Hardness, strength, and toughness are important properties expected in metals and other materials. One or more of these properties will determine their suitability for a specific application.
Stress is the measure of the force that is acting on an object. Stress in a material can be distinguished by the direction in which forces act relative to the material, which could be compressive, tensile, or shear. A strain of a material is a measure of the degree to which it has deformed relative to its original dimensions.
Well, in this reading, we’ll explore what is hardness, toughness, and strength of a material. We’ll also explore the differences between the material properties.
Let’s get started!
What is Hardness?
In general terms, hardness is the ability of a material to withstand friction. It’s well-known and uses abrasion resistance. The hardness of a material is a measure or ability of its resistance to localized deformation.
This deformation in most cases is brought about by abrasion (friction forces) or indentation (constant lateral mechanical forces).
A diamond has a unique characteristic of hardness and it’s known as the hardest substance on earth and incredibly difficult to scratch. In an industrial world or infrastructure, it is rare to use in important applications that require hardness.
However, diamond-tipped drill bits are incredibly common and available in various sizes including the ones for handheld drills used for drilling in metals and solid rock.
On a subatomic level, hardness is determined by the strength of intermolecular bonds. Although there are also a couple of things that come into play. However, the hardness of a material is not responsible for resisting the application of a large amount of force.
Let’s take for instance, despite the hardness of a diamond, it can easily be shattered with a sledgehammer. This is where we can differentiate hardness from toughness.
What is Toughness?
Toughness is the ability to measure the amount of force a material can absorb without fractioning. A material like metal, plastic, or wood can undergo a certain amount of force before it fractures, which means when the material will completely fail.
With this, the material can undergo elastic deformation or deformation that allows it to return to its original form without fractioning. in a situation where a material deforms before fractioning means, it’s ductile. This is to say a ductile material can also be considered as tough.
In addition, the toughness of a material can be determined by its ductility and strength. A good example of toughness is metals and foams being equal in terms of toughness. The materials are extremely different in behavior in terms of stress and strain but they have similar yield points.
Metals can only withstand a very small amount of deformation before they fracture and might require a huge amount of force before they deform. On the other hand, foams will deform with a normal amount of compression force but will only fail at the amount of the same amount of force as metals.
Rubbers are unique in terms of toughness as it has enough strength to withstand a good amount of force before it deforms and it does not suddenly fail upon deformation. Rubbers have both strength and ductility which makes them a unique tough material. Now, let’s get to understand the term strength.
Related: What is Iron (FE)? its Properties, Types & Raw Materials
What is Strength?
Strength is the amount of force a material can contain to resist deformation. This is to say, the higher the force required to change the shape of a material, the stronger the material. Among the above-listed properties of materials like metals, wood, rubbers, etc.
strength is probably the most complex. It can also be said to be the measure of the amount of force an object can withstand before it undergoes plastic deformation. It can also undergo a deformation that is non-reversible, which is also known as unrecoverable strain.
That is, a material that has undergone plastic deformation will no longer return to its original form even after the stress is removed.
For instance, steel is difficult to pull apart, which means it has high strength. But it’s not strong at all as it can quickly deform into all sorts of shapes will very little force.
The difference between strength and toughness is that toughness measures the force needed for a material to completely fail. Whereas strength is only the amount of force a material requires to reach plastic deformation.
In some situations, like a very brittle material such as ceramic, strength and toughness play an important role as they practically overlap.
However, there are different types of material strength, which brought about the types of stress a material can experience. So, a material can have a unique value of compressive strength and tensile strength.
This depends on the material’s ability to withstand compressive stress or tensile stress. Furthermore, all materials have their own shear strength, which measures the ability to withstand shear stress before undergoing a sliding failure.
Stress and Strain
Stress is the measure of the force that is acting on an object. Stresses are distinguished by the direction in which forces act relative to the material, which could either be compressive, tensile, or shear. Compressive stress is stress that acts in longitudinal directions to compress the object.
While tensile stress is the exact opposite, acting to expand the object. Torsional stress is different as it’s the result of forces acting in opposite directions that serve to twist or provide torsional loading.
On the other hand, the strain of a material is a measure of the degree to which it has deformed relative to its original dimensions. For instance, when a material undergoing stress from any direction is bound to respond to the stress by either deforming or outright failing.
Engineers and scientists were able to predict the reaction of materials when subjected to forces at different levels. This has also allowed the classification of materials based on their material properties, which may include brittle, ductile, elasticity, hardness, toughness, and strength.
Related: What is Steel? its Properties & Raw Materials
The difference between Hardness, Toughness, and Strength
The difference between hardness, toughness, and strength is above explained, but in brief, we can quickly say; hardness is the ability of a material to resist friction, while Toughness is the ability to measure the amount of force a material can absorb without fractioning.
Finally, strength is the amount of force a material can contain to resist deformation. Here is a table showing the key differences between hardness, toughness, and strength:
| Property | Hardness | Toughness | Strength |
| Definition | Resistance of a material to deformation, particularly permanent deformation, scratching, cutting, or abrasion. | Ability of a material to absorb energy and plastically deform without fracturing. | Maximum stress a material can withstand while being stretched or pulled before breaking. |
| Measurement | Measured by indentation hardness tests (e.g., Mohs, Rockwell, Vickers, Brinell scales). | Measured by the amount of energy absorbed before failure (e.g., Charpy or Izod impact tests). | Measured by stress-strain tests, yielding ultimate tensile strength or compressive strength values. |
| Indication | Indicates how resistant the material is to surface deformation. | Indicates how well the material can absorb energy and deform plastically. | Indicates the maximum load-bearing capacity of the material. |
| Importance | Important for wear resistance applications (e.g., cutting tools, abrasion-resistant surfaces). | Important for impact resistance and energy absorption applications (e.g., crash components, protective gear). | Important for load-bearing applications (e.g., structural components, bridges, buildings). |
| Examples | Diamonds (very hard, high resistance to scratching), hardened steel. | Rubber (high toughness, able to absorb significant energy), metals like titanium and stainless steel. | Steel (high tensile strength, used in construction), carbon fiber composites. |
| Behavior Under Stress | Hard materials resist deformation but can be brittle and crack under high stress. | Tough materials absorb energy and deform before breaking, offering resistance to fracture. | Strong materials withstand high stress and load without failing, often until a point of fracture. |
| Typical Testing Methods | Hardness tests (Rockwell, Vickers, Mohs scale). | Impact tests (Charpy, Izod). | Tensile tests, compressive tests. |
| Applications | Cutting tools, wear-resistant coatings, armor plating. | Automotive parts, aerospace components, safety equipment. | Bridges, buildings, load-bearing components, cables. |