加载破裂和卸载破裂的焦散阴影区动态特征的实验研究

AN EXPERIMENTAL STUDY OF THE DYNAMIC FEATURES OF SHADOW AREAS OF CAUSTICS IN RESPONSE TO LOADING/UNLOADING FRACTURE

以实时全息干涉计量方法和焦散阴影光学法 ,用有机玻璃试样直观、形象地研究加载过程和卸载过程破裂时的应力 (应变 )场、塑性区或成核区 (阴影区 )不同地动态特征变化 .结果发现 ,在低应力时 ,预制裂缝端点先出现应变核 (条纹密集区 ) ,进而形成焦散阴影区 .这些核和阴影区会随荷载增加变大 ,随卸载而减小 .在一定的压力范围内重复加载卸载 (循环加载 ) ,焦散阴影区则反复增大缩小 ;但在高应力时 ,尤其在将出现大破裂时 ,加载和卸载的焦散阴影区变化是不可逆的 ,其变化也完全不同 .加载破裂时的焦散阴影区由慢到快迅速扩展 ;而卸载时的焦散阴影区则扩展较慢 。

Using a plexiglass sample and by means of real-time holographic interferometry and shadow optical method of caustics, the different features of dynamic variation in stress (strain) field, plastic area and nucleation zone (shadow area) when the sample fractures during loading(loading-fracture) and unloading (unloading-fracture) are strdied visually. The results show that the strain nuclei (zones with dense fringes) appear first at the tips of prefabricated cracks at low stress, and then the shadow areas of caustics form with the increase of load. These nuclei and shadow areas can become larger, or smaller, when the process of loading, or unloading, goes on. When the stress in kept within a certain range, the shadow areas of caustics can become larger and smaller altematively with repeated loading and unloading (cyclic loading). However, when loading and unloading at high stress, in particular with the macrofracture is about to appear, the variations of the shadow areas of caustics are irreversible and quite different. The shadow areas of caustics expand rapidly at an increasing speed when loading-fracture appears. In contrast, the shadow areas of caustics expand at a lower speed when unloading-fracture appears; besides, there is a circular shadow in front of the sharp-angle shaped area.