Dimensional Stability of Composite in a Space Thermal Environment

Abstract

Abstract Composites are steadily replacing metals in weight-critical space structures primarily because of their high specific moduli. Many space structures require not only high stiffness but precision alignment and dimensional stability as well. These parameters are sensitive to thermal strains caused by the mismatch in coefficients of thermal expansion (CTE) of the constituents, anisotropy in ply CTE, and variations in service temperature. The CTE of a laminate can change in service as a result of mechanically or thermally induced microcracking and this may result in undesirable distortion of the composite structure. In this work, changes in the CTE of a representative space structural material-XN-70/RS3 graphite/cyanate-ester were studied using strain gages. The longitudinal and transverse CTEs of unidirectional and crossply ([0/90]2s) laminates were initially determined over the temperature range of −157°C to 121°C. Specimens were then subjected to thermal cycles from −150°F to 250°F to induce microcracking. The specimen CTE was again measured in the same location after 25, 50, 100, 200, 500 and 1000 cycles, and the corresponding crack density (in the vicinity of the strain gage) was determined by microscopic examination of a polished specimen edge. Predictions of CTE variation with crack density were made using the axisymmetric formulation of a large-radius, hollow, layered-cylinder model, which is equivalent to the flat laminate formulation. The experimental results compared fairly well with the predictions for a [0/90]2s cross-ply laminate. This paper also includes a discussion of the statistical variation of the measured CTEs.