Effects of Shear Lag on Thermal Expansion of 3D Carbon-Carbon Composites
JORTNER RESEARCH AND ENGINEERING INC COSTA MESA CA COSTA MESA
Pagination or Media Count:
Thermal expansion of coupons of 3D carbon-carbon is analyzed in terms of the thermo-elastic properties of the mini-constituents yarn bundles and matrix pockets and the stress transfer across their interfaces. The weakness of the interfaces limits the stress transfer by shear. When the shear strength of the interface is exceeded and debonding occurs, stress transfer in the de-bonded region is assumed to be frictional. The frictional shear capability is estimated as a function of the inter-constituent stresses that occur on heating. The analysis, which deals with the stress-transfer using a shear-lag approach, provides estimates of the displacements between mini-constituents. Numerical results are presented to show how elastic and frictional shear lag influences measurements of thermal expansion. The relation of specimen dimensions to the geometry of the composites unit cell, and details of the measurement technique, are predicted to influence the measured expansions for some techniques, the measured expansion is predicted to differ significantly from the true expansion of the bulk composite material. Analytical predictions are shown to compare reasonably well to Landers data on end effects, up to about 4000 F, beyond which the likelihood of creeprelaxation phenomena makes the analysis inapplicbale.