Design and Performance of Novel Aircraft Sandwich Structures with Folded Composite Cores

Attention is focussed on novel composite sandwich structures with energy absorbing cores. These are non-standard sandwich structures, in which a main load-bearing composite laminate is protected from impact damage by an energy absorbing core and an outer cover laminate. The core materials consist of folded composite plate structures (foldcore). The DLR has been concerned with the damage tolerance of foldcore sandwich structures under more critical impact loads. A gas gun impact test programme was carried out on foldcore composite panels with projectiles such as steel cubes, ice balls and tyre rubber fragments at impact velocities in the range 50 – 220 m/s. A wide range of failure modes have been observed, ranging from rebound from the outer skin, outer skin damage, core penetration, inner skin damage and inner skin penetration. NDE methods were applied such as X-ray, lock-in thermography, ultrasonic C-scans and computer tomography (CT) to ascertain the presence and nature of the impact damage. Physical phenomena associated with impact damage and progressive collapse of such structures are complex, and predictive models and simulation tools for design and analysis are being investigated. Key issues are the development of constitutive laws for modelling composites in-ply and delamination failures, suitable models for folding and collapse in the composite cores, materials laws for soft body impactors, and the efficient implementation of the materials models into FE codes. The solution proposed here is to use meso-scale models for composites based on continuum damage mechanics (CDM) in explicit FE codes which provides a framework within which in-ply and delamination failures may be modelled