11.1 INTRODUCTION

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Stress is the force per unit area. This is because the geometry of the parts are not fixed, and we must account for the size of the cross section that is resisting a force or moment.

So far we have dealt with tensions and compressions in members somewhat simply. If we consider a simple member as shown below, we use the force divided by the area to give us the force per unit area.

The normal units for stress are given below.

The material at the section of interest acts like numerous supports, each exerting internal forces. As a result the stress is statically indeterminate, but a reasonable estimate is made by assuming uniform distribution. In the internal forces are not constant over the cross section of the part.

For two dimensional objects the internal forces of interest are shear force, normal force and bending moment.

Using stress we can calculate deformations (strain) and failure loads. This is done by referring to tables of values for particular materials.