Pressure Bar Method

Hopkinson (1914) developed a pressure bar method to facilitate accurate measurement of large pressures subjected to very rapid changes. Mechanical and Piezo-electric gages were found to be inadequate. In this method, pressure to be measured is applied normally at one end of a cylindrical steel bar, and the magnitude of pressure is determined by using end pieces at the other end of the bar. With the right type of steel, pressures upto 60 tons/ sq.in (912.33MPa) could be measured.

Storkmann et al. (1998) included acceleration compensation over three different models and measured aerodynamic forces over these models satisfactorily in the Aachen shock tunnel (TH2). The models used were a pointed cone, an Apollo CM capsule and a delta wing configuration.

It was found that the duration of the stress pulse as it travels along the bar increases due to dispersion. It was found that at a distance of x cm from the origin, the duration of a very short pulse becomes 3.3x μs. Relationship between transient strain and time was determined using a condenser bar setup and an oscilloscope.

Though wire foil gages have the advantages of simplicity and compactness, they cannot provide for large signal amplification. Hence their dynamic application range is limited. Iltinskaya and Podmar'kov (1966) thus advocated the use of semiconductor strain gages for dynamic measurements.

Experiments were carried out by Knapp et al. (1998) testing the performance of accelerometers and strain gages in high frequency shock wave environment. The Hopkinson pressure bar used was designed for two types of pulses; a longitudinal monomode pulse and a longitudinal/radial multimode pulse for calibration and operational tests respectively. The results show that the interpretation of recorded acceleration signals is often difficult. Foil strain gauges were proved to be well suited for shock measurements if the strain levels are sufficiently high. Polyvinylidene fluoride (PVDF) gauges were found to be the best choice for low strain levels due to their excellent signal-to-noise ratio. The application of accelerometers required careful selection of

They concluded that, as in static measurements, the gage factor determines the sensitivity of the gage even a low frequencies ranging to several kHz, while the gage length becomes the dominating factor at higher frequencies.

Experiments performed by Smith and Mee (1996) showed that polymer strain gages could be used to obtain accurate measurements of axial stress waves propagating within the incident bar of a split Hopkinson bar arrangement. In the experiment, the polymer strain gages were mounted in a bending compensation arrangement to minimise the effect of radial inertia.

Experiments conducted by Anderson (1998) showed that the wheatstone bridge circuit suffered from high arm impedance, which makes the signal more non linear with respect to the variable which is being measured. The transfer function of the measuring instruments using the wheatstone bridge was effectively “fixed” after installation, which reduced the flexibility of the circuit. To overcome the aforementioned fallacies of the wheatstone bridge, a dual differential subtractor was introduced. It showed excellent linearity in output signals as well as cost