The Balanced Bending Stiffness Method for Characterizing Interfacial Properties of Overmolded Composites

This study introduces the Balanced Bending Stiffness (BBS) method, a novel experimental approach to measure the intrinsic Mode-I interfacial fracture toughness (GIC) in overmolded hybrid composites. Traditional testing methods for these asymmetric systems are complicated by inherent stiffness mismatches that couple opening and shearing failure modes, requiring complex post-analytical corrections. The BBS method addresses this challenge by engineering physically balanced Asymmetric Double Cantilever Beam (ADCB) specimens through comparative stiffness matching, isolating pure mode-I failure conditions and enabling direct toughness measurement. The method is validated using a glass fiber–reinforced polypropylene (GF/PP) system, with parametric studies investigating the effects of fiber content and processing temperatures on interfacial toughness for short-fiber (SFT) and long-fiber (LFT) thermoplastics. Results reveal that higher fiber content and substrate preheating significantly enhance toughness, with particularly strong results for LFTs due to fiber bridging. This work provides a framework for material characterization and insights into optimizing overmolded composite interfaces.