P.M. Mohite

Local quality of smoothening based a-posteriori error estimators for laminated plates under transverse loading

Abstract The local and global quality of various smoothening based a-posteriori error estimators is tested in this paper, for symmetric laminated composite plates subjected to transverse loads. Smoothening based on strain recovery and displacement-field recovery is studied here. Effect of ply orientation, laminate thickness, boundary conditions, mesh topology, and plate model is studied for a rectangular plate. It is observed that for interior patches of elements, both the estimators based on strain or displacement smoothening are reliable. For element patches at the boundary of the domain, all estimators tend to be unreliable (especially for angle-ply laminates). However, the strain recovery based estimator is clearly more robust for element patches at the boundary, as compared to displacement-recovery based error estimators. Globally, all the estimators tested here were found to be very robust.

Focussed adaptivity for laminated plates

Abstract Accurate computation of the critical response quantities for laminated composite structures has become essential, especially from the design and design certification point of view. Worst case scenario analysis (corresponding to the load envelope) of the structure require computation of local quantities of interest. In such a situation, control of both modelling error and discretisation error for the quantities of interest is required. In this study, for a fixed plate model, a novel adaptive procedure is presented, based on a posteriori estimation of the error in the quantity of interest. This focussed adaptive procedure involves prediction of the desired optimal mesh sizes in the neighborhood of the region of interest and away from the region of interest, based on an a priori estimate of the error in the quantity of interest. The final desired mesh is obtained in one shot. It is found that the error estimator, for the quantity of interest is reasonably robust. Further, the adaptive procedure is very effective in controlling the local error to within the specified tolerances.

Validation of Intralaminar Behaviour of the Laminated Composites by Damage Mesomodel

In this article the enhanced version of the damage mesomodel (DML) developed at LMT is validated with the simulations performed for intralaminar damage mech anisms using User MATerial subroutines in Abaqus Standard. The damage mechanisms validated in the present study include fibre breaking, diffuse damage and matrix microcracking. The efficacy of the proposed model is shown by a series of identification resul ts on industrial test cases for T700/M21 material . The basic identification tests include 0 ° traction and compression, traction on [0 m/90 n]S laminates, monotonic and cyclic traction of [±45] 4S and [±67.5] 4S laminates. The validation test is carried out for a 32 -ply carbon -epoxy laminate with an open hole in traction. It is seen that DML is successful in predicting the initiation and propagation of the above mentioned intralaminar damage mechanisms.

Design and Optimization of a Composite Canard Control Surface of an Advanced Fighter Aircraft under Static Loading

Abstract The minimization of weight and maximization of payload is an ever challenging design procedure for air vehicles. The present study has been carried out with an objective to redesign control surface of an advanced all-metallic fighter aircraft. In this study, the structure made up of high strength aluminum, titanium and ferrous alloys has been attempted to replace by carbon fiber composite (CFC) skin, ribs and stiffeners. This study presents an approach towards development of a methodology for optimization of first-ply failure index (FI) in unidirectional fibrous laminates using Genetic-Algorithms (GA) under quasi-static loading. The GAs, by the application of its operators like reproduction, cross-over, mutation and elitist strategy, optimize the ply-orientations in laminates so as to have minimum FI of Tsai-Wu first-ply failure criterion. The GA optimization procedure has been implemented in MATLAB and interfaced with commercial software ABAQUS using python scripting. FI calculations have been carried out in ABAQUS with user material subroutine (UMAT). The GAs application gave reasonably well-optimized ply-orientations combination at a faster convergence rate. However, the final optimized sequence of ply-orientations is obtained by tweaking the sequences given by GAs based on industrial practices and experience, whenever needed. The present study of conversion of an all metallic structure to partial CFC structure has led to 12% of weight reduction. Therefore, the approach proposed here motivates designer to use CFC with a confidence.

Statistical Strength Determination of Carbon Fibres Using a Generalized Weibull Model

The mechanical and damage properties of individual constituents used in the fibrous composites are very essential in the micromechanical damage modeling. As fibres are the principal load carrying component of a composite, the knowledge of statistical strength distribution of fibres is very essential for estimating the effective properties of the composite and for its damage investigation. Even though various micromechanical models are available in the literature, most of them disregard the effects of diametric irregularities and variation in properties of the fibres. This may be attributed to the discrepancy between theoretical predictions and experimental results. So, in order to model the uncertainties in composites, the statistical strength distribution of the single fibres should be known a priori. In this work various damage parameters like ultimate strength and failure strains of Torayca R

Development and Analysis of Gull Inspired UAV Flapping Wing

Unmanned Aerial Vehicles (UAVs) are the aircraft which are controlled remotely or autonomously. They can be characterized on the basis of type of wings used, namely fixed, flapping and rotary wing UAVs. They can be used for various military and civil applications. Present study is focused on flapping wing UAVs. Natural fliers are the master of flapping flight and can be taken as inspiration for developing an efficient flapping UAV model. In this study a flapping wing system, inspired from Black Headed Gull, is developed and tested for its kinematic and aerodynamic characteristics. Like the original biological structure, the developed model has a shoulder joint, an elbow joint, and a wrist joint. Laser displacement sensor and digital image correlation setups were used for kinematic testing. The aerodynamic analysis was carried out using six component force balance in wind tunnel at different wind speeds and angle of attacks. It was found that the wing performed flapping motion similar to the gull with independent control of each joint. Also, the effect of each joint was observed on the lift, thrust and moments generated by the model during flight. It was observed that the developed model showed similar properties as compared to its biological inspiration.

