Discussion on crack growth behavior in large-scale fatigue tests of carbon and low-alloy steel plates based on fracture surface observation

Abstract

In Japan, the Design Fatigue Curve (DFC) Phase 1 and Phase 2 subcommittees, organized under the Atomic Energy Research Committee of the Japan Welding Engineering Society, have proposed new best-fit fatigue curves and design fatigue curves for carbon, low-alloy, and austenitic stainless steels. To investigate the correlation between the fatigue life by the proposed best-fit curves and that of the actual components, the authors carried out reversed four-point bending fatigue tests for large-scale specimens of carbon steel and low-alloy steel plates with 90 mm thickness and 150 mm width. As a result, the fatigue lives given by the best-fit curve were found to correspond to approximately 3-4-mm-deep crack initiation lives in large-scale specimens. The result implies that only several-millimeter crack initiation defines a large structure. It would be reasonable to consider the crack growth behavior after reaching the design fatigue life. During the fatigue tests, the crack growth behavior was monitored with a plastic replica and a beach mark method. Since the main crack grew with the coalescing of micro cracks, the applicability of fracture mechanics method is uncertain. In this study, a crack growth analysis was performed on the cracks observed in large-scale specimens using a crack growth law specified in a fitness-for-service (FFS) code or literature. With respect to the surface crack length, some experimental data surpassed the analysis results, which might be attributed to crack coalescence. The crack depth data that are not easily affected by the coalescence was plotted within the upper limit of data scatter. The analysis results enabled the prediction of the total fatigue life of the large-scale specimens as the sum of the crack initiation life by the best-fit curve and the growth life from a 3-mm-deep crack.