Abstract
Standard Compression-After-Impact test devices show a weakening effect on thin-walled specimens due to a free panel edge that is required for compression. As a result, thin-walled undamaged samples do not break in the free measuring area but near the free edge and along the supports. They also show a strength reduction due to the free edge which can become potentially relevant for very weakly damaged panels. In order to reduce the free edge influence on the measured strength, a modified Compression-After-Impact test device has been developed. In an experimental investigation with carbon fiber reinforced plastics, the modified device is compared with a standard device. It is shown that thin-walled undamaged specimens investigated with the modified device now mainly break within the free measuring area and no longer at the free edge and along the bearings as it is the case for standard test devices. The modified device does not cause a free edge weakening effect in comparison to standard devices. The modified device is therefore more suitable for determining the compression strengths of undamaged thin-walled composite plates.
Highlights
Thin-walled fiber-reinforced plastics, in particular, carbon fiber reinforced plastics (CFRP), are very susceptible to reductions in strength due to low velocity impact damages
The increase amounts to about 10% which is greater than the resulting standard strength loss for low impact energies is in the order of magnitude of the increase
The weakening effect of the free edge is significant for CAI testing as the thin-walled CFRP laminates with different layer set-ups, e.g., the decrease in CAI strength amounts strength loss for low impact energies is in the order of magnitude of the increase
Summary
Thin-walled fiber-reinforced plastics, in particular, carbon fiber reinforced plastics (CFRP), are very susceptible to reductions in strength due to low velocity impact damages. In aeronautics, this strength loss has to be considered during construction by a sufficient damage tolerance leading to weight penalties. The influence of impact damages on the static behavior is typically investigated by Compression-After-Impact (CAI) testing as the residual compressive strength is one major property that significantly decreases due to impact damages. The residual strength represents a structural strength value which results from the material properties of the plate, the structure of the CFRP, the induced damage mechanisms (such as interlaminar debonding, fiber breakage, matrix cracking, local-sublaminate buckling as well as their interactions) and from the global geometrically non-linear behavior of the plate under pressure, in particular, as a result of geometric imperfections
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