基于微孔贯通细观损伤模型的金属韧性断裂分析

韧性材料断裂过程通常可看作是材料内部微孔洞的形核、扩展及相互贯通的积累。经典的Gurson- Tvergaard (GT)模型能够很好地模拟具有变形均匀、各向同性的孔洞的萌生及扩展过程;但无法模拟由孔洞贯通而引起的局部变形过程,因此需要对其修正,引入相应的孔洞贯通准则。该文采用两种贯通准则对经典GT模型进行修正,即Thomason的塑性极限载荷准则和临界等效塑性应变准则。借助用户自定义程序UMAT将采用这两种贯通准则修正的GT本构关系嵌入至商用有限元软件ABAQUS中,从而可通过对金属材料应力状态和断裂机理的分析控制孔洞的贯通。以一组含有不同缺口根半径的圆棒拉伸试验件为例,分析了该类金属构件自孔洞萌生至最终断裂的整个损伤演化过程,并与试验数据进行了对比,验证了该模型的有效性。该文还讨论了金属断裂过程中应力三轴度对微裂纹萌生与扩展的影响。     

基于DIC的铝合金薄壁缺口件多轴疲劳行为

为探究缺口对材料多轴疲劳性能的影响,以航空用7075-T651铝合金薄壁缺口件为研究对象,进行等效应力幅变量的比例多轴疲劳实验,采用数字图像相关技术进行表征并分析临界平面角度.结果表明:随加载等效应力幅的增加,试样多轴疲劳寿命降低,缺口附近最大轴向、扭向和剪切工程应变增大.不同加载条件下,缺口附近应变集中现象均随着加载周次的增加而逐渐增强.缺口附近的应变变化过程可分为裂纹萌生、裂纹扩展及瞬断阶段.引入临界平面角度和其上最大正应变,提出SWT修正模型,其预测结果均位于2倍分散带内.     

基于损伤力学的疲劳裂纹萌生及扩展规律研究

针对金属构件疲劳裂纹的萌生、扩展寿命及其规律等问题,应用损伤力学理论与有限元相结合的方法,建立了计算疲劳裂纹全寿命的统一模型。引入附加载荷法,通过MATLAB编程计算,实现了对刚度矩阵的连续计算,并给出了编程的流程。通过对单个单元的损伤计算,得到了单元从无损到破坏过程中等效应力的变化;通过计算各个构件损伤单元寿命,进而给出了金属构件总体疲劳寿命。分析得到了微裂纹萌生及扩展寿命占总体疲劳寿命的80%以上,并应用有限元软件ANSYS模拟给出了缺口件裂纹萌生及扩展过程。理论计算的结果与试验数据对比基本一致,验证了本工作方法的准确性。     

基于修正GTN模型的不锈钢管剪切过程韧性断裂准则研究

为了改善修正Gurson-Tvergaard-Needleman模型(修正GTN模型)无法准确模拟不锈钢管剪切过程的缺陷,该文在原模型基础上,构造了一个与位移相关的损伤函数作为新的断裂准则。同时,在ABAQUS中采用隐式向后Euler应力更新算法编制了改进后模型的VUMAT用户子程序,实现了其数值求解。最后进行了SUS304不锈钢管剪切试验,通过分析对比试验数据与改进后模型的模拟结果,验证了该文所提出的方法的准确性。     

TiCp/W复合材料热冲击损伤行为的数值模拟

为了揭示TiC颗粒增强的钨基复合材料（TiCp/W)高温下的失效规律,采用有限元方法从宏观和微观两个方面对该复合材料在氧乙炔冲击中的损伤行为进行了数值模拟,模拟结果表明,复合材料试样宏观损伤行为是裂纹在试样周边萌生,沿径向向心部扩展,微结构损伤行为是微裂纹在TiCp/W界面附近产生,而生在基体中扩展,TiC颗粒含量越高,复合材料超易损伤。TiC颗粒没有阻止裂纹扩展的作用,在基体中增加TiC反而会降低材料的抗热冲击性能。复合材料非稳态温度场的模拟结果,材料的宏观与微观损伤行为的模拟结果都与实验结果吻合。     

