A composite structure fabricated by thermoplastic welding, and a method for producing such structures. First and second composite parts are provided, having first and second surfaces, respectively, that define an interface where the parts are to be joined. The parts include a thermoplastic matrix. An insert is provided, which includes a film portion, which may include the same thermoplastic matrix. The insert is provided with an electric resistance heating element. The first and second parts are urged together using low fusion pressure while the insert is heated using the electric resistance heating element. After a selected period of time, the electrical supply to the resistance heating element is turned off. A high fusion pressure is then applied to urge the parts together. The weldment joint is allowed to cool. After cooling, excess material may be removed, to form a composite structure.