Stress Gauges

Measurements from stress gauges, assuming equal accuracy and time resolution, are equivalent to measurements from particle velocity gauges in exploring a material s equation of state. Both piezoresistive and piezoelectric techniques have been used extensively in shock-compression science. 

The gauge is usually calibrated in well-controlled uniaxial strain experiments by measuring the fractional change in resistance AR/Rq as a function of the shock stress. The results are empirically correlated to the stress through the relation 

The gauge element in the form of a foil (50 ohms) is normally embedded in materials such that the active gauge element is normal to the stress-wave propagation direction. Manganin is the only in situ stress gauge available for 

Of all the piezoelectric crystals that are available for use as shock-wave transducers, the two that have received the most attention are x-cut quartz and lithium-niobate crystals (Graham and Reed, 1978). They are the most accurately characterized stress-wave transducers available for stresses up to 4 GPa and 1.8 GPa, respectively, and they are widely used within their stress ranges. They are relatively simple, accurate gauges which require a minimum of data analysis to arrive at the observed pressure history. They are used in a thick gauge mode, in which the shock wave coming through the specimen is 

The PVFj gauge has been calibrated up to 4 GPa (Bauer, 1984) for both shock loading and release. Graham and Lee (1986) have extended these calibration studies to about 20 GPa, and have measured both a shock loading and release profile in sapphire at 12 GPa, as indicated in Fig. 3.15. 

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DeCarli, P.S., D.C. Erlich, L.B. Hall, R.G. Bly, A.L. Whitson, D.D. Keough, and D. Curran (1976), Stress-Gauge System for the Megabar (100 GPa) Range. Defense Nuclear Agency Report No. DNA 4066F, unpublished. 

Gupta, Y.M. and W.J. Murri (1982), Piezoelectric Shear Stress Gauge for Dynamic Loading, in Shock Waves in Condensed Matter—1981 (edited by W.J. Nellis, L. Seaman, and R.A. Graham) American Institute of Physics, New York, pp. 525-529. 

Numerous resistance measurements have been carried out under high-pressure shock compression [79D01]. Most of the work has been motivated by the desire to develop stress gauges to measure pressures in shock-compressed materials. Other measurements were undertaken to determine critical pressures to induce phase transformations. Although most of the work is not carried out in sufficient detail to relate resistance observations to defect characterizations, excess resistance at given shock pressures is observed in every case compared to comparably loaded static pressure observations. The presence of residual resistance for times after the loading is removed provides explicit evidence for irreversible changes in resistance due to defects. 

The Fastress instrument is not a diffractometer but rather a stress gauge that must be calibrated with specimens containing known stresses. It is specifically designed for measurements on ferritic and martensitic steels (BCC) by observation of the 211 reflection with CrKoc radiation at a 26 angle of about 156°. Counter movements are restricted to a range of 152° to 159°, and only materials that produce... 

Piezoelectric devices are used in the following areas Measurement of pressure, measurement of vibrations, stress gauge, strain gauge, measurement of acceleration, impact detector, position sensors. There is a wide choice of materials and material forms that are actively piezoelectric. Most of them have the ability to convert mechanical strain into an electrical charge when used as sensor, and to do the opposite when used as an actuator. Studies of piezoelectric semiconductors, nonlinear effects and surface waves led to constraction of useful devices. The research of sophisticated electroacoustic devices may one day simplify the dialogue between the user and the computer. There are matty more examples of this kind one coirld mention. 

Takasc et al. (63] have found that rctiumcni polarizatkms of 0.1 Cfm can be achieved in deulcratcd PVDF, and Bauer [64] reported the same for biaxially oriented films. Bauer and co-workers have developed a standard stress gauge using PVDF and in the process have demonstrated [6S] that they can achieve a remanent polarizalioo of 0.092 0.002 C/m using a patented poling process. 

The fitud category to be rfiscuased n a general one which will be denoted measurement instrumentation. Included within this is accelerometers, strain and stress gauges, acoustic emission sensors, fiber optic phase nwdulators. gas emission detectors, and external pressure field flow detection by use of surfsce acoustic wave (SAW) ferroelectric polymer transducer configurations. 

PVDF is useful for a variety of measurement instruments iitcluding acceletometers, stress gauges, strain gauges, shock gauges, turbulence sensors, fiber optic phase modulators. 

FIgurn 25 Cross-sectional view of the impact toading configuration showing the Bauer PVDF stress gauge. (Adapted froni Ref. 38.)... 

R. A. Oraham. L. M. Lee. and F. Bauer, Reapanac piezoelectric polymer stress gauges (PVDF) lo shock loading, Shack Waves in Condensed Matter (S. C Schmidl and... 

Fig. 1. Layout of sensors on experimental floe. A, B, C, D and E are sites of deformations measurement. Harbour, center, bog, frontier, old frontier are sites of CRREL stress gauges. Thick solid line shows location of ridge formed late in September 1993...

The electrodes bonded to the top and bottom surfaces of the polymer film become part of the structure as shown in Figure 16.7. For example, the electrodes are commonly thin strips of gold (for good conductivity) that are much stiffer than the polymer material. Figure 16.7a and b shows the structure of a piezoelectric stress gauge and a bending actuator. 

The stress gauge can be used for compression in the vertical z-direction normal to the thin strips or for measuring tension or compression in the x- and y-directions parallel to the thin strips when glued to the surface of another structure to be analyzed. The thicknesses of the layers are very thin. A typical gauge from one manufacture has thicknesses of 25 pm for both the gold electrodes and the piezoelectric PDVF element. 

FIGURE 16.7 Structure of a piezoelectric stress gauge, (a) Composite structure for stress gauge and (b) structure for bending actuator. 

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