An experimental investigation of brittle failure mechanisms in scratch tests of rock
An experimental investigation of brittle failure mechanisms in scratch tests of rock
复制标题
DOI：
10.1016/j.engfracmech.2022.108827
复制
发表时间：
2022-10
影响因子：
5.4
通讯作者：
He Zhang;J. Le;E. Detournay
中科院分区：
文献类型：
--
作者：
He Zhang;J. Le;E. Detournay
本文报道了三种岩石(Bonne Terre白云岩、阿拉巴马大理岩和Dunnville砂岩)在0.2~2.6 mm深度范围内的划痕试验结果。实验是在厚度略小于刀具宽度的板材上进行的，用摄像机记录下整个切割过程。试验结果表明，随着切割深度的增加，存在延性、破碎性和脆性三种切削区。破坏模式不同，比能ϵ与切割深度d的关系也不同，破片形状也不同。在韧性阶段，由于强烈的剪切作用，ϵ∼d0和碎屑是岩石或粉末的本构颗粒。在碎裂区，ϵ∼d−1/2和碎屑是韧性类颗粒和片状碎屑的混合物，也是剪切破坏的产物。最后，脆性区域的特征是ϵ∼d−1，本质上与片状碎片有关，这些碎片有两种形式，取决于它们是由剪切还是通过拉伸产生的。只有当岩石在实验尺度上足够均匀时，才能观察到比能量的d−1依赖关系。不同的故障模式之间的区别是基于视频记录和扫描电子显微镜断口图像的证据。结果表明，拉伸断裂对剪切断裂的优势与压应力场中破碎带的存在密切相关，破碎带起着楔形体的作用，在压应力场中引发拉伸宏观裂纹。最后，对脆性区域能量分配的分析表明，与楔形体形成过程中所消耗的能量相比，在产生具有拉伸裂纹的新表面时所消耗的能量实际上可以忽略不计。这一结果表明，根据平行于自由面的裂纹模型，不能根据脆性区域的比能来确定I型韧性，这与已发表的关于这一结果的说法相矛盾。最后，这项实验研究澄清了文献中的一些混淆，这些混淆的基础是假设只存在两种破坏模式，即延性和脆性。
This paper reports the results of scratch tests on three rocks (Bonne Terre Dolomite, Alabama Marble and Dunnville Sandstone) conducted at depths of cut ranging from 0.2 mm to 2.6 mm. The experiments were performed on slabs of thickness slightly smaller than the cutter width, with a camera recording the entire cutting process. The test results reveal the existence of three cutting regimes: ductile, fragmentation, and brittle with increasing depth of cut. The regimes differ by the failure mode, the dependence of the specific energy ϵ on the depth of cut d, and the fragment shapes. In the ductile regime, ϵ∼ d 0 and the fragments are constitutive grains of the rock or powder as a result of intense shearing. In the fragmentation regime, ϵ∼ d− 1/2 and the fragments is a mix of ductile-type particles and flake-like chips also the product of shear failure. Finally, the brittle regime is characterized by ϵ∼ d− 1 and is essentially associated with chip-like fragments that come in two varieties depending on whether they are created by shear or by tension. The d− 1-dependence of the specific energy can only be observed if the rock is sufficiently homogeneous at the scale of the experiment. The distinction between different modes of failure is based on evidence from video recording and fractography with a Scanning Electron Microscope. It is shown that the dominance of tensile over shear fracture is intimately related to the existence of a crushed zone, which acts as a wedge to initiate a tensile macrocrack in a compressive stress field. Finally, an analysis of the energy partitioning in the brittle regime indicates that the energy dissipated in creating new surfaces with a tensile crack is actually negligible compared to the energy expended in the formation of the wedge. This result indicates that the mode I toughness cannot be determined from the specific energy in the brittle regime, on the basis of a model of a crack parallel to the free surface, in contradiction with published claims to that effect. Finally, this experimental investigation clarifies some confusion in the literature, rooted on assuming the existence of only two modes of failure, ductile and brittle.