Low Cycle Fatigue of Martensitic AISI 4140 Steel at Low Temperatures
Synopsis
The effects of low temperature and strain range on cyclic deformation behaviour of martensitic AISI 4140 steel was investigated. In total strain controlled low cycle fatigue (LCF) tests, the push-pull type specimens were tested at total strain ranges between 0.015 and 0.060, and at the temperatures between 373 K and 77 K. Crack initiation and growth tests have been made separately in order to evaluate the LCF test results. It was observed that the metal exhibits cyclic softening regardless of temperature and highest fatigue life was obtained at 243 K for all plastic strain ranges. Coffin-Manson relationship was conserved at low temperatures also, and the constant C of the relationship increases with decreasing temperature, reaches a maximum value at 243 K, then decreases considerably while the exponent a remains unchanged at an average value of 0.68. It was also noticed that the crack initiation was delayed by depressed temperature. On the other hand, lowering the temperature increases the crack growth rate of the metal tolerably. The remarkable change in C values can be attributed to the combined effects of crack initiation and growth behaviours, producing an optimum fatigue life at 243 K.
Keywords
Crack Growth Rate Plastic Strain Range Total Strain Range Critical Crack Length Depressed TemperaturePreview
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