加载历史和裂纹闭合对疲劳裂纹扩展行为影响的数值模拟

采用不同应力比条件下的16MnR钢紧凑拉伸试样,设计了三种有限元分析模型,即不考虑加载历史效应的静态裂纹扩展模型,同时考虑加载历史和裂纹闭合的动态裂纹扩展模型以及仅考虑加载历史的伪动态裂纹扩展模型,对疲劳裂纹闭合过程、裂纹尖端的应力-应变迟滞环、疲劳损伤和裂纹扩展速率进行了数值模拟与分析,进而着重探讨了加载历史和裂纹闭合影响疲劳裂纹扩展行为的交互作用机制。结果表明:对于同类分析模型,应力比越大越不容易产生裂纹闭合;而在应力比相同的情况下,加载历史引起的残余压应力对裂纹闭合有明显的促进作用。裂纹闭合效应阻碍了平均应力的松弛,减小了裂纹尖端附近的应力-应变场强度、疲劳损伤和裂纹扩展速率,而加载历史引起的残余压应力则加快了平均应力的松弛和抑制了棘轮效应。与实验结果比较发现,只有同时考虑了裂纹闭合效应和加载历史影响的动态裂纹扩展模型,才能对疲劳裂纹扩展行为进行准确、定量的模拟。     

化学钢化前后玻璃表面裂纹扩展的实验比较与数值模拟

本工作对手机超薄盖板玻璃表面裂纹萌生及扩展过程进行了实验比较和数值模拟。结果表明：对于未经化学钢化处理的玻璃，在载荷为9.80 N的情况下，裂纹萌生时间为压痕出现后30 s；而对于化学钢化玻璃，即便在严苛环境条件下，在9.80 N的载荷作用下，缺陷压痕处未发现裂纹。ABAQUS数值模拟结果表明：（1）最大主张应力位于压印缺陷的四角，并沿径向向外扩展；（2）化学钢化玻璃的最大主张应力比未经化学钢化的玻璃低465 MPa。数值模拟得到的最大主张应力位置与实际裂纹萌生位置一致。对玻璃表面裂纹扩展行为的认识有助于高强度超薄盖板玻璃的研发。     

弹带挤进过程身管内壁损伤的数值模拟研究

针对火炮发射过程中内膛的损伤、裂纹萌生和扩展问题,基于HLC细观损伤本构模型建立了相应的损伤力学有限元数值计算方法,将完全隐式应力更新算法与显式有限元计算相结合,通过用户自定义材料子程序VUMAT将损伤模型嵌入到有限元软件ABAQUS/EXPLICIT模块中。对某型火炮多发射击工况下内膛损伤破坏过程进行了数值模拟计算,分析了弹带挤进内膛过程中身管内壁材料性能随射弹发数变化的规律,并与实验进行了对比。结果表明:HLC细观损伤模型可以有效揭示身管内膛复杂的损伤行为并预测破裂缺陷,为火炮身管安全性设计提供有益的参考。     

T700轴向增强经编织物复合材料力学性能的模拟分析与试验研究

应用一种新型界面元模型研究了复合材料层间剪切损伤。通过引入双线性损伤准则和损伤演化,预报复合材料层间裂纹扩展。轴向增强经编织物复合材料由针织纱线引起的纤维变形(KYD)产生了富树脂区域,基于细观力学理论提出了一种新的研究轴向增强经编织物单胞模型受单向拉伸和剪切时KYD区周围应力分布的方法,得出了单胞在受载时首先在这一区域产生裂纹。对单轴向T700经编织物复合材料进行了三点弯曲性能和层间剪切性能试验测试,分析了经编织物复合材料的力学特性。分别模拟了弯曲性能和层间剪切性能试验,得出了最大预报载荷值与试验值误差小于10%,并基于有限元模型研究了复合材料面内损伤和层间裂纹扩展损伤机制。     

十字叠层梁冲击损伤的有限元模拟

对一类十字叠层梁的冲击损伤机理与演化进行了有限元模拟。按实际叠层结构计算应力分布,根据破坏理论预测了损伤的萌生、位置与模式,并用“损伤材料刚度衰减原理”模拟损伤演化。结果表明,早期损伤以层间剪切为主导,而后将与90°层基体开裂发生互耦作用,最危险的位置是三点弯曲加载点和支撑点。计算了因损伤造成的弯曲刚度下降,与试验结果相符。此外,还定性地分析了损伤的稳态扩展与失稳扩展。     

On the numerical implementation of a shear modified GTN damage model and its application to small punch test

Gurson-type models have been widely used to predict failure during sheet metal forming process. However, a significant limitation of the original GTN model is that it is unable to capture fracture under relatively low stress triaxiality. This paper focused on the fracture prediction under this circumstance, which means shear-dominated stress state. Recently, a phenomenological modification to the Gurson model that incorporates damage accumulation under shearing has been proposed by Nahshon and Hutchinson. We further calibrated new parameters based on this model in 22MnB5 tensile process and developed the corresponding numerical implementation method. Lower stress triaxiality were realized by new-designed specimens. Subsequently, the related shear parameters were calibrated by means of reverse finite element method and the influences of new introduced parameters were also discussed. Finally, this shear modified model was utilized to model the small punch test (SPT) on 22MnB5 high strength steel. It is shown that the shear modification of GTN model is able to predict failure of sheet metal forming under wide range of stress state.     

