Accumulation and evolution of elastically induced defects under cyclic loading: Quantification and subsequent properties

This article quantifies the evolution of bulk material defects in α-iron during cyclic elastic loading and investigates its subsequent effect on quasi-static and dynamic mechanical response. Fatigue test specimens were cycled at a constant stress amplitude, 70% of the yield stress, to cause microscopic defects throughout the material due to a macroscopic elastic load. Afterwards, wire-cut electrical discharge machining was used to obtain multiple sub-tensile specimens from within the gauge section of the fatigued material. X-ray computed tomography of the sub-tensile specimens was used to quantify distribution and location of voids that were induced at different stages of fatigue life. Then, tension tests were performed on the sub-tensile specimens at low (10⁻³ s⁻¹) and high (10³ s⁻¹) strain rates to study changes in mechanical properties. Yield stress and ultimate tensile stress were found to change under both quasi-static and dynamic strain rates based on the amount of prior cyclic loading. Changes in mechanical behavior were correlated to the characteristics of observed defects that exhibited distinct grouping at a certain point during the fatigue life. Mechanical properties of the sub-tensile specimens after fatigue loading displayed different trends depending on the strain rate. Low strain rate deformation resulted in decreases of approximately 20% in the yield stress, ultimate tensile stress, and ductility due to increased fatigue loading. Conversely, during high strain rate deformation some samples with more fatigue loading displayed higher maximum flow stresses. Results demonstrate the significance of bulk material changes which occur at mechanical loads below the yield stress. The systematic experimental procedures developed here for investigating the accumulation, evolution, and effect on subsequent properties due to elastically induced microscopic defects could potentially be applied to a wide range of material systems.