Can anyone offer a clarification of the difference between creep and plastic deformation? In various literature outside of continuum mechanics theory, the concepts of creep and plasticity seem to be used interchangeably, and thus likely erroneously. The basic textbook definitions tell me that creep is a time-dependent, potentially irreversible deformation due to one of several microstructural mechanisms. Likewise, plasticity is a irreversible deformation above some yield stress, often atributed to fracture and slippage behaviour at the microstructural level, and time-independent. My question is two fold:

1. Is time-dependent plastic deformation a subset of creep behaviour? i.e. if a material is observed to creeping in a time dependent manner, but the mechanism is, for example, fiber slippage, can this be called plastic deformation?
2. If the material is a polymer, but shows irreversible deformation above some yield stress, is this plasticity, or is this term restricted to crystalline solids?

Many thanks for the clarification or references leading to it.

Here are the slides for my final presentation for ES 241. During the presentation, a few suggestions were made, which I plan to follow up on. Please check back here or subscribe for updates.

This is a great problem. Getting the boundary conditions right is going to be a real challenge. They should be able to be implmented as slip on the free boundary, possibly as a weak form contribution. Instead of posting an article, I have attached a document about this sort of problem in 2D from a model implemented in Comsol Multiphysics. This document provides some basic analysis of the problem, and may be of some use here.

The biology of the inversion process in Volvox carteri is examined, and a coupled mechanical and kinetic model is proposed. See attached proposal for details. The presentation given is also attached here, as well as the final paper. Also, a movie of the simulation that made all of the work wroth while, the inversion of a half-sphere, is attached here as well. Note: The file inversion2bw.doc is a movie, just download it and change the extension to .avi. This has to be done since iMechanica doesn't allow attachment of .avi files directly.

For students with some mechanics and math background, and some interest in biological problems, I would like to recommend Y.C. Fung's "A First Course in Contiuum Mechanics". This test does a good job of explaining the concepts with both words and mathematical statements. The derivations are not detailed in this book, and you must refer to other texts for more details. Reviews on this book are mixed, with some people feeling like more detail should be provided, and others thinking it is short, to the point, and well written. I am of the latter opinion. With the many thick tomes of detail available on the subject, this text, at a little over 200 pages gives the necessary details and references at each step. The chapters of the book are

1. Introduction
2. Vector and Tensor Calc
3. Stress
4. Principal Stress and Principal Axes
5. Deformation Analysis
6. Velocity Fields and Compatibility Equations
7. Constitutive Equations
8. Isotropy
9. Mechanical Properties of Real Fluids and Solids
10. Derivation of Field Equations
11. Field Equations and BC's in Fluids Mechanics
12. Simple Elasticity Problems
13. Stress, Strain and Active Remodeling of Structures

My previous experiences with mechanics have been through applied research, the basics that are covered in undergraduate physics, and a single survey class on tissue mechanics. As part of my research on limb development, I worked with a viscoelastic model for mesenchymal tissue, however I admittedly did not understand it thouroughly. In fact, this lack of understanding was part of my motivation for taking this class. My undergraduate coursework focused on mathematics and computer science, and I hold B.S. degrees in both. I have applied these tools to biology problems, and in general find biology to be a good problem set to work on. While I feel comfortable with the ideas of calculus, I am looking forward to the practice this course provides in vector calculus, and more so to the practice in manipulating and solving differential equations in several dimensions. My research here at Harvard will continue to explore the ties developmental systems in biology, and the constraints put on them by their mechanical constituents. I will continue my work in limb development, with a goal of creating a model system that both replicates the observed biological phenotypes, but also predicts new phenotypes caused by genetic mutants. Furthermore, I hope to develop a more general scheme for describing cell and tissue level outgrowth, and apply it to roots, pollen, fingers, and more. More abstractly, I am interested in the solutions of the equations of fluid and solid meshanics, and feel that this class will certainly give me experience in solving these equations.