Steel Fracture Types

A link in this steel chain fractured due to ductile overload.

Steel is one of the of the world's most commonly used metals. When steel fractures, identifying the type of fracture is one of the first steps in determining how and why the fracture occurred. Fractures in steel are described by the fracture mode and the fracture orientation. Although there are thousands of different kinds of steel, there are only a few types of fractures that occur in steel. The study of fractures and fracture surfaces is known as fractography.

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Mode and Orientation
- Describing the mode and orientation of the fracture are the two key steps in determining the type of fracture in steel. The mode of the fracture describes the manner in which the fracture propagates through the steel: ductile, brittle or fatigue. The orientation of the fracture describes the loading conditions on the steel at the time of fracture, such as tension, torsion, bending and shear. It's important to note that a fracture can change mode and orientation as it progresses, so a single fracture can encompass several fracture types.
Fracture Modes
- $A failure analyst uses a microscope to determine the mode of fracture.$
  A failure analyst uses a microscope to determine the mode of fracture.
  
  There are three basic fracture modes in steel: ductile, brittle, and fatigue. Ductile, or overload, failures involve flow of the metal and a permanent change in shape known as plastic deformation. Ductile fracture surfaces are dull in appearance, and when you observe them in a microscope they appear rough and pitted with tiny pits known as microvoids.
  
  By contrast, brittle fractures leave no permanent plastic deformation, and have a smooth or woody appearance at low magnification. At high magnification, brittle fractures are either smooth, when the fracture cuts through the metal grains, or faceted like rock candy, when the fracture follows the grain boundaries.
  
  A third mode of fracture in steel is fatigue. Fatigue can occur when steel is subjected to changing loads or stresses. Fatigue fracture surfaces are wavy, with ripples resembling sand dunes or the ripples in beach sand.
Fracture Orientation
- The second item of information in determining the type of fracture is the fracture orientation. There are four basic fracture orientations: tension, shear, torsion, and bending. Tensile fractures occur when a piece of steel is pulled directly apart by the load. Shear fractures are caused by a cutting action, like scissors. Torsional fractures occur from twisting, like a screwdriver, and bending failures occur as described, through bending.
Types of Fractures
- To describe the type of fracture, then, requires a combination of fracture mode and orientation, as in "tensile overload," or "bending fatigue." Because there are three basic modes of fracture in steel and four load orientations, there are twelve basic fracture types. A single fracture can change mode or orientation, however, so different portions of a single fracture can be of different types.