Instrumented impact testing
Brittle materials do not readily accommodate impact because they cannot resist localised stresses without cracking, so understanding more about impact is an important aspect supporting usability.
NPL has refurbished and instrumented a drop-weight impact tester for use on hard and brittle materials. This instrument is based on a pair of vertical air-bearing guides between which the impactor slides. The impactor is equipped with the required type of geometry nose, e.g. a hard ball of given diameter, and is held at a given starting height by an electromagnet, which is released to allow the impactor to fall under its own weight. The test-piece is held in position in a suitable manner, e.g. clamped to a baseplate, or held in a flexural strength jig at the base of the apparatus. Impact forces and accelerations are recorded as a function of time. The velocity before impact can be determined from a photocell system providing an acceleration calibration.
Localised impact into advanced technical ceramic tiles using a ball indenter has demonstrated the development of a combination of radial and hoop cracking, the energy required to initiate this being related to ball diameter, and the total radial crack length obtained being linearly related to drop height or impact energy.
The impact energy to flexural impact failure of notched hardmetal test-pieces of a variety of grades has been found to be correlated closely with the quasistatic strength within the scatter of strengths, indicating that there is probably no additional effect of fast loading rate.
Other applications, such as dynamic edge chipping, are being investigated.
For more information: Ken Mingard