镁合金MB2在高应力三轴度下的拉伸破坏行为研究

通过光滑试件及不同曲率半径缺口圆柱试件的拉伸试验,实现对镁合金MB2的单向及多向应力状态加载。结合数值模拟分析,研究了不同试件在拉伸加载过程中应力状态的变化。以应力三轴度为参数,给出了镁合金MB2等效破坏应变的变化规律,在应力三轴度-等效破坏应变空间建立了镁合金MB2的失效破坏准则。利用扫描电镜对试件断口形貌进行观察,分析了导致材料宏观延性变化的微观损伤机理,对不同应力状态下镁合金MB2的失效破坏行为做出了合理解释。     

真三轴应力状态下岩石损伤本构模型

从岩石微元破坏服从Weibull 随机分布的特点出发,建立了真三轴应力状态下岩石损伤软化统计本构关系;根据应力应变关系的几何条件,建立了本构模型参数与岩石变形过程中应力应变曲线特征参量的理论关系,从而增强了模型的适应性;最后通过试验实测数据验证了模型的合理性.     

基于应力三轴度和罗德参数的6061和7075铝合金材料断裂失效分析

分别对6061铝合金和7075铝合金材料的缺口圆棒试件和凹槽平板试件进行准静态拉伸试验,并采用ABAQUS软件对拉伸过程进行数值模拟。模拟结果与试验测试结果吻合很好,验证了有限元模型的合理性和可靠性。通过有限元模拟,分别给出了不同试件的应力三轴度和罗德参数随等效塑性应变的变化曲线以及两种材料的失效轨迹,并对它们进行了分析讨论。结果表明:形状相同、材料性质不同的试件,应力三轴度的演化规律不同;材料的失效应变受应力三轴度和罗德参数的影响,并且不同性质的材料对罗德参数的敏感性不同。     

基于损伤泊松比的混凝土多轴强度准则

基于两个基本假定并提出损伤泊松比的概念,应用损伤力学理论和最小耗能原理,根据混凝土单元体的耗能率应受到强度准则的约束,建立了混凝土在多轴应力状态下强度准则的一般形式并揭示混凝土的破坏机理,指出损伤泊松比是决定混凝土破坏的关键.根据混凝土破坏包络面的特征和已有试验资料,初步确定了损伤泊松比的表达式.通过与国内外633组多轴应力下混凝土强度试验资料的比较,结果表明该强度准则计算结果与试验结果符合较好.针对二轴应力状态下,新混凝土强度准则可进一步简化,其形式更简单,且偏于安全.     

三轴应力状态下混凝土的侵彻性能研究

研制了三轴应力状态作用下混凝土侵彻实验装置。该装置采用液压伺服控制系统对立方体试件施加三向独立的静载、采用高压气体驱动子弹侵彻混凝土试件;试件六个面的动态压缩信号和侧面摩擦信号可由六根杆上的应变片记录。以混凝土试件为例,对立方体试件施加三向独立的0~100 MPa真三轴静载,研究其在不同应力状态下的侵彻性能,得到了立方体试件在无侧限、单向侧限、双向侧限下的侵彻差异,揭示出应力状态对侵彻性能的影响。     

基于J-C模型的Q235钢的失效准则

使用Instron材料试验机、霍普金森拉杆（SHTB）对Q235钢试件进行了不同温度下的准静态和动态拉伸实验,研究了温度、应变率及应力三轴度对Q235钢失效应变的影响,结果表明:Q235钢失效应变随温度的升高而增加,随应变率的增加而减小,随应力三轴度的增加先减小后增加再减小。基于实验结果对Q235钢J-C失效模型中的温度项进行了修正,并结合数值模拟提出了基于J-C失效模型的应力三轴度三分段式失效准则,通过Taylor撞击实验和数值模拟对给出的模型相关参量进行了验证,实验与模拟结果吻合较好。     

