David Kumar

Crosslinking of agarose bioplastic using citric acid

We report chemical crosslinking of agarose bioplastic using citric acid. Crosslinking was confirmed using Fourier transform infrared (FTIR) spectroscopy. The effects of crosslinking on the tensile strength, swelling, thermal stability, and degradability of the bioplastic were studied in detail. The tensile strength of the bioplastic films increased from 25.1MPa for control films up to a maximum of 52.7MPa for citric acid crosslinked films. At 37°C, the amount of water absorbed by crosslinked agarose bioplastic was only 11.5% of the amount absorbed by non-crosslinked controls. Thermogravimetric results showed that the crosslinked samples retain greater mass at high temperature (>450°C) than control samples. Moreover, while the crosslinked films were completely degradable, the rate of degradation was lower compared to non-crosslinked controls.

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.

Time-Temperature Dependent Creep and Recovery Behaviour of MWCNTs-Polypropylene Nanocomposites

Time-dependent viscoelastic materials are characterized by their creep, recovery and stress relaxation behaviour under different thermo-mechanical conditions. Thermoplastic polymers are lightweight and fatigue resistant, among which polypropylene (PP) is widely used. Its high isotacticity provides excellent physical and mechanical properties. In the present study, multi-walled carbon nanotubes-polypropylene nanocomposites (MWCNTs/PP) were developed by solvent casting method using isotactic PP with MWCNT-COOH (surface functionalized). Surface morphology was observed using scanning electron microscopy to check for a uniform distribution of CNT into PP matrix. Time-temperature controlled short and long-term creep and recovery tests were conducted in creep mode using DMA. The effects of loading time and temperature on creep and recovery behaviour of nanocomposites with CNT loading were studied with respect to structural behaviour and thermal stability using SEM and TGA. It was found that MWCNT-PP nanocomposites show a decrease in creep compliance and creep and recovery strains at 20 °C, and an increase in creep compliance at 50 °C due to the relative motion of entangled polymeric chain and MWCNT.

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.

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.