Determination of effective stress intensity factors under mixed‐mode from digital image correlation fields in presence of contact stresses and plasticity

Abstract

Digital Image Correlation (DIC) is more and more popular to monitor fatigue crack growth and to determine the stress intensity factors. However, the post-treatment of the recorded displacement fields becomes tricky when the crack faces are not stress-free and when crack tip plasticity becomes significant. Several post-treatment methods to locate the crack tip and measure the effective SIFs in such cases are compared, using FEM-computed displacement fields, and then used on real DIC fields. An approach coupling DIC and FEM is proposed to estimate the contact stresses along the crack.

Fatigue crack growth under non-proportional mixed-mode I + II. Role of compression while shearing

“…A 4 hour annealing treatment at 450°C in a vacuum was applied to relax, as far as possible, the residual stresses left by rail manufacturing and subsequent machining. A method based on the projection of the measured displacement field over William's series expansion of the near tip displacement field at order 7 was used to locate both crack tips, using image pairs captured at peak tensile load and after a small elastic unloading (see Appendix B or [21]). Iterations were performed over the crack tip position to minimize the error between theoretical and measured fields.…”

“…The derivation of the effective SIFs in such cases is a problem in itself, which has been discussed in detail in [21]. The method which was specifically developed is just briefly explained below.…”

“…To determine the effective SIFs, the profiles of relative opening and sliding displacement jump amplitudes between two rows of virtual extensometers parallel to the crack, and located at a distance δ = 0. validated/calibrated against FE-generated displacement fields (for which the effective SIFs were known), and compared favorably with other approaches, can be found in [21].…”

Fatigue crack growth in bearing steel under cyclic mode II + static biaxial compression

“…This method has later been improved by Bertolino & Doquet [33] to separate, through numerical simulations, the opposite effects of crack face friction and crack tip plasticity on the measured sliding displacements, and get a more accurate estimate of ΔK , . More recently, Bonniot et al [34] improved the method further, to overcome the limitations associated with the assumption of stress-free crack faces in the traditional post-treatment of Digital Image Correlation (DIC) data to determine ΔK , . A normal compressive stress reduces the effective ΔK because of an enhanced friction between the crack lips [14,23], and thus reduces the mode II crack growth rate.…”

Investigation by Digital Image Correlation of Mixed Mode I and II Fracture Behavior of Metallic IASCB Specimens with Additive Manufactured Crack-Like Notch

“…In general, two main approaches emerges in literature concerning the fitting of theoretical models on the full displacement fields: the first one is based on guessing a general form of an analytical function and fitting this to the displacement experimental data: an example is the complex function analysis of Muskhelishvili [16], which was further developed to calculate mixed mode (I + II) [17] and generalized to cases without restrictions in boundary conditions or symmetry [18]. The second approach is based on the Williams' model [19][20][21][22], where the fitting on the experimental data is done by considering the first n term of the Williams expansion for the displacement. In this work this latter approach was used.…”