2 Linear Elastic Stress-Strain Relations 2.1 Basic Equations Consider a single layer of fiber-reinforced composite material as shown in Fig.2.1.In this layer,the 1-2-3 orthogonal coordinate system is used where the directions are taken as follows: 1.The 1-axis is aligned with the fiber direction. 2.The 2-axis is in the plane of the layer and perpendicular to the fibers. 3.The 3-axis is perpendicular to the plane of the layer and thus also perpen- dicular to the fibers. Fig.2.1.A lamina illustrating the principle material coordinate system2 Linear Elastic Stress-Strain Relations 2.1 Basic Equations Consider a single layer of fiber-reinforced composite material as shown in Fig. 2.1. In this layer, the 1-2-3 orthogonal coordinate system is used where the directions are taken as follows: 1. The 1-axis is aligned with the fiber direction. 2. The 2-axis is in the plane of the layer and perpendicular to the fibers. 3. The 3-axis is perpendicular to the plane of the layer and thus also perpen￾dicular to the fibers. Fig. 2.1. A lamina illustrating the principle material coordinate system