Almost all engineered structures, from gears and valves, to buildings and bridges, must be designed to function safely and effectively. Failure in even the smallest component can often have disastrous consequences.
The key to avoiding failure usually begins with understanding the concepts behind material fracture. It is, therefore, essential for design engineers to have adequate knowledge of material fracture and fracture mechanics.
What is a Material Fracture?
Fracture is the separation of a material into two or more pieces under the action of an applied stress. A material may undergo one of two major types of fracture modes depending on its mechanical properties: ductile and brittle.
Materials undergoing ductile fracture first experience plastic deformation, i.e., the material resists the fracture by stretching itself. Imagine pulling on two ends of a plastic bag. The bag stretches by a considerable amount before it eventually tears. This plastic deformation, which is not limited to polymers, is also seen in metal alloys.
Materials that undergo brittle fracture, on the other hand, will fracture with negligible plastic deformation. In other words, they break without warning.
Regardless of the type of fracture, during failure a material will experience:
- Crack formation, where all fractures start, and
- Propagation of the crack, in response to the applied stress
In ductile fractures, this crack is stable, i.e., it will undergo continuous deformation, only propagating when more stress is applied. As such, ductile materials will typically deflect by a significant amount before they fail, thus giving warning before they fracture entirely.
On the other hand, when cracks form under brittle fracture, they propagate across the material instantaneously; thus, failure can occur with little to no warning. This is one of the characteristics that makes brittle failure so undesirable, especially in applications such as building construction. (Learn more in The Effects of Stress Concentration on Crack Propagation.)
In the next sections, we will dive deeper into the mechanics of these two types of fractures and look at some of the most common ways material fracture is tested in a laboratory environment.
The Mechanics of Brittle and Ductile Fracture
Ductile Fracture (Cup-and-cone Fracture)
Most metals and metal alloys are ductile by nature. One of the main characteristics of ductile fracture is a phenomenon known as necking. During necking, the material's cross-section gradually reduces before fracture occurs. The separated ends of the fractured material adopt a concave and convex shape. Hence, ductile fracture is commonly called the cup-and-cone fracture. The stages in the cup-and-cone formation are:
- The ductile material undergoes necking
- Micro-voids start to form perpendicular to the stress direction
- The micro-voids coalesce to form a crack
- The crack propagates
- The material eventually fractures (a cup and cone form at the top and bottom respectively)
In brittle fracture, no noticeable deformation is observed since crack propagation is instantaneous. This propagation is almost purely perpendicular to the direction of the tensile stress, compared to ductile fracture, which produces cup- and cone-like shapes.
While brittle fracture has a distinctive shape, different materials may exhibit unique characteristics. For example, in brittle steel, V-shaped markings are usually found in the center of the fractured cross-section. On the other hand, for amorphous materials like ceramic glasses, the surface of the fractured cross-section may have a smooth, shiny appearance.
Factors That Affect the Fracture of a Material
Engineers must understand the mechanics of different types of fractures to inform material selection and mitigate potential catastrophic failures. Some of the critical factors that affect material fracture include:
For most brittle materials, the measured fracture strength is much lower than the predicted theoretical value based on the atomic bonding energies in the material. This is due to the presence of microscopic cracks and other flaws in the material's cross-section. These stress risers must, therefore, be accounted for when calculating fracture strength. (Related reading: Calculating Tensile Stress: Why It Needs to Be Done Now.)
The location of the applied load must also be taken into account. Consider a cylindrical material with an elongated crack (flaw) in its cross-section. The maximum stress it can handle before it undergoes fracture is measured on the crack tip and is dependent on the applied stress and the physical properties of the crack. However, applying the load away from the crack may alter the measured fracture strength.
Fracture toughness is the resistance of a material to applied stress when a crack is present. This parameter is dependent on:
- The critical stress for crack propagation
- The crack length
- The crack geometry
For thin specimens, fracture toughness also depends on its thickness. However, fracture toughness becomes less of an influential factor in thick materials. (Learn about The Difference Between Strength and Toughness.)
How to Test Impact Fractures
To perform these tests, a hammer is attached to a pendulum, which applies an impact force to the test specimen. The hammer is first raised to its starting position then released, hitting the specimen during its downward motion. Because some of the hammer's energy is absorbed by the sample, the pendulum swings to a smaller maximum height on the opposite side. The difference between the maximum heights of the hammer before and after impacting the specimen is used to calculate the impact energy. The only difference between Charpy and Izod is the way the specimen is loaded.
A fracture is simply the separation of a material into several pieces due to an applied stress. Fractures are subdivided into two major types: ductile fractures and brittle fractures. The type of fracture that a material will undergo depends mainly on its ability to deform before it cracks.
For both these fractures, failure begins with crack formation, which progresses to crack propagation and eventually separation.
It is crucial for engineers and designers to understand the mechanics behind material fractures to ensure that selected materials perform as intended in their given environment.