A Study on Crashworthiness of CFRP Laminates: Comparison of two Finite Element Models in LS-DYNA and ABAQUS/Explicit

Kurzfassung

Carbon Fibre Reinforced Polymer (CFRP) materials are being widely used for crashworthy structural applications in the aerospace and automotive industry. The effective use of composite materials in energy absorbing structures depends on the ability to reliably predict the onset and propagation of damage in dynamic events such as crash or high velocity impact (HVI). The computational prediction of progressive damage in composite structures remains a challenging area of ongoing research due to the complex nature of various failure modes which are interacting on different length scales. Key issues are the development and implementation of suitable constitutive laws and the determination of relevant parameters from material tests. Since 2015, the German Aerospace Center (DLR) and the University of British Columbia (UBC) are working together in the research cooperation DLR@UBC. Objective of this cooperation is to establish a robust simulation framework using finite element codes to support the effective design of crashworthy composite components with reduced reliance on experiments. Two intra-laminar composite material models with different underlying assumptions currently being used by DLR and UBC are investigated to assess their predictive capabilities and limitations with respect to progressive damage at different levels of the building block from single element analysis over fracture tests at coupon level up to progressive crushing at the structural level (Figure 1). In this work, the sub-laminate based continuum damage model CODAM2 [1], developed at UBC and implemented in LS-DYNA as the material model MAT_219 is compared with the ply-based damage model ABQ_DLR_UD using the Ladevèze theory [2] developed at DLR and implemented as a user subroutine (VUMAT) in ABAQUS/Explicit. In previous work, the two intra-laminar damage models were investigated in tension loading on the coupon level. Over-height Compact Tension (OCT) and a wide range of scaled Center-Notched Tension (CNT) specimens were used to evaluate characteristic quantities [3] [4]. In a next step, the damage models were investigated in compression loading using Compact Compression (CC) specimens, these results were presented in the 2017 edition of the ASIDI Conference [5]. Based on these previous simulation results at the coupon level, the simulation framework as well as the two damage models were further refined and are used to numerically study progressive axial crushing of flat coupon specimens (Figure 2A). Different material model specific options and simulation approachesare investigated and a best practice simulation methodology is defined to predict the mass-specific energy absorption (SEA) of flat coupon specimens made from IM7/8552 CFRP. The sensitivity of different CFRP layups on the SEA is numerically studied to investigate the capability of the two intra-laminar damage models to predict those layup variations. The numerical results are compared with experimental crush test data provided by the University of Utah [6] and presented in the Crashworthiness Working Group (CWG) of the Composite Materials Handbook CMH-17. The overall goal of the CMH-17 CWG is the numerical prediction of a lower lobe aircraft fuselage section subjected to crushing as shown in the top Detail Level of the building block pyramid in Figure 1. As an intermediate step to move up the building block towards the numerical prediction of the lower lobe fuselage structure, the joint DLR and UBC simulation methodology is used for the prediction of progressive axial crushing on the element level. Pre-test simulations are performed for a self-supporting C-section structure fabricated from IM7/8552 CFRP. The average crush force and the mass-specific energy absorption are used for the comparison of the CODAM2 and ABQ_DLR_UD damage models. The sensitivity of different CFRP layups on the characteristic crash-related parameters is also investigated. This study demonstrates the capabilities, effects of various parameters and material model specific options and limitations of both damage models, thus contributing to further understanding and improvement of the structural analysis of composites under dynamic loading conditions such as crash or high velocity impact events.