Strain rate distribution and localization band width evolution during tensile test

The main objective of this paper is to study the evolution of the necking zone in a flat specimen during a tensile test. Two approaches are used and compared:

• –An experimental investigation of the strain rate distribution with Electronic Speckle Pattern Interferometry (ESPI).
• –A numerical analysis with a thermodynamically consistent constitutive model that couples strongly isotropic continuum damage (CDM) and the elastoplastic behavior.

It is shown that strain rate maps are, for both approaches, relevant to investigate the development of the X-shape pattern that occurs during necking evolution. In particular, this pattern can be clearly observed on maps of the minimum determinant of the acoustic tensor. It appears even when damage values are low and the problem is still elliptic.It is shown that ESPI and CDM modeling are able to give a coherent picture of the phenomena that occur during neck development from the onset of instability to localize necking, in particular on localization bands angles and widths. In particular, physically meaningful information which is seldom considered such as band width evolution or strain rate distribution will be extracted from the analysis.     

单个弹丸撞击316L不锈钢引起的变形场

运用金属材料表面纳米化试验机对单个弹丸撞击316L不锈钢表面进行了撞击实验;采用激光共聚焦显微镜观察了弹坑的三维形貌,测量不同振动频率下弹坑的直径及离面位移;采用云纹干涉法对弹坑周围的面内应变场进行测量,并分析振动频率及撞击方式对弹坑尺寸、塑性应变大小以及塑性应变区范围的影响;采用有限元方法对单个弹丸垂直撞击试件表面的应变场进行数值模拟,与实验结果进行比较,分析了弹坑周围残余应力的分布。结果表明:随振动频率的增加,弹坑直径和离面位移都增加,频率在50~55Hz,弹坑直径有突变,离面位移和振动频率呈线性关系;振动频率越大,塑性应变越大,塑性应变分布范围均大于弹坑直径的2倍;同一振动频率下弹丸垂直撞击比倾斜撞击的塑性应变大,而塑性应变分布范围相差不大;面内残余应变场的数值模拟结果和实验结果吻合较好,最大误差小于10%。     

Strain rate dependency of uniaxial tensile strength in Gosford sandstone by the Distinct Lattice Spring Model with X-ray micro CT

Mechanisms causing strain rate dependency of the uniaxial tensile strength of Gosford sandstone are studied using the Distinct Lattice Spring Model (DLSM). The DLSM is built to have a microstructure which resembles aspects of the microstructure in a sample of the sandstone observed through 5 μm resolution X-ray micro CT scanning. Numerical dynamic uniaxial tensile tests on the sandstone are performed using both X-ray micro CT based and homogenous particle models. The results indicate that there is an only negligible strength increase with increasing strain rate for the homogenous particle model. However, a significant strength increase is observed with increasing strain rate for the X-ray micro CT based particle model. Therefore, it must be the microstructure that causes a strain rate dependency. Moreover, the influence of viscosity and rate dependency of springs are also studied. Results reveal that the rate dependency of the springs rather than their viscosity is also a main cause of the rate dependency.