单轴拉伸作用下层状岩石表面裂纹的形成模式及其机制研究

RESEARCH ON FORMATION MODE AND MECHANISM OF LAYERED ROCK SURFACE FRACTURES UNDER UNIAXIAL TENSION LOAD

层状岩石的表面裂纹发端于表面自由边界,而后向着层与层之间的交界面扩展,最后止于交界面或沿着交界面继续扩展。为更深入地理解这一表面裂纹扩展现象的机制,采用数值模拟方法,再现微裂纹从萌生、扩展、贯通、成核、填充到饱和的全过程,以及层间剥离现象。首先以表面预制裂纹的模型为例,对其进行单轴拉伸加载,发现不同预制裂纹间距的情况下两裂纹间的应力分布不同,这将直接影响到新裂纹的产生模式,即裂纹是插入到表层(与已有裂纹平行),还是不再有新裂纹插入(裂纹饱和),而是产生层间剥离,或已有裂纹向下扩展,导致模型破坏。其次,对比分析模型均质和非均质情况下不同裂纹间距时的应力分布,结果表明非均质材料情况下产生相同间距的裂纹所需的荷载较均质材料情况下更小,裂纹饱和对应的裂纹间距与层厚比值的临界值基本一致。最后,提取等间距裂纹形成过程中若干具有代表性的截面应力,通过分析其应力转化机制,解释裂纹饱和现象的力学机制。数值计算得到的应变与裂纹间距之间的关系拟合结果良好。研究结果还表明:产生层间开裂的裂纹间距比没有层间开裂的裂纹间距大。通过对层间的应力传递过程随着裂纹间距变化规律的探讨,研究此过程的应力传递模式。

Fractures initiated from free surface of layered rocks,which are called surface cracks,often terminate at the interface which divides the fractured layer and the matrix layer,but can continue to propagate along the interface.A numerical simulation is carried out to study the mechanism of surface fracturing process including micro-fracture formation,propagation,coalescence,nucleation,fracture infilling,fracture saturation,termination,and interface delamination.A model with a fractured surface subjected to uniaxial tension is simulated firstly.It is found that the stress distribution between fractures depends on the fracture spacing;and the stress distribution will directly affect the fracture mode.New fractures can either appear in the surface layer（parallel to existing fractures）,or appear between the layers（interface peeling）.Furthermore,an existing fracture can propagate to the underlying layer with no fracture inserting（fracture saturation）.Secondly,material homogeneity is considered in the simulation;and the results show that the stress distributions in homogeneous and heterogeneous models are different.A smaller stress is required to produce the same fracture spacing in the heterogeneous model;and the critical values of the spacing to layer thickness ratio at fracture saturation are almost the same.Lastly,stresses in a few sections during fracture formation are analyzed;and it is found that stress transformation can be used to explain the mechanism of fracture saturation.A fitting curve of the relationship between strain and spacing to layer thickness ratio is obtained.It is found that fracture spacing in the case of interface delamination is greater than that without interface delamination.Stress transition between the two layers on fracture spacing in the fracture process is also investigated,with a focus on stress transfer mode.