Skip navigation

Structures and Materials

Structures & Materials Lab Facilities
W. Chen

Dr. Chen's research activities mainly involve the development of novel dynamic material characterization techniques and the determination of dynamic responses of engineering materials at high loading rates. He built dynamic material characterization laboratories at California Institute of Technology, University of Arizona, and Purdue University. He also assisted the development of such laboratories at Sandia National Laboratories in Albuquerque, NM and Livermore, CA; Army Research Laboratory in Aberdeen Proving Ground, MD; U.S. Army Waterway Experiment Station in Vicksburg, MS; National Institute of Standard and Technology in Gaithersburg MD; and a number of university and industrial laboratories. The techniques he developed are focused on ensuring valid testing conditions during dynamic experiments to obtain accurate material properties at high rates of loading. These techniques, summarized in over 15 journal articles, have been well accepted in the research community. Two of top five, four of top ten "most cited papers of Experimental Mechanics" are from Dr. Chen's group.
Using the novel techniques, Dr. Chen and his students have obtained accurate and reliable material behavior at high rates for soft rubbers, glassy polymers, polymeric foams, gelatins, glass/epoxy composites, soy-bean based clay nanocomposites, biological tissues (muscles, skins, bones), shape memory alloys, high-strength steels, geomaterial, masonry materials, textile materials, and armor ceramics. For each class of the materials under dynamic tension, compression, or multiaxial compression, at various temperatures, his group examined the valid dynamic testing conditions to obtain valid experimental results. Microstructural characterization was carried on some of the materials. Based on the experimental results and microstructural observations, material constitutive models were developed to describe the recorded material behavior. Over forty journal articles have been published based on the results from these research programs.
The research accomplishments demonstrate that Dr. Chen has established himself with unique contributions in the field of experimental solid mechanics. He has developed an independent and well funded research program investigating the dynamic mechanical behavior of materials and the necessary experimental techniques, and has established a national and international reputation in his field.

W. A. Crossley

Professor Crossley's major research interests are in the area of design methodologies and optimization, with emphasis on the use of genetic algorithms for aerospace engineering design problems. Techniques like the genetic algorithm will allow optimization-like techniques to be applied in the conceptual phase of design, which traditionally has been dominated by qualitative or subjective decision making. There are two major areas of research being pursued by Professor Crossley and his students - genetic algorithm roles in aerospace design and optimization, and genetic algorithm methodology development. Other research topics related to aerospace design are also investigated.

Topology Design of Rotor Blades for Aerodynamic and Structural Concerns. This computational research effort strives to develop a rotor blade design strategy with the potential to improve the aerodynamic, structural, and dynamic performance of advanced rotorcraft. This work investigates the Genetic Algorithm (GA) as a means to combine aerodynamic and structural concerns for topology design of rotor blades. Inverse airfoil design and optimal airfoil design are receiving much attention in both industry and academia; the same holds true for structural optimization. The combination of the two concerns for topology design has not been fully addressed. A multidisciplinary approach combining structural and aerodynamic concerns for optimal topology design of rotor blades provides potential benefit to the rotorcraft design process. The aerodynamic optimization portion of this research was cited in the technical research highlights of the NASA Ames Research Center, Rotor Aeromechanics Branch for 1999. Contributions in the structural portion of the research have demonstrated capabilities for discrete (on/off) topology, most notably handing connectivity issues and performing design of sections under combinations of bending and torsion, that several authors had previously claimed were not possible.

