Structures and Materials
Structural mechanics is the study of the mechanical behavior of solids and structures. Aerospace structures differ from other structures due to their high demands for performance and lightweight. Modern aerospace structures typically require the use of composite materials, advanced multifunctional materials and thin-walled constructions. To obtain the level of performance required from flight structures, thorough knowledge of material limitations, structural stability and strength considerations are needed. Current research in the Department emphasizes the characterization of advanced materials, material and structural stability, computational material/structural design, thermo-mechanical and electro-mechanical interactions, structural dynamics, multiscale modeling, multifunctional structures, morphing structures, aeroelasticity, structural health management and design optimization. This specialization covers theory, computations, experiments and implementation issues, as well as the study of specific cutting edge aerospace vehicles.
Selected Research Projects of the Four Faculty Members of the Structural Mechanics Group:
Director of the Active Aeroelasticity and Structures Research Laboratory
Research interests: Multifunction active aeroelastic structural modeling, analysis and design; structural health management
Coupled Nonlinear Flight Dynamics/Aeroelasticity of Very Flexible Aircraft
The mission profile of high-altitude long-endurance (HALE) UAV's requires new structural design paradigms to achieve effective weight and corresponding vehicle performance. By taking advantage of the flexibility of lifting surfaces instead of fighting it, our research has been uncovering fundamental nonlinear mechanisms that couple aeroservoelasticity and flight dynamics.
Aerothermoelastic Modeling of Hypersonic Vehicles for Control Design
Air-breathing hypersonic vehicles (HSV) present strong interactions among aerodynamics, elastic airframe and control effector deformations, heat transfer and propulsion system (itself tightly integrated into the lifting body). As part of a collaborative center with the AFRL and in collaboration with NASA, our research focuses in two main areas: (i) development and validation of simple (low-order) control models that can characterize the main aerothermoelastic effects coupled with propulsion in a 6 DOF flight dynamics simulation of HSV; and (ii) determination on how to appropriately modify vehicle configuration to improve its dynamics controllability without compromising vehicle performance.
Active Vibration and Noise Control of Helicopters
Helicopters have severe vibration and noise problems primarily coming from the rotor blades immersed in an unsteady aerodynamic environment. The acoustic noise signature of helicopters can be affected by altering the blade-vortex interaction pattern, which reduces their detectability. Vibrations have also serious consequences to equipment and troop fatigue, and equipment readiness. Our research has addressed active blades that dynamically reshape themselves to reduce both vibration and acoustic noise. His work has defined the modeling framework in which active twist rotor blades employing embedded piezoelectric materials can be aeroelastically tailored. It has been used to design, build and test active twist rotor blades with very low vibration levels. The design and prototype blade have culminated in two first-of-a-kind forward flight wind tunnel tests (open and closed loop) of a 1/6th Mach-scale, fully-active, four-bladed rotor system that experimentally demonstrated the concept. Currently, simultaneous vibration and acoustic noise reduction studies are under way.
Micro Air Vehicles (MAVs) structural dynamics and aeroelasticity
MAVs can meet evolving asymmetric threats and support homeland security by providing the ability to fly in urban settings, tunnels and caves, maintain forward and hovering flight, maneuver in constrained environments and "perch" until needed. Our current research is utilizing insights gained from biological flight and focuses on hovering and forward flight modes of MAVs with an emphasis on the intrinsically unsteady environment due to wind gust and flapping motion. This is part of a bigger effort with other colleagues from the Department and AFRL which overall objective is to develop the fundamental scientific foundation necessary to enable agile, autonomous computational aeroelasticity and nonlinear structural dynamics modeling for flapping wing MAVs.
Structural Health Management
Integrated systems health management and particularly structural health monitoring will enable the readiness required for future Space Force reusable launch vehicles. The real time monitoring of the damage development on primary and secondary structures subjected to a wide range of environmental conditions demand special transducers, data processing and fusion. Our research has been addressing the fundamental mathematical formulation of guided wave (GW) propagation generated by surface-mounted and/or embedded piezoelectric actuators. The theoretical development has been motivated and validated through environmentally controlled experiments. From the theoretical formulation, guidelines for sizing actuators and sensors in view of the system's architecture have been established. New transducer concept based on radar scanning techniques is been pursued. Further studies on high-temperature carbon-carbon composite development to determine a suitable failure index of the structural component. The self-sensing characteristics of those structures further support the notion of multifunctional structural concepts.
