Damage Tolerance of Composites Sandwich Airframe Structures—Additional Results
Tomblin, J.S., Raju, K.S., Walker, T.H., Acosta, J.F.
The behavior of sandwich panels with open holes subjected to in-plane tensile and compressive loads were investigated experimentally. The objective of this study was to establish whether the open-hole damage was more severe compared to an impact damage of equal planar size. Hole diameters of 1, 2, and 4 inches were selected as they represented the planar dimensions of impact damage that produced contrasting failure modes in sandwich panels. The experimental results indicated that the open holes are more severe when compared to impact damage created with different impactor sizes. Comparison with past data revealed that residual strengths of impact damaged sandwich panels tend to approach that of the open hole with increasing residual indentation depth. Similar to impact damage, the single facesheet holes in sandwich panels under compression produced distinct failure modes, which was a function of the hole diameter. A facesheet fracture failure mode was observed for small diameter holes (and through holes of all sizes investigated), while a local instability mode was observed for the 4-inch-diameter hole. The buckling mode seen in sandwich panels with 4-inch-diameter single facesheet holes appeared to blunt the strain concentration due to facesheet bending, resulting in higher compressive strengths compared to smaller size holes. Finally, the open-hole compressive strengths were significantly lower than the open-hole tensile strengths for [(90 / 45)2 / CORE]s sandwich panels.
The impact damage resistance and damage tolerance of sandwich panels with facesheets fabricated using Toray P707AG-15 Unitape/epoxy and Toray T700S-12K plain weave fabric/epoxy systems were investigated. This study was intended to augment the observations on damage tolerance characteristics reported in previous studies. Three different facesheet configurations [(45F / 90F) / CORE]s, [(45F / 0T / 0T / 45F) / CORE]s, and [45F / 0T / 0T / 45F / 0T / 0T /45F / CORE]s were investigated. The sandwich panels exhibited damage resistance and tolerance characteristics that were similar to the earlier findings. The damage states were typical of blunt impactors, and thus, the behavior of these panels under compression was dominated by facesheet stiffness and core properties.
The FAA damage tolerance program was independently analysed by Mr. Tom Walker of NSE Composites. The primary emphasis of the evaluation was the residual strength prediction development effort, but also included a review of the residual strength test results. Recommendations addressing the program scope and the analysis method development and verification are in appendix C.