Application of Crack Front Marking Technique in Fatigue Fracture Experiment for High Strength Steel
降载勾线法在高强钢疲劳断裂试验中的应用

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.