Micro-crack Initiation Life and Micro-fractographic Analysis in Strain Cycling Fatigue of a 60 kg/mm2 High Strength Steel

Abstract

Reversed diametral strain controlled low cycle fatigue test was carried out on hour-glass type specimens of which elastic stress concentration factor was 1. 06. Specimens of 10 mm in diameter were machined out parallel to the rolling direction from a 25 mm thick plate of 60 kg/mm2 high strength steel. Surface of test section was polished mechanically at first, and lastly by electrolytic polishing.Fatigue test was conducted at ambient temperature with natural strain amplitude ranging from 0.14 to 0.005. Cycling rate was ranged from 1 to 5 cpm when the failure life was expected to be less than 100 cycles, and otherwise 5 to 10 cpm. Micro crack developed on specimen surface was replicated on a strip film of cellulose acetate by stopping the fatigue testing machine about five minutes. Microfractographic analysis was applied on fracture surfaces of all specimens.The following conclusions were obtained : (1) Initiation life of micro crack 0.01 to 0.05 mm in length N0.01 and initiation life of visible crack of 0.2 to 0.5 mm long Nc were related to complete failure life Nf with the following relations.N0.01=0.0735Nf1.134Nc=0.626Nf1.016(2) Constants Cp and Ce in the expression, _??_ta=Cp·Nc-kp+Ce·Nc-ke, were obtained as 0.41 and 0.0050 respectively for the present steel tested. The following relations were also re-confirmed.Cp=0.35_??_fCe=0.000107 σu-0.00214(3) Dimples were observed on fatigue surfaces fractured in the failure life range of 1/4 to 90 cycles.(4) Rub marks appeared abruptly as the failure life was exceeded about 90 cycles, and in like manner tire tracks appeared at failure life of 350 cycles. Tire tracks and rub marks are considered to be formed as the results of any specific number of cycles of compressive load.(5) Spine pattern was observed on fatigue surfaces of failure life over 690 cycles.(6) Microstriations of 1 to 3 it spacing appeared when the failure life was exceeded about 700 cycles.(7) Peculiar fractographic features to the fatigue surface were obtained on replicas taken from a severely damaged fracture surface pounded by compressive load.