Abstract
This paper discusses how the strain gradient influences the fatigue life of carbon steel in the low-cycle and high-cycle fatigue regimes. To obtain fatigue data under different strain distributions, cyclic alternating bending tests using specimens with different thicknesses and cyclic tension–compression tests were conducted on carbon steel for pressure vessels (SPV235). The crack initiation life and total failure life were evaluated via the strain-based approach. The experimental results showed that the crack initiation life became short with decreasing strain gradient from 102 to 106 cycles in fatigue life. On the other hand, the influence of the strain gradient on the total failure life was different from that on the crack initiation life: although the total failure life of the specimen subjected to cyclic tension–compression was also the shortest, the strain gradient did not affect the total failure life of the specimen subjected to cyclic bending from 102 to 106 cycles in fatigue life. This was because the crack propagation life became longer in a thicker specimen. Hence, these experimental results implied that the fatigue crack initiation life could be characterized by not only strain but also the strain gradient in the low-cycle and high-cycle fatigue regimes.
Highlights
Fatigue failures in engineering components are a serious problem in machines and structures
This study investigated the influence of the strain gradient on the fatigue life of the steel used for pressure vessels
Alternating bending fatigue tests were carried out using specimens with different thicknesses, and cyclic tension– compression (T/C) tests were done
Summary
Fatigue failures in engineering components are a serious problem in machines and structures. To increase the structural integrity of components, many studies have investigated fatigue mechanisms, fatigue life, and life prediction techniques for structural materials. Stress–strain hysteresis at a notch root that is plastically deformed due to an applied load is analyzed, and the crack initiation life is estimated based on the LCF data of a material obtained on smooth (unnotched) specimens under cyclic uniaxial loading, such as tension– compression (T/C). A fracture mechanics parameter, such as stress intensity factor K or J-integral for a crack propagating from the notch root, is analyzed, and the crack propagation life is estimated based on the crack propagation properties of the material (da/dN-∆K or da/dN-∆J relations). A few studies related to LCF life evaluation considering a strain gradient have been recently reported [2,3], the understanding of the influence of strain
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