The effect of aging on the mechanical properties of bulk molding compound with different fiber lengths

Bulk molding compound (BMC) manufactured using fiber reinforced polymers (FRPs) has attracted extensive attention and is widely used because of its capability to fabricate structures with complex shapes. This study investigated the effects of aging on the mechanical properties of BMC composites made using an epoxy matrix and discontinuous carbon fibers of varying lengths. Tensile, compressive, and flexural tests were conducted. The results showed that longer fibers did not necessarily increase the moduli and strengths of BMC composites due to stress concentrations resulting from the curling and entanglement of the longer fibers. When aged, BMC composites using shorter carbon fibers exhibited more significant reductions in moduli and strengths due to higher void contents, resulting in more severe matrix epoxy degradation. When comparing epoxy and BMC, BMC experienced greater reductions in moduli and strengths even though carbon fiber, a type of artificial fiber, should be less affected by aging. This is because aging not only degraded the matrix epoxy but also affected the adhesion between the fibers and the matrix, leading to a larger displacement of the fibers, resulting in a more severe reduction in the mechanical properties of the BMC composites.Highlights BMC composites were manufactured using epoxy and discontinuous carbon fibers. Longer fibers did not necessarily increase strengths due to entanglement. BMC using shorter fibers had more voids and hence degraded more severely. BMC showed greater strength reductions than epoxy due to matrix degradation. Degradation of matrix epoxy affected the adhesion between fibers and matrix.