塑性变形中应力状态与应力状态参数的关系

材料的变形性能不仅与其化学成分和组织状态有关,还与变形时的应力状态有关,应力三轴度、软性系数和罗德参数是研究金属材料变形、损伤、开裂时常用的应力状态参数,本文计算和分析了塑性变形过程中9种不同应力状态和不同应力幅值下各参数值的变化情况,结果表明,应力三轴度与应力状态间有较好的对应关系;软性系数和罗德参数只有在平面屈服应力状态下才与应力状态具有很好的对应关系;在三向屈服应力状态下,软性系数和罗德参数不能直接反映不同的应力状态和变形类型。     

C-Mn结构钢塑性变形过程中空穴增长规律的研究——结构钢韧性断裂机理研究之二

在试验分析的基础上,本研究提出了一种程序,以建立C-Mn结构钢在韧性断裂过程中的空穴增长与塑性应变及三轴应力状态之间的关系,并确定了空穴增长系数。由光滑拉伸试样和预制裂纹侧槽三点弯曲试样试验所确定的空穴增长系数,可能适宜较宽的试样拘束变化范围。空穴增长的实验关系式表明,当三轴应力状态(σm/(?))约大于1.2时,在该结构钢中的实际空穴增长速率低于由Rice-Tracey理论所确定的空穴增长速率,然而当三轴应力状态(σm/(?))约小于1.2时,由Rice-Tracey理论所确定的空穴增长表现为对实际材料中的空穴增长估计略为不足。另外,根据实验空穴增长研究,对预制裂纹侧槽三点弯曲试样在韧性起裂时钝化裂纹前端的塑性应变、相对空穴体积和应力状态三轴性的变化进行了测量和估计。     

ON THE VOID GROWTH OF C-Mn STRUCTURAL STEEL DURING PLASTIC DEFORMATION——Ductile Fracture Mechanisms in a Structural Steel (Ⅱ)

在试验分析的基础上,本研究提出了一种程序,以建立C-Mn结构钢在韧性断裂过程中的空穴增长与塑性应变及三轴应力状态之间的关系,并确定了空穴增长系数。由光滑拉伸试样和预制裂纹侧槽三点弯曲试样试验所确定的空穴增长系数,可能适宜较宽的试样拘束变化范围。空穴增长的实验关系式表明,当三轴应力状态(σm/(?))约大于1.2时,在该结构钢中的实际空穴增长速率低于由Rice-Tracey理论所确定的空穴增长速率,然而当三轴应力状态(σm/(?))约小于1.2时,由Rice-Tracey理论所确定的空穴增长表现为对实际材料中的空穴增长估计略为不足。另外,根据实验空穴增长研究,对预制裂纹侧槽三点弯曲试样在韧性起裂时钝化裂纹前端的塑性应变、相对空穴体积和应力状态三轴性的变化进行了测量和估计。     

基于修正GTN模型的双相钢断裂失效判据研究

在屈服函数中引入剪切损伤参数对GTN模型进行修正,对较广应力三轴度的失效进行预测;设计不同切口的剪切试验,并将修正的GTN模型以及双相钢DP780的材料参数和损伤参数编入VUMAT子程序,用ABAQUS进行仿真,获得双相钢失效时的临界应力三轴度T与其对应的剪切损伤,拟合出极限剪切损伤值与应力三轴度的函数关系曲线;然后结合颈缩断裂时的临界孔洞体积分数与应力三轴度的关系,建立适用于较广应力三轴度的DP780断裂失效细观判据;设计了拉弯成形试验并带入ABAQUS中进行仿真,证明得出的细观断裂判据能预测双相钢在冲压成形过程中出现的不同断裂现象。     

应变路径变化对材料韧性断裂的影响

为研究非比例加载条件下应变路径的变化对材料韧性所产生的影响,设计了一组包含宽应力三轴度的非比例加载试验,得到相应的载荷-位移/扭角曲线。运用ABAQUS/Explicit及用户子程序VUSDFLD,基于试验与模拟相结合的方法确定断裂起始点,研究复杂加载条件下断裂起始点的平均应力三轴度、罗德系数和等效断裂应变的变化趋势,分析应变路径变化对断裂成形极限的影响。结果表明:预加载的存在加快了断裂进程,且随着预加载程度的增加,材料的韧性断裂出现的愈早。对于拉-扭试验,在预扭阶段应变沿着纯扭趋势发展,而进入拉伸阶段后应变并非沿着单拉的应变路径进行变化;对于压-扭试验,在预压阶段应变沿着单向压缩的应变路径变化,而扭转阶段则沿着平行于纯扭的应变路径变化。     

