Failure Theory

Failure theory is the science of guessing the conditions, in which the solid materials will lose their strength under the external loads. Brittle failure or ductile failure is the classification of the failure of the material.  The materials will fail in brittle or ductile manner or may be both, is truly depends on the conditions such as temperature, state of stress, loading rate. Even though, the failure theory has been in the development for more than 200 years, the level of acceptability is not yet reached continuum mechanics.  The exact physical definition of a failed state is not easily calculated and various working definitions are used in the engineering community. Often the failure criteria of the same form can be used to guess brittle failure and ductile yield.

Material failure modes

These includes lot, some are Thermal shock, creep, fatigue, buckling, etc.

Thermal Shock

As the result of rapid temperature changes, thermal shock is the name given to cracking. Due to the low toughness, low thermal conductivity, and high thermal expansion coefficients; glass and ceramic objects are particularly susceptible to this form of failure theory. They are engaged with many high temperature applications all because of their high melting point. When the thermal gradient causes various parts of an object to expand by different amounts, thermal shock occurs.  This difference in expansion can be understood in terms of stress and strain. At certain circumstances, this stress will overcome the strength of the materials and thus produces cracks.

Preventing thermal shock

Thermal shock can be prevented by

• Reducing the thermal gradient by changing its temperature slowly and increasing the materials thermal conductivity.
• By reducing the material coefficient of thermal expansion.
• By increasing its strength
• Decreasing its Young’s modulus
• Increasing its toughness by crack tip blunting (plastically), crack deflection.

Fatigue

When the material is subjected to cyclic loading, fatigue is the progressive and localized structural damage will occur. The maximum stress value are supposed to be less than ultimate tensile stress limit and may be below the yield stress limit of the materials. When the material is subjected to repetitive loading and unloading will cause fatigue. The microscopic cracks will begin to form at the surface, if the loads are above a certain threshold. Finally the crack will reach the critical size and the structures will suddenly failure. The important thing that tends to increase the fatigue strength of the structure is round holes and fillets.

Questions:

• What is failure theory?
• Explain fatigue?