Evaluation of Fatigue Properties and Effect of Stress Concentration on Fatigue Life of Dual Phase Steel Grade DP 800 2011-26-0120

Dual Phase (DP) steels with their high energy absorbing capacity are fast emerging as materials for automotive body applications with improved crashworthiness. The unique combination of high strength and good ductility associated with the DP steels originates from its specially developed microstructure that consists of martensitic islands in ferritic matrix. The high strength and good ductility are expected to give very good resistance to fatigue crack generation and propagation respectively. This paper discusses the fatigue properties determined for a DP steel grade viz. DP 800. The strain controlled fatigue tests were carried out on the un-notched specimens prepared from 1.2 mm sheets to generate E-N curves. The force controlled axial fatigue tests were carried out on two types of specimens prepared from 1.2 mm sheets to generate S-N curves for two Stress Concentration Factors (SCF) viz. 2.5 and 4.4. A special anti-buckle fixture was developed in-house in order to avoid bending of the sheet metal specimens during the axial fatigue tests in tension-compression mode. The results indicate that the DP 800 steel is sensitive to the presence of notch as the fatigue ratio (ratio of endurance limit to the static UTS) decreased with increasing SCF. However, for the DP 800 steel tested, the fatigue stress concentration ratio (ratio of endurance limit of smooth specimen to the endurance limit of notched specimen at 106 cycles to failure) was found to be less than the corresponding SCF.