The strength of a metal or any material for that matter, is how it responds to different types of loads. Since loads can be applied to a material in different ways, there are different ways of talking about a material’s strength. Let’s look at some of the ways that loads can be applied to materials, and these loads generate a thing called stress.
One of the most obvious ways of loading a metal part is to pull on it. Pulling on an object puts it in tension. Tensile strength is how much pulling a material can withstand without failing. But what kind of units should we use to talk about tensile strength? Let’s say that a metal bar can hold about 2000 pounds before breaking in two. We could say that we had a material that could hold 2000 pounds. But how useful would that be? You see, units of force like pounds are good if we are talking about doing a specific job, like picking up an engine block. But what if we wanted to talk about how strong the material was in general.
Think about this, If we knew that the metal bar was this big (3/16 of an inch in diameter), and that it could hold 2000 pounds, that tells us a lot more about the material. When we consider the load divided by the cross-sectional area of the supporting object, we are talking about a thing called stress. Stress is force per unit area. That means force divided by area. In our example, the load was 2000 pounds, and the area was about .03 square inches. In this case, the stress is 2000 pounds divided by .03 square inches which is about 66 thousand pounds per square inch. The units of stress are pounds per square inch. For large values of stress you may hear people talk about KSI, the term kip is short for kilopounds or 1000 pounds, so one ksi is thousand psi. The SI unit of stress is the Megapascal. A pascal is one newton per square meter. 66,000 psi is equal to about 455 Megapascals.
Always remember it is the force per unit AREA and not the DIAMETER or length we are talking about. Inches are not the same thing as square inches. It is important to know the formulas for area by heart. Especially the formulas for the area of a circle, square and triangle.
Now, we’ll need to take a closer look at the tensile strength of a material, because there is actually a lot going on there that we are not talking about. But we’ll cover that soon.
So resistance to pulling forces is tensile strength. Another way to load a material is by pushing on it. A pushing force is called a compressive force. Let’s take a look at this punch. If we load this half-inch diameter punch with 8,000 pounds, what how much stress is being developed in the punch? Again we will use the formula force per unit area to calculate the stress. The stress in the punch is 8,000 pounds divided by .196 square inches. 8,000 divided by .196 is a little more than 40,800 pounds per square inch. The compressive stress is 40,800 psi.
One more way of loading a material is in shear. In shear the forces are equal and opposing, and they are parallel to the surface resisting the load. Here is a picture of what shear loading looks like. Again, In shear, we will be dividing the load in force units by units of area. But what area? It is the area doing the resisting. For our example it is here, along this surface. Punching is a type of shear loading. The punch itself is in compression, but the metal sheet is in shear. You see, the punch is pushing like this, and the die is pushing in the opposite direction. The area is here, around the circumference of the hole being punched out. To find the area, we take the distance around the punch and multiply it by the thickness.
So, there is 8,000 pounds of force applied, and the area is .1256 square inches. The shear stress developed in the metal being punched is a little more than 63,600 PSI
So in summary, there are three main ways of loading a workpiece: tension, compression, and shear. Loads applied to objects develop stresses in those objects. This stress can be tensile stress, compressive stress, or shear stress. The units for stress can be psi, ksi or Megapascals.