Compressive Yield and Postyield Response of Epoxy Resins Modified With Short Carbon and Aramid Fibers: Effects of Fiber Geometry, Content, and Curing System

This study investigates the effects of short‐fiber geometry, fiber content, and curing system on the compressive yield and postyield behavior of epoxy resins. Short carbon fibers (CFs) and short aramid fibers (AFs) were incorporated into two anhydride‐cured epoxy systems based on maleic anhydride (MA) and methyl tetrahydrophthalic anhydride (MTHPA, denoted as MT). Compressive yield strength, elastic modulus, and postyield load‐retention behavior were evaluated, together with macroscopic failure and SEM observations. CFs showed greater potential for enhancing strength and stiffness. Within the investigated range up to 80 phr, the maximum compressive yield strength increased by 32.4% and 25.3% in the MA and MT systems, respectively, while the corresponding maximum increases in elastic modulus were 40% and 37%. MT‐CF‐L3‐80 phr exhibited the highest compressive yield strength and elastic modulus, reaching 142.31 MPa and 3.24 GPa. AFs produced smaller increases in strength and stiffness but exhibited a more gradual postyield response. At 40 phr and an additional postyield displacement of 7 mm, the minimum load‐retention ratio among the investigated fiber‐length groups remained 0.945. At high fiber contents, longer fibers tended to overlap, entangle, or agglomerate, thereby reducing dispersion uniformity and interfacial load transfer. SEM observations showed fiber fracture, pull‐out, interfacial debonding, and crack deflection in CF‐modified systems, whereas AF‐modified systems mainly exhibited matrix peeling, shear tearing, localized pull‐out, and interfacial debonding. Overall, fiber geometry, content, and curing system exerted distinct effects on compressive yielding, stiffness, and postyield load‐retention behavior.