6063铝合金韧性断裂机制的研究

设计可实现不同应力状态的原位拉伸试样,在SEM下进行原位拉伸试验,对断裂过程做了详细的研究和分析.试验表明,不同应力状态下的试样表面在拉伸过程中都产生了大量的滑移带,但其韧性断裂机制不同.随着三轴应力度的降低,断裂从韧窝剪切机制向纯剪切断裂机制过渡,试件断口也由韧窝断裂模式向剪切断裂模式演变;6063铝合金的晶界最薄弱,微裂纹形核于晶界,随载荷增大,微裂纹之间通过扩展或剪切连接导致试样断裂;试样最小截面上的三轴应力度越小,试样断口的两个面上韧窝的取向越明显,而且断口越光滑.     

金属韧性断裂准则的实验研究

韧性断裂准则作为材料成形极限中的重要指标,对于准确预测金属内部和表面韧性破坏发生的时间和位置、工艺制定和模具设计具有重要的意义。以45钢为材料,采用不同形式的试件进行了拉伸、压缩、剪切、扭转等实验,并利用金属塑性成形模拟专业软件对实验过程进行数值模拟,对工程中广泛使用的11种韧性断裂准则进行分类对比研究。指出Normalized Cockroft & Latham和Brozzo断裂准则比较适用于高应力三轴度成形过程,而Ayada和Rice-Tracy断裂准则更适用于较低应力三轴度成形过程。     

基于临界空穴扩张比理论的不锈钢管高温韧性损伤研究

针对不锈钢管在高温塑性加工过程中出现的裂纹缺陷，在Gleeble-3800热模拟试验机上对TP321奥氏体不锈钢光滑棒材进行高温拉断试验，得到了TP321在温度850～1180℃、应变速率0.01～10 s^-1下的真应力-真应变曲线和试样拉伸断裂瞬间的等效塑性应变。结合有限元仿真，对试验结果进行了数值模拟，得到了应力三轴度的数值，从而计算出了临界空穴扩张比参数V_GC。在此基础上，验证了临界空穴扩张比参数韧性断裂准则在高温下的适用性。发现在特定温度范围内TP321断裂瞬间的V_GC与应变速率相关性较小，与温度呈线性关系，并建立了奥氏体不锈钢断裂瞬间的V_GC与温度的数学表达式。将得到的V_GC准则应用于TP321管坯的不同辊形斜轧穿孔过程中的韧性损伤，对比分析了不同辊形斜轧穿孔仿真结果，发现V_GC准则可准确预测缺陷情况。     

Q460D结构钢在不同应力状态下的断裂机制

用扫描电镜和金相显微镜对Q460D高强度结构钢不同几何形状试件断口处的微观形貌和组织变化进行了分析。结果表明：在高应力三轴度纯拉状态下,Q460D结构钢断裂呈空穴聚合型穿晶断裂,微观组织发生抛物线状的聚合变形;在应力三轴度为零的纯剪状态下,断裂呈脆性穿晶解理断裂,微观组织变形成长条状且铁素体变形较大;在低应力三轴度拉剪状态下,断裂呈抛物线形韧窝穿晶断裂,微观组织中铁素体变得粗大,珠光体变得细小;而在高应力三轴度平面应变状态下的断裂呈韧窝型沿晶断裂。     

考虑应力三轴度影响的X80管线钢延性断裂研究

为了得到X80管线钢在不同应力三轴度下的延性断裂特征，通过标准拉伸试件的准静态拉伸试验标定了X80管线钢的Johnson-Cook本构模型参数。通过不同应力三轴度下的准静态试验和有限元分析得到了各试件的平均应力三轴度和断裂应变，并得到了应力三轴度对断裂应变、抗拉强度和延性的影响规律。结果表明，建立的X80管线钢本构模型与失效模型能够较好地表征X80管线钢在不同应力三轴度下的力学行为与断裂特征。在不同的应力三轴度区间内，X80管线钢的断裂应变与平均应力三轴度的关系满足不同的函数类型，当应力三轴度为0.80～1.56时为递减的Johnson-Cook失效模型函数，当应力三轴度为0～0.80时为递增的线性函数，当应力三轴度为-1/3～0时为递减的幂函数。X80管线钢的抗拉强度与应力三轴度的函数关系满足二次函数的分布形式并单调递增，而延性随着应力三轴度的增加而降低。     

The characterization and interpretation of ductile fracture mechanisms in AL2024-T351 using X-ray and focused ion beam tomography

