Mixed Mode Fracture Of The Thin Sheet Metal

With the sustainable development of economy and society in China, people seek for a much better standard of living, electrical equipment and vehicles such as cars is facing a good opportunity to update. The equipment contains high amounts of thin sheet metals that mainly reclaim and utilize by the method of shredding, so it has the important scientific significance and the practical significance to study the crushing mechanism as well as the recovery and rational utilization of thin sheet metals. From the perspective of fracture mechanics, this thesis found the relationship between the single failure model and mixed failure model of thin sheet metals by digital image correlation technique. The mode Ⅰ、Ⅱ、Ⅲ fracture, mixed mode Ⅰ/Ⅱ、Ⅰ/Ⅲ fracture and their influence and contribution rule of thin sheet metals were studied. In this paper, we tried to seek for the influences of sensitive factors on ductile fracture of thin sheet metals and use the CTOA at stable crack extension to describe the fracture toughness of these metals, so as to make a good foundation for establishing the shredding theory system of thin sheet metals.The main contents and conclusions of this thesis are as follows:(1) A multipurpose and angle tunable experiment device, and testing a specimen as well were successfully designed for investigating the mixed mode crack fracture. The experiment device was an entirely new invention for testing the fracture behavior of mode Ⅰ、Ⅱ、Ⅲ crack and Ⅰ/Ⅱ、Ⅰ/Ⅲ mixed mode crack at the universal material testing machine. The loading conditions can be transferred to any angle between0°and9°when testing mixed mode crack fracture.(2) With the protective steel plant from scrap automatic teller machine, we successfully completed the relevant experiment, gathered correlative data and discussed the comparison of fracture behavior by the multipurpose and angle tunable experiment device we designed.(3) Real-time displacement in stable crack tearing was measured by digital image correlation technique. Quantitative analyses were provided on mixed mode Ⅰ/Ⅱ fracture and critical CTOA for recycled thin sheet metals. The variation curve of P-Δa and CTOA-Δa were obtained and analyzed. It was shown from the results that the recycled thin sheet metals were not easy to fracture due to the large plastic deformation, and the mode Ⅰ crack was the most difficult to extend and the mode Ⅲ crack was the easiest, meaning that the loading path of mode Ⅲ was the best way to shred the metals that we used. The critical CTOA of Ⅰ/Ⅱ mixed mode crack at stable crack tearing was constant and could be used as a fracture criterion for the thin sheet metals.(4) From the aspect of the stress triaxiality and the directions of principal stresses nearby the crack tip, we made discussions of the appearance that the crack initiation point failed to pass the crack tip in the Ⅰ/Ⅱ mixed mode crack fracture experiment. The article pointed out that crack opened at the direction of maximal value of stress triaxiality nearby the crack tip and except for mode Ⅰ crack, the else failed to pass the crack tip.