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ASTM Selected Technical Papers
Composite Materials: Testing and Design (Fifth Conference)
SW Tsai
SW Tsai
Senior Scientist
Air Force Materials Laboratory, Wright-Patterson Air Force Base
Ohio 45433, editor
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ASTM International
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This investigation was undertaken to evaluate the damage tolerance of typical graphite/epoxy structure to service damage. AS/3501 graphite/epoxy-faced honeycomb panels were used for this study to measure their tolerance to low-velocity impacts. Three impact threats were investigated: runway stones, a blunt impactor with a 0.63-cm tip radius, and a blunt impactor with a 2.54-cm tip radius.

Impact parameters that would just cause visual damage were selected. This worked well for the runway stones, but visual damage was not easily observed on panels impacted with the 2.54-cm penetrator. Damage was easily detectable by C-scanning the panels. Specimens were cut from damaged panels and tested for residual strength. Each specimen contained one damage site. The induced damage was at the visual threshold or slightly higher or lower.

For the damaged specimens, the more extensive the damage, the lower the strength (tensile or compression) and the shorter the fatigue life. Tension tests showed that the sharper the impactor, the more severe the strength degradation for a given impact energy. In most cases, damage that would degrade the honeycomb is not visable but must be detected by nondestructive investigation techniques.

Design implications of these findings are only beginning to be understood. Damage, even at the visual threshold level, can cause a substantial loss of load-carrying ability for small specimens. More work must be performed to understand better the mathematical relationships between area of damage and degradation of large parts. In this case, the damage at the visual threshold could be aproximated by a 0.6-cm hole.

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