Introduction and Outline

There are many classifications of materials properties: thermal, electrical, chemical, optical etc. Of these, probably the most uniformly applicable in different fields is mechanical properties - which tell us about materials respond to applied forces. This is because all materials are exposed to applied forces in some way, and therefore mechanical response is nearly always an important consideration.

The importance of mechanical properties in construction, aerospace, automotive, and manufacturing is clear. But what about in the materials used in electronic applications? Well, the computer or phone gets thrown in a backpack and jiggled around. The transistor, which is comprised of many different materials, changes temperature and may develop thermal stresses. There are mechanical loads during fabrication. The performance of the semiconductor does, to some degree, depend on its mechanical behavior. You'll find that no matter the application, mechanical properties are important.

In this chapter we'll again connect microscale phenomena to macrosopic behaviors. Why do some materials stretch and then return to their original shape under a specified load while others remain permanently deformed? How much energy can materials absorb during these deformations? Can we recover that energy and put it to use? What's happening to atoms and microstructures when these deformations occur? If we understand the underlying reasons for these mechanical behaviors, can we then engineer materials to be stronger, stiffer, more impact-resistant? We'll discuss some of these behaviors in this chapter and the next.

Outline

Section 11.3- Tensile Behavior of Materials: An introduction to perhaps the simpliest mechanical response: tensile behavior. We'll use tensile behavior to beginning learning about important concepts such as load, stress, strain, and displacement. We'll then investigate what happens to an Lennard-Jones assembly of atoms when a tensile load is applied.

Section 11.4- Elastic Properties: In the case where relatively small loads are applied to materials, we have a reversible (or elastic) mechanical response. In this section we'll define those elastic properties, including the elastic modulus and the Poisson ratio.

Section 11.5- Inelastic Properties: When sufficient load is applied, materials undergo a permanent (also called inelastic or plastic) mechanical response. Here, we'll investigate this behavior using a computational model and define the materials properties associated with plastic deformation.

Section 11.6- Common States of Stress: While tensile mechanical response are some of the simplest to describe, test, and model, one can apply loads to materials in many ways. This section describes additional common (or simple) states of stress including compression, shear, and hydrostatic. For many of you, such as mechanical, civil, and biomedical engineers, you'll go much further in later classes and consider combined stresses in which a body experience many types of stress at the same time!

Outcomes

Differentiate between different states of simple stress in a mechanical system. Utilize definitions of stresses and strains in calculations.

Understand and apply simple atomistic models for mechanical deformation.

Apply Poisson’s formula and Hooke’s law (linear elasticity) to engineering problems.

Connect interatomic energy curves to Young’s modulus.

Analyze stress-strain curves to derive values of interest with respect to mechanical properties. Use the data to evaluate materials for their suitability in structural applications.