Fracture Experiments And Numerical Analysis Of High Strength Steel At Room Temperature And Low Temperature

In the Northridge earthquake in America,1994 and Hyogoken-Nanbu earthquake in Japan,1995, there were number of brittle failures of beam-to-column connections, and that indicated that people's cognitions on the stress behavior and damage mechanism of structural steel were rather inadequate. As an important problem in many industries, crack play a vital role in bearing capacity and operational life span of structural members, and there is no exception in steel structure. In order to avoid the personnel casualty and the property damage, the United States and Japan set up specialized research institutions and proposed a lot of modified programs. But whether these programs are of enough effectiveness only after another large earthquake can we know the result.In order to study crack mechanism in stress triaxiality and fracture design of structure steel, by comparing the results of fracture experiments and numerical simulations were performed on different thickness of notched plates, plates with hole and notched bars made of high strength steel Q345, including ten notched plates of 4mm, fifteen notched plates of 10mm, ten notched plates of 16mm, ten plates with hole of 16mm, eighteen notched plates of 26mm and twenty notched bars in the room temperature, but in the low temperature, ten notched plates of 16mm and twenty notched bars were test. The experimental results reveal that the crack initiates at the curved edge of the notch plates or the curved bars, and forms a through-thickness flaw, which leads subsequently to very rapid growth of the crack across the remaining width of the plates.Fracture section was dark gray and had many holes at the center. To increase the thickness of specimen, the fracture ductility decreases. To change the gap size to sharper, the cracking ductility less than the flat specimen and vice versa, which indicates fracture load depending primarily on the net sectional area of the root notch and shows the stress field in the carved section affecting to the notch root fracture strength.The numerical simulation analysis of notched plates, plates with hole and notched bars based on Generalized Yield Model and Mises yield model showed that crack initiated at the center of notched section where had strong stress triaxiality and expanded towards two sides along the thickness of the plates. The 3-D stress field near the notch of the plate specimens prior to fracture, which was computed from a large-deformation analysis using the generalized hydrostatic yield model, proves the accuracy of the proposed fracture criterion. By comparing the Ellipsoid Metal Fracture Criterion, Hancock Equivalent Strain Fracture Criterion, Wierzbicki Equivalent Strain Fracture Criterion and the first Principal Stress Fracture Criterion based on Generalized Yield Model showed that the formulas for predicting the initial crack location were high precision, However, as the Hancock Equivalent Strain Fracture Criterion and the Wierzbicki Equivalent Strain Fracture Criterion which lacked the critical value of equivalent strain were very sorry, because 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 time of fracture occurrence. The first Principal Stress Fracture Criterion based on ductile materials can be used to predict micro-crack formation and fracture in the stress triaxiality space, while the Ellipsoid Metal Fracture Criterion was proposed for High-strength steel, based on the coincidence of yield and fracture in the state of three-way stretch, So the first Principal Stress Fracture Criterion has high accuracy and wider universality.