This is one reason why these types of alloys are used in cryogenic applications. In addition to the impact energy there are two further features that can be measured and may be found as a requirement in some specifications. These are percentage crystallinity and lateral expansion. The appearance of a fracture surface gives information about the type of fracture that has occurred - a brittle fracture is bright and crystalline, a ductile fracture is dull and fibrous.
Percentage crystallinity is therefore a measure of the amount of brittle fracture, determined by making a judgement of the amount of crystalline or brittle fracture on the surface of the broken specimen. Lateral expansion is a measure of the ductility of the specimen. When a ductile metal is broken the test piece deforms before breaking, a pair of 'ears' being squeezed out on the side of the compression face of the specimen, as illustrated in Fig 4.
The amount by which the specimen deforms is measured and expressed as millimetres of lateral expansion. ASME B The next article in this series will look at the testing of welds, how the impact strength can be affected by composition and microstructure and some of its limitations and disadvantages.
For information on this article please contact us. Mechanical testing - Notched bar or impact testing - Part II. Support for SMEs. However the introduction of this kind of specimen has never supplanted the use of the traditional Charpy V notch.
The initial U notch recommended by Charpy has been progressively discarded. In the s, the adoption of the fracture mechanics concept led to the evaluation of fracture toughness via the use of Charpy impact tests in terms of the critical stress intensity factor or the critical strain energy release rate. This approach requires that the critical load is suitably evaluated and the free oscillations experienced by the specimen during impact are understood.
The recent introduction of Notch Fracture Mechanics has provided a promising tool for this problem. In this approach, there is no fundamental difference between fracture emanating from a notches or a crack. However, the effective fracture stress is sensitive to stress field conditions and particularly to relative stress gradient.
It is interesting to notice that Charpy implicitly suggested that the specific work for fracture is a measure of fracture toughness. Once a specimen reaches the precise temperature, it is quickly placed into a special holder with the notch oriented vertically and toward the origin of impact. The specimen breaks at its notched cross-section upon impact, and the upward swing of the pendulum is used to determine the amount of energy absorbed notch toughness in the process.
It is still a question if Charpy and others knew about the ductile to brittle transition that occurs with temperature in steel during these early years of impact tests. Ignorance of the ductile to brittle transition in steel was again evident in the numerous Liberty ships that literally fractured in half during WW II.
The over-stressed steel welds no post-weld heat treatment became brittle at icy water temperatures and catastrophic crack propagation took place even when the ships were at dock. The standard Charpy-V notch specimen is 55mm long, 10mm square and has a 2mm deep notch with a tip radius of 0. In some circumstances, sub-size Charpy specimens can be tested with thickness less than 10mm.
The specimen is supported at its two ends on an anvil and struck on the opposite face to the notch by the pendulum. The amount of energy absorbed in fracturing the test-piece is measured and this gives an indication of the notch toughness of the test material. The pendulum swings through during the test, the height of the swing being a measure of the amount of energy absorbed in fracturing the specimen.
Conventionally, three specimens are tested at any one temperature and the results averaged. Alternatively, tests are carried out at a range of temperatures in order to generate a ductile to brittle transition curve. Charpy tests show whether a metal can be classified as being either brittle or ductile.
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