Exploring the Concept of Brittleness
When considering brittleness, one might envision a delicate flower that crumbles at the slightest touch or a delicate crystal goblet that shatters with a gentle tap. Brittleness refers to the property of a material that tends to fracture under stress, exhibiting little to no ability to deform before breaking. Unlike ductility, which allows a material to be shaped and stretched, a brittle material lacks the capacity for significant plastic deformation.
Compared to ductile materials that can be drawn into thin wires or undergo substantial deformation before failure, brittle materials experience minimal deformation prior to breaking. This characteristic, known as brittle behavior, holds vital importance in the realm of de
sign and engineering. Understanding brittle behavior is crucial for creating effective designs that can withstand stress and prevent catastrophic failures.
Pro-Tip: If you’re interested in refreshing your understanding of ductility, explore the topic of Engineering Fundamentals Refresh: Ductility, Strain, and Toughness.
What Makes a Material Brittle?
Strain is characterized by a material’s change in length divided by its original length under loading conditions, and brittle materials have a lower strain value before they fail. You calculate strain with the following equation:
For brittle materials, there is very little change in length or deformation before failure, so we can characterize brittle materials by:
strain at failure point = ∈ f < 0.05
For ductile materials, the strain at failure is greater than or equal to 0.05, and this relationship provides a method for testing whether or not a material is ductile or brittle.
Because of this low strain value before failure, brittle materials do not exhibit plastic deformation before failure — they fail only after a small amount of elastic deformation as compared to ductile materials, which do exhibit plastic deformation before failure. You can see the stress vs strain curves of brittle and ductile materials below:
Some brittle materials always behave in the manner above, but other materials are susceptible to brittle fracturing. Ductile materials may fail due to a brittle fracture if exposed to certain conditions, such as cyclic loading, impact loads (rapid static loading), low-temperature loading, or parts with defects in their material structures (inclusions, contaminants). Also, certain materials that have undergone processing like welding, hydrogen embrittlement, or hardening may be more susceptible to brittle fracture even though they are considered ductile.
While all of the above are causes of brittleness, the common theme is that ductile materials may fail due to a brittle fracture if exposed to high stress concentrations. Brittle materials are more susceptible to brittle fracture, but it’s still necessary to evaluate stress concentrations for ductile materials, depending upon processing and environmental conditions. Brittle fracture is the failure mode you should be most familiar with when designing and utilizing ductile materials, because this instantaneous failure causes irreparable damage and has a devastating impact on any mechanical system.
Examples of Brittle Materials
Here are some examples of brittle materials:
1. Gray cast iron
2. White cast iron
3. Ductile materials exposed to high stress concentrations under certain environmental conditions (i.e., materials at low temperatures)
4. Glass
5. Ceramics
6. Graphite
7. Concrete
Pro-Tip: If you want to see if your material is more susceptible to brittle fracturing, look at the ratio of the yield strength to the ultimate strength — a higher ratio means a higher likelihood of brittle fracture since the material is not as able to absorb energy in the plastic deformation region of the stress vs strain curve.
Unveiling the Metallurgical Aspects of Brittleness
At the atomic level, the behavior of materials sheds light on the phenomenon of brittleness. In the case of ductile materials like metals, the presence of metallic bonds allows atoms to slide past each other, imparting flexibility and ductility to the material. These metallic bonds create a network of metal atoms interspersed with movable electrons, explaining the excellent conductivity of metals. With a delicate balance of strength and flexibility, metals can undergo plastic deformation.
In contrast, brittle materials such as ceramics possess atoms that are held together by ionic bonds, which offer resistance to deformation. Ionic bonding arises from the attraction between positively and negatively charged particles, resulting in electrostatic forces that impede movement. This resistance, referred to as electrodynamic repulsion, prevents significant atom displacement and leads to rapid fracturing without substantial plastic deformation.
Fracture criteria theories differ between ductile and brittle materials. Ductile materials adhere to general yield criteria theories like Maximum Shear Stress, Distortion Energy, and Ductile Coulomb-Mohr. However, when examining the failure of brittle materials, fracture theories become necessary. Some of these theories include Maximum Normal Stress (although not entirely accurate), Brittle Coulomb-Mohr (providing a conservative estimate), Modified Mohr (offering the highest accuracy), and Smith-Dolan locus (applicable to fatigue failure in the first quadrant).
It is important to note that brittle materials do not exhibit the same yield strength properties as ductile materials. Hence, when dealing with brittle materials, considering compressive and ultimate tensile strength becomes crucial, as they may differ. Unlike most ductile materials, which exhibit similar yield strength in compression and tension, brittle materials demonstrate distinct behaviors in these two loading conditions.
How Do Brittle Materials Behave Under Tensile Loading?
Brittle materials behave slightly differently under compressive and tensile loading, and also behave differently than ductile materials under tensile loading. It’s important to note that brittle materials are stronger in compression than in tension, as their compressive strength is often several times higher than their ultimate tensile strength.
When a tensile test is performed on a brittle material, the material exhibits elastic deformation before failure and no plastic deformation, so the material is unlikely to exhibit necking before failure. Instead, brittle materials fail in a plane that is normal to the applied tensile forces. This is evident from the stress vs strain curves referenced earlier in this article for brittle vs ductile materials.
How Do Brittle Materials Behave Under Compression Loading?
When under compression, a brittle material will not deform significantly before failure, and the failure for a brittle material will occur at an angle that is 30-45 degrees from the axis where the compressive stress is applied. This failure occurs along cleavage lines, aka where crystalline materials split along defined crystal structure planes. Alternatively, a ductile material under compressive loading will start to exhibit cracks parallel to the applied loading.
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