Mechanics of High-Temperature Composites

Several projects in this general area are concerned with identification of deformation mechanisms and development of constitutive relations for fibrous and particulate composite matericals made of ceramic, intermetallic, and metal constituents. Of particular interest is the behavior at high-service temperatures where time-dependent deformation may become significant. Response to both monotonic and cyclic loads and to thermal changes is considered.

Modeling of Hot Isostatic Pressing and Thermomechanical Processing

Many high-performance composite systems are fabricated by hot isostatic pressing of elemental powders and fibers at elevated temperature. Cooling to room temperature causes dissimilar thermal deformation of the phases and thus creates a residual stress state thate may reduce the load-bearing capacity of the finished product. This research explores modifications of the processing variables to obtain more favorable initial stress states.

Thermomechanical Fatigue of Sic/Ti Systems

Titanium matrix composite laminates are considered as candidate materials for advanced aerospace vehicles and propulsion systems. Thermomechanical cyclic loading is of concern as it may lead to fatigue damage and failure. Micromechanical modeling and experimental simulation of the intermal cyclic strain states is conducted to gain understanding of the admissible loading conditions for these systems.

Optimum Design of Submersible Composite Structure

Carefully manufactured carbon fiber-epoxy matrix laminates exhibit high compressive strength that is desirable in structures supporting high pressures. Optimized, minimum weight design procedures are being studied for improved performance at reduced cost.

Transformation Field Analysis of Heterogeneous Media and Composite Laminate

In addition to extermal load, composite materials and heterogeneous media in general are often loaded by internal deformations that may be caused by thermal changes, phase transformations, moisture absorption, or inelastic deformation. A general method for analysis of these effects has been developed for application to diverse practical problems.

Thermomechanics of Functionally Gradient Materials

Composite materials with prescribed distributions of elastic moduli and coefficients of thermal expansion are of interest in applications involving large temperature or stress gradients. Micromechanical modeling of the local gradient fields and overall thermomechanical properties, and design of such systems, are in progrss.

Experimental Characterization of Composite Materials

Measurement of overall mechanical properties, such as stiffness, inelastic deformation as a function of loading path and time, and strength, are conducted under axial and biaxial monotonic or cylic loading, on plate and tubular specimens, at temperatures up to 1200 degress C.

Mechanics of Piezoelectric Composites and Active Materials

Composite materials made of electroelastic constituent materials exhibit coupled properties. Micromechanical modeling of such systems reveals the intermal structure of the macroscopic constitutive relations that need to be considered in design. Related studies of composites with shape memory alloy constituents are in progress.

Damage Development in Composite Laminates

Internal damage states, such as fiber debonding and matrix cracking, can reduce stiffness, open the material to environmental attack, and inhibit strength. Failure maps that indicate the overall stress states leading to onset of damage in ceramic and intermetallic systems are in development.