Damage Tolerance of Sandwich Structures – Modelling, Analysis and Testing –

Abstract

The finite element based tool, CODAC, has been developed for efficiently simulating the damage tolerance of composite structures subjected to low-velocity impacts. It is applicable to monolithic and also sandwich based structural concepts. The considered sandwich shell structures consist of two thin composite face sheets separated by a lightweight core. Due to their high mass specific stiffness and strength, a very weight efficient design is achievable. Moreover, the core can provide damping and insulation, while the outer face sheet can act as an impact detector. However, impact damage in sandwich structures can provoke a significant strength and stability reduction. To achieve a rapid and accurate deformation and stress analysis, two suitable three-layered finite shell elements have been developed. Both account for shear deformation. However, one formulation bases on a plane stress approach while the other one additionally accounts for transverse compressibility. Since an accurate approximation of the transverse stresses is an important requirement for detecting impact damage, transverse stresses are improved by the so-called Extended 2D-Method, which is an equilibrium approach that has been applied to the three-layered shell formulations. To predict damage growth during the impact event, a progressive damage mechanics approach is applied. Stress-based failure criteria are used to detect damage initiation. Subsequently, material resistance is reduced by applying appropriate degradation models. An experimental impact test program on honeycomb sandwich panels is used to validate the impact simulation of the FE-tool CODAC. Concerning the residual strength numerical investigations were conducted to describe the damage growth process which is measured in “Compression after Impact” (CAI) tests of unsymmetrical sandwich coupons. Comparison between test and simulation showed a good agreement with respect to the maximum bearable compression load. Finally, it is described how different non-destructive testing methods (basing on ultrasonic’s) are applied in order to detect and characterize different failure modes of face sheets and core material of sandwich structures.