Design considerations in the strengthening of composite lap joints using metal z-pins

Through-thickness reinforcements are an effective technique to improve the structural properties of bonded fibre-polymer composite lap joints. However, there is a lack of understanding of the effectiveness of this technique for internal step-lap joints. This study examines the strengthening effects of z-pins on different types of composite step-lap joints. Co-cured single, quadruple and multi-step lap joints made of carbon-epoxy laminate were reinforced with steel z-pins. The results reveal that z-pins were highly effective at increasing the ultimate strength (+20%) and elongation limit (+250%) of the single lap joint under tensile loading by suppressing catastrophic failure through plastic shear deformation following fracture of the co-bonded interface. However, the z-pins were less effective at improving the strength (+7.5%) and maximum strain (+18%) for the quadruple lap joint and did not provide any strengthening for the multi-step lap joint. The mutually exclusive relationship between the number of bonded interfaces and the z-pin strengthening efficacy is discussed.