缺口件表象疲劳极限的定量化分析

测定了淬火、高温回火35CrMo钢光滑试样和三种缺口试样的三点弯曲疲劳极限,并利用ANSYS有限元软件计算了缺口试样加载时缺口截面上的应力分布.用“疲劳源形成的微细观过程理论”对试验结果进行了分析,认为,疲劳源的形成虽然发生在个别薄弱晶粒内部,是其中位错往返运动及交互作用的结果,但还必须满足一定的形变协调条件和概率条件,因而必须形成由相当多晶粒组成的“细观屈服区”.对试验结果的分析表明,如取“细观屈服区”临界尺寸(xW)等于11个奥氏体晶粒平均直径,则计算得到的缺口试样的表象疲劳极限,与实测值相比,其综合误差最小.据此计算的三种缺口试样的表象疲劳极限,其误差都小于5％.该xW可认为是材料的特征参量.

QUANTITATIVE ANALYSIS OF APPARENT FATIGUE LIMITS OF NOTCHED SPECIMENS

In this work, the apparent fatigue limits of notched specimens were analyzed quantitatively from a comprehensive point. Three-point bending fatigue limits of smooth specimens and three kinds of notched specimens of quenched and then high temperature tempered 35CrMo steel are determined. The stress distributions of notched specimens are calculated by finite element analysis software ANSYS. The experimental results are analyzed with the "micro-meso-process theory of fatigue source generation": though the generation of fatigue source appears in individual weak grains as a result of dislocation movements, it must satisfy certain deformation harmonization condition as well as probability condition, therefore a "meso-yielding region" containing quite a few grains must be formed. According to above analyses, the apparent fatigue limit of specimens is the maximum stress (expressed as nominal stress) required to create a critical meso-yielding region on the dangerous section of specimen when it is bearing alternating loads. The analyses also show that when take the critical dimension of the meso-yielding region, xW,as 11 grains, the compositive error between calculated apparent fatigue limits of notched specimens and their measured values reaches the least. Under this condition, the errors of calculated ones are all under 5%, so xW can be considered as a characteristic parameter of material. The study above also elucidates the physical essence of concentration factor of fatigue stress kw. In engineering practice, apparent fatigue limits of notched specimens can be predicted according to fatigue limits of material (W), xW and the stress distributions on the notch section.