A New Method of Measuring the Plastic Deformation
一种检测塑性变形的新方法

Roughness on the metal specimen polished surface will be induced when it yields plastic deform or fatigue damage for its interior microscopic crystal grain slip dislocation,and this is very different when it yields elastic deform.Its specula reflection weakened while scattering reflection reinforced.This paper presented a new technique to determine the elastic-plastic boundary and plastic strains on the surface of metal specimen.The metal specimen subjected uniform tension is projected by Slant incident white-light;loads increase as preseted.These distributions of the brightness can be recorded as real-time images by CCD camera.The following simple method we advanced is used to analysis the image data.According the practical situation,one threshold is chosen.The gray level which is greater that the threshold is defined the bright point.Define BI as the reciprocal value of the number of the bright points.In the experiments,BI keeps stable during the period of elasticity;while the material is yielding,the increase of the surface roughness leads to the stronger ability of the random reflection;and the BI goes down sharply.BI keeps monotone decreasing during the period of plasticity.So BI can be taken as a feature parameter.BI can not only be used as a judgment of the status but also be the measuring of the plasticity.Typical experiments can verify the method.The conclusion matches the traditional correlation method very well.This method can be prospectively applied in the engineering with the merit of simple operation and low environmental condition.This paper also discusses a parameter which is very important to the accuracy of this method.Based on the analysis of the data,a method which is called confidence interval was developed to determine the threshold value.And also find that the better results can be obtained with the little of the confidence interval.