There seems to be some confusion out there about the difference between strength and stiffness in metal. After all, isn’t strong metal stiff, and stiff metal strong?
While there is often a correlation between strength and stiffness in metal, for manufacturing purposes, the two terms really should be clearly separated.
To answer this, here are a few of the differences in meaning between stiffness and strength in metal forms:
Stiffness is the rigidness of any object or material. Objects with a high stiffness will resist changes in shape from being acted on by a physical force.
For example, loose, wet clay has a low stiffness, changing shape with just a few pounds of pressure. Steel alloys, on the other hand, have a very high stiffness, taking thousands of pounds per square inch (PSI) of pressure without deforming the surface of the metal.
The ability to take pressure without bending sounds like it should be used as the basis for the strength of a metal object such as a wire basket, but it isn’t. This is for a good reason, too.
As an article on this very subject from the Fabricator states, “stiffness depends on the modulus of elasticity, also known as Young’s Modulus… Unlike strength, which can vary from grade to grade or even coil to coil, Young’s Modulus is constant for a given metal and is independent of heat treatment, processing, or cold work.”
In short, the strength of a metal can change throughout the object, but its stiffness will remain constant.
Now, here comes the confusing part. In the Fabricator article referenced above, strength is defined as “a measure of the stress that can be applied to a material before it permanently deforms (yield strength) or breaks (tensile strength).”
This can be confusing because resistance to physical deformation is a key part of defining both strength and stiffness. One of the key differences is the word permanent. A strong material can bend from force that is below its maximum yield strength, but will return to its original shape once that outside force is removed. This means that a strong material can bend without breaking, and return to its normal shape.
Another key difference is that a material’s strength is, as stated by the Fabricator, “a function [of] its chemical composition, the thermo-mechanical processing route (such as converting from a thick slab to a thin sheet) and subsequent heat treatments.” In short, strength can vary in a metal depending on the processes that metal has been subjected to.
What Does the Difference Mean for My Materials?
As stated earlier, there’s often a strong correlation between strength and rigidity. But, the difference can be a major one.
For example, an extremely rigid material can also be very brittle, prone to breaking as soon as its threshold for pressure is exceeded. On the other hand, strong materials tend to bend before they break, and rebound once the load is removed, unless said load exceeded the yield strength of the material.
So, when you ask about how strong a custom metal form manufacturer’s products are, make sure that they know the difference between strength and stiffness.