Investigation of small crack behaviour under cyclic loading in a dual phase steel with an FIB tomography technique

Abstract

A B S T R A C T Damage accumulation is investigated in the early stage of fatigue life in a ferritic martensitic dual phase steel. Microcrack initiation and propagation are influenced by microstructure, such as grain boundaries, grain orientation and/or phase distribution morphology.The dominant crack initiation pattern is one in which microcracks are generated in a ferrite grain along slip bands inclined at a certain angle with respect to the loading direction. Subsurface observation with a focused ion beam (FIB) device and additional crystallographic characterization by means of the electron backscatter diffraction (EBSD) technique show that a slip system having a high Schmid factor value is activated and results in a crack nucleus. The FIB tomography technique with the help of EBSD measurement allows a three-dimensional investigation of small crack behaviour to be performed.

Grain Boundary Deformation at High Temperature Tensile Tests in ODS Ferritic Steel

“…The driving force for the grain-subdivision could be accumulation of the deformed strain at near grain boundaries. Similar observation by FIB was reported by Motoyashiki et al,14) showing that cracks are formed along slip bands in a dual phase steel. These evidences clearly demonstrate an occurrence of the grain boundary sliding.…”

Inclined defects and their effect on the fatigue limit and small crack growth

“…[2][3][4][5][6] For these tests we mill sharp notches in varied angles and sizes leading to identical projected square root areas as shown in Fig. 1.…”

High-resolution Observation of Steel Using X-ray Tomography Technique

“…In fact, the underlying phases have been identified, as shown in Fig. 5(b), as the ductile ferrite phase and the less ductile martensite phase, 20) which can be associated with the significant mode III tilting shown in Fig. 5(c).…”