Uzair Ahmed Dar

FE Analysis of Dynamic Response of Aircraft Windshield against Bird Impact

Bird impact poses serious threats to military and civilian aircrafts as they lead to fatal structural damage to critical aircraft components. The exposed aircraft components such as windshields, radomes, leading edges, engine structure, and blades are vulnerable to bird strikes. Windshield is the frontal part of cockpit and more susceptible to bird impact. In the present study, finite element (FE) simulations were performed to assess the dynamic response of windshield against high velocity bird impact. Numerical simulations were performed by developing nonlinear FE model in commercially available explicit FE solver AUTODYN. An elastic-plastic material model coupled with maximum principal strain failure criterion was implemented to model the impact response of windshield. Numerical model was validated with published experimental results and further employed to investigate the influence of various parameters on dynamic behavior of windshield. The parameters include the mass, shape, and velocity of bird, angle of impact, and impact location. On the basis of numerical results, the critical bird velocity and failure locations on windshield were also determined. The results show that these parameters have strong influence on impact response of windshield, and bird velocity and impact angle were amongst the most critical factors to be considered in windshield design.

Optimization of Composite Material System and Lay-up to Achieve Minimum Weight Pressure Vessel

The use of composite pressure vessels particularly in the aerospace industry is escalating rapidly because of their superiority in directional strength and colossal weight advantage. The present work elucidates the procedure to optimize the lay-up for composite pressure vessel using finite element analysis and calculate the relative weight saving compared with the reference metallic pressure vessel. The determination of proper fiber orientation and laminate thickness is very important to decrease manufacturing difficulties and increase structural efficiency. In the present work different lay-up sequences for laminates including, cross-ply [0m/90n]s, angle-ply [±θ]ns, [90/±θ]ns and [0/±θ]ns, are analyzed. The lay-up sequence, orientation and laminate thickness (number of layers) are optimized for three candidate composite materials S-glass/epoxy, Kevlar/epoxy and Carbon/epoxy. Finite element analysis of composite pressure vessel is performed by using commercial finite element code ANSYS and utilizing the capabilities of ANSYS Parametric Design Language and Design Optimization module to automate the process of optimization. For verification, a code is developed in MATLAB based on classical lamination theory; incorporating Tsai–Wu failure criterion for first-ply failure (FPF). The results of the MATLAB code shows its effectiveness in theoretical prediction of first-ply failure strengths of laminated composite pressure vessels and close agreement with the FEA results. The optimization results shows that for all the composite material systems considered, the angle-ply [±θ]ns is the optimum lay-up. For given fixed ply thickness the total thickness of laminate is obtained resulting in factor of safety slightly higher than two. Both Carbon/epoxy and Kevlar/Epoxy resulted in approximately same laminate thickness and considerable percentage of weight saving, but S-glass/epoxy resulted in weight increment.

A research methodology for crashworthiness evaluation of aero-structures under impact loading

The present work describes a methodical approach of material characterization and constitutive modeling with intent to apply it in finite-element (FE) solver to simulate the impact behavior of critical aircraft structures. The dynamic response analysis of a military aircraft windshield and canopy manufactured from Poly-methyle-methacrylate (PMMA) polymer was undertaken. The work is divided in three integral phases. In the first phase the material characterization was carried out at low and high strain-rates using universal testing machine and Split Hopkinson pressure bar (SHPB) respectively. In second phase of work, the material testing results were used to constitute a material model that can predict the deformation behavior of polymer under different loading conditions. The last phase of work deals with numerical implementation of constitutive model by incorporating a user-defined material subroutine in LS-DYNA, an explicit finite-element program. After successful implementation, the model was employed to determine the impact behavior of a military aircraft windshield and canopy against bird impact. The close experimental and simulation results encourages that the proposed model can effectively be used to assess the dynamic behavior of polymer based aero-structures under high velocity impact applications.

Modeling the perforation failure of honeycomb sandwich structures through numerical homogenization

In this study, the perforation failure of honeycomb sandwich structures is numerically simulated by using homogenized equivalent model. The high velocity impact behavior of aluminum honeycomb core with reinforced carbon/epoxy face sheets is modeled by using commercial finite element (FE) analysis code AUTODYN-3D. It is observed that the detailed three dimensional FE modeling of honeycomb core is complex, time consuming and computationally expensive. A simplified hexagonal honeycomb equivalent numerical model with relatively less computational time and acceptable degree of accuracy is proposed in this paper. The equivalent numerical model is based on P-alpha (Pα) equation of state for porous materials. In this model, it is assumed that honeycomb core is isotropic homogeneous porous medium in which all the pores are uniformly distributed. For the purpose of validation, the simulation results of detailed and equivalent honeycomb numerical models are compared with available experimental results in terms of ballistic limit, energy absorption, residual velocity and contact time. The results show that the equivalent honeycomb model closely predicts the perforation behavior for various impact velocities and takes considerably less computational time than detailed honeycomb model.