FRACTURE MECHANICS

Through the ages the application of materials in engineering design has posed difficult problems to mankind. In the Stone Age, the problem was mainly in shaping of the material. In the early days of the Bronze Age and the Iron Age the difficulties were both in production and shaping. For many centuries metal working was laborious and extremely costly. Estimates goes that the equipment price of a knight and horse in the 13^th century was of the equivalent price of a centurion tank in Second World War.

       With the improving skill of metal working, applications of metal in structures increased progressively. Then it was experienced that structure built of these materials did not always behave satisfactorily and unexpected failures occurred. The vastly increased use of metals in the 19^th century caused number of accidents and casualties to reach unknown levels. Some of these accidents were due to poor design, but it was gradually discovered that material deficiencies in the form of pre-existing flows could initiate Cracks and Fractures.

           The failure often occurred under conditions of low stress(several ships failed suddenly while in the harbor) which made them seemingly inexplicable. As a result extensive investigation were initiated in many countries and especially in USA. This occurrence of low stress fracture in high strength material induced the development of Fracture Mechanics.                  

What is crack?

Partial breaking of a material is called crack. Initiation of a crack begins due to repeated cyclic loading which causes localized changes in the structure of the material further leading to fracture.

What is fracture?

A    fracture is the local separation of the body into two or more pieces under the action of stress. The word fracture is often applied to bones of living creatures, or to crystals or crystalline materials, such as gemstones or metal.

Types of Fracture:

Fractures can be mainly classified into two types they are as below:

1.    Ductile fracture

2.    Cleavage fracture

1. Cleavage Fracture:

Cleavage fracture is the most brittle form of fracture that can occur in crystalline materials. Brittle cleavage fractures in ships, bridges and tanks have made it a notorious type of failure. Cleavage Fracture is encountered in cases like low temperatures and high strain rates. Cleavage Fracture of metals occurs by direct separation along crystallographic planes due to simple breaking of atomic bonds. Its main characteristics are that it occurs with a particular crystallographic plane.

2. Ductile Fracture:

                 Fracture occurring under the single application of continuously increasing load can be either brittle cleavage fracture or fracture associated with plastic deformation, which is essentially ductile the most familiar type of fractures by overload in tension, which produces the classic cup and cone fracture.

Dynamic Fracture:

How does it happen?

           We have seen cases in which fracture has occurred in static condition after prolonged loading conditions and stress. But here is another case where in the fracture has occurred in the dynamic state or in simple words the fracture has occurred during motion.

            A bike which looks as if its normal but just view the crack in the rear wheel, its due to Dynamic Fracture which has occurred when the bike was in motion. Now let’s analyse how it has actually happened?

           Here instability occurs due to crack extension, the elastic energy release rate remains larger than the crack resistance “R”. The surplus of released energy (G-R) can be converted into kinetic energy .The kinetic energy is associated with rapid movement of the material at each side of the crack path, during the passage of a high velocity crack the difference between “G” and “R” determines how much energy can become available as kinetic energy and consequently it governs the speed at which the crack will propagate through the material.

                                Both G and R represent the energies associated with crack extension. The crack resistance is the function of the plastic behavior of the material at the crack tip and of its fracture characterstics.The yield stress increases and the fracture strain decreases at higher strain rates at the tip of crack moving at high velocity the strain rates are very high, and it must be expected that the material behaves in a more brittle manner the higher crack speed. Fractures are analyzed using the latest scanning electron microscopy (SEM) and other metal testing techniques.

Prevention:

Ø  Fractures can be prevented by proper quality control techniques and suitable design.

Ø  Methods like Non- Destructive testing and X –ray analysis can prevent fracture.

Ø  Excessive loading should be avoided and if possible repeated cyclic loadings should be minimized.

Ø  Manufacturing defects like blow holes should be avoided and suitable manufacturing process should be adopted.

Ø  Research and Developnnent will further provide scope in prevention of fracture.    

Conclusion:

Fracture is an unpredictable phenomena,the only solution to this is by proper design and using of modern manufacturing techniques. Quality of material plays an important role in robustness of the product.