Experimental observation of microcracking of composite laminates under cryogenic temperature

Abstract

The objective of this work was to develop a predictive capability for the onset of ply failure in composite laminates subjected to combined thermal and mechanical loads. The material system studied was a carbon fiberreinforced toughened epoxy composite, IM7/5250-4. Thermomechanical properties required for the analysis were obtained from tests on unidirectional laminates. These laminates were tested at 73°F and -321°F. Coefficients of thermal expansion (CTE) were measured using strain gages, over the temperature range of 300°F to -321°F (liquid nitrogen). Laminates, [0/902/0]T? [0/903/0]T, [02/902]s? and [30/-30/90]s> were used to experimentally determine the onset of ply failure under mechanical and thermal loads. The onset of ply failure was detected from acoustic emission and incremental loading and unloading experiments, and confirmed from microscopic examination of polished specimen edges. Ply stresses were calculated for the corresponding conditions from laminated plate theory, using the appropriate experimentally generated thermomechanical properties and the applied load. The maximum stress failure theory was applied to predict failure. Transverse tensile strength increased at lower temperatures, while strain to failure decreased (indicating increased brittleness). The stress level at the onset of ply failure decreased significantly at lower temperatures, mainly due to an increase in the curing residual stresses. Lamination theory in conjunction with the maximum stress failure criterion tends to overestimate the onset of the transverse cracking in the laminates considered.