Realization and Dynamic Studies of CNTs-PDMS Membranes for Biomimetic Flapping Wing Applications

Aerial and aquatic animals including bats, insects and fish use their wings/fins to generate propulsive forces. Natural fliers deform their wings, actively and/or passively, in bending and twisting modes to generate lift and thrust. Within a flapping cycle, wing skin interacts with surrounding fluid and transfers dynamic loads to the internal stiffening structure. Biomimicking of such complex natural flapping wings is possible if the development involves both materials and structural aspects. In the present study, thin PDMS films are chosen for developing the skin of the biomimetic flapping wings. The films are first characterized for dynamic mechanical properties (storage modulus, loss modulus and loss factor) using a dynamic mechanical analyzer. The tests are done in frequency and strain sweep modes to analyze the effect of strain-rates and strain-amplitudes on the dynamic mechanical properties and generate experimental data for constitutive modeling. The dragonfly and cicada wings are taken as the bioinspiration for developing the biomimetic wings. The fabrication of wing skeletons and their integration with the PDMS membranes are achieved through advanced manufacturing techniques including laser micromachining, photolithography and casting. Two types of composite materials are used for making the wing skeletons, i.e., carbon nanotubes (CNTs)/polypropylene (PP) nanocomposite sheet for cicada inspired wing and carbon fiber/epoxy composite strands for dragonfly inspired wing. Structural dynamic analysis of such light, flexible and small size biomimetic structures are interesting and useful for evaluation of biomimicking performance of used materials and manufacturing methods, but difficult to perform. A real-time high-speed non-contact dynamic testing method based on DIC-FPGA coupling (3D digital image correlation technique coupled with real-time data acquisition system, developed at our lab) is used for determining the natural frequencies and corresponding mode shapes of fabricated wings.

Redesigning of a Canard Control Surface of an Advanced Fighter Aircraft: Effect on Buckling and Aerodynamic Behavior

Abstract A redesign of canard control-surface of an advanced all-metallic fighter aircraft was carried out by using carbon fibre composite (CFC) for ribs and panels. In this study ply-orientations of CFC structure are optimized using a Genetic-Algorithm (GA) with an objective function to have minimum failure index (FI) according to Tsai-Wu failure criterion. The redesigned CFC structure was sufficiently strong to withstand aerodynamic loads from stress and deflection points of view. Now, in the present work CFC canard structure has been studied for its buckling strength in comparison to existing metallic design. In this study, the existing metallic design was found to be weak in buckling. Upon a detailed investigation, it was revealed that there are reported failures in the vicinity of zones where initial buckling modes are excited as predicted by the finite element based buckling analysis. In view of buckling failures, the redesigned CFC structure is sufficiently reinforced with stringers at specific locations. After providing reinforcements against buckling, the twist and the camber variations of the airfoil are checked and compared with existing structure data. Finally, the modal analysis has been carried out to compare the variation in excitation frequency due to material change. The CFC structure thus redesigned is safe from buckling and aerodynamic aspects as well.

Modal Analysis of Hummingbird Inspired MAV Flapping Wings

Natural flyers are the best source of inspiration for making successful MAVs. Hummingbirds are known for their excellent flight characteristics such as long duration hovering, backward flying, high agility, etc. Giant hummingbird is chosen as the bio-inspiration for designing the wing. Wings are required to be light, strong, and fatigue resistant to be used for MAV applications. Carbon nanotubes (CNTs)/Polypropylene (PP) composite is chosen as the wing membrane material whereas carbon fiber (CF)/epoxy (E) composite is chosen for wing frame. Two types of wings are fabricated, one is CNTs/PP wing and another is CF/E frame with CNTs/PP membrane wing. Kinematics, structural dynamics, and aerodynamics are the main component of flapping flight studies. Modal analysis of fabricated wings is done using 3D visual image correlation (VIC-3D) and laser displacement sensor setup. In the end, the results of both type wings are compared with experimental results and a good correlation has been seen. The validation of results is done using Ansys.

Design and Kinematic Analysis of Gull Inspired Flapping Wing Model

A flapping wing mimicking the Black Headed Gull was developed and tested for its kinematics. All the individual joints in the gull wing, namely the shoulder, elbow and wrist joint were mimicked with their corresponding functionalities. The shoulder joint is designed to control the flapping frequency, flapping amplitude, flapping plane and the speed of the upstroke-downstroke. Similarly, the elbow and wrist joints control the upstroke span reduction and twisting of the wing, respectively. Geometry, inertia, mass and frequency data of the gull were used to model the wing. A control input program was designed for the independent control of all the 6 joints (3 per wing). The motion of the manufactured wing system was verified using LASER Displacement Sensor.