Crack initiation and propagation in molded gypsum specimens with pre-cracked single flaws

• Main Author: Chong, Wanling.
• Other Authors: School of Civil and Environmental Engineering
• Format: Final Year Project
• Language: English
• Published: 2012
• Subjects: DRNTU::Engineering::Civil engineering::Geotechnical
• Online Access: http://hdl.handle.net/10356/49196
• Institution: Nanyang Technological University

This paper summarises the results of an investigation on the influence of flaw inclination on crack initiation and propagation for molded gypsum specimens under compression. Molded gypsum specimens with 1.3mm wide flaws of inclinations 75°, 80°, 85° and 90° were tested. Uniaxial compression was conducted using two INSTRON Uniaxial Compression Machines and the entire process was filmed by a SONY HDR-XR150 digital camcorder and a PHANTOM v310 high speed camera. Cracking initiation and propagation sequences were analysed using high speed video footage. A total of 20 specimens were tested. Due to experimental errors, results for 9 out of 20 specimens were not utilized. For the remaining 11 specimens, the first cracks usually initiated from the top edge of the specimen and propagated downwards. In three out of 11 specimens, no cracks were observed initiating and propagating from the flaw tips. Failure of the specimens was generally characterized by a widening of the failure crack aperture which split the specimen into numerous blocks. In some cases, relative movements of these blocks occurred. Based on the results obtained from this experiment, it is proposed that at steep flaw inclinations, the main mode of failure is due to tensile splitting of the specimen and not due to tensile stress concentration on the periphery of the flaw. The non-homogeneity of specimen, unevenness of the loading surface and eccentricity of loading were cited as possible reasons for tensile splitting of the specimen being the main failure mode. Numerous experimental challenges, such as overexposure of images captured and erroneous triggering of high speed video recording, were also faced in the course of this experiment and addressed in this paper with the hope of serving as a guide for similar experiments in the future.