Genetic Algorithm Issues for Optimal Smart Actuator Placement. This research is investigating approaches for smart actuator placement to provide aircraft maneuverability without requiring hinged flaps or other control surfaces. The effort supports many of the goals of the Multidisciplinary Design Optimization focus efforts in NASA's Aircraft Morphing program. Computational studies are being conducted to allow comparison and selection of appropriate techniques for posing and solving an actuator placement problem. The work began with a geometrically simple wing model, but the approaches identified during this research have been applied to complete aircraft configurations. The problem statement and algorithm application are being used at NASA Langley by researchers working on the Aircraft Morphing Program. Research in this area has been cited twice as technical highlights for the NASA Langley Multidisciplinary Optimization Branch; once in 1998 and once in 1999. Improved Satellite Constellation Design and Optimization. Improving satellite constellation design is of great interest to any users of satellite communication (e.g. cellular phones, television), location (e.g. global positioning system) and/or observation (e.g. weather). Many of today's satellite constellation designs rely on the "Walker Constellations," a series of designs developed in 1970, which have rarely been improved upon. These constellations make use of symmetric constellations with circular orbits. Using the genetic algorithm to search the constellation design space has begun to yield constellation designs not previously envisioned but with performance equal to or greater than comparable Walker or "streets of coverage" constellations. Research is ongoing for sparse coverage constellations, constellation build-up problems, multiobjective constellation concerns and elliptic orbit constellations. The Aerospace Corporation performs satellite constellation design for its US Air Force customers using the design techniques developed as part of this research. In one of these studies, a multiobjective GA approach was able to generate constellation designs that outperformed constellations that had been under development for several months. The GA was able to do this in a matter of days.

Development of a Genetic Algorithm for Conceptual Design of Aircraft. Air vehicle conceptual design appears to be a promising area for application of the genetic algorithm as an approach to help automate part of the design process. Because the GA-based approach to conceptual design helps to reduce the number of qualitative decisions needed from the design team, this appears to have great potential for application to aircraft design. Work has been extensively conducted for helicopters, some additional work has been conducted for high-speed VTOL rotorcraft (e.g. tilt-rotor and tilt-wing aircraft), and work is currently underway for fixed-wing aircraft. The Systems Analysis Branch at NASA Langley Research Center supports this research.

Methods to Assess Commercial Aircraft Technologies. Increasing competition in the commercial aircraft industry requires that airframe manufacturers be judicious with technology research and development efforts. Currently, technology development strategies for commercial aircraft appear to be lacking; this research presents a methodology to assess new technologies in terms of both cost and performance. This methodology encompasses technologies that can be applied to the aircraft design and technologies that improve the development, manufacturing, and testing of the aircraft. This differs from past studies that focused upon a small number of performance-based technologies. The method is divided into two phases. The first phase evaluates technologies based on cost measures alone. The second phase redesigns an aircraft with new technologies, assesses the relative importance of performance-based technologies, and recognizes technology interactions using Taguchi's Design of Experiments. For a wide-body transport aircraft example, the methodology identifies promising technologies for further study. Recommendations and conclusions about the methodology are made based on the results. This work was done in collaboration with the Configuration Engineering and Analysis group at Boeing Commercial Aircraft. Response Surface Methods as Approximation Models for Optimization. Approximation techniques, particularly the use of response surfaces (RS), have achieved wide popularity in engineering design optimization, especially for problems with computationally expensive analyses. The chief aims of using RS is to lower the cost of optimization and to smooth out the problem (e.g., for analyses solved iteratively, with a convergence tolerance). In one part of this research effort, an investigation of RS methods to minimize drag of a turbofan nacelle is being pursued in conjunction with engineers at Allison Advanced Development Company. This approach can improve the nacelle design practices at AADC by providing a formalized optimization framework for this CFD-based design exercise. The use of RS raises practical questions about the solution accuracy and computational expense. In particular, building response surfaces may involve a prohibitively large number of high-fidelity function evaluations, depending on problem dimensionality. In another part of this research effort, a computational study to address questions of expense and accuracy was undertaken with researchers in the Multidisciplinary Optimization Branch at NASA Langley Research Center. Important observations about the impact of constructing and using response surfaces for moderately high-dimensional problems were made. NASA researchers are using the RS models constructed during this portion of the research to further investigate techniques to manage approximation models in engineering optimization.

