Stress is usually expressed in pounds per square inch (psi). It represents the load in pounds for every square inch of cross-sectional area. For instance, if a 1-inch square bar is pulled with a 1,000-pound load that tends to stretch the bar, the bar is stressed to 1,000-pound divided by 1 square-inch = 1,000psi. Changing the load changes the stress in direct proportion. Changing the cross-sectional area changes the stress inversely—halving the load halves the stress, but halving the area doubles stress. and vice versa. In simple formula form: S – P divided by A; where S is in psi, P is expressed in pounds, and A is in square-inches.
Although there are different types of stress, I purposely specified a pulling load because it creates a tensile stress—a stress that results when a material is being pulled. Why tensile stress? Because stress ratings for metal are tensile stress.
The two types of stress typically listed in metal-stock catalogs are yield point and tensile strength. Understand that any load applied to any metal will cause it to deform—some more than others. As load increases, so does deformation.
When the metal is unloaded, it returns to its original shape— until it is stressed beyond its yield point.
When a metal reaches its yield point, it will continue to deform, or yield, without any corresponding in-crease in load. Some metals will continue to yield, even though the load may reduce slightly! When this load is re-moved, the metal will not return to its original shape, but will remain permanently deformed.
Often called ultimate strength. tensile strength is the maximum stress a metal can withstand before it fails.
Percent elongation is the ratio of the deformation of a metal, immediately before it fails, to its original length. For instance, if a 2-inch length of tube is stretched to 2.40-inch before it fails, its elongation is 20-percent (0.40 divided by 2) X 100 20-percent. Percent elongation is important because it is an indication of a metal’s toughness.
Toughness is the ability of a metal to absorb an impact load, which is obviously very important in automotive applications. This is particularly true of many race car components: roll bars, cages and frames, for example. Rather than breaking, the material gives, absorbing energy in the process. As a result, the stressed component will not be as highly loaded or stressed as a less-tough component with a higher tensile strength.
An indicator of toughness that’s similar to percent elongation is percent reduction of area. When metal fails in tension, it necks down or pinches in. Pull a piece of putty apart and you’ll see what I mean. The cross-sectional area through the break is smaller than it was originally. Generally, the more a metal necks down—has a higher percentage reduction area—the tougher it is.
Yield Point vs. Tensile Strength
Now that you have an idea of what per-cent elongation and reduction of area are, you probably won’t find it in any metal catalogs. However, another good toughness indicator is the proximity of a metal’s yield point to its tensile strength. The farther away they are, the tougher the material.
For example: Mild steel has a yield strength of about 35,000 psi, and its tensile strength is 63,000 psi-1.80 times higher in tensile than yield. At the other end of the spectrum, a popular high-alloy steel has a yield of 60,500 psi and a tensile strength of 95,000 psi-1.57 times higher in tensile. These materials have an elongation of 38-percent vs 26-percent, and a reduction of area of 62-percent vs. 52-percent, respectively. The high-alloy steel is stronger, but the mild steel is tougher.