What is Brittleness? – Definition, and Meaning

What is a Brittleness?

Brittleness refers to the tendency of a material to fracture under stress with minimal or no prior deformation. In other words, brittle materials tend to break suddenly rather than bend or stretch when subjected to force.

You’ll often notice that brittle materials hardly deform at all before breaking. They typically have a poor ability to withstand impact or vibration, even though their compressive strength might be quite high. At the same time, their tensile strength tends to be much lower. This is why many inorganic, non-metallic materials—like glass or ceramics—are classic examples of brittle substances.

When a brittle material is stressed, it fails with almost no visible elastic or plastic deformation. If you’ve ever heard the sharp snap of something breaking, you’ve probably witnessed brittleness in action. These materials simply don’t absorb much energy before they fracture, no matter how strong they might be otherwise.

In the context of materials science, the term “brittle” is reserved for substances that give way with little or no plastic deformation. There’s a simple test for this: after the break, the two pieces should fit together perfectly, since the material hasn’t deformed.

It’s interesting to note that some materials—like metals and polymers—can become brittle when cooled below a certain temperature. For metals, this is called the ductile-to-brittle transition temperature (DBTT), and for polymers, it’s the glass transition temperature (Tg).

Below these thresholds, even materials that are usually quite tough can shatter unexpectedly. When that happens, cracks tend to spread quickly and often move perpendicular to the applied force, cutting through the material’s internal structure.

Temperature plays a crucial role here because it affects the molecular arrangement within a material. When the molecules lose their ability to move or stretch, the material can no longer retain its flexibility and is prone to failure.

To sum up, while all materials will eventually fail if pushed beyond their limits, a material is considered brittle if it breaks before undergoing any noticeable change in shape or size.

Brittleness material failure occurs when there are two conditions:

• Stress acting on the surface of the material
• Surrounding temperatures below the melting point of a material

Brittleness in different materials

1. Polymers

The mechanical properties of polymers are notably influenced by temperature fluctuations, even within ranges close to room temperature. Take poly(methyl methacrylate), for instance: at just 4˚C, it is remarkably brittle, yet as the temperature rises, its ductility noticeably improves.

Amorphous polymers, in particular, show a range of behaviors depending on the surrounding temperature. At lower temperatures, they resemble glass—hard and inflexible—reflecting what’s called the glassy region.

As temperatures move upward, these polymers shift into a state that’s more rubbery or leathery, which is known as the glass transition region. Push the temperature higher still, and they begin to behave much like a viscous liquid, entering what’s termed the rubbery flow or viscous flow region.

This entire spectrum of responses is referred to as viscoelastic behavior. When in the glassy region, amorphous polymers tend to be rigid and brittle, but as the temperature increases, they lose that brittleness and become more pliable.

2. Metals

Some metals tend to behave in a brittle manner, and this often comes down to the nature of their slip systems. Generally, when a metal has a greater number of slip systems available, it’s less likely to be brittle; this is because it can undergo plastic deformation along several different directions.

On the other hand, if there are only a few slip systems, the opportunities for the metal to deform plastically are much more limited, which makes brittleness more likely.

Take hexagonal close-packed (HCP) metals as a classic example—they only have a small number of active slip systems, so they often fracture rather than deform when put under stress.

3. Ceramics

Ceramics tend to be brittle, and much of this comes down to how difficult it is for dislocations to move within their structure. In most crystalline ceramics, there simply aren’t many slip systems available for these dislocations to travel along. Without enough pathways for movement, the material can’t easily deform under stress, so instead, it tends to fracture.

The bonding in ceramics usually has a strong ionic character. Because these materials are made up of ions carrying electric charges, there’s a significant repulsion between ions of the same charge. This repulsive force makes it even harder for atoms to slip past one another, further limiting deformation and reinforcing that characteristic brittleness we see in ceramics.

How does the material change to brittle?

Materials can be changed to become more brittle or less brittle.

Toughening

When a material approaches the limit of its strength, it faces a choice: it can either undergo deformation or give way to fracture. Take a metal that’s naturally malleable, for example. You can increase its strength by making it harder for it to deform plastically.

But if you push this too far, you actually run the risk of the metal becoming brittle—suddenly, instead of bending, it breaks. This is why improving a material’s toughness is always a balancing act.

With materials that are naturally brittle, like glass, toughening them isn’t as complicated as it might seem. Most methods follow one of two ideas: either you try to stop or absorb a crack as it starts to grow, or you set up internal stresses in a way that actually closes cracks from certain sources before they can spread.

That first approach is what’s at work in laminated glass. Here, two glass sheets are separated by a thin layer of polyvinyl butyral, a polymer that can absorb energy from a crack as it develops. On the other hand, toughened glass and pre-stressed concrete make use of the second method, where residual stresses are introduced intentionally. A classic demonstration of this is the Prince Rupert’s Drop.

If you look at brittle polymers, these can also be toughened, often by adding metal particles that cause localized stretching or “crazing” under stress—a good example of this strategy is high-impact polystyrene (HIPS). Among ceramics, materials like silicon carbide and transformation-toughened zirconia stand out for being less brittle than most.

Composites take a slightly different route. Imagine embedding brittle glass fibers within a matrix of ductile polyester resin. When the material is put under strain, cracks tend to start at the glass–matrix boundary, but there are so many of these micro-cracks that a lot of energy is absorbed before anything catastrophic happens. This idea isn’t limited to glass and resin—it also shows up in metal matrix composites, using the same principle to boost toughness.

Effect of pressure

It is commonly observed that applying pressure tends to enhance the brittle strength of a material. A clear illustration of this can be seen in the Earth’s crust, specifically within the brittle-ductile transition zone, which lies at an approximate depth of 10 kilometers (or about 6.2 miles). At this depth, rocks are less prone to fracturing and instead begin to deform in a more ductile manner.

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