Impact testing becomes essential in order to study the behavior of welded objects under dynamic loading. An impact test determines the behavior of welds when subjected to high rates of loading, usually in bending. An impact test gives an indication of the relative toughness of the material. Toughness is defined as the resistance of a metal to fracture after plastic deformation has begun. The purpose of impact testing is to determine the amount of impact a specimen will absorb before fracturing.
In an impact test, a specimen machined or surface ground and notched is struck and broken by a single blow in a specially designed testing machine. The quantity measured is the energy absorbed in breaking the specimen by a single blow. The ideal impact test would be one in which all the energy of a blow is transmitted to the test specimen.
Charpy impact test specimens are cut from metal samples, and a V-notch is placed in the center. Charpy impact testing can be performed at ambient and low temperatures (down to -196°C).
Low temperature charpy testing involves placing the specimens into a cooling chamber bath, and leaving specimens while a calibrated thermocouple records the temperature required for the test. Charpy test specimens are then removed from the chamber, and immediately tested for joules/lbs force by a swinging arm pendulum (which records the value to propagate the cut V-notch).The energy transferred to the material can be inferred by comparing the difference in the height of the hammer before and after a big fracture.
The notch in the sample affects the results of the impact test, thus it is necessary for the notch to be of regular dimensions and geometry. The size of the sample can also affect results, since the dimensions determine whether or not the material is in plane strain. This difference can greatly affect conclusions made.
The "Standard methods for Notched Bar Impact Testing of Metallic Materials" can be found in ASTM A370, ASTM E23 and BS EN ISO 148-1, where all the aspects of the test and equipment used are described in detail.
Table above shows charpy impact specimen size according to ASTM A370 and BS EN ISO 148-1. Standard size specimen for impact test is 55.0mm x 10.0mm x 10.0mm for both standards
The quantitative result of the impact tests the energy needed to fracture a material and can be used to measure the toughness of the material and the yield strength. Also, the strain rate may be studied and analyzed for its effect on fracture.
The ductile-brittle transition temperature (DBTT) may be derived from the temperature where the energy needed to fracture the material drastically changes. However, in practice there is no sharp transition and so it is difficult to obtain a precise transition temperature. An exact DBTT may be empirically derived in many ways: a specific absorbed energy, change in aspect of fracture (such as 50% of the area is cleavage), etc.
The qualitative results of the impact test can be used to determine the ductility of a material. If the material breaks on a flat plane, the fracture was brittle, and if the material breaks with jagged edges or shear lips, then the fracture was ductile. Usually a material does not break in just one way or the other, and thus comparing the jagged to flat surface areas of the fracture will give an estimate of the percentage of ductile and brittle fracture. The figure below shows the surface of broke Charpy specimen.