Have you ever asked why a glass rod snaps easily, but a steel rod just bends? Or why does a rubber band stretch so easily, while a wooden ruler barely gives at all?
This is a question that gets to the core of how things are made. In the field of mechanical engineering, we frequently discuss material properties.
Two of these properties, which are also often confused, are stiffness and strength. They may sound similar, but they describe very different things. Getting the stiffness vs strength concept right is pertinent to designing anything, from a bridge to a phone case.
So, what is the difference between stiffness and strength? Well, that’s what we are going to talk about in the next couple of minutes.
Let’s get to it!
What is Stiffness?
Let’s say you are pushing on a wall. It doesn’t move, correct? This is because the wall has very high stiffness. Stiffness is a measure of the resistance of a material to bending or stretching when a load is applied. It is all about resisting deformation.
A material that has high stiffness, a stiff material, will bend or deflect very little when loaded. Think about a ceramic plate. It is very stiff. A material that has low stiffness, such as a silicone spatula, will bend very easily. The key here is that this is usually elastic deformation.
That is a fancy term for saying that once you remove the load or force, the material will return to its original shape. When we refer to stiffness, we’re not referring to the ultimate strength; we’re just referring to compliance.
The stiffness vs strength argument typically begins with the concept of temporary bending.
How is Stiffness Calculated?
So how do you get a value for this? You certainly do not have to be a math expert to get the rough idea. The principal property we’ll use is called the modulus of elasticity, or perhaps just Young’s Modulus. This modulus is an important number for any material, acting literally as a measure of the stiffness of the material.
For example, the relationship that steel is three times stiffer than aluminium is realized by looking at their Young’s Modulus numbers. The higher the modulus, the higher the elasticity and therefore the stiffer the material.
But the material stiffness is only part of the equation. Your design effectively conveys the amount of stiffness, which is of critical importance. A thick beam will be far stiffer than a thin beam with the same material.
This geometrical property is usually computed using the area moment of inertia. Thus, when considering the real-world stiffness of a part, we take into account both the material stiffness and the rigidity of its shape.
Types of Stiffness
You might think that stiffness is just stiffness; however, there are actually several different types of stiffness based on how the force is applied. Understanding these concepts will help you comprehend how an object functions and the types of loads it can withstand, which is essential for proper design. Let’s look at the main ones.
1. Axial Stiffness
This one is by far the easiest. Axial stiffness is a resistance to bending longitudinally (stretching or compressing). Imagine pulling on a rope or standing on a column.
The pulling (or standing) is the axial force, and the opposition to that force is axial stiffness. Axial stiffness is directly related to Young’s modulus of the material.
2. Rotational Stiffness
Rotational stiffness is the resistance to a bending moment or a force that acts to rotate/bend an object. Picture a diving board where you walk out to the end of the board.
If you stand, you create a downward force that generates a moment, attempting to bend the board at its base. The diving board’s ability to resist bending under that moment is the rotational stiffness.
3. Lateral Stiffness
Lateral stiffness is the resistance to forces that act off to the side of the main axis. Picture a tall skinny building in the wind; the wind pushes sideways on the building, creating a lateral force that mentally attempts to cause the building to rotate or lean over from the top.
The combination of a building material’s horizontal strength and the building’s lateral stiffness reacts against that lateral force.
4. Torsional Stiffness
Ever tried to twist a ruler? If you have, you will know what torsional stiffness feels like. Torsional stiffness is the resistance of a material to a rotational or twisting force.
An example of an application that employs torsional stiffness in its design is a car’s driveline shaft. In this example, you have a driven part that is required to transmit power while it twists.
5. Bending Stiffness
Bending stiffness is basically the same as rotational stiffness. However, bending stiffness refers to how a beam or plate resists bending when you load Ø material is applied.
The amount of deflection we observe when you stand in the middle of a plank is a direct measure of its bending stiffness. Both bending stiffness and deflection requirement are functions of the material and section (moment of inertia).
6. Shear Stiffness
To accurately describe shear stiffness is to describe resisting forces that will eventually cause one part of the material to slide past the other part of the material.
Shear stiffness is best conceptualized with a deck of playing cards; should you apply a force onto the top card, the entire deck leans over. A solid block resists this much better because it has a much higher shear stiffness. Shear stiffness is sometimes considered in the overall design of beams and plates.
What is Strength?
Now, to switch gears a little and look at strength. Where stiffness resists bending, strength resists breaking. In the context of manufacturing and CNC machining, strength is defined as the maximum sustainable stress before it either fails completely or is deformed permanently.
If you apply a force that exceeds a material’s strength, it will not come back – it will either bend and stay bent (plastic deformation) or it will break (fracture). Strong materials are likely very good plastics at sustaining very high load levels before they fail.
When you extract a strength from a material data sheet, you are getting the ultimate limit of that material. The entire strength vs stiffness discussion conducted earlier was based on the same principle: one is breaking, one is bending.
Types of Strength
Just like with stiffness, strength isn’t a single thing. We measure strength in different ways based on the force applied to the material. Strength is just one of a material’s mechanical properties, among others, that can collectively tell us how the material would hold up in real life. So, we’ll try to touch on the key types.
