Experimental validation of theoretical traction law for inclined through-thickness reinforcement

In this study, a revised model for determining the traction law of a stitch, pin or other through-thickness reinforcement in a composite laminate has been proposed and validated experimentally. The model has a number of physical and geometric input parameters, which need to be measured for an accurate validation. Parameters used in the model include the stitch geometry, stitch modulus, laminate punch strength, interfacial shear strength and friction coefficient. These parameters, along with the traction law relating to the case of vertical pull-out of inclined stitches, have been measured for Kevlar threads embedded in cold cured resin. The validation was conducted for two thread diameters and three incline angles. The model for inclined reinforcement was validated with knowledge of the measured individual parameters. The validation yielded differences between 5% and 29%. The accuracy of the model will be affected by the material system, manufacturing method and loading case. For the experiments considered, the model is most accurate for small diameter stitches. A combination of bending effects and lateral fibre compression reduces the accuracy as the diameter is increased. In addition, the thread material and its construction method were found to have a significant effect on the physical properties. Thread twist can increase the interfacial shear strength but will decrease the breaking load and modulus, all of which affect the fracture toughness.