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[September 2004]
Flexural Strength and Edge Chipping Using Impact Tester
NPL - National Physical Laboratory (Teddington, Middx, England) - has re-instrumented its drop-weight impact tester to allow it to record accurately impact velocity, impulsive forces and timing of the impact events. NPL added that its outstanding headache was accurate calibration of load cells under impact conditions, where the timescale is of the order of a few microseconds.
Despite this, the team has successfully used the instrument for correlation of fracture phenomena with drop height - and hence impact energy - using incremental drop heights until failure.
In one example, it was shown that the quasistatic flexural strength of notched pieces of various grades of hardmetal (fracture time 20s to 30s) is very similar to that obtained in impact testing (fracture time ~10-6s). This result indicates that simpler quasistatic testing is a good indicator of material strength under impact conditions.
In another example, in a test involving dynamic edge flaking of an alumina ceramic using a weighted ball impactor, an approximately linear relationship was found between the drop height (and hence energy) required to cause failure and the distance of the impact site from the edge.
Furthermore, in similar testing of other types of material from glass to tungsten carbide, the slope of this relationship correlates with the material's fracture toughness in a similar manner to that found for quasistatic testing.
More from Roger Morrell at: roger.morrell@npl.co.uk
ENDS