A Matter of Tensile Stress and Pressure
Pretty much everything in the world gives you some sort of stress or pressure. In physics, everyone probably knows that pressure is what you get when you divide a force by an area. We’re familiar with the units “psi”, or pounds per square inch. Note that pound in this case is “pound-force” and not “pound-mass”. Yes, there is a distinction, but I will not go into it here. For now, it is enough to know that the pounds you are used to when you weigh something is also a pound-force.
Pressure Formula
Stress, it turns out, is exactly the same. It has the same units, and it conceptually and intuitively has the same behavior. They go hand in hand, so I’ll refer the pair interchangeably. While stress isn’t derived from engineering, it is a fundamental science tool that nearly all engineers use.
So, what is it exactly? In every day life, we deal with pressure in our car and bike tires. We have tire gauges to tell us the air pressure inside the tire, in “psi”. The metric equivalent is the Pascal (or Pa), where the force is in Newtons, and the area is in squared meters. I guess to follow the “psi” format and for purposes of demonstration only, it’ll supposedly be Npm, or Newton per square meter.
For now, we’ll simplify the whole deal by looking only at positive axial normal stresses. It’s positive because we’re stretching and not compressing. (It is also called tensile stress.) It’s axial and normal because the area we look at is perpendicular to the direction we’re pulling; for now, we’re not pulling at some angle to the cross-sectional area in the general formula.
In everyday engineering, we tend to deal instead with ksi (kilopounds per square inch) and MPa or GPa (megapascals or gigapascals, respectively). One psi or one Pa is far too small to deal with because a pound-force is too small to act over a square inch and a square meter is too large for a Newton to act upon. Don’t worry. This’ll make more sense when we discuss really strong bars and beams made out of steel and aluminum, as opposed to rulers made out of plastic.
The next time to go walking or driving over a bridge, for example, you can hopefully realize that all the metal cables and beams are working in tandem with huge amounts of tensile stress to support you, the other cars, and itself. The cabling of a suspension bridge, for example, is completely engineered with the fundamental principle of tensile stress of force divided by area.
That’s it for now.
But check out the video below. It shows what happens to a steel bar when there is too much tensile stress in the bar from too much pulling force.
If you notice at 0:27, the steel actually appears to stretch, a process called necking. At 0:30, there is too much force and stress to the point that the steel broke in two. In future posts, we’ll discuss these magical stretching and breaking points, and how we can calculate how much force it took to fracture this piece of steel.
Take care.
Leave a Comment