Experimental validation of efficient impact simulation methodologies of sandwich structures

Abstract

Aircraft design calls for weight efficient shell constructions. Composite sandwich structures satisfy this demand by the combination of two thin, stiff face sheets and an intermediate lightweight core. Furthermore, the outer face sheet can act as an impact detector while the core provides damping and insulation. Thus, sandwich structures are increasingly aspired for application as fuselage and wing panels. However, impact damage in sandwich structures can provoke a significant strength and stability reduction. Therefore, methodologies are sought-after, which reliably simulate impact events and accurately predict impact damage sizes. The finite element based damage tolerance tool CODAC has been developed for efficiently simulating the damage resistance of sandwich structures subjected to low-velocity impacts. Since frequent design loops require a quick analysis, efficient deformation and failure models are needed. To achieve a rapid and accurate stress analysis, a recently developed three-layered finite shell element is used [1]. The element accounts for shear deformation and 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 a three-layered shell theory [2]. To predict damage growth during the impact event, a progressive damage mechanics approach is applied [3]. 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, which was conducted at the Department of Aerospace Technology at Dresden University, acts as referee for the validation of the impact simulation methodologies used by the damage tolerance tool CODAC. Force-time histories and damage sizes are examined, and the influence of distinct failure models on the impact response is analyzed. Comparisons between impact tests and simulations show that CODAC is capable of accurately and rapidly simulating low-velocity impact events.