Damage Tolerance of Honeycomb Sandwich Composite Panels

During this study, a number of 8.5-inch by 11.5-inch flat honeycomb sandwich panels were inflicted with low energy impact damage, inspected non-destructively, and tested for residual in-plane compressive strength. Each panel had either a 3/8-inch or 3/4-inch low density Nomex honeycomb core, and either 2-ply, 4-ply or 6-ply face sheets. The face sheets were either carbon or Eglass (prepreg) fabric. The panels were either clamped or simply supported in a test fixture during impact from a gravity assisted drop mechanism, and impacted with either a 1-inch or 3-inch diameter spherical indenter. After impact the damage to each panel was characterized by (1) ultrasonic through-transmission to obtain a c-scan representing planar damage area, (2) indentation volume and depth, and finally (3) visual inspection to rate the damage according to a predetermined rating scale. The panels were then tested for in-plane compressive strength. After data analysis, a number of conclusions were reached about the interaction of various parameters. The two most significant conclusions are: (1) residual strength correlates well with planar damage area (c-scan) and may decrease to 40-45% of virgin strength, and (2) visual inspection is unreliable for evaluating damage inflicted by blunt objects (3-inch spherical indentor).