Experimental determination of the structural properties and strengthening mechanisms of z-pinned composite T-joints

This paper presents an experimental investigation into the efficacy of z-pins to improve the structural properties of stiffened joints made of carbon/epoxy composite. Pull-off tests were performed on T-joints without z-pins or reinforced along the skin–stiffener bond-line with z-pins to volume contents of 0.5%, 2% or 4%. Testing was performed at different pull-off load angles between 0° and 45° to the stiffener to induce different proportions of normal (through-thickness) tensile and in-plane secondary bending stresses along the skin–stiffener bond-line. It was found that z-pins do not improve the stiffness or failure initiation load of T-joints, but they are effective at raising the ultimate failure strength, failure displacement, and absorbed energy capacity. These properties increase rapidly with the z-pin content, and maximum improvements of about 75% to the ultimate strength and over 600% to the total absorbed energy capacity were achieved at the highest pin content (4% by volume). The percent improvements to the structural properties are approximately the same for the different load angles, revealing that z-pins are equally effective at resisting bond-line cracking under normal tensile or secondary bending stresses. Fractographic analysis revealed that z-pins increase the joint properties by creating bridging tractions across the bond-line crack between the stiffener and skin. The z-pins ultimately fail by a combination of debonding/pull-out from the adherends.