Hopkinson Pressure Bar Technical History

The purpose for SHPB testing is to retrieve reliable stress wave measurements so accurate high strain rate material analysis can be achieved. Energy from the stress wave is measured by the strain gauges located on both the incidence and transmission bars. Before conventional strain gauges, use of condenser microphones and capacitive sensors were used for measuring stress.

The means to calculate and produce a stress vs. strain curve has evolved with technology. The earliest calculations used the Pochhammer-Chree elastic wave equations which assume waves are trapezoidal and periodic. To calculate a frequency domain from time, the Fast Fourier Transform (FFT) algorithm was introduced. Later, the inverse FFT is included in the calculation to reduce noise on the calculated stress vs. strain curve. With the inverse FFT it is no longer assumed that the pulse is trapezoidal and periodic. To create an actual phase velocity graph, rather than theoretical, the stress wave is captured from both the incidence and transmission bar simultaneously. Use of and oscilloscope and analog strain recorder allow simultaneous readings. Current gauges are routed through amplifiers and an oscilloscope to a computer where data is stored.

The data collected by the computer needs to be processed through a series of formulas. What previously took a half hour to calculate by hand now takes just a few moments due to developed data management programs. The Split Hopkinson Pressure Bar designed and manufactured by REL includes all leading technologies to simplify the process for the tester. With cutting edge data recording devices and strain gauges, a SHPB from REL is a great investment if you’re business includes material analysis or engineering.