数字图像相关的不锈钢真实应力应变关系测量

工程上广泛应用的名义应力应变曲线,在材料发生大变形时,由于应力和应变不再沿着试件均匀分布,因而不能真实反映材料在塑性阶段的本构关系。在不锈钢标准板的拉伸试验中连续采集试件散斑表面的数字图像,采用数字图像相关技术对序列数字图像进行相关运算以获得其每个变形状态下的变形场,得到了整个拉伸过程中试件的变形形态。以光学虚拟引伸计的方法分别测量横向和竖向的实时应变数据,结合体积不变理论计算得到真实应力应变曲线。采用Johnson-Cook材料模型对真实应力应变曲线进行拟合,得到了塑性本构关系;最后根据以上实验结果和真实应力应变曲线获取了材料的力学特性参数。实验结果表明了基于数字图像相关方法对不锈钢真实应力应变关系测量的有效性和可靠性,对实验结果的真实应力应变曲线后处理,获得不锈钢材料的弹性模量为169 GPa,屈服强度为236.8 MPa,并获得了Johnson-Cook简化模型的所有三个参数A、B和n的数值。

Measurement of true stress-strain relationship of stainless steel based on digital image correlation technique

The nominal stress-strain relationship is used widely in engineering. The stress and strain is no longer uniform distribution when the large deformation, so it can not reflect the true constitutive relation with the stage of plastic. Tensile testing of stainless steel is produced and series of digital images of specimen speckle surface were recorded, and processed by digital image correlation technique to obtain the deformation field of each recording step, so the deformation process of tensile test is revealed; the optical extensometer technique is also used to measure the real time horizontal and vertical strain of the specimen, and the real stress-strain curve is obtained with combination of the volume invariant theory, then the Johnson-Cook model is employed to match the real stress-strain curve to extract the plastic relationship; finally based on the above experimental results and the real stress-strain curve, the mechanical properties could be obtained. The experimental results shows that the true stress-strain relationship obtaining by digital image correlation is effective. Through the post processing of true stress-strain relationship the elastic modules and yield strength are calculated as 169 GPa and 236.8 MPa respectively, and moreover, all the three parameters of the simplified Johnson-Cook model A, B and n are obtained together.