Estimation of Fatigue Limit and Strength Degradation Mechanism of a High Strength Steel in Petrol Environment by Means of a Small Sized Specimen with Radial Notch

Abstract

Strength of structural materials used in corrosive environment will be lowered compared with that of materials used in air due to the effect of corrosion fatigue. In the previous study, fatigue tests on a type 440C stainless steel, which has been used as a part of automobile fuel injection system, in air and petrol environment were conducted by means of a small sized smooth specimen and a fatigue testing machine for the specimen performable in corrosion and flammable liquid under push-pull loading at high frequency region. However, it was difficult to investigate the effect of corrosion due to the difference of fracture morphology. In this study, fatigue tests on the same material were conducted by means of a small sized specimen with notch for matched fracture origin in air and deteriorated petrol under the same test condition. The fracture on fatigue tests with notched specimens initiated from specimen surface regardless of the test environment. In petrol that contains methanol, fatigue fracture was caused on lower stress level because minor intergranular cracks were originated by the effect of corrosion. Fatigue lives of flat specimen were correlated using the size of the inclusions and they were correlated with high accuracy. Fatigue limit was also estimated. It was confirmed that fracture origin was changed to the surface due to immersion to petrol that contains methanol. The relationship with the stress state in notch part and fatigue failure mechanism was discussed in detail.