Experimental Research On Crack Propagation In Gypsum Boards Containing Pre-existing Crack Under Compression

It is an important subject of rock fracture mechanics to study the law of crack propagation failure in rock masses under compression. So laws of the wing crack initiation, propagation and failure have important significance for the study on the failure mode and failure mechanism of the rock masses with crack under compression. In this paper, the testing methods were combined and the laws of open crack propagation under compression were mainly studied in detail. In similarity with the rocks to meet a certain basis, based on two characteristics of rocks: characteristics of brittle fracture and dilatancy under compression, we used calcined gypsums as rock-model materials. A series of experiment approach on crack propagation by using the gypsum boards containing pre-existing cracks as rocklike materials was carried out, and the new laws of crack propagation under uniaxial compression and biaxial compression, respectively, are mainly studied.The elliptic open cracks with different angles and different thickness in the center of gypsum boards are made by embedding pieces of thin metal. The whole experimental research under uniaxial compression was performed using a universal testing machine which could record load-displacement curves through the computer. A HD video camera is used to record the cases of crack propagation of the specimens. The results show that:①The initiation angles of the wing cracks are mostly about 79°, which are above the theoretical value of 70.5°, and the initiation angle maintains at around 70.5°only when the width of crack comes to 2mm;②The initiation angle is irrespective with the angle of crack;③The initiation load is near to the ultimate load, and the larger the initial angles, the greater the initiation loads, in which the growth rate is more than Linear relationship with the angle of crack;④The angle of crack has a great impact on the elastic modulus of the board, while the width of crack has less. The strain of failure stays 8% or so in most of boards.An experiment research on crack propagation of gypsum boards containing pre-existing cracks under biaxial compression is carried out. All the experiments are tested on self-made biaxial loading apparatus, the initial confining pressure is 0.2MPa, and maintain the horizontal pressure constant but axial loading. A HD video camera is used to record the cases of crack propagation. From the experiments the following conclusions are drown:Only in the confining pressure of 0.2MPa, the initiation load and ultimate load are 2.5-3 times as large as that under uniaxial compression; The wing crack initiation angles under biaxial compression were between 72°and 75°, initiation load and ultimate load vary considerably; When the thickness of initial crack is certain, the larger the initial crack angle is, the greater the overall slope of the stress-strain curve is, basically in proportion. The experimental study on gypsum boards containing pre-existing cracks by using self-made biaxial loading apparatus are investigated in detail, which will provide experimental basis for the mechanisms of rock failure under biaxial stress states.