Failure analysis of zinc-plated self-drilling screws used to assemble metallic grille panels on a building façade

Zinc-plated AISI C1022 steel self-drilling screws, used to fasten steel frames containing grille panels to I-shaped beams on a building facade, fractured after a just few months in service. Metallographic techniques and scanning electron microscopy were used to analyse the microstructure and the fracture surface of the screws. Tensile, torsion, shear and hardness tests were performed to verify compliance with the manufacturer’s specifications. The stress on the screws in the considered service conditions was estimated. The mechanical properties of the screws were found to be in agreement with the manufacturer’s specifications. Fractographic analysis of the fracture surfaces revealed no fatigue features, but indicated intergranular fracture on the periphery and ductile features (microvoid coalescence) with some patches of transgranular cleavage in the core. Intergranular fracture was found to be hydrogen embrittlement (HE) induced. However, HE was not the only cause of screw failure, which was also partly due to in-service overloading.