Brittle Failure: Definition, Causes, and Prevention
Learn more about how to manage brittle failure with various materials.
Brittle failure is the sudden fracture of a material with minimal plastic deformation. This can be a dangerous situation since there is little to no warning that failure is about to occur, and cracks in the material propagate quickly. Brittle failure can occur in material for several reasons, such as: environmental factors like extremely low operating temperatures, imperfections in the material and its grain structure, or inadequately designed parts and structures. Engineers need to understand the mechanisms behind brittle failure so appropriate measures can be taken to prevent it. Such measures include: operating structures and parts at suitable temperatures, conducting regular maintenance to remedy cracks and flaws that may weaken the material, and improving and optimizing designs. This article will discuss brittle failure, from what it is, to what causes it, to how to prevent it.
Brittle failure is the sudden breakage of a material characterized by little to no plastic deformation of the material, and rapid crack propagation. While brittle materials may exhibit high strength, they absorb less energy before fracture than ductile materials. Small impact loads can cause a brittle fracture.
In materials science, brittle is a term that describes a material that, when subjected to stress, fractures with no or minimal plastic deformation. In contrast to ductile materials, which experience varying degrees of plastic deformation before breakage with applied loads, brittle materials break almost instantaneously with a load beyond the yield point.
In brittle materials, failure begins with the creation of a crack at a defect or point of concentrated stress in a part. Cracks propagate quickly in a direction nearly perpendicular to the direction that the stress is applied. This is called cleavage, or the splitting of the atomic bonds in the crystal structure of the material along definite planes. Once cracks have propagated to the point that the integrity of the material is compromised, the material finally breaks, and a loud snap is heard. This entire process is nearly instantaneous.
Brittle failure look like cracks expanded in a nearly perpendicular direction to the applied stress and there is minimal deformation in the material. This makes the fracture surface of the material relatively flat compared to the fracture surface of a part that experienced ductile failure. In brittle failure, V-shaped chevron marks form near the center of the cross-section of the fractured surface and point toward the area where the cracks are initiated. Figure 1 is an example of a brittle failure:
An example of brittle failure.
Image Credit: Shutterstock.com/lbrumf2
While some materials are inherently brittle due to their chemical composition, some normally ductile materials become brittle under certain conditions. The four main causes of brittle failure are listed below:
Material thickness is a major contributing factor to brittle failure. This is because higher thickness correlates to higher tension, compression, and shear stresses. Each of these stresses is perpendicular to the other and the material will deform along each of these axes when loaded. For example, a material that is under tension will be stretched in the direction of the applied force but will shrink in the directions perpendicular to the force.
In thicker materials, there are more atoms that must be dislocated for the material to deform. Consequently when under tension, the material cannot effectively deform in the directions perpendicular to the applied force, since the material’s subatomic and grain structure prevents the atoms from moving. A brittle fracture occurs, and a flat surface oriented perpendicular to the load is created when the tensile force is sufficiently high to cause a fracture.
At colder temperatures, the thermal energy of a material is reduced. This means that the atoms of a material are less excited and their movement is restricted. Therefore, the material is less likely to plastically deform, since its atoms tend to resist dislocating when a load is applied. This contributes to brittle behavior and is a common reason why normally ductile materials experience brittle failure.
There are many factors that contribute to metallurgical degradation. Examples include thermal effects, hydrogen embrittlement, and intergranular corrosion. Cold temperatures can embrittle a material as explained before. Hydrogen embrittlement is the reduction of a material’s ductility due to absorbed hydrogen atoms. Because hydrogen atoms are small, they dislocate the material’s atoms and compress them together. This makes the material more brittle since it makes the material more resistant to deformation. Intergranular corrosion can also embrittle a material. This is because of the corrosion of the grain boundaries of the material. Corrosion causes the boundaries to weaken which then makes cracks propagate much easier through the material’s grains. Each of these factors contributes to metallurgical degradation and can embrittle a material.
The alloying elements present in steel and its grain structure have a big impact on its brittleness. Alloying elements affect the metal’s microstructure since the atoms of the alloying element (impurities) squeeze between the atoms of the metal. Metal can be modified to have desired properties that it normally wouldn’t have if it was in its pure form. The modification will depend on the concentration of the impurities, the alloying element, and the temperature at which the impurities are introduced. The introduction of impurities increases the grain sizes of metals which consequently can make the metal more brittle. Cracks propagate more quickly through large grains since there is less resistance by the material.
To prevent brittle failure, consider the items listed below:
- Control the Operating Temperature: Maintaining operating temperatures that are below the material’s critical temperature but above the freezing point of water will ensure the material does not become embrittled.
- Increase Material Toughness: Material toughness can be improved by various heat treatment methods. Examples of heat treatment include: tempering, annealing, quenching, and precipitation hardening. The toughness of the material will increase to varying levels depending on the type of heat treatment process used. In general, however, heat treatment processes reduce the material’s grain size and make it less susceptible to cracking.
- Design Structures to Have Smaller Cross-Sections: Designing parts with smaller cross-sections the material a chance to plastically deform before breakage. Consider a 3-dimensional plane: a material stretched in one direction will shrink in the other two directions. This is called the Poisson effect. When stretched in one direction, structures with thicker cross-sections do not deform much in the other two directions. This leads to brittle failure.
- Avoid Using Parts That Are Inherently Brittle: If possible, avoid parts that are overly brittle. Brittle materials are sometimes preferred in abrasive applications where parts rub against each other or in applications where compressive strength is ideal. This is because brittle parts are typically harder which makes them more wear-resistant. Additionally, brittle materials are often strong when subjected to compressive loads due to the atoms’ resistance to dislocation. If it can be achieved, use tougher and more ductile materials that have sufficient strength to fulfill the application to prevent brittle failure.
- Minimize Defects in the Part: All defects can increase stress concentrations in parts and make it easier for cracks to form. For example, marks and nicks leftover from machining processes and voids in castings can allow cracks to form more easily.
If catastrophic brittle failure has occurred, it is not possible to fix the broken component. The component must be replaced. However, in larger structures such as bridges that are comprised of several structural and load-bearing members, the failure of one member does not compromise the integrity of the entire structure. Cracks may exist in structural members due to brittle failure or fatigue, but propagation is stopped, or arrested, by a crack arrestor. These arrested cracks can be repaired temporarily by welding. Design modifications of the structure are required to prevent the cracks from appearing again.
Some examples of brittle materials are listed below:
- Cast iron
- Martensitic steel
- High-carbon steel
- Alkali metals when sufficiently cold
There are two types of brittle failure as described below:
- Transgranular: Transgranular brittle failure occurs when a crack does not follow material grain boundaries but travels right across the grain. As the crack travels, it changes direction to follow the planes of least resistance through the crystal lattice of each grain.
- Intergranular: Intergranular brittle failure occurs when the crack follows the boundaries of the grains of the material. This is common when the boundaries of the grains are weak and brittle.
The main difference between ductile failure and brittle failure is the amount of plastic deformation the material will experience before breaking. In ductile failure, obvious signs of deformation can be observed around the breakage point. Necking, or localized deformation near the breakage point, is easily observed and occurs in ductile failure. The opposite ends of a material experiencing ductile failure taper down to the breakage point. In brittle failure, no such deformation occurs and the area of breakage looks more like a straight, horizontal line rather than a single point. For more information, see our guide on Ductile Failure.
This article presented brittle failure, explained what it is, and discussed how to manage it. To learn more about brittle failure, contact a Xometry representative.
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