The steel industry is one of the largest in the world and is integral to numerous adjacent industries such as construction, transportation, energy, and manufacturing. Steel is widely-used for its strength, but it must also meet certain requirements for stiffness, depending on the end product it is used to make. The terms stiffness and strength are often confused; an article on thefabricator.com explains the difference:
Strength: Strength is a measure of the stress that can be applied to a material before it permanently deforms (yield strength) or breaks (tensile strength). If the applied stress is less than the yield strength, the material returns to its original shape when the stress is removed. If the applied stress exceeds the yield strength, plastic or permanent deformation occurs, and the material can no longer return to its original shape once the load is removed.
Stiffness: Stiffness relates to how a component bends under load while still returning to its original shape once the load is removed. Since the component dimensions are unchanged after load is removed, stiffness is associated with elastic deformation.
A material can have high strength and low stiffness. If a metal cracks easily, it has low strength, but if it has low stiffness, it can deflect a high load. The article explains that stiffness depends on the modulus of elasticity, also known as Young’s Modulus, which is constant for a given metal. Because Young’s Modulus for steel is three times that of aluminum, an aluminum part under load will deflect three times as much as a similarly loaded steel part. The thickness and shape of the formed part also contributes to its stiffness.
All steel has approximately the same stiffness, but comes in many different strengths depending on the alloying metals used. Stainless steel comes in more than 100 grades which are created by adding alloys such as chromium, silicon, nickel, carbon, nitrogen, and manganese to impart properties such as heat resistance, strength, flexibility, and ductility. Martensitic or semi-austenitic steels are the strongest due to the addition of elements such as aluminum, copper and niobium.
As an example of the importance of differentiating between strength and stiffness, consider the Corporate Average Fuel Economy (CAFE) standards. Car manufacturers are meeting CAFE requirements by using lighter-weight, higher-strength steel components that provide improved fuel economy. These new steel grades, called advanced high-strength steels (AHSS), make the car body structures stronger but lighter in weight. Car manufacturers are also considering the less stiff aluminum as a lighter weight alternative to steel. In the event of a crash, strength is obviously a critical safety consideration in car components, but so is stiffness depending on the function of the car body part. If a steel part on a car is replaced with an identical (shape, thickness) aluminum part, it may deflect more due to its lower stiffness. This could be an issue if a part ‘sags’ and rubs against another part. However, adjusting the design (shape, thickness) of the aluminum part, it is possible to obtain the strength and stiffness needed, while lowering the mass of the part.
Steel starts out as flat sheet metal or plates and must be manufactured to precise thickness specifications depending on the application for which it is used. It must also be easily machinable so that it can be formed into its permanent shape without cracking. While strength is an advantage in many applications, adding strengthening alloys may contribute poor machinability, meaning the material is difficult to cut and wears down the tooling. Accurate thickness measurement of process-line steel ensures the finished products have specific mechanical properties, including the appropriate strength and stiffness for their application. An excellent way to accomplish this is by processing the material through a cold rolling mill. Cold rolling is a metal forming process in which a sheet of metal is pressed through a pair of rolls to reduce thickness, increase strength and improve surface finish.
Recommended reading to learn more about manufacturing and processing steel to meet strength and stiffness requirements:
- X-ray-based Sensor System Enhances Process-Line Steel Operations
- Cold-Formed Steel Shakes Up Building Construction
- What is Stainless Steel? Part I
- What is Stainless Steel? Part II
- Download our free eBook: A Practical Guide to Improving Steel Manufacturing Processes and Production Methods
- Visit our center for Improving Steel Manufacturing Processes and Production
Zequek Estrada says
Words like strength and stiffness are so similar that it seems easy to not know the difference. Unless people are in the steel industry, I’d doubt that they would know the difference. It’s pretty interesting thinking about how the terminology changes depending on the industry.
Marlene Gasdia-Cochrane says
Thanks for your comment Zequek. Feel free to ask us about any other terminology in the metals analysis industry.
If you were to have a credit card sized piece of metal, what type of metal would it need to be made of to make it the hardest to bend? Asked another way, if you wanted to make the world’s strongest credit card, what metal would you make it out of so that even superman would struggle to bend it in half? Not concerned about how it would stand up to a bullet or other force, but specifically being as rigid as possible, if “rigid” is even the right word? Thanks
Marlene Gasdia-Cochrane, Editor says
Let’s ask our readers! Comment below.
What about pins that ard attached to shackles they seem to be small in relation yo the loads they carry 72mm diameter and 50tonne wll at 6:1 safety. That must be reall tough steel.what would the metal be in those pins
Marlene Gasdia-Cochrane says
The pins would have to be analyzed with an XRF analyzer to know the content, but you might be interested in this article which talks about small metal fasteners:
XRF Provides Quality Assurance of Critical Fasteners
Greta James says
Thank you for informing me that stiffness is how well a shape holds that shape after pressure is applied. My brother is thinking of building a large metal barn that would hold hay for the horses. It needs to be pretty strong so that it can withstand high winds. We will have to look into aluminum sheets that would fit the job!
Alice Carroll says
It’s interesting to learn that stiffness pertains to a metal’s resilience to bending. I’ve been slowly building up my own workshop recently since I’ve gotten the hang of machining as a hobby. Having a good stock of stainless steel fasteners on hand at all times will make sure that my creations would be as durable as possible by reinforcing weak points and making them resistant to bending.
Darrien Hansen says
I didn’t realize that some types of metals are especially sturdy since they contain copper. My brother is interested in hanging some wall art outside of the bakery he plans on opening next month, but he needs to make sure that the material is sturdy since he lives near an area that experiences frequent storms. He should think about hiring a copper cutting service to help him.
It is, unfortunately, a great deal more complicated than this. Copper is resistant to certain things – some kinds of corrosion and it bends before it breaks, but, but many copper alloys are soft enough that large, thin, sheet metal sections could be bent out of shape in a high wind, and some of them are weak enough that I personally would worry about it coming apart in a serious storm unless designed carefully, but I live where there are hurricanes, I admit. On the other hand, there are also high strength copper containing alloys, and ones both stiffer and harder than most aluminum alloys (and with superior corrosion resistance to the high strength aluminum alloys). It is really something that it’s a good idea to research a bit before starting a project. You can find pretty good info on the internet now.