Characterisation of fatigue crack growth using digital image correlation measurements of plastic CTOD

Abstract

In this work an experimental study of the crack tip opening displacement (CTOD) is performed to evaluate the ability of this parameter to characterise fatigue crack growth. A methodology is developed to measure and to analyse the CTOD from experimental data. The vertical displacements obtained by implementing digital image correlation (DIC) on growing fatigue cracks are used to measure the CTOD. Fatigue tests at stress ratios of 0.1 and 0.6 were conducted on compact tension (CT) specimens manufactured from commercially pure titanium. A sensitivity analysis was performed to explore the effect of the position selected behind the crack tip for the CTOD measurement. The analysis of a full loading cycle allowed identifying the elastic (CTODel) and plastic (CTODp) components of the CTOD. The plastic CTOD was found to be directly related to the nonlinear zone (i.e., plastic deformation) generated at the crack tip during fatigue propagation. Moreover, a linear relationship between da/dN and ΔCTODp independent of the stress ratio was found. Results show that the CTOD can be used as a viable alternative to the stress intensity factor range (ΔK) in characterising fatigue crack propagation since the parameter considers the fatigue threshold and crack shielding in an intrinsic way.

Numerical tool for the analysis of CTOD curves obtained by DIC or FEM

“…For the experimental analysis of the CTOD, commercially pure titanium CT specimens (Figure 6A) with a thickness of 1 mm were tested at constant amplitude fatigue loading 29 . Fatigue tests were conducted at a stress ratio of 0.1, applying 750 N as maximum load level.…”

“…For the experimental analysis of the CTOD, commercially pure titanium CT specimens ( Figure 6A) with a thickness of 1 mm were tested at constant amplitude fatigue loading. 29 Fatigue tests were conducted at a stress ratio of 0.1, applying 750 N as maximum load level. The surface of the CT specimen was sprayed with a random black speckle pattern over a white background to enable the measurement of the displacement field by DIC.…”

“…A sensitivity analysis was performed to explore the effect of the position of the two points behind the crack tip, and it showed that the CTOD is accurately characterized by using data corresponding with horizontal and vertical distances behind the crack tip of 150 μm. 29…”

“…A sensitivity analysis showed that the variation and this empirical limit has a minimum impact on the elastic and plastic CTOD ranges. Vasco‐Olmo et al 29 proposed an alternative procedure, based on the Appendix X2 of the ASTM E647 standard, 30 that deals with the determination of opening load from compliance. Segments of three data points are used to determine a slope, which is compared with the elastic slope.…”

A Numerical Study of the Effect of Isotropic Hardening Parameters on Mode I Fatigue Crack Growth

“…The parameters studied were isotropic saturation rate, C Y , and isotropic saturation stress, Y sat . A numerical analysis based on the finite element method was followed to calculate the plastic crack tip opening displacement (CTOD), δ P , which was found to be linearly related to FCG rate in numerical [19,20] and experimental [21] studies. This approach has already been used in previous works to predict the effect of a material's yield stress, Y 0 [22], and Young's modulus [23].…”