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

通过圆缺口拉伸试样的断裂试验，研究了等效塑性应变与应力三轴度的关系。根据断裂试验结果和有限元分析，建立了由上述关系表示的临界延性曲线，试验中使用的基体材料为SM 490 B钢。室温下的屈服强度和极限强度分别为353和543 MPa。压缩预应变高达30%的塑性应变，在一些标本。研究了预应变和缺口半径对材料韧性断裂起裂行为的影响，利用扫描电镜（SEM）观察了材料的断口形貌，并利用声发射（AE）传感器检测了材料在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的产生并降低塑性。断口扫描电镜分析表明，原始材料的断口为100%韧窝，而10%预应变材料的断口为少量解理，30%预应变材料的断口为100%解理。

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.