Failure mode for SS400 notched specimen under repeated impact loading

Abstract

We conducted impact fatigue tests, wherein the magnitude and duration times of impact stresses were varied for JIS SS400 notched specimens to evaluate the effect of strain amplitudes on the impact fatigue strength. The transition of fracture mode from ductile to crack propagation both in impact fatigue and in standard fatigue was experimentally investigated in a range of cyclic loadings up to about 50,000 times using two types of fatigue testing machines. One machine was used for electro-hydraulic servo loading, and the other was for drop-weight cyclic impact loading. To obtain the variation in true strain of the specimen during the impact fatigue tests, the minimum diameter of the specimen was measured using a two-dimensional displacement measurement system. The main results were summarized as follows. A comparison of the nominal stress and the number of cycles to failure between the smooth specimen and the notched specimen showed that the impact fatigue strength of the notched specimens was lower than that of the smooth specimens when the number of cycles to failure was around 8,000 times or more. A comparison of the strain growth and the number of cycles to failure showed that the U-notched R=2.0 specimens fractured with a smaller amount of strain growth in the impact fatigue tests than in standard fatigue tests. Impact fatigue test results indicated the following equation could represent the relationship between the strain growth Δεp and the number of cycles to failure Nf. ΔεpNkf = C, where the exponent k and C are material constants.

Keywords

• impact fatigue tests
• strain growth
• ductile fracture
• crack propagation fracture