Brittle fracture is one of the major concerns in Industrial Damage Mechanism and characterized by a sudden fracture in the material under stress. Normally all materials of construction are susceptible to damage and efforts have been made to prevent failures in equipment caused by fracture mechanism.
Brittle fracture occurs when the following four factors are present;
- The material has flaws inside it.
- High-Stress Level.
- Susceptible microstructure.
- Transition (ductile to brittle) temperature
Brittle fracture in material
Factors affecting Brittle Fracture:
Normally a flaw within a material tends to develop in the form of bigger cracks under high concentration of stresses. Stress may be residual as a result of 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 which offers resistance to fracture. It means material with less toughness will provide less resistance and vice versa. The grain size in microstructure has a significant effect on material’s toughness, and that is why a susceptible microstructure will offer less resistance to fracture.
Failures have been seen in Petrochemical and Refining industries in equipment which are 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 down 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 serious considerations 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 temperature and maintain sufficient toughness to bear stresses. Materials with lower toughness at the high-stress level are subject to failure. To prevent this, use tougher materials.
2. Suitable Material Composition and Microstructure
As it is stated above that a susceptible microstructure is more likely to prone 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 to occur 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 access the probability of failure three factors must be considered; the level of stress, the size of flaw, and toughness of material. Controlling one or more of the above three factors is a best preventive method. If the toughness and size of flaw are unable to sustain stress level, the material will subject to fracture.
This can be best controlled by monitoring damage mechanism, operating conditions and inspection of parts under high stress 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 of time to find out any crack-like flaw if present. Use streamline 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 tougher material which can sustain lower temperature. Special attention should be given to thicker materials because they are more prone to damage due to the development of triaxial stresses on cracks. Crack tends to propagate under stress and sooner or later may result in brittle fracture.
