Jet engine cowlings use 5 micrometer diameter whisker filaments of SiC that are coated in 3 micrometer of aluminum using a chemical bonding process. The coating process is carried out at 650 degrees C before the sample is cooled to room temperature. Elastic stresses due to thermal expansion mismatch do not being to appreciably accumulate until they alloy has cooled to approximate 200 degrees C due to alloy viscosity. If the stresses necessary to maintain whisker/coating compatibility are assumed to only be acting in the direction of the filament axis, what is their sign and magnitude at 20 degrees C?
Givens: E(SiC) = 476 Gpa ----- α(SiC) = 4.7 x 10^.6
E(Al) = 70 Gpa -------- α(Al) = 21 x 10^-6