Ductile to brittle transition

Causes of Ductile to Brittle transition, Ductile to brittle transition temperature, Types of Impact tests

1. Ductile to brittle transition

2. Background During the world war II attention was dragged by the brittle failure of welded Liberty ships and T-2 Tankers. Some of these ships completely broke into two while in some instances ships were not completely disabled. Most of the failures took place in the months of winter and as a result of application of sudden loading. These brittle fractures drew attention towards the brittle fracture of mild steel which in actual showed/shows ductile fracture. This was not the only case where brittle fracture was a problem. Brittle fractures in tanks, pressure vessels, pipelines and bridges were also documented even in the year of 1886.

3. Factors for brittle fracture It was evident and studied that three basic reasons destined to the brittle fracture of ductile materials. A low temperature. A high strain rate or rapid rate of loading. Presence of stress concentrated region. It is not necessary that brittle fracture will occur when all of these three conditions coexist. A rapid rate of loading multiplies the contribution of above two effects towards brittle fracture. That is why “impact tests” are carried out to study the brittle behavior of ductile materials. In impact testing, rapid loading applied to the specimen in which have produced the stress concentrations, intentionally. Or rapid loading is applied to the material at varying temperatures in the presence of stress concentrated regions.

4. Basically ductile materials have high toughness in normal conditions. They can absorb large amount of energy in the plastic range. While, brittle materials has lowest values of toughness. In case, a ductile material shows brittle fracture means it is not more able to absorb energy in the plastic range. Its toughness is reduced. To measure the ability of material to absorb energy, or toughness, impact testing is carried out.

5. Impact test Impact test is carried out to measure the ability of the material to absorb energy in the presence of notch (stress concentration) by the application of rapid rate of loading. Two types of impact tests are commonly employed. Charpy impact test Izod impact test

6. Charpy impact test A specimen of specified dimensions having a V notch of 45 o at the center of the specimen, is used and supported at both ends as a simple beam. The specimen is struck on the opposite side and directly behind the notch, with a pendulum released from a known height. By knowing the mass of the pendulum and the difference between its initial and final heights, the energy absorbed by fracture can be measured.

7. Specimen dimensions for charpy impact test Charpy impact testing

8. Izod impact test In this type of testing, specimen of specified dimension is used with 45 o V notch present near one end instead of the center of the specimen. Specimen is clamped vertically like a cantilever. Struck with the pendulum hammer on the side of the notch at the corner of the beam.

9. Ductile – Brittle transition temperature In both types of impact testing presence of notch is studied alone, so far and resulting transition from ductile to brittle behavior is studied. The effect of varying temperatures can also be studied by providing a range of temperatures to the notch specimen. So that the “low temperature” can be found out for the given material at which it will start behaving completely brittle.

10. Ductile-Brittle transition temp

11. To estimate the DBTT of the given material. The material is undergone notched impact testing over a range of temperatures. At high temperatures impact energy is relatively large. As temperature drops the impact energy drops suddenly over a very narrow temperature range. Below that temperature, impact energy has almost constant but very small value.

12. Ductile – brittle transition takes place over a range of temperature instead of a sharp value of temperature. Therefore it is difficult to define a single temperature. Usually DBTT is taken that temperature at which charpy V notch (CVN) energy value is 20J.

13. Note: BCC metals have transition temperature, but most FCC and HCP metals do not undergo this transition. Impact Energy v/s Temperature