应力三轴度的有限元计算修正

对某高强度钢制成的光滑圆棒和缺口圆棒进行了系列准静态拉伸实验,采用ABAQUS对每个试 件进行了数值模拟,得到了该材料的真实应力应变曲线,拟合出了J-C本构模型和失效模型的部分材料常数。 最后,对该高强度钢制成的平板进行了撞击实验,并用得到的J-C模型对平板撞击实验进行了数值模拟,计算 结果与实验结果吻合很好,证明利用数值模拟并修正应力三轴度的方法是可行的。     

一个可区分破坏模式的新应力三轴度参数

三轴应力状态对破坏模式影响很大,但是根据应力三轴度的传统定义难以定量地判断破坏模式。本文根据准脆性断裂和塑性屈服的发生条件,从二者被满足的先后顺序角度,提出了一个新的应力三轴度定义以及临界三轴度这一材料常数。区分破坏模式的判据可简单地表示为:当应力三轴度大于材料的临界三轴度时,则发生准脆性断裂;当应力三轴度小于临界三轴度时,则先发生屈服和塑性变形。通过对平面应力和平面应变状态下椭圆孔孔边应力三轴度的理论分析,提出了有限厚度板中裂尖应力三轴度的数值计算方法,考察了不同板厚下单边裂纹裂尖应力三轴度沿板厚方向的分布特点,并给出了相应的经验公式。     

45钢的J-C损伤失效参量研究

为了在结构碰撞效应的有限元分析中描述材料行为,通过开展45钢在不同应力状态和温度下的准静态材料力学性能实验及拉伸SHB实验,考察了应力状态三轴度、温度和应变率对材料失效应变的影响。由实验数据得到了Johnson-Cook失效模型参量,并通过出现失效的Taylor撞击实验和数值模拟进行了一定的验证,表明模型描述与实验结果的趋势一致。     

金属材料在复杂应力状态下的塑性流动特性及本构模型

工程应用中,金属材料和结构往往处于复杂应力状态。材料的塑性行为会受到应力状态的影响,要精确描述材料在复杂应力状态下的塑性流动行为,必须在本构模型中考虑应力状态效应的影响。然而,由于在动态加载下材料的应变率效应和应力状态效应相互耦合、难以分离,给应力状态效应的研究和模型的建立造成很大困难。通过对Ti-6Al-4V钛合金材料开展不同加载条件下的力学性能测试,提出了一个包含应力三轴度和罗德角参数影响的新型本构模型,并通过VUMAT用户子程序嵌入ABAQUS/Explicit软件。分别采用新提出的塑性模型和Johnson-Cook模型对压剪复合试样的动态实验进行了数值模拟。结果表明,新模型不仅在对材料本构曲线的拟合方面具有较强的优势,而且由该模型所得到的透射脉冲和载荷-位移曲线均更加准确。因此,该模型能够更精确地描述和预测金属材料在复杂应力状态下的塑性流变行为。

     

STRENGTH CRITERION FOR PLAIN CONCRETE UNDER MULTIAXIAL STRESS BASED ON DAMAGE POISSON＇S RATIO

A new unified strength criterion in the principal stress space has been proposed for use with normal strength concrete （NC） and high strength concrete （HSC） in compressioncompression-tension, compression-tension-tension, triaxial tension, and biaxial stress states. The study covers concrete with strengths ranging from 20 to 130 MPa. The conception of damage Poisson＇s ratio is defined and the expression for damage Poisson＇s ratio is determined basically. The failure mechanism of concrete is illustrated, which points out that damage Poisson＇s ratio is the key to determining the failure of concrete. Furthermore, for the concrete under biaxial stress conditions, the unified strength criterion is simplified and a simplified strength criterion in the form of curves is also proposed. The strength criterion is physically meaningful and easy to calculate, which can be applied to analytic solution and numerical solution of concrete structures.     

