Seminar - Cyclic Deformation Response and Fatigue Crack Initiation of Polycrystalline OFHC Copper
Under Pure Compression Fatigue Condition by Dr. Zhirui Wang
Date: 26 February 2014 (Wednesday)
Time: 11:00 am - 12:00 pm
Venue: EF305, The Hong Kong Polytechnic University
An investigation was set out to examine the cyclic deformation response and crack initiation mechanisms under pure compressive fatigue using polycrystalline OFHC copper. It is generally believed that fatigue cracks can only initiate under the influence of tensile or multiple-direction load spectrums. However, practical examples demonstrate that cracks may also initiate under pure compressive fluctuating loads such as the failures in aircraft landing gear frames. As the knowledge on pure compressive fatigue is very limited up to date, the understanding of cyclic deformation response under such conditions leading to the eventual crack formation becomes critical in preventing the future failure of such components. In the present systematic investigation, it is observed that the cyclic stress strain response and microstructural evolution of copper under pure compression fatigue exhibits rather dissimilar responses compared to those under general push-pull fatigue condition. Under pure compression fatigue, the specimen hardened rapidly in the early cycles unlike the gradual hardening behaviour in symmetrical fatigue. Compressive cyclic creep behaviour was also observed as another feature in the tests. More interestingly, it was found that the cyclic plastic strain accommodated by the sample was not in any major way through dislocation activities, as there was only moderate slip activities observed on the surface and no PSB features were detected from TEM observations. Instead, the specimen surface continuously roughened by grain rotation as cycling progressed. Such phenomenon eventually led to crack formation along the grain boundaries when the applied stress amplitude was high.
Dr. Zhirui Wang obtained his Bachelor degree of Engineering at Xi’an Jiao-Tong University, China, in 1968. He then worked as an engineer first and then the head of technical department in a manufacturing company in Shaanxi province. In 1980, Dr. Wang went to USA to continue his advanced education and obtained Master and Ph. D. degree in 1982 and 1985, respectively, in physical metallurgy at Polytechnic University of New York, and then conducted research activities as a post-doctorate fellow at the University of Pennsylvania from 1986-1988.
From 1989, he joined the Department of Materials Science and Engineering at the University of Toronto and became a full professor there in 1996. His major research interests are in the area of plastic deformation, cyclic deformation, fatigue and fracture of various materials from single crystals, to materials for manufacturing, to nano-grained materials. A number of his theories and/or findings on cyclic deformation and fatigue of materials have been adopted in classic text books. These include such as cyclic deformation of multiple-slip-oriented crystals, residual stresses in sheet metal forming, a new criterion on wave–planar slip transition, and fatigue of low stacking fault energy materials… To date, he has supervised more than 80 Ph. D. and M. ASc. students, authored/co-authored more than 250 papers including more than 130 papers in refereed journals. He has given more than 30 invited keynote speeches in various major international scientific conferences. He is the recipient of a number of prestigious awards and has 4 US and Canadian patents.
Dr. Wang has been serving in various international/national professional committees, and served as the organizer/session chair for many international conferences. He is also the invited reviewer for major international journals such Acta Materialia、Scripta Materialia、Philosophical Magazine、International Journal of Fatigue、Materials Science and Engineering…