Drawing

DEPCOllc

3305 Airport Drive | Pittsburg, KS 66762

Force, Deflection, Stress and Yield

The loads or forces applied to the axle are called stress. Stress is a measure of force per unit area. An

example might be pounds per square inch or Newtons per square centimeter. A given amount of

stress (Force per unit area) will cause a given amount of change in the shape or length of the axle.

This change in length is called strain. Put in plain terms, if you apply consistently greater force

(Stress) the axle material will deform (Strain) to a greater degree.

Stretching a Steel Axle

Figure B9 is a graph illustrating the elongation (strain) of the axle due to increasing stress. This is a

common way to test the strength of materials and components. In this test the axle is held securely

on both ends. The axle is subjected to increasing tension (stretching) loads. The force of tension is

divided by the cross sectional area of the axle. This yields the stress value in pounds per square inch

(psi). Example:

A 0.250 inch diameter axle has an area = 0.05in . If that axle is placed under a tension force of

1500 lbs, the stress would be:

in0.05

lbs.1500

Stress

= 30,000 psi

During the test the axle is stressed and the change in length or strain is measured. A plot of stress vs.

strain is produced. The example graph indicates that stress is proportional to strain until the stress

reaches approximately 30,000 psi. Beyond that point, an increase in stress produces an increasingly

larger increase in strain. The material has begun to yield, and has suffered permanent deformation

(stretch). The axle will no longer return to its original length when the applied stress is removed. The

material has failed.

Engineers and designers do not like to design components to function at or even close to the yield

point. It is common practice to design components with large enough cross sections to withstand two

and even three times the applied loads without reaching the yield point.

10K

30K

20K

Stress (psi)

.01 .02 .03 .04 .05

The Proportional Limit

(The point below which stress and strain are proportional)

Yield Point

(The point of permanent deformation)

Strain (inches)

Fig B9

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