Effect of Specimen Thickness and Stress Intensity Factor Range on Plasticity-Induced Fatigue Crack Closure in A7075-T6 Alloy

Kenichi Masuda; Sotomi Ishihara; Noriyasu Oguma

doi:10.3390/ma14030664

Effect of Specimen Thickness and Stress Intensity Factor Range on Plasticity-Induced Fatigue Crack Closure in A7075-T6 Alloy

Materials (Basel). 2021 Jan 31;14(3):664. doi: 10.3390/ma14030664.

Authors

Kenichi Masuda¹, Sotomi Ishihara^{1

2}, Noriyasu Oguma¹

Affiliations

¹ Department of Mechanical Engineering, University of Toyama, Gofuku 3190, Toyama 930-8555, Japan.
² National Institute of Technology, Toyama College, Toyama 939-8630, Japan.

Abstract

Fatigue crack growth experiments are performed using A7075-T6 compact tension (CT) specimens with various thicknesses t (1-21 mm). The stress intensity factor at the crack opening level K_op is measured, and the effects of t and the stress intensity factor range ΔK on K_op are investigated. In addition, the change in K_op value due to specimen surface removal is investigated. Furthermore, we clarify that the radius of curvature of the leading edge of the fatigue crack decreases as t becomes thinner. Using the three-dimensional elastoplastic finite element method, the amount of plastic lateral contraction (depression depth d) at the crack tip after fatigue loading is calculated quantitatively. The following main experimental results are obtained: In the region where ΔK is 5 MPam^1/2 or higher, the rate of fatigue crack growth da/dN at a constant ΔK value increases as t increases from 1 to 11 mm. The da/dN between t = 11 and 21 mm is the same. Meanwhile, in the region where ΔK is less than 5 MPam^1/2, the effect of t on da/dN is not observed. The effects of t and ΔK on the da/dN-ΔK relationship are considered physically and quantitatively based on d.

Keywords: 3D elastoplastic finite element method; CT specimen; aluminum alloy; fatigue crack growth behavior; plane stress and plane strain; plastic lateral contraction at the fatigue crack tip; plasticity-induced fatigue crack closure; specimen thickness.