Brittle Fracture: Mechanics and Considerations

Brittle fracture is one of the significant concerns in Industrial Damage Mechanism and is characterized by a sudden fracture in the material under stress. Typically, all construction materials are susceptible to damage and efforts have been made to prevent failures in equipment caused by fracture mechanisms.

The brittle fracture occurs when the following four factors are present:

  1. The material has flaws inside it.
  2. High-Stress Level.
  3. Susceptible microstructure.
  4. Transition (ductile to brittle) temperature
Brittle fracture in material

Brittle fracture in material

Factors affecting Brittle Fracture:

Typically a flaw within a material tends to develop in more significant cracks under a high-stress concentration. Stress may be residual due to manufacturing and fabrication processes or externally applied by structural/wind loadings. These crack-like flaws propagate and eventually lead to catastrophic failure.

Also, it is a material toughness that offers resistance to fracture. It means material with less toughness will provide less resistance and vice versa. The grain size in a microstructure significantly affects the material’s toughness. Due to this, it is a susceptible microstructure that will offer less resistance to fracture.

Failures have been seen in Petrochemical and Refining industries in equipment operating below the transition temperature (the temperature below which the flexible nature of material tends to transit into brittle nature) because at this temperature, the toughness of materials rapidly decreases and offers less resistance to brittle fracture.

Railway tracks joined by thermite welding were subject to brittle fracture because thicker materials offer less resistance due to triaxial stresses on a flaw if present. So the thick numerous walled items should be given severe consideration against fracture damage.

Prevention:

1. Adequate Toughness

The brittle fracture can be best controlled by using suitable materials which offer sound resistance even at low temperatures and maintain sufficient toughness to bear stresses. Materials with lower toughness at the high-stress level are subject to failure. To prevent this, use more rigid materials.

2. Suitable Material Composition and Microstructure

As stated above, a susceptible microstructure is more likely to damage, so a material with suitable chemical composition and microstructure should be used. If equipment is fabricated by using welding, the equipment must be post-weld heat-treated and other thermal treatments should be done to prevent failure during service. As we all know, weld metal has a brittle heat-affected zone with susceptible grain size and a proper post-weld thermal treatment is required to reduce residual stresses and modify the grain size.

3. Monitoring Damage Mechanism

To assess the probability of failure, three factors must be considered; the level of stress, the size of the flaw, and the material’s toughness. Controlling one or more of the above three factors is the best preventive method. If the toughness and size of the flaw cannot sustain stress levels, the material will be subject to fracture.

This can be best controlled by monitoring damage mechanisms, operating conditions, and inspection of high-stress parts during equipment service.

Factors to control Brittle Fracture:

  • The size, shape, and depth of flaw.
  • Material Toughness
  • Material Chemical Composition
  • Micro-structure
  • Stress Concentration
  • Operating Conditions (temperature, pressure, etc.)
  • Transition Temperature

Last Words

Parts with material more likely prone to damage must be checked at regular intervals to find out any crack-like flaw if present. Use streamlined designs to prevent stress concentration at sharp edges and joints. Materials with sharp notches and cutting edges are stress raisers and may result in cracking. Use stricter materials that can sustain lower temperatures. Particular attention should be given to thicker materials because they are more prone to damage due to the development of triaxial stresses on cracks. A crack tends to propagate under stress and may result in a brittle fracture sooner or later.

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Caroline is a mechanical materials engineer and a professional welding consultant. She loves exploring new ways to improve industrial fabrication and construction works.

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