This paper describes an experimental investigation into the mechanism of ductile fracture in the aluminum alloy AL2024-T351 using a combination of synchrotron X-ray and focused ion beam tomography. Microstructural features that influence fracture at the micro- and nanoscale were characterized in virgin material in three-dimensions. The nature and volume fraction of ductile damage was then quantified as a function of distance below the fracture surfaces of tested notched and fatigue pre-cracked laboratory specimens. In both specimens the ductile fracture process initiates with the brittle fracture of large irregular intermetallic particles at low levels of plastic strain. With increasing plasticity, the resulting voids grow and combine with pre-existing porosity to increase the overall void volume fraction. Once a critical void volume fraction is attained, final failure occurs by the fracture or decohesion of dispersoids from the matrix, initiating a second population of nanoscale voids, which interlinks the larger voids. The critical void volume fraction for coalescence and the distribution of ductile damage below the fracture surface is markedly different between blunt-notched and pre-cracked specimens, with notched specimens exhibiting a significantly lower critical void volume fraction and a more extensive distribution of ductile damage below the fracture surface than is observed in pre-cracked specimens. This observation, related to the gradients in stress triaxiality and plastic strain in each specimen type, has important implications for the calibration of ductile damage mechanics models against notched-specimen data and their use to predict crack behavior in engineering structures.     

空穴长大延性损伤新模型

以45中碳钢为研究对象,进行了拉伸、压缩、扭转、精冲实验,并结合有限元对各实验过程中的应力三轴度和延性损伤进行了分析,归纳了金属材料的3种延性损伤机理:无空穴影响剪切损伤、剪切型空穴损伤和拉伸型空穴损伤.在上述工作基础上,提出了一个新的基于空穴长大的延性损伤模型,能够较准确地预测材料拉伸、精冲过程中延性断裂的出现,扩大了损伤模型的预测范围.     

Failure characteristics of a dual-phase steel sheet

Failure in ductile sheet metal structures is usually caused by one, or a combination of, ductile tensile fractures, ductile shear fractures or localised instability. In this paper the failure characteristics of the high strength steel Docol 600DP are explored. The study includes both experimental and numerical sections. In the experimental sections, the fracture surface of the sheet subjected to Nakajima tests is studied under the microscope with the aim of finding which failure mechanism causes the fracture. In the numerical sections, finite element (FE) simulations have been conducted using solid elements. From these simulations, local stresses and strains have been extracted and analysed with the aim of identifying the fracture dependency of the stress triaxiality and Lode parameter.     

Effect of specimen thickness on the creep response of a Ni-based single-crystal superalloy

Creep tests on Ni-based single-crystal superalloy sheet specimens typically show greater creep strain rates and/or reduced strain or time to creep rupture for thinner specimens than predicted by current theories, which predict a size-independent creep strain rate and creep rupture strain. This size-dependent creep response is termed the thickness debit effect. To investigate the mechanism of the thickness debit effect, isothermal, constant nominal stress creep tests were performed on uncoated PWA1484 Ni-based single-crystal superalloy sheet specimens of thicknesses 3.18 and 0.51 mm under two test conditions: 760 °C/758 MPa and 982 °C/248 MPa. The specimens contained initial microvoids formed during the solidification and homogenization processes. The dependence of the creep response on specimen thickness differed under the two test conditions: at 760 °C/758 MPa there was a reduction in the creep strain and the time to rupture with decreasing section thickness, whereas at 982 °C/248 MPa a decreased thickness resulted in an increased creep rate even at low strain levels and a decreased time to rupture but with no systematic dependence of the creep strain to rupture on specimen thickness. For the specimens tested at 760 °C/758 MPa microscopic analyses revealed that the thick specimens exhibited a mixed failure mode of void growth and cleavage-like fracture while the predominant failure mode for the thin specimens was cleavage-like fracture. The creep specimens tested at 982 °C/248 MPa in air showed the development of surface oxides and a near-surface precipitate-free zone. Finite-element analysis revealed that the presence of the alumina layer at the free surface imposes a constraint that locally increases the stress triaxiality and changes the value of the Lode parameter (a measure of the third stress invariant). The surface cracks formed in the oxide scale were arrested by further oxidation; for a thickness of 3.18 mm the failure mode was void nucleation, growth and coalescence, whereas for a thickness of 0.51 mm there was a mixed mode of ductile and cleavage-like fracture.     

