降载勾线法在高强钢疲劳断裂试验中的应用

制作高强钢特征试件,在压、弯应力共同作用下进行疲劳断裂试验;结合疲劳辉纹产生机理,采用降载勾线法在试件断面制造出“海滩状花纹”;利用CAD软件描绘断面形貌并测量裂纹长度,用Newman-Raju理论进一步研究表面裂纹扩展的规律,并与试验值比较.结果表明降载勾线法可以解决高强度钢表面裂纹不易测量及断面难以观测到疲劳辉纹的问题;采用Newman-Raju公式模拟压弯组合应力下表面裂纹扩展形貌是合适的,但由此计算的疲劳寿命偏于保守.另外,当压弯应力比值较小时,计算扩展形貌时压应力可以忽略,而计算疲劳寿命时压应力不可忽略.

Application of Crack Front Marking Technique in Fatigue Fracture Experiment for High Strength Steel

Fatigue fracture experiments were performed for characteristic samples made of high strength steel and subjected to compressive and bending combined stress. Crack front marking technique was adopted in the fatigue experiments, in which slightly smaller load was exerted on the samples to produce a "beach-like pattern" on specimen's section. CAD software was adopted to depict the fractography and to measure the crack length. Newman-Raju equation was used for further study of the surface crack growth. The obtained result was compared with experimental data. Results show that crack front marking technique can solve the problems of hard measurement of surface crack and hard observation of sectional fatigue striations for high strength steel. Crack growth in both depth direction and surface direction is conformed to Paris formula when Newman-Raju stress intensity factor is adopted, but estimated fatigue life is rather conservative. In addition, when compressive stress is relatively small, it can be ignored in calculation of crack growth morphology; but it cannot be ignored in calculation of fatigue life.