A unidirectional graphite/epoxy lamina having the properties described in Problem 5.10 is to be designed to have a CTE of zero along a particular axis. Determine the required lamina orientation for such a design.

Problem 5.10

A carbon/epoxy lamina is clamped between rigid plates in a mold (Figure 5.17), while curing at a temperature of \(125^{\circ} \mathrm{C}\). After curing, the lamina/mold assembly (still clamped together) is cooled from \(125^{\circ} \mathrm{C}\) to \(25^{\circ} \mathrm{C}\). The cooling process occurs in moist air and the lamina absorbs \(0.5 \%\) of its weight in moisture. The lamina has the following properties:

Assuming that the lamina properties do not change over this temperature range and that the lamina is initially dry and stress free, determine the residual hygrothermal stresses in the lamina at \(25^{\circ} \mathrm{C}\) for angles \(\theta=0^{\circ}\) and \(45^{\circ}\).

E = 140 GPa E = 10 GPa =-0.3x106/K = 28106/K V12 = 0.3 = 0 G12 = 7 GPa 2 = 0.44