用DEFORM2D软件对A—1100冲裁工艺进行了有限元数值模拟，利用Cockrofl＆Latham准则和Rice＆Tracey断裂准则对冲裁过程徽裂纹产生和最终断裂进行了模拟，分析比较了两种断裂准则下的开始断裂时的静水压力及载荷行程曲线，得到结果与实际基本一致。     

Resistance spot welding and the effects of welding time and current on residual stresses

A 2-D finite element model is developed based on fully coupled electrical-thermal and incrementally coupled thermal-mechanical analysis. The growth rate of the weld nugget as a function of welding time and current is studied. Comparison of the predicted results with the experimental data shows good agreement. Contact area variations and pressure distribution between the sheets’ faying surface and electrode-sheet interfaces during the welding process are studied. Compressive radial residual stress on the surface of the specimen obtained in the center region of the nugget while it becomes tensile and rises toward the nugget edge. The maximum tensile residual stress occurs outside of the nugget, near the edge region. The effects of welding time and current on distribution and magnitude of welding residual stresses are also investigated. The magnitudes of radial residual stresses in the inner and outer areas of the weld nugget grow with increasing the welding time and current while they decrease slightly in the edge regions of the weld nugget. The growth rate of the maximum residual stress reduces with increase in the welding time and current. This fact is more tangible for welding time.     

Extrusion-like chip formation mechanism and its role in suppressing void nucleation

The mechanism of chip formation transforms from concentrated shearing to an extrusion-like behavior at a critical combination of undeformed chip thickness and tool edge radius. Finite element analysis shows that material is removed by severe deviatoric stress within the boundary of elastic-plastic deformation during extrusion-like chip formation while this boundary is constantly redistributed to accommodate chip growth. Simultaneously, the deformation region is contained within active compressive components and hydrostatic pressure as chips are extruded. Under such operating conditions, void nucleation is prevented according to the Le-Chatelier's principle. Exceptional surface finish was produced experimentally through the extrusion-like chip formation mechanism.     

高应力比下HQ－60钢焊缝的疲劳裂纹扩展速率

通过对ＨＱ－６０新钢种的试板和试样焊缝的残余应力实测得到，试样焊缝中的残余应力很小，在研究高应力比下焊缝的疲劳裂纹扩展时可忽略其影响。根据焊缝的裂纹扩展试验结果，得到了在线弹性范围内扩展速率公式。在小范围屈服区提出了一个应力强度因子修正方法，扩大该公式的应用范围，为焊接结构寿命分析提供了方便。     

激光冲击强化对2524铝合金疲劳寿命的影响

目的探索激光冲击工艺参数对2524铝合金疲劳寿命的影响。方法开展不同激光能量、不同冲击次数下的激光冲击强化实验,测试其残余应力和表面硬度,并进行裂纹扩展实验和显微组织观察。结果激光冲击强化能显著提高材料的表面硬度,且材料的硬度值随着冲击能量和冲击次数的增加而递增,但硬度增长率随冲击次数增多而降低。激光冲击强化在试样表层形成较大的残余压应力,使用6.25 J的激光能量冲击1次,最大残余压应力可达-222MPa,并且残余压应力随着激光能量和冲击次数增加而增加,但冲击强化次数存在阈值。相较于未冲击试样,激光冲击1次的试样的疲劳寿命提升32%,冲击2次的疲劳寿命提升41%。对试样断口进行微观形貌观察,在裂纹长度为28 mm处,未冲击试样、激光冲击1次和冲击2次试样的疲劳条带间距分别为1.06、0.628、0.488μm,裂纹扩展速率分别为1.06×10^-3、6.28×10^-4、4.88×10^-4 mm/N。结论激光冲击强化能显著提高2524铝合金的表面硬度,并在表面产生较大的残余压应力。激光冲击强化能够有效迟滞2524-T3铝合金的疲劳裂纹扩展速率,进而有效延长疲劳寿命。     

