Structural Mechanics

Multiscale Modeling

Multiscale modeling is a research thrust area with a wide variety of potential applications in the engineering arena. Airplane components are subjected to cyclic loading conditions throughout their service lives. The associated phenomenon of fatigue inherently involves multiple scales due to the presence of micro-size cracks or inclusions with respect to the large size of the component of interest. It is necessary to scale multi-physical behavior from the micro-level to the full-scale system level while developing a clear understanding of the material performance/ degradation in operational environments to assess the survivability of engine fan containment structures through improved blade-out event simulation and damage evaluation. Given these tremendous disparity of scales, any attempt at life predictions is a great challenge to mechanics and materials communities. Advancements made in the area of multiscale modeling in recent years make it possible to attempt such predictions with a reasonable level of accuracy.

With the development of new processing techniques capable of analyses involving multiple scales, such as nanotechnology dealing with multiply spatial scales, multiscale modeling is making inroads into a wide range of applications in fields such as engineering, physics, and computer science. The nanomechanics theory incorporates the atomistic information from the interatomic interactions and nano-scale structures and can overcome the limitations of atomistic simulation methods on length and time scales. Complicated nano-scale structures, such as nano-composites can be studied using these methods. Current research activities in multiscale modeling and related applications are funded by AFOSR, ARO and NSF.

Stretchable Electronics

To make electronics stretchable is one of the top priorities for next-generation electronics as it provides fully reversible mechanical stretchability in electronic interconnects or in the active devices themselves such as field effect transistors. Integrated electronics that use such stretchable components could be important for devices such as flexible displays, eye-like digital cameras, conformable skin sensors in artificial muscle, intelligent surgical gloves, and structural health monitoring devices.

 Damage Mechanics

One of the key applications for multiscale modeling is in the area of damage propagation predictions in structures such as aircraft, high pressure vessels etc. The need for such a tool is especially acute in dealing with the issue of aging aircraft in both civilian and military transportation sectors. Coupled with the fact that composite materials are increasingly being used in aerospace, mechanical as well as infrastructure applications, this problem and its solution are proving to be more complex than ever before. Research in this area is funded by agencies such as AFOSR, ARO, DOE and NSF.

Material Characterization

Mechanical Behavior and Structure Property Relations of Materials
Work is focused on basic and applied aspects of mechanical behavior and structure property relations of materials, which are a natural it for the center. Interest is centered on deformation and fracture of metals, intermetallics and ceramics under a variety of load conditions, including cyclic loading, quasi-static, dynamic (shock), as well as blunt and sharp contact. Projects are typically undertaken with a “vertical” approach that includes: (i) Materials processing: single crystal and bicrystal growth, directional solidification, alloy preparation, heat treatment under controlled atmosphere, etc. (ii) Sample preparation: electro- discharge machining, high precision polishing, materialography, electro-polishing, focused ion beam (FIB) machining, etc. (iii) Testing: uniaxial testing, instrumented indentation, fracture toughness, fatigue crack growth, three- and four-point bending, environmental (air, inert gas) and high temperature (up to 1700 °C) capabilities Characterization: optical and electron microscopy (SEM and TEM), Orientation Imaging Microscopy, Atomic Force Microscopy, FIB imaging, serial sectioning, digital image correlation, in-situ testing with SEM, etc. (iv) Modeling: analytical and semi-analytical analysis of elastic, inelastic and hydrodynamic behavior and large deformation numerical analysis using finite elements and hydrocodes.

Fatigue Crack Growth Characterization
Plastic deformation ahead of a fatigue crack tip is related to crack advance. The blunting has to be accommodated by strain ahead of the crack tip; this implies that there is a close relationship between crack advance and strain. In-situ experiments in the SEM along with digital image correlation are in use for measuring displacement and strain fields ahead of stage II fatigue cracks n nickel, which are found to be localized in strong deformation bands that usually carried pure shear.

Structural Property-Relations in Surrogate Nuclear Fuel Materials
The next generation of nuclear reactors being planned will require advanced nuclear fuels that can burn hotter and longer with higher structural reliability. Advanced fuels based on actinide mononitrides (UN and PuN) are being investigated as potential advanced fuels. ZrN is being used as a “surrogate” for actual fuels, due to its similarities with actinide nitrides in terms of crystal structure, microstructure and mechanical behavior. Failure of nitrides under load is being related to its microstructure, including grain geometry, grain orientation, pore size and location. Microstructurally explicit finite elements are being carried out in 2-D and will be performed in 3-D to study these effects.