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

通过圆缺口杆拉伸试样断裂试验，研究了等效塑性应变与应力三轴性的关系。根据断裂试验结果和有限元分析，构建了上述关系所代表的临界延性曲线。试验中使用的基材为SM490B钢。室温屈服强度和极限强度分别为353和543 MPa。部分试件的预压应变可达30%的塑性应变。研究了预应变和缺口半径对韧性起裂行为的影响，并通过扫描电镜观察了断口表面。在试验过程中，通过使用附着在试样上的声发射传感器来检测损伤的起裂行为。结果表明，预应变导致了解理微裂纹，降低了塑性。sem断口形貌发现，接收材料的断口表面100%为韧窝，10%预应变材料的断口表面有少量解理，30%预应变材料的断口表面解理达到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.