Notch stress intensity factor for center cracked plates with crack stop hole strengthened using CFRP: A numerical study

Abstract

It is well known that drilling a hole ahead of crack tip is one of the most common techniques to prevent crack propagation in structures subjected to fatigue load. An adequate size crack stop hole is necessary to convert a sharp crack into a blunt notch thereby preventing crack propagation. However, fatigue cracks typically occur at locations where drilling a crack-stop hole of required dimensions may not be possible due to geometrical constraints. In such situations, the crack may initiate again from the hole within its service life. Hence, there is a need to strengthen undersized crack-stop holes. In the present study, a combination of crack-stop hole and carbon fiber reinforced polymer (CFRP) overlays under static loads are studied numerically using finite element analysis (FEA) to evaluate its potential as a viable repair technique. A steel plate with an initial central crack subjected to static tensile loading (mode-I) is considered. A hole is modelled ahead of a crack tip and CFRP patches are applied on either side of the crack. The numerical analysis is performed using general purpose FEA ANSYS. A total of 1008 models, i.e 112 unstrengthened and 896 strengthened specimens are used to evaluate the stress intensity factor at notch tip. The material behaviour is assumed to be elastic in case of linear analysis and as a multi linear isotropic hardening material type for nonlinear analysis. The results from linear analysis are used to compare Crack Stress Intensity Factor (CSIF,KI/ρ) and Notch Stress Intensity Factor (NSIF, KI/ρα). The results indicate the need to include the stress gradient α in arriving at adequate crack stop hole radius for both bare steel and CFRP patched specimens. Nonlinear FEA, which takes into account the post yield material behaviour, is carried out to propose a modified NSIF expression by including a Reduction Factor (RF) that is a function of the ratio of the radius of crack stop hole to crack length (ρ/2a), the ratio of stiffness of CFRP to steel (SR) and the ratio of applied stress to yield stress (σapplied/σys). Numerical examples are provided to demonstrate the applicability of the proposed equation.