Mechanical and chemical consequences of through thickness thermal gradients in polyimide matrix composite materials

Composite material parts may be exposed during their service life to through thickness temperature gradients where the difference between the inner and outer faces may exceed 150°C. The effect of these conditions on 4,4′-bismaleimidodiphenylmethane and diallylbisphenol-A based BMI resin reinforced with glass fabrics was investigated as a function of location through the laminate thickness. It was found that a gradient of chemical, physical and mechanical properties exists through the laminate thickness. A decrease of interlaminar shear strength and an increase of glass transition temperature follow high temperature exposure that causes resin chemistry changes. The material becomes less flexible and more sensitive to microcrack development as a result of these changes.