Fundamental investigation of cure-induced microcracking in carbon fiber/bismaleimide cross-ply laminates

Transverse microcracks are present in carbon fiber/bismaleimide (BMI) cross -ply composite laminates composed of 4.4'-bismaleimidodiphenylmethane (BMPM )/ 0,0'-diallyl bisphenol A (DABPA) matrices after standard cure and fabric ation conditions, and grow in width upon subsequent postcure. This investig ation characterizes cure-induced microcracking in terms of the critical fun damental macroscopic, microscopic, and molecular damage mechanisms and thre sholds, and a cure cycle modification that prevents microcrack formation un der standard processing conditions for [0 degrees /90 degrees](S) laminates is examined. A unique in-situ technique is utilized in which cure of the l aminate is performed inside the chamber of an environmental scanning electr on microscope (ESEM), allowing for (i) physical observation of microcrack g rowth and formation mechanisms and (ii) characterization of microcracking o nset time-temperature thresholds. The cure cycle modification that prevents microcracking is an extended initial cure time at 177 degreesC prior to hi gher temperature cure regimes. Effects of this modification are examined th rough network structure-property-processing interrelationships by way of (i ) dynamic mechanical analysis (DMA), (ii) optical and electron microscopy, (iii) differential scanning calorimetry (DSC), and (iv) our previous work o n carbon fiber/bismaleimide composites. From the aforementioned analysis it was concluded that an extended initial cure time at 177 degreesC prior to higher temperature cure steps prevents microcracking under standard fabrica tion postcure conditions for [0 degrees /90 degrees](S) laminates: no micro cracking was observed until an additional postcure of 6 h at 300 degreesC. This microcrack resistance was independent of initial BMPM:DABPA monomer st oichiometry for the two monomer ratios examined and associated with an impr oved fiber-matrix interface and lower composite residual stress.