确定动态劈裂裂纹起裂和扩展特性的简便方法

利用直径100 mm的分离式霍普金森压杆对大理岩巴西圆盘进行了动态劈裂试验。在试样表面加载直径方向粘贴一系列电阻应变片,用来记录试样加载直径上各点的应变随时间的变化曲线,通过记录的应变历程,可以判断劈裂试样的起裂应变、起裂位置和时刻,以及裂纹的扩展顺序和裂纹扩展的速度。试验结果表明,裂纹的起裂对应着该点的应变历程上应变值的突变。裂纹起裂一般并不在试样中心,而是偏向中心和入射杆撞击端之间的某一点,然后裂纹沿加载直径向两个相反方向发展,直至试样完全劈裂为比较大的两部分。裂纹扩展的速度大约为400~800 m/s,试样从开始起裂到完全裂开的破坏时间约为几十个μs。当应变率比较低时,裂纹在扩展过程中的分岔点比较少,因而破坏形态较好,当应变率比较高时,裂纹在扩展过程中的分岔点比较多,破坏形态也比较多。因此,在巴西圆盘加载直径上粘贴一系列应变片来研究脆性材料的动态起裂和扩展特性,是一种简便高效的试验方法。

A Simple Method for Determining Crack Initiation and Propagation in the Dynamic Splitting Test

The dynamic splitting tests of marble using Brazilian disc and flattened Brazilian disc were performed at high strain rate with split Hopkinson pressure bar(SHPB) of 100 mm diameter.A series of electrical resistance strain gauges were stuck on the samples along the loading diameter in order to record the strain histories,from which the strain,the time and position of the crack initiation,and the consequence and the velocity of the crack propagation could be detected.Experimental results indicated that crack initiation time was in accordance with the sudden change of the strain.Generally speaking,the crack does not initiate from the center of the sample,but from a certain point between the center and the interface where the incident bar impacts the sample.Then the crack propagates in opposite directions along the loading diameter till splitting into two halves.The velocity of the crack growth ranges from 400 m/s to 800 m/s,and the failure time is about dozens of microseconds.With low strain rate loading,the failure pattern of the sample is good and there are fewer turnoffs during the growth of the crack.On the contrary,with high strain rate loading,the failure pattern of the sample is diversified and there are more turnoffs at the fracture path.It is a brief,effective method to stick a series of strain gauges along the loading diameter of the sample to gain the characteristics of the dynamic crack initiation and propagation for brittle materials.