On the influence of void clusters on void growth and coalescence during ductile fracture

Based on the behavior of a three-void cluster embedded within a representative volume element, this study utilizes three-dimensional finite element analyses to examine the sensitivity of void growth and coalescence to strain hardening, multiaxial stress state and inter-void spacing. The strain-induced growth of voids within the cluster is accelerated when the voids are closely spaced in a low strain-hardening material subject to high levels of stress triaxiality. Far-field deformation causes strain to concentrate within the inter-void ligament, and the resulting behavior induces a load–loss response of the inter-void region. Based on the load–loss criterion for the onset of void coalescence, the results show that coalescence is accelerated by increasing stress triaxiality and decreasing strain hardening and inter-void spacing. A straightforward analysis is then presented that relates void coalescence to (a) the strain-hardening exponent and (b) the dependence of the plastic constraint factor within the inter-void ligament on strain, the latter being sensitive to far-field stress triaxiality and void geometry.     

考虑材料韧性损伤的中厚板半冲孔成形过程分析

为研究半冲孔成形过程中韧性损伤的演化以及部分工艺参数对成形质量的影响规律,本文在ABAQUS有限元模拟软件中建立了半冲孔轴对称有限元模型,并通过VUMAT用户子程序引入GTN(Gurson-Tvergaard-Needle-man)损伤模型,结合同时考虑空穴形状与体积变化影响的韧性断裂准则,进行弹塑性大变形有限元分析.基于该有限元模犁,预测了半冲孔工艺中静水压力、等效应力、等效应变、应力三轴度以及损伤断裂的产生和发展趋势,分析了反顶力、压边力和冲裁间隙对零件的影响规律,并与实验结果进行对比分析,验证了数值模拟的准确性.     

Fracture behaviour of diffusion bonded bimaterial Ti–Al joints

Abstract

Failure modes of constrained metal foils between two elastic solids are rather different from those in the unconstrained condition. If the interface adhesion is strong between materials, a lower strength thin metal (plastic) foil between two much higher strength metals (elastic) can undergo substantial plastic deformation and fail with high triaxiality induced ductile fracture. Experiments have been conducted to explore the modes of failure and the factors governing fracture in such a constrained metal interlayer. In the present work, the effects of soft inter layer thickness and brittle reaction layer on the fracture behaviour of four point bend specimens have been investigated. A series of solid state diffusion bonds were produced between 25 × 25 mm section titanium bars using pure aluminium foils of different thickness (50, 457, 914, and 2000 μm) as the soft constrained inter layer. All four point bend specimens containing an ～ 2 μm thick intermetallic reaction layer TiAl₃ between the titanium and aluminium failed in ductile fracture mode within the soft aluminium interlayer next to the interface. A number of void formations were observed ahead of the crack tip next to the interface. No evidence of interface debonding was observed. However, the specimens containing an 8 μm thick TiAl₃ layer failed by brittle fracture along the interface between the titanium substrate and the TiAl₃ layer. It was found that decreasing the soft interlayer thickness from 2000 to 457 μm increased the load carrying capacity and decreased the fracture toughness caused by constrained plastic deformation (high triaxiality) of the interlayer.     

Effect of triaxiality on void growth and coalescence in model materials investigated by X-ray tomography

Void growth and coalescence/linkage, which play significant roles during ductile fracture processes, are strongly influenced by stress triaxiality in a deforming solid. The stress state can be changed by cutting notches in a tensile sample. In the current paper, void growth and linkage of an artificial void array embedded in a notched model material was studied by X-ray computed tomography, coupled with in situ tensile deformation. The cross-sectional shape of the tensile specimens was square, and a pair of notches was cut along only one direction. Thus, the lateral principal stress does not have an isotropic distribution: the principal stress along the notch direction is considered to be higher. This technique allowed us to explore the entire process of growth and linkage events of a void array embedded in a metal matrix. The notch effect creates a marked acceleration in void growth, leading to a large reduction in the linkage strains, as compared with similarly fabricated unnotched samples. The standard models for coalescence could not provide consistent predictions of the measured notch effect.     

Small Punch Test of U-Shaped Notch of TC4 Titanium Alloy and Its Numerical Simulation

In order to investigate the mechanical properties of small TC4 titanium alloy specimens under high stress state, a U-shape notched small punch test and its corresponding simulation were carried out. Meanwhile, the influence of the friction coefficient, specimen thickness, the diameter of the hole, and the punch rate on the calculated results were also discussed. The results showed that upon increasing the specimen thickness and punch rate, the peak load obtained by calculations increased as well. The void volume fraction and stress triaxiality along path 2 decreased at first and then increased. However, the void volume fraction and stress triaxiality along path 1 lowered. Research showed that the U-shape notched small punch test was an effective way to test the performance of the small specimen under a high stress state.