J. F. Doyle

The main research area is Experimental Mechanics with an emphasis on the development of a new methodology for analyzing impact and wave propagation in complicated structures with the goal of being able to extract the complete description of the wave and the dynamic system from limited experimental data. Special emphasis is placed on solving inverse problems by integrating experimental methods with computations methods (primarily Finite Element based methods).

This research is based on the assertion that, in modern analyses, constructing the model is constructing the solution—that the model is the solution. But all model representations of real structures must be incomplete; after all, we cannot be completely aware of every material property and every aspect of the loading and every condition of the environment, for any particular structure. Therefore, as a corollary to the assertion, we posit that a very important role of modern experimental stress analysis and experimental mechanics in general is to aid in completing the construction of the model.

That the model is the focal point of modern Experimental Mechanics has a number of significant implications. First, collecting data can never be an end in itself. Invariably, the data will be used to infer indirectly (or inversely) something unknown about the system. Typically, they are situations where only some aspects of the system are known (geometry, material properties, for example) while other aspects are unknown (loads, boundary conditions, behavior of a nonlinear joint, for example) and we attempt to use measurements to determine the unknowns; these are partially specified problems. The difficulty with partially specified problems is that, far from having no solution, they have a great many solutions. The research question revolves around what supplementary information to use and how to incorporate it in the solution procedure. Which brings us to the second implication. The engineering point to be made is that every experiment or every experimental analysis is ultimately incomplete, there will always be some unknowns and at some stage the question of coping with missing information must be addressed. This is not a question of collecting more data, "re-do the experiment," or design a better experiment. This is not a statistical issue where if the experiment is repeated enough times, the uncertainty is removed or characterized. We are talking about experimental problems which inherently are missing enough information for a direct solution.

These ideas with the accompanying algorithms and examples are presented in the book:

  1. J.F. Doyle, Modern Experimental Stress Analysis: completing the solution of partially specified problems, Wiley & Sons, UK, 2004.
The problems tackled cover the complete range of topics and include static/dynamic, linear/nonlinear applied to a variety of structures and components.

A final point: current technologies are exploring the possibility of using nanostructures in a variety of applications. These do not lend themselves to traditional methods of experimentation; the inverse methods being developed here offer great potential to overcome the challenges.

A. F. Grandt

General technical interests deal with assuring the safe operation of aerospace and other complex structures through damage tolerance analyses and nondestructive inspection. Particular emphasis is on basic research to predict critical and subcritical crack growth under static and cyclic loads (i.e. fracture and fatigue). The influence of corrosion on structural integrity is also of interest. This research employs numerical and experimental approaches, and impacts both the design of new components as well as the continued operation of older structures (i.e. aging aircraft).
Experimental research is conducted in the School of Aeronautics Fatigue and Fracture Laboratory. This facility has two computer-controlled electro-hydraulic test machines and associated equipment needed to measure fatigue crack formation, propagation, and fracture in specimens subjected to simulated aircraft or spacecraft load histories. A scanning electron microscope is on hand to examine fracture surfaces and equipment are also available to artificially corrode specimens in connection with corrosion and/or corrosion-fatigue related research.

P. Imbrie

Engineering epistemologies and assessment of teaming and teamwork, of student success and retention, and of global awareness, values, and competencies. Development of reliable data preservation, access, integration, and analysis capabilities for engineering education. Outcome-based assessment.

