Dent depth visibility versus delamination damage for impact of composite panels by tips of varying radius

Abstract

The ability to visually detect damage from impact events is a common concern for carbon/epoxy composite laminates, particularly because composite materials can sustain impact damage in forms such as delamination and backside fiber breakage that is not directly visible at the site of the impact. Therefore, a relationship drawn between the internal damage level and visibility of surface dents would be useful. An experimental investigation is presented on the relationship between visual detectability and the extent of internal damage created by low-velocity impacts. The specimens examined were 8, 16, and 24 ply carbon/epoxy panels impacted by tips of radii: 12.7, 25.4, 50.8, and 76.2 mm. In order to quantitatively describe the visibility of the damage, the depth and span of the surface dents were considered. The results indicate that there are direct relationships between visible and internal damage, but that these relationships are strongly dependent on the radius of the impact tip. Small radius impactors can leave significant visual evidence (i.e. surface dent) without creating internal damage, while the larger radius blunt impactors can create large-area internal delamination without leaving any visual evidence. Therefore, the visual detectability of impact damage is highly dependent upon tip radius, and the level of internal damage cannot be determined based on visual observation alone, especially since in practice, the radius of an impacting object is not always known.