A Study on the Micro-mechanisms for the Crack Initiation and Growth Ductile Fracture.

通过圆槽钢筋拉伸试件的断裂试验，研究了等效塑性应变与应力三轴度之间的关系。根据断裂试验结果和有限元分析，建立了上述关系所代表的临界延性曲线。试验所用的母材为SM490B钢。室温屈服强度为353 Mpa，极限强度为543 Mpa。在一些试件中，预压应变高达30%塑性应变。研究了预应变和缺口半径对延性断裂起裂行为的影响，并用扫描电子显微镜(SEM)对断口进行了观察，利用连接在试件上的声发射传感器检测了试验过程中的损伤起裂行为。结果表明，预应变会导致解理微裂纹，降低塑性。从扫描电子显微镜断口观察发现，原始材料断口为100%韧窝，10%预应变材料断口有少量解理，30%预应变材料断口解理达100%。

Relations between equivalent plastic strain and stress triaxiality were studied by fracture tests of round notched bar tensile specimens. The critical ductility curves represented by above-described relationship were constructed by fracture test results and FEM analyses.Base material used in the tests is as received SM490B steel. The yield and ultimate strength at room temperature were 353 and 543 MPa respectively. Compressive prestrain up to 30% plastic strain was given in some specimens. The effects of prestrain and notch radius on ductile fracture initiation behavior were investigated and observed the fractured surface by scanning electron microscope (SEM).Damage initiation behavior during the tests was detected by use of AE sensors attached to the specimens. It is shown that prestrain leads to cleavage micro-cracking and reduces ductility. From SEM-fractographs, it is found that fractured surface is 100% dimple in as-received material, in the meantime, tiny amounts of cleavage in 10% prestrain material, up to 100% cleavage in 30% prestrain material, respectively.