Unbonded strain gauge

Small pressure transducers with flush-mounted diaphragms are commercially available for the measurement of either steady or fluctuating pressures up to 100 MPa (about 15,000 Ibftin ). The metallic diaphragms are as small as 4.8 mm (%6 in) in diameter. The transducer is mounted on the apparatus containing the fluid whose pressure is to be measured so that the diaphragm is flush with the inner surface of the apparatus. Deflection of the diaphragm is measured by unbonded strain gauges and recorded electrically. 

FIGURE 2.5 Structure of an unbonded strain gauge pressure sensor. (Reproduced from Neuman M.R. 1993. In R.C. Dorf (Ed.), The Electrical Engineering Handbook, Boca Raton, FL, CRC Press. With permission.)... 

Typically strain and stress are measured by unbonded strain-gauge transducers, the signals from which are then fed to a phase meter, which provides a direct reading of the relative amplitudes and the phase difference, hence giving values of the modulus and tan <5 [9]. 

Unbonded. The resistance wire may also be wrapped around pins on the test structure, or affixed to pins on some type of motion-amplifying device, which in turn is attached to the test structure. In this case, the wire is not cemented, but only held in place mechanically. This forms the unbonded strain gauge (see Fig. 8.1b). 

FIG. M-20 Unbonded strain gauge. (Source Demag Delaval.)... 

Fig. 7.10. Stress versus strain characteristics of three lap joints and an unbonded test coupon. Joint A, 25 mm X 12 5 mm overlap B, 25 mm X 190 mm overlap C, 25 mm X 25 0 mm overlap. Adherend mild steel, 16 gauge. Adhesive heat cured toughened epoxide (Permabond ESP105). Test coupon 25 X 75 mm, 16 gauge mild steel of the type used to fabricate the joints represented by continuous line. The classical form of the elastic/plastic deformation of the unbonded test coupon is clearly seen. (A) This specimen does not fail until after the test coupon has become plastic. (B) Although possessing 50% more bond area, the load required to fail specimen B is not very much greater than that needed to fail A however, the toughness of the adhesive and the spare capacity of the initially unloaded central area are clearly illustrated by the ability of the joint to sustain a load even though it has cracked. (C) This example emphasises the point made with specimen B. A performance such as this makes it difficult to say whether the adhesive has failed the steel or the steel has failed the adhesive.

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