Tensile Strength
This is probably the most important and the most common type of strength you will encounter. Tensile strength is a measure of how much pulling force, or tensile load, a material can endure before being pulled apart. The tensile strength can be determined by examining a stress-strain curve, which represents a material’s behavior under tension and stress.
Impact Strength
Impact strength indicates how well a material will endure a sudden shock and load. Impact strength is a measure of toughness – for example, a strong material (by means of tensile load) but shatters as you hit it with a hammer, might be considered brittle and therefore have low impact strength.
Compressive Strength
As you may suspect by now, compressive strength is opposing tensile strength, and it measures how much the material can take before it is squished back or compressed. Concrete is a great example of this. Concrete has great compressive strength (it can withstand heavy loads) and poor tensile strength (it cracks easily if subjected to tension).
Yield Strength
This is it. Yield strength is how much stress you can add to a material before you cause it to deform. Yield strength is easy to confuse with tensile strength (or any strength values) for that matter. Up to this point (the yield point), the material is experiencing elastic deformation.
If you stop loading the material below the yield stress, it will return to its original shape. But once you cross the yield strength loading, you have caused permanent deformation. The part is now bent for good.
Ultimate Strength
This is the big one. The ultimate tensile strength (UTS) is the absolute maximum stress the material can withstand before it begins to fail and break. A stress-strain graph will show that this is the very top of the curve.
After this point, the material will begin to neck (get thinner in one spot), followed by a complete fracture. The difference in yield strength and ultimate strength tells you how much the material can stretch before it breaks.
Difference Between Stiffness and Strength of Material
So, let’s answer the big question: What is the difference between stiffness and strength?
Here are four main differences.
1. Resistance to Deformation vs. Resistance to Failure
The clearest difference in strength vs. stiffness is what they resist: stiffness resists temporary, elastic deformation, and answers the question “How much will it bend?”, while strength resists permanent failure.
It answers the question, “How much force can it take before it fails or bends forever.” And this is the essence of the stiffness vs strength topic.
2. Point of Measurement
We measure these two quantities at different points. Stiffness is a measurement that involves the elastic phase, which occurs long before any permanent damage has been done.
We measure stiffness using Young’s Modulus. Strength measurements, yield strength, ultimate tensile, etc., are measured at the moment the material has been permanently deformed or breaks.
3. Material Characteristics
A material can be relatively stiff and yet not be very strong. Glass is a great example. It is stiffer than many plastics (it won’t bend easily), but it is brittle and weak (shatters easily).
On the flip side, some materials are relatively strong but not very stiff; some polymers fall into this category. The relationship of stiffness vs strength may not always be a direct correlation.
4. Influence of Geometry
Also, this is an important point related to the strength vs stiffness conversation. Strength is an intrinsic property of a material. However, the stiffness of a component is a function of the material and shape (geometry).
You can increase the stiffness of a part by varying the shape (like using an I-beam) without changing the strength of the material at all. This concept of the area moment of inertia is a plumbing trick in mechanical engineering. Best
Practices for Designing for Stiffness & Strength
So how do we leverage our understanding? When designing a part, you want to consider both strength and stiffness. You don’t want your part breaking, but you also probably don’t want it bending too much. The following best practices are designed to help you strike the challenging balance between strength and stiffness.
1. Calculate the Expected Stress on Each Material
Before you look at anything else, you want to understand the loads that your part will be subject to. Is it going to be subjected to a constant tensile load? Is it going to be compressed? Will it see intermittent loads or sudden impacts? You’ll want to calculate the expected stress and strain, and whether the force will be below the yield strength of your material.
2. Run tests on a range of materials before selection
Don’t just choose a material from a catalog. Test the material if possible. This is an excellent opportunity to see the trade-off between stiffness and strength in practical terms. Depending on your process, you might be able to test several different materials and see how they perform in “real-world” conditions. Material selection is one of the most important parts of the overall design process.
3. Identify essential factors at the beginning of your design
Consider other factors than loading alone. What is the operating temperature? Humidity? How long will it take to load? These can introduce additional problems to solve and even consider factors such as creep and fatigue. These can cause a loss in stiffness and fracture strength over your part’s life cycle.
4. Verify your design is solving what you intended before you make a prototype
Instead of putting significant money into a prototype, you could iterate on the design in computer simulations (Finite Element Analysis or FEA) to quantify/visualize your design. Programs like this can identify areas of high stress and provide a quantified deflection of your part. Collecting this data can verify whether you’re geometry in your design performs functionally before you create a part.
Conclusion
So, we have taken a journey through the world of mechanical properties. At the end of the day, the whole strength vs stiffness is not about which is better.
The whole point is to understand that they are two different characteristics, equally important. Stiffness gives you an answer to the question, “will this part hold its shape while under a load?” Strength answers the question, “will it survive the load without breaking.”
The great engineer knows how to strike a balance between these two. They select specific materials and design specific geometries that give just the right amount of strength and stiffness for the application.
To fully understand the distinction between strength and stiffness is to hold one of the fundamental keys to good design. The entire stiffness vs strength conundrum is a cornerstone of why we make things operate and last.