A lap shear test fixture and method of use to test the performance of the weld interface in welded thermoplastic composite parts such as infrared welded or resistive implant welded or adhesive bonded composite parts. The specimens to be tested are provided from production-ready parts with typical geometry. The test fixture has at least two aligned blocks with at least two lateral support blocks that operably hold the specimen including two welded substrates. The test is performed in compression-mode at a predetermined crosshead speed until the maximum load produced by a universal testing machine makes the weld interface between the two thermoplastic substrates fail in shear mode.