THE IMPACT TEST
Impact tests are used to indicate the ability to resist the rapid propagation of a crack conditions. It measures the energy necessary to fracture a standard notched bar by an compact load, which is indication of the notch toughness of a material under shock loading.
A pendulum released from a known height, strikes and breaks the sample as it continues its upward swing. From the knowledge of the mass of the pendulum and the difference between the initial and final heights, the energy absorbed in fracture can be calculated.
The impact test indicates the notch sensitivity of a material resulting from the presence of internal stress raiser, such as grain boundary inclusions, internal cracks and secondary phases. It is also useful as a production tool in comparing manufacturing materials with others which have proved satisfactory in service. Impact test is often used to assess the temperature of the transition from the ductile to brittle state which occurs as the temperature is lowered. The transition temperature is also dependent on the shape of the notch in the specimen. For identical materials, the sharper the notch, the higher the apparent transition temperature.
Resistance to rapid propagation of a crack at a specified temperature level.Tests for the impact toughness, such as the Charpy Impact test, were developed before fracture mechanics theory was available.The Charpy impact test continues to be used nowadays as an economical quality control method to assess the notch sensitivity and impact toughness of engineering materials. It is usually used to test the toughness of metals. Similar tests can be used for polymers, ceramics and composites.
The charpy impact test can be used to assess the relative toughness of different materials, e.g. steel and aluminium, as a tool for materials selection in design. It may also be used for quality control, to ensure that the material being produced reaches a minimum specified toughness level.
The Charpy V-notch impact test has limitations due to its blunt notch, small size, and total energy measurement (i.e., no separation of initiation and propagation components of energy). However, this test is used widely because it is inexpensive and simple to perform. Historically, extensive correlation with service performance has indicated its usefulness.
The Charpy V-notch test commonly is used as a screening test for evaluation notch toughness changes influenced by chemical composition (alloying and impurity elements, including gases) and physical and mechanical properties of materials. Typical physical properties of interest are microstructure and grain size, which are influenced by fabrication procedures (temperature of working, cross-rolling, surface treatment, etc.). Mechanical properties generally considered are yield and flow properties and hardness.
In many structural steel applications, the Charpy V-notch test is used in procurement and quality assurance for assessing different heats of the same type of steel. Also correlation with actual fracture toughness data is often devised for a class of steels so that fracture mechanics analyses can be applied directly. Heat-to-heat differences in Charpy properties can be quite large, and when conditions involving human safety are concerned, failure prevention is imperative. Two such examples are the nuclear pressure vessel and the structural steels.
AEIS can perform Impact testing from 30 to 240ft-lb at temperatures ranging from 150˚F upto room temperature or higher for both Charpy and Izod methods, this is in addition to Drop_Weight Tear Test, Dynamic Tear Tests and CTOD. Some of the standards utilized at AEIS for Impact testing are:
|ASTM TESTING CAPABILITIES|
|A 370||Standard for Test Methods and Definitions for Mechanical Testing of Steel Products|
|E-23||Standard Test Methods For Notched Bar Impact Testing of Metallic Materials|
|E-208||Standard Test Method For Conducting Drop-Weight Test to Determine Nil-Ductility Transition Temperature of Ferritic Steels|
|E-436||Standard Test methods for Drop-Weight Tear Tests of Ferritic Steels|
|E-604||Standard Test Method for Dynamic Tear Testing of Metallic Materials|
|E-812||Standard Test Method for Crack Strength of Slow-Bend Pre-cracked Charpy Specimens of High Strength Metallic Materials|