Application of the cohesive zone model for analysing the edge crack propagation of steel sheet in the cold rolling process

In the cold rolling process, the expansion and coalescence of micro‐defects can make steel sheet quality descend and create edge crack in the steel sheet. And the edge crack can cause the strip rupture completely. In this research, the cohesive zone model (CZM) was used to analyse the initiation and propagation of edge crack in the cold rolling process with the non‐reversing two‐high mill. A bi‐linear traction–separation law was utilized which is primarily given by the CZM parameters including the cohesive stress, T, and the cohesive energy, Γ. Compared with other popular models such as the Gurson–Tvergaard–Needleman (GTN) model, the CZM presents certain advantages because it requires a smaller number of parameters to be defined. Comparison results of the experiments and simulation illustrated that the CZM can provide accurate prediction for the propagation of edge crack in the cold rolling process. Parametric analysis was carried out and showed that the extent of the crack propagation increases with the increasing of the reduction ratio.     

Mechanical and damage analysis along a flat-rolled wire cold forming schedule

Numerical simulation is used to study patented high-C steel flat-rolled wire cold forming processes. An elasto-plastic power law, identified from mechanical tests, is used into Forge2005? finite element (FEM) package in order to describe the material behaviour during wire drawing followed by cold rolling. A through-process approach has been favoured, transferring residual wire-drawing stresses and strain into the flat-rolling preform. This mechanical analysis, associated with a triaxiality study, points to dangerous areas where fracture may initiate due to high tensile stresses. Lema?tre’s isotropic damage criterion, including crack closure effect, a -1/3 cut-off value of stress triaxiality, and tension/compression damage asymmetry, has been used and has confirmed the previous analysis. A number of non-coalesced voids nucleated on inclusions have been observed in the Scanning Electron Microscopy (SEM), especially in high-deformation zones (“blacksmith’s cross”). Their evolution has been simulated in the FEM model using spherical numerical markers, which deform into oblate or prolate ellipsoids. The deformation-induced morphological evolution of voids observed in the SEM compares well with the geometrical evolution of the markers, which suggests that the morphologies observed do not result from micro-crack propagation, but from material transport of the nucleated voids.     

Ductile rupture of A508 steel under nonradial loading

This study deals with the effect of loading path on the strain to failure of a C-Mn-Ni-Mo steel. The tests are carried out at 373 K on axisymmetric, notched tensile specimens calculated by the finite element method. Specimen geometries containing different notch radii and leading to widely different stress triaxiality ratios are investigated. The effect of nonradial load path is studied using relatively sharply notched specimens. It is shown that a simple linear damage rule does not account for the experimental results. A model based on the critical cavity void growth rate calculated from the Rice and Tracey model is shown to give results consistent with the experiments provided that the effect of prestrain on constitutive equation and on the stress triaxiality ratio is properly taken into account. The experimental results are also compared to a model based on continuum damage mechanics.     

Modelling of damage development and failure in notched-bar multiaxial creep tests

Finite‐element predictions of creep rupture in notched specimens are presented in this work. A damage model linked to the creep strain rate and stress triaxiality has been used to predict creep life under multiaxial stress conditions and the predictions have been compared with experimental data for a C–Mn steel. Finite‐element analyses have been conducted using primary–secondary (PS) and primary–secondary–tertiary (PST) creep laws. As expected a PST analysis gives a shorter predicted rupture life than a PS analysis. An additional term was included in the model to allow for an increase in hydrostatic strain due to creep damage. The incorporation of this term improved the agreement between the experimental data and the finite‐element predictions. A further enhancement to the model was to model the initiation and growth of a sharp crack in the vicinity of the notch, through the use of a nodal release technique linked to the damage evolution. It was found that the predictions obtained using the nodal release technique were very similar to those from the PST creep model incorporating the hydrostatic damage term. The effect of mesh size has also been examined and the finite‐element predictions were seen to be quite mesh sensitive with a finer mesh generally giving a shorter predicted life.     

