Blunt impact damage initiation in composite laminates

As composites are increasingly used for primary structures in the commercial aircraft industry, it is necessary to understand the development of damage when a laminate is subject to low velocity impacts from sources related to in-service conditions, maintenance, and other ground-equipment mishaps. In particular, collisions with ground vehicles and other large equipment can present a wide area, blunt impact source. Therefore, the effects of bluntness of an impactor are of interest as this is related to both the external visual detectability of an impact event, as well as the development of any internal damage in the laminate. The objective of this investigation is to determine the effect of impactor radius on the initiation of damage to composite panels. A low-velocity pendulum impactor was used to strike 200 mm square woven glass/epoxy composite plates of 3.18 and 6.35 mm thickness. Hemispherical steel impactor tips of radius 12.7mm to 152.4 mm were mounted to a piezoelectric force sensor which measures the contacting force history during the impact event. Distinct threshold energy levels for the onset of delamination and backside fiber breakage have been measured. These threshold energy levels increase significantly with increasing impactor tip radius.