Predicting failure of cracked aluminum plates with one-sided composite patch

Composite patch repairs are an alternative to weld repair methods to arrest crack growth in metal plates. While these repairs have been effective, it has been difficult to use basic linear elastic fracture mechanics (LEFM) theories and design methods based on LEFM to predict the efficacy of patch repairs because the repaired plate exhibits bending and significant crack tip plasticity. In this investigation, we have studied the effect that one-sided low modulus composite patch repairs have on the development of crack tip plasticity on the free surface and through the thickness using large strain area and volume, total axial stress balance, and the free surface J-integral. Digital Image Correlation (DIC) and three-dimensional finite element analysis with first order elements were used to understand the evolution of crack tip plasticity and strain energy release rate to develop a simplified prediction method identifying a characteristic crack opening displacement prior to crack tip behavior of a one-sided composite patch repaired Center-Cracked Tension (CCT) specimen transitioning from Small Scale Yielding (SSY) to Large Scale Yielding (LSY), and eventually the re-initiation of crack growth. This method was then validated by comparing the predicted characteristic transition load to observed ultimate patched CCT specimen load capacity. Results correlated well with predictions when the composite patch was perfectly bonded to the aluminum and predicted characteristic transition loads more than 90 $$\%$$ greater than LEFM based failure predictions and 25 $$\%$$ lower than observed ultimate failure loads, demonstrating the potential to use this method for patch repair optimization.