Microstructure–Mechanical Response Relationship in Discontinuous Fiber‐Reinforced SMC Composites: Damage Mechanisms

The mechanical performance of sheet molding compounds (SMCs) is dependent on microstructural characteristics determined by material composition and processing conditions. The study investigates standard SMCs, advanced SMCs (A‐SMCs), and low‐density SMCs (LD‐SMCs) under quasi‐static and fatigue loading to elucidate the influence of microstructure on damage evolution. Damage evolution is characterized from the microscale to the macroscale using a range of analytical techniques, including microscopy, anisotropy measurements, and interrupted mechanical testing. Key damage mechanisms—including fiber‐matrix interface debonding, matrix cracking, fiber fracture, and pseudo‐delamination—are systematically identified, with particular emphasis on the effects of hollow glass microspheres and fiber orientation. The results reveal distinct fatigue and quasi‐static (QS) responses among the SMC variants: Compared to the standard SMC, the A‐SMC exhibits ~258% higher tensile strength and delayed fiber‐matrix debonding, while the LD‐SMC shows moderate quasi‐static strength (~49% higher than the standard SMC). Correlating microstructural damage features with macroscopic mechanical responses provides critical insights for multi‐scale modeling and optimized design of SMC components under realistic service conditions.