Crash energy absorption of Kevlar® fabric composite structures [electronic resource]
Keywords: Mechanical Engineering

Fiber reinforced composites currently being used in myriad applications including aerospace, defense and wind industries due to their light weight, higher specific strength and stiffness, compared to traditional metallic structures. For crashworthiness applications, advanced textile composites such as fabrics and braids, are showing superior performance over traditional tape laminated composites structures. Kevlar fabric composites, in particular, are offering a great potential as the next generation energy absorbing material for the rotorcraft applications. Unfortunately, none of the existing material models can predict the behavior of Kevlar fabric composites for the crashworthiness applications. The prime objective of this investigation is to develop a computationally efficient and robust material model based on a unified unit-cell approach to simulate the crush response of tubular and honeycomb structures made of Kevlar fabric composites under quasi-static and dynamic loading conditions. Tests are conducted at various levels to understand the constitutive behavior of Kevlar fabric composites. Based on experimental observations, a physics based material model is developed. This material model is then implemented in commercial finite element software such as LS-DYNA and ABAQUS as a User Material (UMAT) Routine. The material model is built by considering majority of length scales in the composites: Kevlar composite tows at the meso-scale, a fabric unit cell at macro-scale and overall structural level. The material model identifies a smallest repetitive unit (i.e. unit-cell) within the composite material. The tow geometry within the unit-cell is represented using a simplified three-dimensional description. An elastic-plastic constitutive law is developed to simulate the behavior of Kevlar composite tows. Efficient homogenization scheme along with appropriate failure criteria is implemented to calculate the effective quantities such as stiffness and stress and predict the response of the unit-cell. Coupon tests are conducted on unidirectional Kevlar and plain woven Kevlar fabric flat plaques. The unidirectional coupon tests are conducted to characterize the material properties such as stiffness, strength and plastic parameters necessary as input to the material model. The plain woven fabric coupon tests are conducted to verify the initial stiffness, strength and damage progression predictions of the model at the ply-level. Several tubes with square and circular cross-section are crushed quasi-statically using both flat-plate and plug-type crush initiators. During crushing, Kevlar fabric composite tubes buckled locally and failed progressively by forming folds similar to aluminum tubes. Test results show that the failure mode and overall energy absorption is not affected significantly by the change in fabric angle or the method of crushing. Dynamic crush tests on Kevlar fabric honeycomb structures also exhibited global folding failure mechanism with very little fiber fracture. During crushing, honeycombs typically reach a peak load and then crush at constant, sustained load which is a characteristic of an ideal crash energy behavior. The material module is verified by carrying out simulations at different levels. Coupon test data is initially validated for both the initial stiffness and the strength predictions. Tubular and honeycomb crush data is validated for the deformation behavior, the failure mechanism and the crash energy absorption characteristics. Strong correlation between simulation and experiments suggests that the model can be used as state-of-the-art computational tool for predicting the crushing behavior of tubular and honeycomb structures made of Kevlar fabric composites.