A Numerical Study on Combination of Crack Stop Hole and Carbon Fiber Reinforced Polymer (CFRP) Laminates as a Repair Technique

Vutkuru, J (2014) A Numerical Study on Combination of Crack Stop Hole and Carbon Fiber Reinforced Polymer (CFRP) Laminates as a Repair Technique. Masters thesis, Indian Institute of Technology, Hyderabad.

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Abstract

Drilling a hole ahead of crack tip is one of the most common techniques, to prevent further crack propagation in structures subjected to fatigue load. Fatigue cracks are typically occur at locations where drilling a crack stop hole of desired dimensions is not possible due to geometrical constraints. In such cases, crack may reinitiate from the hole after few loading cycles. Techniques such as Static mechanical cold working are used to strengthen the undersized crack stop hole mentioned in the literature. In this study, application of Carbon Fiber Reinforced Polymer (CFRP) patches laminated over cracks is proposed to delay the crack initiation from under sized crack stop hole. Although extensive literature exists for crack stop hole and CFRP patches independently to prevent the crack growth, few literatures are available for combined effect. In the present study combination of crack stop hole and CFRP overlays as a repair technique is studied. This combination helps to reduce the stress at edge of the hole, which delays the crack initiation. The combined effect of crack stop hole and CFRP patches is studied numerically using ANSYS. Steel plate subjected to tensile loading (mode-I) with initial central crack is considered. Material behaviour is assumed to be elastic. A hole is modelled ahead of crack tip and CFRP overlays are applied over it. Holes of various radii are considered to understand the effect of hole radii on stress at the edge of the hole. The entire setup is modelled in ANSYS. Ratio of stress intensity factor to square root of radius is KI / considered as a parameter for all cases.By the application of CFRP patches, the KI / is reduced by 70% at a stiffness ratio of 0.16. It is also observed that at a stiffness ratio 0.95, the KI / value at the edge of the hole is reduced by 90%. A reduction parameter is proposed for KI / as a function of stiffness ratio. In this study, it is observed that the parameter KI / used for crack initiation, which is based on a constant stress gradient 0.5.This leads to smaller radius crackstop hole and hence is un- conservative. Therefore, another parameter called notch stress intensity factor (N-SIF) which takes into account the effect of stress gradient (á) is considered in determining the appropriate hole radius to halt crack propagation. This approach conservative compared to KI / approach.

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