Method development for finite element impact simulations of composite materials

Simulations of structural impact utilizing explicit finite element codes have high requirements for computational efficiency. Woven fabric composites have high impact resistance and toughness but complex microstructure. A micro-mechanical model of woven fabric composites is developed here. The geometrical simplifications and the low discretization of the microstructure lead to computational efficiency. The model is based on the stiffness homogenization technique. The elastic property prediction of the model is in good agreement with the experimental data. A computationally efficient fiber reorientation technique is developed and applied to unidirectional composites. An investigation of the effect of the fiber reorientation on the elastic properties of the woven composite model is carried out. The reorientation technique, material non-linearity, and stiffness degradation are implemented in progressive failure model development. The model is used in structure survivability simulations. Loosely woven neat fabrics of high modulus fibers are used as protectors against impacting bodies because of their flexibility and high strength. The homogenization and fiber reorientation techniques are implemented in a loosely woven fabric model. The model is validated in ballistic impact simulations and demonstrates reasonable behavior in airbag inflation simulations. The viscoelasticity of polymeric materials increases their strength and Young's modulus during impact. The viscoelasticity is included in another model of loosely woven fabrics. The model is based on a mechanism representing the crimped yarns of the fabric. The viscoelastic model of loosely woven fabrics is validated in ballistic impact simulations. The viscoelasticity of the fibers, the viscoplasticity of the matrix material and the progressive damage with strain softening are essential for energy dissipation in structure crashworthiness simulations. They are implemented in another model of woven fabric composites based on the same micro-mechanics but using simple strain and stress transformation. The model is validated by comparison with experimental strain-rate dependence tests and impact simulations. The strain softening causes strain localization and severe mesh sensitivity of finite element simulations. The strain gradient method is implemented in high order elements to remove the mesh sensitivity. Collapsible high order shell element is developed for impact simulations of thin walled structures when strain-softening material models are used.