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

Abstract

Transverse microcracks are present in carbon fiber/bismaleimide (BMI) cros: composite laminates composed of 4, 4′‐bismaleimidodiphenylmethane (BMPM)/diallyl bisphenol A (DABPA) matrices after standard cure and fabrication condit and grow in width upon subsequent postcure. This investigation characterizes cure‐induced microcracking in terms of the critical fundamental macroscopic croscopic, and molecular damage mechanisms and thresholds, and a cure cycle modification that prevents microcrack formation under standard processing conditions tions for [0°/90°]s laminates is examined. A unique in‐situ technique is utilized which cure of the laminate is performed inside the chamber of an environim scanning electron microscope (ESEM), allowing for (i) physical observation of microcrack crack growth and formation mechanisms and (ii) characterization of microcracking onset time‐temperature thresholds. The cure cycle modification that prevents microcracking is an extended initial cure time at 177°C prior to higher temperature; cure regimes. Effects of this modification are examined through 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 on carbon fiber/bismaleirnide composites. The aforementioned analysis it was concluded that an extended initial cure time 177°C prior to higher temperature cure steps prevents microcracking under standard; fabrication postcure conditions for [0°/90°]s laminates; no microcracking observed until an additional postcure of 6 h at 300°C. This microcrack resist was independent of initial BMPM:DABPA monomer stoichiometry for the monomer ratios examined and associated with an improved fiber‐matrix interface and lower composite residual stress.

Accelerated ageing versus realistic ageing in aerospace composite materials. III. The chemistry of thermal ageing in bismaleimide based composites

“…In contrast to the strong surface chemical changes in the spectra, the center of the laminates did not change significantly even at 170°C except for a strong, broad OH region (3440 cm −1 ) increase in both materials to give a spectrum similar to that of the original surface. This center increase (rather than a decrease) again indicates that the OH decreases at the surface are probably due to extensive oxidative degradation of previously formed alcohols and acids and not crosslinking, as previously suggested 5, 27, 28. The surprising result is the low level of change that is seen in the 1770–1100 cm −1 region of the spectra at the center of the composite when the surface is so heavily degraded.…”

“…Figure 3 displays the percentage mass change of the System A BMI composite as a function of the square root of time (hours [1/2] ) at all ageing temperatures over the 7500 h of experiment time. If only one mechanism of oxidation/degradation had been involved the lines would have been simple straight lines 28. As plotted in this figure, only the 250°C line has a significant deviation from a straight line, with a strong change in slope at about 400 h. However, the loss of material is so great after this time that it is obvious that the 250°C ageing temperature is much too high an accelerating temperature for a long‐term study.…”

“…The progression of this change can be seen partially in the 170°C aged samples as shown in Figure 8 and even ageing at lower temperatures shows the beginnings of the change. This type of effect is normally viewed as the material showing further cure, or crosslinking,28, 29 but FTIR evidence presented later, and some literature evidence,19 indicated significant functional group changes (oxidation) from the surface, but not necessarily extra crosslinking.…”

High performance self‐healing bismaleimide/diallylbisphenol a/poly(phenylene oxide) microcapsules composites with low temperature processability

“…The cured BMI resins possess excellent mechanical properties, high temperature stability, excellent electric properties, low water absorption, and so on, their outstanding properties of cured BMI allow BMI resins to be used for multilayer printed circuit boards, electrical insulation, semiconductor industries, advanced composites for the aerospace industry, and structural adhesives [1,2]. However, the cured BMI resins are extremely brittle, during the processing of BMI composites, damages such as microcrack and delamination are always caused by heat, force load, and so on, leading to a significant reduction in strength, stiffness, and stability [3][4][5][6][7][8]. References have been reported that polymeric microcapsules containing healing agent can heal microcracks within polymer matrix to maintain the properties of polymer composites as well as toughen polymer composites [9][10][11].…”

Electron‐beam curing of bismaleimide‐reactive diluent resins

“…Thermal cure reactions of BMI systems4–9 and related resin modifications,10–17 and structure–property characterization of BMI and BMI composites, have already been widely studied 17–24. However, conventional thermally cured fabrication has many limitations, such as long curing time, thermal stress and induced shrinkage and microcrack formation, expensive tooling, hazardous chemical volatiles, and size limitations for autoclave processing.…”