FATIGUE BEHAVIOUR OF A LOW-CARBON STEEL AND PATERNS OF FRACTURED SURFACES

Fatigue tests are carried out using a rolled low-carbon, low-alloyed 09Mn2-steel for offshore applications with microstructure revealing a sequence of long and uniform perlite and ferrite bands. Investigations are based on short fatigue crack growth and its monitoring by replication of surface crack propagation from initiation to failure. The investigations include short fatigue-crack rotation-bending experiments and length measurements of propagating crack on smooth specimens divided into two groups of hour-glass specimens original and with modified surface obtained at special conditions of electrical-discharge treatment in electrolyte, followed by high-speed cooling. Tests were carried out on a table model Fatigue Rotating Bending Machine, FATROBEM-2004, designed and assembled in Fracture and Fatigue Laboratory in UCTM Sofia. The microstructure and the fractured surfaces were observed under scanning electron and 3D microscopes. The results of investigation are presented in microstructure photos and plots of fatigue data: "Crack Length, a Cycles, N"; "Crack growth rate, da/dN - Crack length, a.