A comparative experimental study for determination of residual stress in laminated composites using ring core, incremental hole drilling, and slitting methods

Selecting suitable methods for determining residual stress in laminated composites is of great importance because each method shows different results due to its nature. In this study, for the first time, three methods of hole-drilling, ring core, and slitting were used to estimate residual stress and their results were compared to each other for layered composite materials. The material used in this study is a Glass/epoxy composite with one-direction E-GLASS 92145 fibers and EPON 828 resin along with EPI-CURE 3234 (TETA) hardener. Samples were created in several symmetric cross-ply composite layers and one-direction fibers using hand layup method. Vacuum bagging technique was used in order to level-out the samples and remove extra resin. Samples were then placed at room temperature for one day and were post-cured for 4 h at 150 °C. In slitting method, strip loads method and in ring core and hole-drilling techniques, incremental hole drilling method was used for calculations. The results of residual stress calculations showed that ring core method can at least release 17% more stress compared to drilling method. The results of experimental tests showed that with increase in depth, the strain increases in all three methods and comparison between the three methods showed that slitting, ring core and drilling methods have highest to least amount of strain. This strain in slitting method is 15% higher than ring core method, which in turn has 40% more strain compared to drilling method. In ring core method, increase or decrease in residual strain follows an almost constant trend through the entire length, which shows that this method is more stable compared to other methods.