STRENGTH AND STRESS

Текст A

Part I

All construction materials must resist force. A force has a value and a direction. Gravity causes most of the forces in construction. There are other causes such as wind. Unit stress (stress) is force per unit area over which the force acts. We obtain it by dividing the force by the area on which it acts and show it as pounds per square inch, or psi.

Strength of a material is the ability to resist a force. That ability depends on the size and shape of the object and its material. Strength of a material is equal to the unit stress that the material can resist. Strength has the same units as unit stress.

The useful strength of a material is equal to the unit stress at failure. Failure takes place when an object can not serve its purpose. The material may fail if it breaks or if deformation is excessive. A change in the outside dimensions of an object that a force has caused is deformation.

The amount of deformation depends on the size and shape of the object and its material. When we divide the total change in dimension by the original dimension we obtain unit strain (strain). Unit strain is the result of unit stress.

We can see unit strain when we stretch a rubber band or compress or twist a piece of rubber hose. A rubber band which we subject to a compressive force becomes much shorter and a little wider. A sample which we subject to a tensile stress becomes much longer and a little narrower.

There are three kinds of unit stresses and corresponding strengths - compressive, tensile, and shear. They depend on the position of the forces which act on the object. The three are shown in Fig. 1.

a b c

(a) Compression (b) Tension (c) Shear

In each case unit stress = P/A

FIGURE 1. Illustration of stresses

Notes:

ability n. способность

hose n. шланг

pound n. фунт (0,454 кг)