Damage evolution in experiments and simulation in a construction steel

Results from the modelling of the initiation and the evolution of microvoids governed by triaxiality and effective plastic strain in heterogeneous materials are presented. In particular, the damage evolution in a microalloyed thermomechanically treated steel consisting of brittle hard phases embedded in a ductile matrix is studied. Results obtained by numerical simulations using the finite element method are compared with experimental investigations of a notched cylindrical tensile bar. For this, two kinds of continuum damage models are used: the model of Rousselier, which assumes the yield stress as function of damage, and the concept of effective strains and stresses proposed by Lemaitre.     

Ductile shear failure or plug failure of spot welds modelled by modified Gurson model

For resistance spot welded shear-lab specimens, interfacial failure under ductile shearing or ductile plug failure are analyzed numerically, using a shear modified Gurson model. The interfacial shear failure occurs under very low stress triaxiality, where the original Gurson model would predict void nucleation and very limited void growth. Void coalescence would therefore be largely postponed. However, using the shear modification of the Gurson model, recently introduced by Nahshon and Hutchinson (2008) [1], failure prediction is possible at zero or even negative mean stress. Since, this shear modification has too large effect in some cases where the stress triaxiality is rather high, an extension is proposed in the present study to better represent the damage development at moderate to high stress triaxiality, which is known to be well described by the Gurson model. Failure prediction and tensile response curves for an interfacial shear failure or a ductile plug failure, are here compared when using either the original Gurson model, the shear modified model, or the extension to the shear modified model. The suggested extension makes it possible to use the shear modified model as a simple way of accounting for damage development under low triaxiality shearing, without further increasing the damage rate in regions of moderate to high stress triaxiality.     

Study on creep mechanical behaviour of P92 steel under multiaxial stress state

The creep mechanical behaviour of P92 steel at 650°C has been studied by experimental research and finite element analysis. During the creep of P92 steel, there existed the notched strengthening effect, which was influenced by the shapes of the notch and the nominal stress. Under the condition of the same notch depth, the creep life enhancement factor increased with decreasing notched radius or the increase of stress. The multiaxial stress caused by the notch effect had a significant influence on the evolution of the microstructure and resulted in a transforming tendency from ductile to brittle at the root of the notch. The fracture position varied with the shapes of the notch: the U shaped notch started to fracture at the root of the notch, while the C shaped notch in the centre of the specimen. The creep process of notched specimens was simulated by embedding Kachanov–Rabotnov creep damage constitutive model into the interface program of finite element software. The result showed that damage distribution of notched specimens varied during the process of creep. The maximum damage location at the end of creep depended on the notch shape: with larger notch radius the maximum damage location was in the centre, while smaller radius of notch specimens was near the notch root, which was consistent with the analysis of the fracture morphology.     

基于应力三轴度的钢框架循环加载损伤分析

为了研究在低周大应变循环作用下钢材损伤对钢构件及钢框架抗震性能的影响,该文在Bao-Wierzbicki的等效塑性应变与应力三轴度路径的基础上,提出了符合钢材微观机制的应力三轴度起始损伤准则。随后通过10组Q235钢板拉伸试验确定了钢材的损伤演化准则。为了验证这些准则在描述钢构件及框架损伤破坏方面的正确性与适用性,该文采用有限元软件ABAQUS对一组型钢梁、钢框架梁柱节点及一榀钢框架的典型循环加载试验进行了损伤模拟分析。分析结果显示,考虑损伤准则的有限元曲线与试验曲线吻合一致,能较为准确的描述钢构件及钢框架在循环加载下承载力与刚度退化的现象,并能直观地模拟各结构的损伤部位及损伤程度。该文研究为分析钢构件及钢框架在低周大应变循环作用下的性能退化提供较为便捷的方法。     

Numerical modeling of damage evolution of DP steels on the basis of X-ray tomography measurements

In order to couple the damage evolution and the stress state of DP steel grades, a new advanced GTN (Gurson-Tvergaard-Needleman) model was developed and implemented into a finite element code. This model is an extension of the original one. It takes into account the plastic anisotropy and the mixed (isotropic + kinematic) hardening of the matrix. Two different methods to compute the void volume fraction were developed and used within the constitutive equations. The first method is new and allows the accurate modeling of the observations of damage initiation and growth in DP steels measured using high-resolution X-ray absorption tomography ( [Bouaziz et al., 2008] and [Maire et al., 2008]). The second method is classic and assumes the additive decomposition of the total void volume fraction into a nucleation and a growth part. A parametric study is carried out to assess the effect of the kinematic hardening on some mechanical parameters such as the equivalent plastic strain, the triaxiality and the porosity. The numerical predictions are favorably compared to the experimental results.