基于数字图像相关方法的单轴压缩煤样应变局部化过程试验

为了进一步揭示煤样的应变局部化过程,利用数码相机记录单轴压缩煤样一个观测表面的散斑场,采用数字图像相关方法计算应变场,根据清晰应变局部化带位置布置相互垂直的测线,获得了最大剪切应变的变异系数与纵向应力的依赖关系。研究发现:(1)子区尺寸及测点间距不会对最大剪切应变的变异系数-纵向应变曲线的发展趋势产生影响;(2)应变局部化带法向的最大剪切应变分布左右并非严格对称,应变局部化带切向的最大剪切应变分布并非均匀,前者应与煤的非均质性有关,后者反映了应变局部化带的传播;(3)随着纵向应变的增加,应变局部化带宽度有变窄的趋势,应变局部化带法向的最大剪切应变剖面有变陡峭的趋势;(4)只有应变局部化带经历由模糊到清晰的过程才能引起最大剪切应变的变异系数的突增,此时,应变局部化出现时的纵向应力与单轴抗压强度之比为0.56。

Experiments of strain localization processes of coal specimens in uniaxial compression based on the digital image correlation method

To further reveal the strain localization processes of coal specimens in uniaxial compression,the authors record the images of a surface of each specimen by use of a camera.The digital image correlation method is used to obtain strain fields.The authors arrange monitored lines orthogonal each other according to positions of clear strain localization bands.The relation between the variation coefficient of the maximum shear strain and the longitudinal stress is established.The study found that ① the subset size and spacing of monitored points do not have an influence on the developing tendency of the relation above;② the distribution of the maximum shear strain in the normal direction of the strain localization band is not strictly symmetric,which is related to heterogeneities of coal,and the maximum shear strain in the tangential direction is not uniform,reflecting the strain localization band propagation;③ with an increase of the longitudinal strain,the thicknesses of strain localization bands become narrow and the profile of the maximum shear strain in the normal direction becomes steep;④ the rapid increase of the variation coefficient is due to the fact that unclear strain localization bands are converted into clear bands,which corresponds to the ratio of the longitudinal stress to the uniaxial compressive strength of 0.56.