Development of finite element model for approximating composite structures crushing characteristics

A finite element model of a composite keel beam web was developed to approximate its crushing behavior and to provide estimates of its energy absorption characteristics.The model development was done as part of NASA Langley Impact DynamicsResearch Facility development of an energy absorptive composite keel beam for use in general aviation aircraft subfloors intended to increase occupant survivability in crashimpacts with vertical sink rates up to 30 ft/s2. The finite element model was developed by modeling the impact test of a block core keel beam web specimen referred to asB1.4 [8] and establishing correlation with energy absorption parameters measured during testing. SDRC Ideas® Master Series was used to generate the specimen geometry and finite element mesh. MSC/Dytran™ was used to define the test kinematics and dynamics, the specimen material constitutive relationships and failure criteria and to solve the model. Initially the glass fabric/epoxy and kevlar fabric/epoxy material wrapping the foam core was modeled as an orthotropic laminate and assigned a Tsai-Wu failure criteria. The foam was modeled as crushable material with a stress-strain and failure relationship defined by the foam stress-strainThe impact dynamics was modeled as a free fall onto a rigidly supported specimen.This specimen/test representation resulted in premature failure of the laminate and collapse of the specimen model. The premature failure was due to unrealistically high accelerations and impact forces being generated by the free fall impact. The free fall curve.Ill Impact was eliminated and the impact acceleration was defined to be that of the actual acceleration pulse generated in the test. With this representation the model’s crushing behavior similar to that of the test. However the energy absorption parameter magnitudes were low. The model crash initiation load was 2.8 kLb (12.5 kN) and its sustained crash load was .25 kLb (1.1 kN) which were 63.7% and 91.9% below recorded test values respectively. The low magnitudes were a result of the orthotropic constitutive relationship not representing the material nonlinear shear stress-shear strain behavior and the brittle failure mode of the Tsai-Wu failure criteria. Constitutive relationship and failure criteria that better approximates the glass fabric/epoxy and kevlar fabric/epoxy material are needed to improve the model\u27s estimation of energy absorptive parameters. An approximation of these materials behavior was made by representing them as a laminate of isotropic elastic-plastic laminate with strain based failure criteria. With this representation the model crashinitiation load was 6.7 kLb (30.0\u27kN) and its sustained crash load was 3.1 kLb (13.8kN) which are within 5% and 1.5% of recorded test values respectively. Thus the model closely approximates the specimen crashing behavior and its energy absorption parameters for the defined impact acceleration when constitutive/failure relationships similar to the material are used