Application of the Method of Caustics to Fiber-Reinforced Materials

Abstract

Cracks are studied in cross-ply carbon fiber-reinforced long, thin epoxy plates under tension while they are being loaded quasi-statically. The emphasis of this work lies on the measurement of the strain field at the crack tip. The shadow-optical technique has been applied for the first time to fiber-reinforced materials. The method is additionally used to obtain experimental results about the fracture and damage behaviour of fiber-reinforced plastics.

When the load is clearly below the fracture load, the shadow pattern shows a caustic as it is known from isotropic materials. The observed shape of the caustic is strongly dependent on the ratio of the stiffnesses in the two fiber directions. If the load is increased quasi-statically a damage zone appears at the crack tip which influences the shadow pattern. At higher loads the damage zone dominates and no caustic in the classical sense can be observed. Reducing the load one obtains a smaller but not vanishing damage zone. Once the crack starts running, disturbances due to secondary effects in the loading direction are superimposed and influence the running shadow pattern strongly.