A Consistent Plate Theory
D. A. Tortorelli*
National Science Foundation, DDM 93-58132 NYI

Much work has been performed to develop finite elements for the analysis of plates. These theories, primarily based on the Kirchoff assumptions, contain certain inconsistencies. In this study, a plate theory consistent with 3-D elasticity is developed using the theory of internal constraints. The basic assumptions of the Kirchoff plate theory are enforced as internal constraints. In this way, no inconsistencies are introduced. A 3-D, 8-noded brick element is developed and yields satisfactory results in the analysis of some simple plate problems. The applicability of the element to more complicated problems is being investigated.

Application of Strain Gradient Plasticity-Modeling and Experiments
Y. Huang,* M. X. Shi, Z. Xue
National Science Foundation, 98-96285; Campus Research Board; National Science Foundation of China

The purpose for this research project is to develop a microscale plasticity theory for applications from 0.1 to 10 microns such as in nano- and microindentations, microelectronic devices, and MEMS.

Computational Methods for Mechanism-based Higher-Order Continuum Theories
Y. Huang,* P. Zhang
National Science Foundation, CMS-9983779

The objective of the proposed work is to develop novel computational methods for the recently proposed virtual internal bond (VIB) method. The VIB method extends the application range of classical continuum mechanics to modeling cohesive elasticity at much smaller length scales (e.g., lattice spacing in elastic deformation). A major obstacle to the application of such theory is the lack of efficient numerical methods for higher order or nonlocal continuum theories involving multiple distinct materials length scales. We propose to develop a robust and reliable numerical method to analyze multiscale phenomena that cannot be addressed by classical continuum theories.

Evaluation of Stress Redistribution and Notch Sensitivity in Ceramic, Metal, and Polymer Matrix Composites
T. J. Mackin,* G. Horn
National Science Foundation, CMS 96-24705; Stress Photonics

The use of high-temperature CMCs requires holes, notches, attachments, and various joining procedures, all of which lead to stress concentration. Consequently, these stress concentrators are the most likely locations of failure in the material. Fortunately, fiber reinforcements impart a degree of ductility to CMCs that mitigates the stress concentration. Various CMCs are being investigated to determine their performance and notch sensitivities. Thermoelastic stress analysis (TSA) is being used to quantify damage progression; the results are used in the development of materials models and the subsequent development of new materials.

Full-Field Strain Mapping Using Image Analysis
T. J. Mackin,* G. Horn, D. Gardner
AT&T; National Science Foundation, CMS 96-24705; U.S. Air Force Office of Scientific Research

The advent of relatively small, high-powered computers are creating new opportunities for real-time processing of experimental data. Video and photographic images of test specimens are captured during loading. Key features on the specimens are identified and tracked throughout the loading history, enabling full-field displacement measurements as a function of applied load. The displacement fields are converted into strains, followed by constitutive transformation into stresses. This procedure provides a new, noncontacting tool for experimental stress analysis.

Interface Design of Composites for Improved Damage Tolerance
T. J. Mackin,* N. R. Sottos* (Theoret. & Appl. Mech.), V. Damljanovic, M. Brandi
U.S. Air Force Office of Scientific Research

A detailed investigation of composite constituent properties as related to stress redistribution, notch sensitivity, and damage evolution. Model ceramic, polymer and cement matrix composites are fabricated with a range of constituent properties. These properties are measured using standard tensile tests as well as fiber pushout tests. The micromechanical mechanisms are then related to the macromechanical response. Damage evolution is quantified using infrared imaging and Moire interferometry.

Stresses under Contact Loading and Material Ratchetting
H. Sehitoglu,* Y. Jiang
Association of American Railroads

Based on a stress invariant hypothesis and a stress/strain relaxation procedure, an analytical approach is forwarded for approximate determination of residual stresses and strain accumulation in rolling contact. For line rolling contact problems, the proposed method produces residual stress distributions in favorable agreement with the existing finite-element findings. We study ratchetting behavior of 1070 steel under uniaxial tension-compression and axial-shear loadings experimentally. Strain ratchetting direction exhibits a complex dependence on the previous loading history, including nonconsistence with the mean stress direction. Different models to predict this phenomenon are proposed and compared to experiments.