金属的后继屈服面研究—单试样与多试样测试

采用退火45号钢薄壁试样进行拉扭组合试验，测试材料经历预拉伸后的系列后继屈服面。试验采用偏移应变法定义屈服应力，讨论了不同的偏移应变、预变形程度对后继屈服面测试结果的影响；试验分别用单试样法和多试样法进行，分析讨论了两种方法的合理性与局限性。试验研究得到以下结果：（1）单试样法和多试样法确定的初始屈服面的形状与Mises圆很接近；（2）单试样法屈服点的测试顺序对所测得的屈服面的形状有很大影响，第一测试点与预加载方向相反时，所得屈服面会出现“内凹”现象，而若改变测试顺序，可不再出现“内凹”；（3）单试样法测得的后继屈服面与测试的屈服点数密切相关，当测试的屈服点数较多时，所得结果因累积塑性应变引起的应变强化作用而出现较大的偏离；（4）单试样法与多试样法测得的后继屈服面形状和大小有较明显的差异，测试过程中塑性变形的积累对单试样法的测试结果有明显影响，采用多试样法研究屈服面演化更为合理；（5）多试样法若采用较小目标偏移应变定义屈服，测得的后继屈服面也出现轻微内凹。

COMPARATIVE STUDY ON SUBSEQUENT YIELD SURFACES TESTED BY USING SINGLE AND MULTIPLE SPECIMEN

Through tension-torsion test with thin-walled tubular specimens of 45 annealing steel, the subsequent yield surfaces under different pre-tension deformation are investigated after the prescribed pre-strain. The yielding is defined by the several levels of offset strain, the effects of the chosen plastic strain offsets and the prescribed pre-strain levels on subsequent yield surface are explored. The initial and the subsequent yield surfaces are determined by the probe methods of multiple identical specimens method and single-specimen method, the rationality and limitations are discussed by the two probe method. The experimental results of yield surface for 45 steel are as follows: (1) The shape of initial yield surface tested by using multiple specimens and single-specimen method were found to be close to the von-Mises surface; (2) The shape of yield surface is significantly affected by test sequence with single-specimen method, the first yield point tested in opposite directions of preloading, the yield surface appears "inner concave"，if the test sequence is changed, the " inner concave" will no longer appear ; (3) The shape of yield surface is affected by test amounts of yield points through the single-specimen method, when the yield surface is tested by many yield points, the subsequent yield point will deviate from the yield surface because the strain hardening is caused by the cumulative plastic strain,; (4) Subsequent yield surface shape and size have the obvious difference between using the single-specimen method and the multiple-specimen method respectively，the shape of yield surface tested with single-specimen method is affected by cumulative plastic strain, the results by the probe method of multiple identical specimens are proved to be more accurate; (5) Subsequent yield surface is defined by a smaller offset strain with multiple-specimen method, the " inner concave" will also appear, but the phenomenon is not obvious.