Low‐cycle Fatigue Investigations and Numerical Simulations on Dual Phase Steel With Different Microstructures

Abstract

Fully reversed low cycle fatigue tests were carried out on 7 mm diameter cylindrical specimens of a dual phase steel treated to give five different microstructures, namely F-ferrite ( + little carbide particles), CF-ontinuous ferrite + 8.2% martensite, FM-49.1 YO ferrite + 50.9% martensite mixed structure, CM-continuous martensite + 22.4% ferrite and M-I00% martensite. A finite element program was developed based on Eisenberg's cyclic plasticity theory and the low cycle stress--strain response of the steels with duplex phase microstructures was calculated from the low cyclic curves of the single ferrite (steel F) and martensite (steel M) phase. The experimental results show that fatigue performance of dual phase steel improves as martensite content is increased up to about 50%, thereafter it obviously deteriorates. During cyclic loading, the calculated plastic strain accumulated in ferrite is higher than that in martensite. Inhomogeneity of the plastic strain accumulation in steel C F is more pronounced than that in steel CM. In steel FM a relatively uniform strain distribution was found. Controlling the size of particle phase can result in an optimum strain distribution. Thus, the plastic deformation capability of the constituent phases can be enhanced leading to fatigue performance improvement. Crack initiation occurs easily at the ferrite/martensite interface with a coarse particle phase size, regardless of phase continuity. In steel CF, a crack initiates at the interface perpendicular to the stress axis and propagates in the ferrite matrix by deflecting around coarse martensite particles or by cutting fine martensite particles. In steel CM, a crack initiates at the interface along the stress axis and propagates in the martensite matrix through ferrite particles.

The Development of Stresses and Their Distribution in Two‐phase Alloys During Cyclic Loading

“…The low cycle fatigue behaviour of two-phase alloys has been extensively studied [l-71. When studying two-phase alloys, it is necessary to rationalize the inhomogeneity of the stresses and strains produced by the misfit strain between the component phases [4][5][6]. Since it may not be experimentally possible to give a quantitative description of this inhomogeneity, it is therefore attractive to develop a numerical method to visualize this problem.…”

“…Beatty and Shiflet [6] employed the strain distribution in a ferrite-martensite microstructure (obtained from monotonic finite element method calculations) to explain the low cycle fatigue behaviour of F d -M o dual-phase steels. A direct numerical simulation was made by the present authors [5], in which a finite element program was developed based on Eisenberg's cyclic plasticity theory. The stress-strain response of the dual phase steel was calculated from the cyclic stress-strain curves of the constituent phases, while the plastic strain accumulated in the in situ phases was calculated during cyclic loading, which provide an explicit physical picture of the microfracture mechanisms.…”