基于J-C模型的镁合金MB2动静态拉伸破坏行为

为了研究不同应力状态和应变率条件下镁合金MB2的拉伸破坏行为,利用材料试验机和分离式Hopkinson拉杆(SHTB),对镁合金MB2的光滑及缺口圆柱试件进行了动静态拉伸加载;拟合得到了镁合金MB2的动静态拉伸本构关系,建立了其修正的Johnson-Cook失效破坏准则,并对不同试件的拉伸破坏行为进行了数值模拟;利用SEM对宏观破坏模式对应的微观损伤机理进行了分析。结果表明,随着应力三轴度的增加,镁合金MB2的等效破坏应变先增大后减小,宏观破坏模式由剪切转为正拉断,微观损伤机制由混合断裂转变为韧窝断裂;而随着应变率的增加,等效破坏应变不断减小,破坏模式不发生改变。Johnson-Cook本构关系和修正后的Johnson-Cook失效破坏准则能较好地拟合动态静态拉伸实验结果并预测不同试件的杯锥形破坏特征。

     

A local viewpoint for evaluating the influence of stress triaxiality and Lode angle on ductile failure and hardening

All damage and failure models, describing either the evolution of microvoids, the development of shear bands or local rupture, rely on the knowledge of the hardening function at large plastic strains which, then, becomes an essential prerequisite for any failure prediction.     

Failure behavior of composite sandwich structures under local loading

Usually when analyzing the mechanical response of foam-cored fiber-reinforced composite sandwich structures to localized static loading, the face sheets are treated as a linear-elastic material and no damage initiation and growth is considered. However, practice shows that at higher indentation magnitudes damage develops in the face sheet in the area of contact with the indentor, which could lead to local failure of the face laminate due to the loss of bending stiffness and strength. Therefore, the main objective of the present study is to develop a damage model for predicting the local failure in the composite face sheet and its influence on the load–displacement behavior of sandwich structures under local loading. For this purpose, the Hoffman failure criterion is incorporated into a finite element modeling procedure using the ABAQUS program system. Results deducted from the modeling procedure are compared with experimental data obtained in the case of static indentation tests performed on sandwich beam specimens using steel cylindrical indentors. It is shown that taking into account the damage in the face sheet leads to a substantial improvement in the performance of the model when simulating the mechanical behavior of the sandwich structures at higher indentation values.     

Fatigue limit under multiaxial loadings: on the definition of the equivalent shear stress

The goal of the present paper is to propose a criterion to predict the fatigue strength of hard metals under conditions of multiaxial, non-proportional loadings. It is very simple to compute, but still provides very good results for a wide range of in-phase and out-of-phase cycling loads.     

Failure of spot welds under in-plane static loading

Under in-plane loading conditions, two independent modes contribute to the failure of a spot weld: the in-plane shear mode and the in-plane rotational mode. In this work, the failures of both modes under large static load are examined individually. To study the combined failure of these two modes, two special test coupons are designed. The first coupon contains one spot weld. The second coupon contains five spot welds. Tests conducted in this work show that a very simple force-based failure criterion can be used to predict the failure of a spot weld under large in-plane combined static loads. Current multiaxial failure theory cannot explain this combined failure. This force-based spot weld failure criterion fits current automotive industry needs for body shell finite element application very well.     

A continuum damage model for the high-cycle fatigue life prediction of styrene-butadiene rubber under multiaxial loading

In the present contribution, the relationship between the fatigue life of styrene-butadiene rubber (SBR) and the stretch amplitude was established. Focusing on the multiaxial loading effect on the life duration of SBR, experimental tests were conducted using cylindrical specimens subjected to tension and torsion loadings under constant and variable amplitudes. Based upon the continuum damage mechanics approach, a three-dimensional model was derived and coupled with the cracking energy density criterion to predict the fatigue life of SBR. The capabilities of the model, which requires only three damage parameters to be identified, were analysed and a good agreement between predicted values and experimental data were clearly highlighted for tension and torsion loadings both in constant and variable amplitudes.