One of the most important recent developments in materials science is the ability to engineer material microstructures at the atomic level. While in its early stages of development, this capability offers the potential for significant increases in the performance capabilities of structural materials. One of the first applications for this new technology is a microscopic clay based nanocomposite developed by NASA to reduce hydrogen permeability at cryogenic temperatures in fuel storage tanks. Now that these materials are becoming economically viable, it is very important that appropriate materials characterization techniques be developed to quantitatively evaluate the microstructural features of nanocomposites.

Technical Approach

We propose to develop materials characterization techniques suitable for determining the nature and composition of nanocomposites at the microstructural level. In order to be useful, any proposed technique must meet several stringent criteria. In particular, the approach must be:

The approach proposed here employs a specialized laser UT system to accurately measure material properties on a local basis along with a composite micrmechanics model to extract quantitative microstructural features.

Laser UT

A specialized high frequency laser based ultrasonic inspection system is proposed to obtain property information on a localized basis in the particle reinforced nanocomposite. With this approach, laser signals are introduced into the structures and can be used either to generate acoustic waves in a material or to detect their presence. As a generator, the laser source is used to locally heat a point in the test region. The resultant thermal expansion launches an acoustic wave into the material, which can be used to probe structural integrity or analyze material properties. As a sensor, the laser light is coupled to an interferometer to sense the waves generated by the laser source after they have interacted with the structure. Analysis of these wave patterns can be used to determine small differences in material stiffness or density needed to quantitatively characterize the local composition of the nanocomposite. This information is critical in evaluating the degree of homogeneity present in the nanocomposite and the effectiveness of the particulate dispersion phase of the processing.

Micromechanics Modeling

Micromechanics models are commonly utilized in working with composites as they quantitatively relate global properties ( strength, stiffness, density, etc ) to its microstructural constituents. Here, we desire to develop the appropriate mathematical relations, which relate the measured stiffnesses and density of the nanocomposite to its constituent properties and composition. Of particular concern in this regard is the applicability of a continuum mechanics approach to a nanocomposite structure. One of the major goals of this modeling effort is the determination of the frequency range where continuum mechanics assumptions are valid.

Integrated Characterization

In order to quantitavely determine the microstructure of the nanocomposite, it is necessary to couple the laser UT results with the appropriate micromechanics models of material behavior. In this way the problem is posed as a nonlinear inverse problem where microstructural parameters ( particulate volume fraction, porosity, reinforcement homogeneity ) can be obtained from the local ultrasonic velocity measurements from the laser UT system.