载荷速率对充氢SA508-3钢断裂韧性的影响

采用高温高压气相热充氢方法将氢充入SA508-3钢,采用J积分方法比较不同载荷速率下未充氢与充氢钢的断裂韧性,考察氢对SA508-3钢断裂韧性的影响。结果表明,在相同载荷速率下,与未充氢SA508-3钢相比,充氢钢断裂韧性明显降低,充氢断口均为韧性和脆性混合断口形貌。随着载荷速率的降低,断裂韧性损失逐渐增加,准解理所占面积增加,脆性提高。在三向应力的作用下,氢与静水应力的交互作用能大于氢与可动位错的交互作用能,静水应力更易捕获到氢。SA508-3钢断裂韧性测试过程中,在三向应力的诱导下会促进氢富集在裂纹尖端碳化物和基体的界面处,从而降低了碳化物和基体的结合强度,致使阻碍裂纹扩展的能力减弱,因此钢充氢后断裂韧性降低。随着载荷速率的降低,三向应力作用在裂纹尖端的时间增加,氢富集在碳化物和基体界面浓度增加,氢压增强,加速裂纹扩展,钢的脆性提高,断裂韧性损失增加。     

高应力比下HQ-60钢焊缝的疲劳裂纹扩展速率

通过对ＨＱ－６０新钢钟的试板和蔗样焊缝的残余应力实测得到，试样焊缝中的残余应力很小，在研究高应力下焊缝的疲劳裂纹扩展时可忽略其影响。根据焊缝的纹扩展试验结果，得到了在线弹性范围内扩展速率公式，在小范围屈服出提出了一个应力强度因子修正方法，扩大该公式的应用范围，为焊接结构寿命分析提供了方便。     

2050铝合金的疲劳裂纹扩展行为研究

研究取样方向、应力比以及微观组织对2050铝合金疲劳裂纹扩展速率的影响。结果表明:取样方向和应力比对疲劳裂纹扩展速率在近门槛区和快速扩展区的影响显著,而在稳态扩展区的影响则相对较小;L-T取样方向的速率最低,T-L取样方向的速率次之,S-L取样方向的速率最高,这主要与合金的晶粒取向、织构取向和第二相粒子取向有关;应力比为0.5的裂纹扩展速率高于应力比为0.1的速率,这在近门槛区主要与裂纹闭合效应有关,在快速扩展区主要受最大应力场强度因子Kmax的影响。     

Factors governing hole expansion ratio of steel sheets with smooth sheared edge

Stretch-flangeability measured using hole expansion test (HET) represents the ability of a material to form into a complex shaped component. Despite its importance in automotive applications of advanced high strength steels, stretch-flangeability is a less known sheet metal forming property. In this paper, we investigate the factors governing hole expansion ratio (HER) by means of tensile test and HET. We correlate a wide range of tensile properties with HERs of steel sheet specimens because the stress state in the hole edge region during the HET is almost the same as that of the uniaxial tensile test. In order to evaluate an intrinsic HER of steel sheet specimens, the initial hole of the HET specimen is produced using a milling process after punching, which can remove accumulated shearing damage and micro-void in the hole edge region that is present when using the standard HER evaluation method. It was found that the intrinsic HER of steel sheet specimens was proportional to the strain rate sensitivity exponent and post uniform elongation.     

Cavity growth in a superplastic Al–Mg alloy: II. An improved plasticity based model

The extent of cavity growth estimated from a combination of diffusional and plasticity based growth models generally underestimates the actual cavity growth in superplastic alloys. It has been shown that in a fine grain Al–Mg alloy, cavity growth begins by matrix/particle debonding at grain boundary particles (Mat. Sci. Forum, Trans. Tech. Pub. 304–306 (1999) 609), and also from pre-existing voids. In this study, cavity growth beyond interface decohesion is modeled in which deformation of the matrix surrounding the cavity is free from interface constraint, but it still experiences an accelerated local deformation rate. Stress and strain-rate in this region are intensified due to the perturbed flow field near the cavity, and not relaxed during the time frame for superplastic forming. This local deformation around the cavity is a function of strain-rate sensitivity, m, the level of strain concentration, and the cavity spacing. Two important effects not previously considered: (i) local stress concentration around the cavities, and (ii) continuous nucleation of new cavities, have been included in this work. Using this model that is suitable for low overall cavity volume (i.e. no cavity coalescence), faster growth rate is predicted for single cavities when strain-rate sensitivity is low and/or the population density of cavities is low (generally at slow strain-rates). By combining the predicted growth rate of individual cavities with the emerging cavity population density determined experimentally, a quantitative understanding of the various complex dependencies of cavitation has been obtained.