François-Xavier Bagnound Professor
Director, FXB Center for Rotary and Fixed Wing Air Vehicle Design
Research interests: Helicopter and fixed wing aeroelasticity, active control, aero-servoelasticity, active materials, rotary-wing aerodynamics.
Blade Vortex Vibration Alleviation in Helicopters Using Actively Controlled Trailing Edge Flaps
Blade Vortex Interaction is a source of vibration and noise due to the rotor blade interacting with the tip vortex shed from the preceding blade. Novel means to reduce vibration are developed using actively controlled rotor blade flaps.
Vibration Reduction in Helicopters Using the Active Control of Structural Response Approach and Improved Aerodynamics Modeling
A flexible fuselage, at selected locations, is excited by controlled forcing inputs, such that the combined response of the fuselage, due to rotor loads and the applied excitation, is minimize
Rotary-Wing Aeroelastic Scaling and its Application to Adaptive Materials-Based Actuation
A novel approach to aeroelastic scaling produces aeroelastic scaling laws by a judicious combination of the classical approach with simulations.
Aeroservoelasticity of a Hypersonic Vehicle
The aeroservoelasticity of a generic hypersonic vehicle is studied to predict vehicle flutter boundaries and develop means for flutter suppression.
Director of the Adaptive Materials and Structures Laboratory
Research interests: Active materials, smart structures, material instabilities, thermo-mechanical constitutive modeling.
Experimental and theoretical studies of shape memory alloys, such as NiTi, providing an understanding of the coupling between the mechanical and thermal behavior of the material.
Design, fabrication, and testing of novel cellular forms of SMAs to develop new types of thermal actuators and highly resilient structures.
Thermo-mechanical Constitutive Modeling
Constitutive modeling at the continuum and atomic lattice scales to capture the complexities of therm-mechanical coupling and material level instabilities in shape memory alloys and elastomeric materials.
High Temperature Degradation of Elastomeric Components
Experimental and theoretical studies of the chemorheological behavior of elastomeric materials at elevated temperatures to develop service life predictions for elastomeric components.
Development of infrared imaging, custom-built temperature control facilities and photogrammetry techniques to enable constitutive thermomechanical experiments.
Director of the Multi-Scale Structural Simulations Laboratory
Research interests: Computational mechanics, computational material science, multi-scale modeling, atomistic modeling of materials, computational materials design.
Multi-scale homogenization and finite element sensitivity analysis techniques are developed to optimize microstructures of aircraft materials so that mechanical properties are enhanced. With this approach, polycrystalline materials with tailored elastic modulus (see figure), yield strength and magnetic hysteresis loss distributions have been developed. Simulations are performed on a 200 processor Beowulf cluster at the Center for Advanced Computing. Mathematical model reduction and statistical learning methods are used to facilitate rapid exploration of the space of material variability and generation of property closures with a goal to optimize properties in aircraft materials.
Multiscale approaches for simulation of composite property degradation:
We are interested in multiscaling (continuum-atomistic, finite element homogenization) and coarse graining (multi-body expansion, cluster expansion) techniques for linking simulations at different length scales. Such techniques are used to compute property degradation of composites in high temperature (eg. propulsion, reentry) environments in the presence of oxidation and matrix-fiber interface separation (atomistic phenomena) and bulk transport of gases in the matrix and thermal stress development (continuum phenomena).
Atomistic modeling of deformation and failure:
Deformation and failure of structures and interfaces at atomistic length scales are studied. We employ molecular dynamics (MD) and more accurate first-principles simulations (DFT) for modeling phenomena such as grain boundary behavior (see figure), interface sliding and separation. Non-linear constitutive relations that describe complex microscopic processes are subsequently employed in continuum FE simulations. Features such as stress concentrations at the tips of the distributed cracks that cause plastic deformation in the grain interiors are studied using such approaches.