INTRODUCTION
Micro-crack formation zones developed near a crack tip in rocks and ceramics play a significant role in fracture behavior of these materials. In order to study microcrack formation near a crack tip and its effects on crack behavior, a mica-epoxy composite material was developed. Multi-layer mica flakes were embedded in an epoxy matrix. Under high stress in the vicinity of a crack tip the mica delaminated, simulating the micro-cracking along grain boundaries or crystal planes in rocks and ceramics. A compact tension specimen was fabricated from micaepoxy composite material. During the test the load and crack opening displacement were measured following the ASTM E561 (R-curve determination) procedure. Through the compliance calibration method the effective crack length and the K R were computed. The experiment clearly demonstrates the formation of micro-cracks and their effects on the overall behavior of the specimen. The paper discusses the significance of micro-crack formation on fracture toughness characterizations, and the advantages of the use of mica-epoxy composite for detailed studies of interaction between the micro-crack formation zone and the specimen geometry and size.
One of the major differences in fracture behavior of rocks and ceramics from that of metals is the formation of micro-cracks around a crack tip (1-5). This phenomenon poses an interesting problem in the application of linear elastic fracture mechanics to these materials. Linear elastic fracture mechanics assumes that a zone near a crack tip exists in which stresses and strains are characterized by a single parameter: the stress intensity factor, K. This zone is called a singularity dominated zone, and its size may depend upon such parameters as specimen dimensions, crack size and loading. Another parameter which has to be considered is the size of a fracture process zone such as micro-crack formation zone in rocks and ceramics. When the singularity dominated zone size is sufficiently large compared with the fracture process zone, a stress intensity factor characterization is quite appropriate; however, when the size of the fracture process zone approaches or exceeds that of the singularity dominated zone, a linear elastic fracture mechanics characterization may no longer be valid. It is, thus, imperative to understand the role of micro-crack formation in fracture mechanics characterization of rocks and ceramics. In order to study micro-crack formation and its effect on crack behavior and to study the effects of specimen geometry, specimen size, and loading systems, a mica-epoxy composite specimen was developed. Multi-layered natural mica flakes were embedded in an epoxy matrix. Under high stress near a crack tip, delamination in mica flakes took place, simulating micro-crack formation in rocks. An advantage of this system is that the zone in which delamination of mica flakes occurred is clearly visible. This paper describes the mica-epoxy composite fabrication technique and preliminary test results.
The mica flakes employed in this work were purchased from Mica Products Co. of Balboa Island, California, as natural mica- sheet of 0.2 mm to 1.5 mm in thickness and approximately 75mm X 150mm in size.
