Effects of Processing Conditions on Mechanical Properties of Quartz/BMI Laminates

Bismaleimide (BMI) resin with Quartz (AQ581) fiber reinforcement is a hightemperature composite material that is frequently used in aerospace structures, such as engine cowlings and radomes. Variations in laminate fabrication conditions, such as prepreg conditioning and applied cure pressure, can have a significant effect on microstructural features of fiber-reinforced composites, such as fiber volume fraction and void content, and consequently influence mechanical properties of composites. In this paper, flexural properties of Quartz/BMI laminates manufactured by prepregs conditioned at different relative humidity, to generate process-induced microvoids, and subsequently cured at different pressures are presented. Flexural test specimens were prepared in accordance with the testing standard ASTM-D790 and were not subjected to any degradation prior to flexural testing. Prepregs were conditioned in an environmental chamber at 2%, 40%, 70%, or 99% relative humidity at room temperature for a period of 24 hours prior to laminate fabrication. To alter the fiber volume fraction, the laminates were manufactured at cure pressures of 68.9 kPa, 206.8 kPa, 344.7 kPa, or 482.6 kPa via a hot-press. The laminates are shown to have different levels of microvoids and fiber volume fractions based on the processing conditions, which are also observed to affect the mechanical properties of Quartz/BMI. Flexure stiffness increased from 23 GPa to 29 GPa, or approximately 25%, for quartz/BMI laminates as the laminate fabrication pressure increased. An increase in flexure strength from 455.4 MPa to 555.9 MPa was observed for increasing laminate fabrication pressure. As the relative humidity level of prepreg conditioning increased, a decrease in flexure strength of approximately 50 MPa to 100 MPa was observed for the quartz/BMI laminates.