R. Pipes
Dr. Pipes' graduate students work in composites manufacturing simulation focuses on the influence of manufacturing processes on the development of composites microstructure and the resulting implications upon structural performance. Two primary manufacturing processes are the subject of the majority of the scholarship underway and these include composites additive manufacturing (cAM) through fused filament fabrication and discontinuous prepreg platelet composites. For cAM, the research is directed at the prediction of the free-surface flow of fiber-reinforced polymer extrudates and the multi-physics phenomena that determine extrudate-substrate adhesion, shrinkage deformation, mechanical and transport properties and performance of the printed geometry. The discontinuous prepreg platelet studies examine the rheology of these highly anisotropic systems to determine fiber orientation distribution within a molded geometry, micro-CAT scan methods for determination of fiber orientation and prediction of the strength properties of structural elements molded from these materials systems.
M. Sangid

Professor Sangid’s research activities combine knowledge of materials science, solid mechanics, and advanced manufacturing to solve complex problems in materials behavior and processing. His research group employs physics-based computational modeling and design tools, which are experimentally validated and verified. The goal of this work is to improve our understanding and our tools for designing, processing, and lifing materials through simulation-based modeling of the microstructural defects. His research specifically focuses on (micro)structure to performance modeling, via the use of atomistic simulations to inform multi-scale models for plasticity, fatigue, and fracture of metallic alloys and high temperature composites. Both material systems have direct applications in Aerospace Engineering. Many times, it is necessary to start at the atomistic level to gain a quantifiable understanding of the deformation pathways and failure mechanisms at component scales. Further, there is also an experimental component to his research as he does advanced materials testing and characterization including digital image correlation, advanced microscopy, and high-energy x-ray diffraction. Thus, the most advanced characterization and interrogation methods are exercised at each scale to verify and validate model predictions, including four dimensional mapping of ‘defect’ features, strain fields, and complex stress states within the material.

Professor Sangid is the principal director of the Advanced Computational Materials and Experimental Evaluation (ACME) Laboratory. The ACME group’s philosophy is to simultaneously address fundamental research needs and implement this knowledge into integrated models that can directly aid in and transform our design methodologies providing pragmatic engineering solutions. The overarching goal and uniqueness of the ACME group’s modeling strategy is to avoid fitting parameters prevalent in classical engineering models, while providing a general framework, which allows: (1) Easy integration and modification of software, (2) Uncertainty quantification and probabilistic lifing, and (3) Pragmatic tools to answer production questions.

C. Sun

Current research interests include the following areas:

Composite Materials and Structures – Advanced fiber composites have gained wide applications in aircraft and aerospace structures. Our research programs cover a broad spectrum in mechanics and design of various composite materials and structures. Research topics include developing methods for testing and modeling high strain rate and fracture behavior of polymeric composites, unconventional modeling of heterogeneous solids, exploring the use of nano particles in reinforcing composites, developing self-assembly methods for processing nanocomposites, improving methods for joining composite structures using adhesives, and developing multifunctional composite materials and structures.

Fractures Mechanics – Fracture mechanics is an important tool in analyzing failure in materials and structures. Our current research focuses on (1) the inadequacy of LEFM in brittle fracture due to the small size of K-dominance zone that renders stress intensity factor alone to be incapable of characterizing fracture toughness; and (2) the physical foundation of cohesive zone modeling of fracture with the attention centered on the cohesive traction law: its physical meaning and conditions it must satisfy.

Nanomaterials – Many nanostructured materials possess highly desired physical and mechanical properties and offer tremendous potentials in many applications. Our research is concentrated on developing multiscale modeling techniques for nanomaterials and their composites and on the use of molecular mechanics to study the mechanical behavior of nanomaterials including nanocomposites.

Acoustic Metamaterials – Acoustic metamaterials are materials with man-made microstructures that exhibit unusual mechanical properties such as negative effective modulus or negative mass density that is not found in natural materials. Many of the recently developed metamaterials are in the form of composites. If modeled as a homogeneous classical elastic solid, the effective mass density of a metamaterial would have the form of a second order tensor and is frequency-dependent. Moreover, the microstructure can be designed so that the representative homogenous elastic solid would possess an anisotropic mass density and may become negative in certain frequency range. This unusual property may be used to tailor the wave motion and produce an anisotropic band gap structure of the metamaterial. Many novel applications can be realized using the anisotropy and negativity of the mass density of this metamaterial. Potential applications of acoustic metamaterials are still being explored.

V. Tomar