Determining Orthotropic Coefficients From, and Thermoelastic Stress Analysis of, Diametrally Loaded Composite Disk

Abstract

Thermoelastic stress analysis measures temperature variations in loaded solids and relates these to associated stresses. For orthotropic materials, the measured signal is proportional to a linear combination of the normal stress changes in the directions of material symmetry, and K1 and K2 are thermo-mechanical coefficients. Quantitative thermoelastic stress analysis of orthotropic composites necessitates (i) determination of the above two thermo-mechanical coefficients (i.e. calibration), and (ii) separation of the stresses. Although calibration procedures can take different forms, K1 and K2 can be experimentally determined most reliably and easily from calibration specimens of the same material, paint coating, loading frequency and ambient conditions as the test structure. Such calibration specimens typically employ a geometry and loading for which the state of stress or strain is known theoretically, or independently determined. Loaded beams, a diametrically-compressed disk, or uniaxial tensile coupons have been used for isotropic materials. Orthotropic materials usually necessitate testing two calibration specimens, with their principal material directions interchanged respectively. The present paper demonstrates the ability to determine both K1 and K2 from a single diametrally-loaded orthotropic composite (graphite/epoxy) disk. To be able to determine both coefficients from a single calibration specimen is advantageous. Disks are also easy to machine and load, rendering them very convenient for calibration.