Structural mechanics is the study of the mechanical behavior of solids and structures. Aerospace structures differ from other structures due to the highest demands for performance and light weight. 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, 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.

Faculty
Carlos E. S. Cesnik
Peretz P. Friedmann
Daniel J. Inman, Chair
John A. Shaw
Veera Sundararaghavan
Anthony M. Waas
Peter D. Washabaugh

Selected Research Projects of the Seven Faculty Members of the Structural Mechanics Group:

Multifunction Active Aeroelastic Structural Modeling, Analysis and Design; Structural Health Management - Carlos E. S. Cesnik, Associate Professor and Director, Active Aeroelasticity and Structures Research Laboratory

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 1/6^th 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.

Helicopter and fixed wing aeroelasticity, active control, aero-servoelasticity, active materials, rotary-wing aerodynamics - Peretz P. Friedmann, FranÇois-Xavier Bagnound Professor and Director, FXB Center for Rotary and Fixed Wing Air Vehicle Design

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 minimized.

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.

Active materials, smart structures, material instabilities, thermo-mechanical constitutive modeling - John A. Shaw, Professor

Active Materials
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.

Active Structures
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.

Experimental Mechanics
Development of infrared imaging, custom-built temperature control facilities, and photogrammetry techniques to enable constitutive thermomechanical experiments.

Computational mechanics, Computational material science, Multi-scale modeling, Atomistic modeling of materials, Computational materials design. - Veera Sundararaghavan, Assistant Professor [Lab website]

Micro-structure-sensitive 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.

Nonlinear continuum mechanics, finite elements, bifurcation-stability of solids, microstructured media - Nicholas Triantafyllidis, Professor

Solids with Regular Micro-structures
Modem aerospace materials have periodic micro-structures (composites, honey-combs, foams); of interest is their nonlinear microscopic behavior and failure.

Stability of Large Flexible Structures
Lightweight flexible aerospace structures, i.e. space trusses, have buckling instabilities with many modes which interact.

Mechanics of Manufacturing Processes
Modeling of cold forming (stamping, hydroforming) and failure problems such as wrinkling, puckering, spring-back.

Nanomechanics of Crystalline Solids
Atomistic-level studies of thermo-mechanical stability of shape memory alloys.

Mechanics of composites, stability, biomechanics, nanocomposites - Anthony M. Waas, Professor

Progressive Failure of Composites and Composite Structures
Advanced fiber-reinforced composites exhibit a variety of failure mechanisms that must be understood to utilize the full potential that these materials offer. Modern experimental and analytical techniques are used to investigate failure mechanisms of continuous fiber-reinforced composites as a function of loading and environment.

Mixed Mode Damage Growth in Laminates
Through-the-thickness damage growth in laminated fuselage panels is investigated in collaboration with Boeing researchers. The objective of this research is to develop suitable computational techniques that can predict damage growth in fuselage type structures that can contain large area damage as that caused by, for example, by a blade throw out. Data obtained from laboratory scale specimens are used to obtain the input data that is necessary for the large structural scale models. As a result, size effects due to specimen scaling are also simultaneously studied with a view to developing scaling laws that can be used for the computational studies.

Structures and Materials for Extreme Environments (SMEE)
High performance composite materials are being incorporated into many aerospace structures, where their specific stiffnesses and strengths are advantageous. It is critical that these materials are proven in the extreme environments to which they will be subjected. Furthermore, it is vital that modeling techniques keep pace or exceed advances in the materials, so that engineering analysis can drive the design of structures that are enabled by these materials.

A critical area currently being investigated at the University of Michigan is analysis of adhesively bonded joints. Bonded joints offer improved load distributions over the fasteners that they are replacing. However, techniques for designing these joints are immature, particularly in the arena of extreme environments. The constitutive and structural responses are highly non-linear, and require special attention in order to provide a converged predictive solution.

Current efforts have focused on the Discrete Cohesive Zone Model (DCZM), a technique, developed in the Aerospace Engineering Department. Using this technique, researchers have had significant success in predictive modeling of joint behavior at the coupon level over a broad range of temperatures. Furthermore, recent advances have shown that the technique generalizes to the structural level, where good correlation has been achieved with experimental results. The current effort is at the leading edge of bonded joint analysis, and is expected to continue to provide industry leading solutions for analysis of joint failure and composite delamination.

Stiffened Composite specimen subjected to compression and buckling. Adhesive failure is predicted at the termination of the stiffener.

Textile Composites for Aerospace and Automotive Applications
The mechanical behavior of braided and woven resin infused textile composites are studied with a view to establishing micromechanics based modeling tools for predicting compression strength, crush energy absorption, fracture and damage growth behavior. Experimental, analytic and numerical studies are conducted for evaluating the performance of large structures made of textile composites with modeling strategies that span several length scales (from the fiber/matrix scale to the layered structural scale).

Ultra strong and Stiff Nanocomposites
While nanoscale building blocks are individually exceptionally strong, their properties are difficult to transfer to the macroscale composites made from them. Assembly of a clay (aluminosilicate )/polymer composite one nanoscale layer after another allowed for preparation of a homogeneous material with planar orientation of aluminosilicate sheets. Its stiffness and tensile strength are similar to those of steel and are at least an order of magnitude greater than those for similar nanocomposites that do not have the nanoscale precision in particle alignment. These properties are combined with transparency and low temperature processing. Besides obvious technological importance of such composites, the realization of nearly perfect load transfer from individual sheets to the macroscale material is fundamentally important for the design of ultrastrong materials. This collaborative work between the Waas group (aero), Arruda group (ME) and the Kotov group (Chem. Eng.) is currently investigating fracture properties of the nanolayered films and also the high strain rate mechanical performance for a variety of applications.

Experimental solid mechanics, fracture mechanics, non-destructive testing, optimal design. Peter D. Washabaugh, Associate Professor

Experimental Solid Mechanics
Recent advances in technology are used to enhance mechanical measurements, including high-speed cinematography, interferometry, and fiber optic displacement gauges.

Fracture Mechanics
Static, fatigue and dynamics loading on structures containing cracks are studied and compared to models.

Non-Destructive Testing
High-resolution X-ray tomography is used to differentiate materials in fibrous carbon-based composites.

Optimal Design
Designs of Micro Air Vehicles, adaptive space structures such as large-aperture telescopes, and minimal energy self-gravitating solids, are studied.