The adhesives used are of the epoxy-type, consisting of a number of ingredients like resin, hardener, accelerator, metallic fillers, thixotropic agents, antioxidants, etc. Prior to adhesive bonding, a protective film (primer) is applied on the surface of the substrate. Before use, the freshness of the adhesives is checked by means of chemical and mechanical tests. Quality control of the faying surfaces is performed. Several non-destructive testing methods are available: ultrasonic or acoustic resonance testing, radiography, holographic interferometry