A test piece configured for the implementation of a method for evaluating an assembly by welding of parts made of thermoplastic materials, wherein the test piece includes at least two parts based on thermoplastic materials having surfaces to be welded that are at least partially placed opposite one another and respective free surfaces, wherein at least one of the parts is a perforated reference part including at least one perforation.

There is provided a technique to strongly integrate a galvanized steel sheet and a resin molded article. A hot-dip galvanized steel sheet is immersed in an aqueous solution for aluminum degreasing to form a specific roughness on the surface. The surface is covered with convex protrusions having a diameter of about 100 nm, and a chromate treatment layer appears in the surface. A resin composition comprising 70 to 97 wt % of polyphenylene sulfide and 3 to 30 wt % of a polyolefin resin is injected onto the surface. The resin composition penetrates into ultra-fine irregularities and is cured in that state, and thereby a composite in which the galvanized steel sheet and the resin molded article are strongly integrated can be obtained. The shear rupture strength of the composite is extremely high.