What’s the Difference Between Hardness, Toughness, and Strength?

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When discussing any material – whether it’s plastic, metal, or a composite – a distinction is often made between hardness, toughness, and strength. The terms are also very frequently encountered when shopping for 3D printing filaments. To the common consumer, these terms sound very similar. After all, shouldn’t a strong and tough object also be hard? However, in the field of engineering and materials science, hardness, toughness, and strength are different, although related, properties.

So, what’s the difference between hardness, toughness, and strength? Which one should I prioritize when buying a 3D printing filament? Read on as we demystify this seemingly complex matter.

A brief background on stress vs. strain

When discussing the properties of materials, it is important to differentiate between stress and strain. Most material properties can be defined by how the material reacts to different combinations of stress and strain.

1. Stress

Stress is a measure of the force that is acting on an object. Different types of stress are distinguished by the direction in which the forces act relative to the object: a stress could either be compressive, tensile, or shear. As its name implies, compressive stress refers to stress that acts at the longitudinal directions to compress the object. The tensile strength is the exact opposite, acting to expand the object. Torsional stress is unique and is the result of forces acting in opposite directions that serve to twist or provide torsional loading.

2. Strain

As you can imagine, an objecting undergoing stress from any direction is bound to respond to it by either deforming or outright failing. The strain of an object is a measure of the degree with which it has deformed relative to its original dimensions.

By analyzing the relationship between stress and strain, scientists and engineers have been able to predict how materials will react when subjected to forces at different levels. This has also allowed for the classification of materials based on their material properties, leading to qualifiers such as brittle, ductile, elasticity, hardness, toughness, and strength.

What is hardness?

Simply put, the hardness of a material is a measure of its resistance to localized deformation. In most cases, this deformation is brought about by abrasion (due to friction forces) or indentation (due to a constant lateral mechanical force). On a subatomic level, hardness is determined by the strength of intermolecular bonds, although there are also a couple of minor details that come into play.

The unique characteristic of hardness is elegantly displayed by diamond, a substance known to be one of the hardest in the planet. Due to the hardness of diamond, it is practically impervious to getting scratched. In the world of infrastructure, it is not rare to find diamond used in some important applications that require hardness. Diamond-tipped drill bits are incredibly common, whether it’s for handheld drills are industrial-sized drills for drilling in solid rock.

However, hardness does not determine how a material will react to sudden application of a large amount of force. For instance, diamond can be easily shattered with a single smash from a sledgehammer, despite being one of the hardest materials. This ties right into the next material property – toughness.

What is toughness?

Toughness is a measure of the amount of force that a material can absorb without fracturing. Take note that the key word here is fracture, which constitutes a total failure of the material. This means that a material can undergo elastic deformation, or a deformation that allows it to return to its original form, without fracturing.

The condition that allows a material to deform before fracturing means that ductile materials can also be considered tough. Therefore, the toughness of a material depends on both its strength (which we shall get to shortly) and its ductility.

To understand this in more practical terms, metals and foams are just about equal in terms of toughness. In terms of stress and strain, these two materials exhibit very different behaviors but have similar yield points. Metals can only withstand a very small amount of deformation before they fracture, but even getting it to deform will require a huge amount of force. On the other hand, foams deform with a minimal amount of compression force but will only fail at almost the same amount of force as metals.

Rubber reigns supreme in terms of toughness. It has enough strength to withstand a good amount of force before it deforms but does not fail right away upon deformation. It is this combination of strength and ductility which makes rubber a uniquely tough material.

What is strength?

Of the three material properties we have discussed here, strength is probably the most complex. Strength is a measure of the amount of force that an object can withstand before it undergoes plastic deformation or the kind of deformation that is non-reversible. Also known as unrecoverable strain, an object that has undergone plastic deformation will no longer return to its original form even after the stress has been removed.

Take note of the distinction between strength and toughness: while toughness measures the force needed for the complete failure of the material, strength is only a measure of the force it takes to reach plastic deformation. In some cases, such as very brittle material such as ceramics, strength, and toughness practically overlap.

There are different types of material strength, which ties right back to the types of stress that an object can experience. Thus, a material can have unique values compressive strength and tensile strength, depending on whether it measures the material’s ability to withstand compressive stress or tensile stress. All materials also have a shear strength, which measures its ability to withstand shear stress before undergoing a sliding failure.

Which characteristic should I prioritize?

Now that we have the definitions out of the way, it’s time to use this knowledge to make smarter decisions when buying products. When is hardness important? In which products should I prioritize tensile strength or toughness?

1. Prioritize toughness for rigid prints

Rigid prints, or prints designed to be durable, will benefit from having high toughness. These can include replacement machinery parts, toys, hinges, or outdoor signs. A high toughness means that these parts can withstand a great deal of stress before complete failure. Take note that an object can undergo plastic deformation before it fails, which you should take as a sign to create another replacement part. Still, having a wide plastic deformation window gives you a wide margin before the object breaks or fractures.

2. Prioritize tensile or compressive strength for flexible items

If you want to print using a flexible filament, such as TPE, then you’re better off choosing a filament with a high tensile or compressive strength rating. This means that the object has a wider elastic deformation region, allowing it to deform under stress but return to its original form once the stress has been relieved. For flexible items, plastic deformation is practically tantamount to failure – after all, you wouldn’t want your custom-made phone case or bracelet to be in a perpetually stretched form.

Final thoughts

Developing a deeper understanding of the inner workings of 3D printing technology, including the filament materials, is crucial in becoming an expert in the field. After years of buying and printing with various filaments, many of us have started to take concepts such as hardness and tensile strength for granted. When harnessed properly, the knowledge of these material properties can help us in choosing the right filament material for the perfect application.

Hardness, toughness, and strength are just some of the physical properties that are important to 3D printers. The elasticity of the filament, how prone it is to moisture absorption, and how it reacts to heat are also equally important properties. The journey to mastery is long, but it’s littered with loads of great tidbits of knowledge.