Proposal for New Hardness Concept Using Herbert Hardness Tester

A Herbert hardness tester uses an inverted pendulum and performs measurements under loading conditions different from those used in conventional indentation and rebound hardness tests. Hardness is conventionally measured as a resistance to monotonic deformation. In contrast, a Herbert hardness tester oscillates on a test specimen, and provides information on resistance to cyclic deformation. The original Herbert hardness tester yielded three kinds of hardness data: (1) Scale hardness (indicated by the position of the bubble in a spirit level at the end of the first swing); (2) Time hardness (based on the pendulum swing time measured using a stopwatch); and (3) Flow hardness (defined as the scale hardness/time hardness ratio, and which is an indicator of work hardening). However, this test method did not become popular due to the time and effort involved compared with other techniques. Nevertheless, the present authors have continued to work towards making improvements in hardness measurement methods based on freely damped oscillations. These improvements included the use of rotary potentiometers, laser displacement meters, and image processing. And a new concept of “Damping hardness” has also been proposed. This paper covers the following topics: (1) A new hardness measure given by the natural angular frequency/damping factor; and (2) Development of a new Herbert hardness tester incorporating a wireless acceleration sensor.