Determination of the Elastic Constants of a Unidirectional Fiber Composite Using Ultrasonic Velocity Measurements

Ultrasonic velocity measurements have been made to obtain the dynamic elastic stiffnesses necessary to determine fully the elastic properties of a unidirectional glass‐reinforced epoxy‐fiber composite. In units of 106 psi, these stiffnesses are C11=6.01, C22=C33=2.58, C12=C13=0.70, C23=1.42, and C44=0.49, where the subscript 1 refers to the fiber direction. Since more velocities were measured than were necessary to obtain the five constants required by the symmetry of this composite, the extra measurements were used to check on the experimental method. Analysis shows the ultrasonic technique to be satisfactory for measurement of the elastic stiffnesses of a fiber composite. The experimental results are compared with the elastic constants predicted for this composite from expressions based upon several theoretical models. Good agreement is obtained when the theoretical calculations are made using the dynamic (as opposed to the static) modulus of the epoxy matrix.