Fracture Mode And Fracture Experiments Of Thin Metal Plate At Room Temperature And Low Temperature

Fracture of metal material was the interdisciplinary propositions which mechanic scientists and material scientists struggled for nearly a century. The consequences of brittle failure were catastrophic, all engineering were required to avoid brittle fracture. Aluminum alloy were widely used in building structures due to its own advantages. In order to avoid the personnel casualty and the property damage due to brittle fracture, it has theory significance and engineering value to explore fracture mechanism of metal plate under low stress triaxiality.In order to study crack mechanism in low stress triaxiality and fracture design of aluminum alloy and structure steel, the results of fracture experiments and numerical simulations performed on different materials of plates with hole and notched plates were compared, including seventeen aluminum alloy plates with hole of 3mm, ten plates with hole of 4mm made of Q235structure steel, ten plates with hole of 8mm made of Q345 high strength steel in the room temperature, and in the low temperature, eighteen aluminum alloy notched plates of 3mm, ten notched plates of 4mm made of Q235structure steel, ten notched plates of 8mm made of Q345 high strength steel were tested. The experimental results revealed that the macroscopic cracking was initiated at the curved edge and expanded towards the thickness of the plates, and then it expanded towards the vertical directions of load quickly until it ruptured. The fracture ductility was lower for the specimen with the sharper notch. Fracture load was primarily determined by the net sectional area and had a less connection with the sharpness of notch. Yield strength and ultimate strength of standard part in low temperature were slightly higher than it in room temperature, but the rate of elongation decreased. There was obvious brittleness in low temperature experiments.The numerical simulation analysis of plates with hole in the room temperature and notched plates in the low temperature showed that crack initiated at the center of notched section where had strong stress triaxiality. The Stress triaxiality was higher for the specimen with the sharper notch, and the fracture ductility was lower. By comparing the Ellipsoid Metal Fracture Criterion, First Principal Stress Fracture Criterion, Hancock Equivalent Strain Fracture Criterion and Wierzbicki Equivalent Strain Fracture Criterion,it showed that the formulas for predicting the initial crack location were high precision. However, the Hancock Equivalent Strain Fracture Criterion and the Wierzbicki Equivalent Strain Fracture Criterion lacked the critical value of equivalent strain, but the Ellipsoid Metal Fracture Criterion and the first Principal Stress Fracture Criterion had the critical value of equivalent strain, they can be used to determine the fracture occurrence. The first Principal Stress Fracture Criterion can be used to predict micro-crack formation and fracture in the stress triaxiality space and the Ellipsoid Metal Fracture Criterion has high accuracy. Fracture of metal material has always been a research difficulty, and up to now, there has been a large number of fracture criterions and fracture models. However, they could not be fully accepted. This is a great inconvenience in engineering. It is necessary to study on fracture characteristics and failure criterion of metal material in different stress triaxiality.The stress state had significant effect on the fracture mode of materials, equivalent plastic strain was an important parameter in measuring the fracture ductility. The equivalent plastic strain and stress triaxiality were extracted in acture location when aluminum alloy plates fractured in low temperature experiment. Equivalent plastic strain fracture criterion was obtained in using least square method. The equivalent plastic strain fracture criterion was used in analyzing the notched plates of aluminum alloy, it showed that the criterion had high precision in predicting the initial crack location. It has reference value in theoretical research and engineering practice.