Comparative analysis of bismaleimide matrix composites reinforced with carbon fibre and quartz fibre: mechanical properties, anisotropy, and damage mechanisms

Two bismaleimide (BMI) matrix composite systems, unidirectional carbon-fibre/BMI (CF/BMI) and woven quartz-fibre/BMI (QF/BMI), were fabricated via vacuum-assisted resin transfer moulding and evaluated under tensile (ASTM D3039), three-point flexural (ASTM D7264), Charpy impact (ASTM D6110) and short beam shear (ASTM D2344) testing in the 0° and 90° ply orientations. The CF/BMI system exhibited pronounced mechanical anisotropy across all loading modes: tensile strengths of 876 ± 36 MPa (0°) versus 17.9 ± 3.3 MPa (90°), flexural strengths of approximately 955 MPa (0°) versus 34 MPa (90°), and Charpy impact energies of 10.0 ± 0.8 J (0°) versus 1.8 ± 0.2 J (90°). The QF/BMI system displayed a near-isotropic in-plane response under quasi-static loading, with anisotropy ratios not exceeding 1.4:1 in both tensile and flexural testing; however, the Charpy impact anisotropy ratio (∼2.1:1; 3.8 ± 0.3 J versus 1.8 ± 0.3 J) was markedly higher, a trend whose underlying mechanism could not be determined from the present data set and which requires dedicated rate-sensitivity investigations. Under transverse (90°) loading, both systems converged to comparable impact energies (∼1.8 J), indicating that 90° dynamic resistance is governed predominantly by the BMI matrix rather than fibre type. Short beam shear testing yielded interlaminar shear strength values of 60.1 ± 0.3 MPa and 4.64 ± 0.07 MPa for the CF/BMI system in the 0° and 90° orientations, respectively, reflecting a strongly orientation-dependent response, whereas the QF/BMI system displayed a near-isotropic interlaminar response of 19.4 ± 0.5 MPa (0°) and 16.1 ± 0.6 MPa (90°), consistent with the balanced plain-weave architecture. Fractographic analysis identified fibre pull-out and interlaminar delamination as the dominant damage mechanisms in CF/BMI composites, whereas QF/BMI specimens exhibited matrix-dominated fracture with a preserved woven-ply architecture across all loading modes.