Measuring stress and strain can be extremely difficult and often requires use of sensitive equipment able to measure very small values of strain. Generally, direct stress measurement methods are not available for commercial applications and thus stress generally is determined by measuring strain. Several types of sensors are employed for measuring strain and include strain gauges, e.g. piezo-resistive strain gauges, magnetoelastic devices, optical sensors, acoustic sensing devices, eddy current devices, rings under load, load cells, and diaphragms. However, existing strain gauges and other strain measurement sensors are extremely sensitive to external conditions such as drift (permanent movement of the sensor after strains occur), residual stresses or strains, temperature effects, electric noise, other environmental factors, and/or defective mechanical bonding of the sensor to the material being tested for strain. Accordingly, measuring strain accurately is difficult in downhole applications, such as wellbore drilling applications.
Various approaches have been employed to correct for these conditions. For example, signal amplification devices, e.g. operation amplifiers, may be employed; or the sensitivity of the gauge may be electrically increased through the use of Wheatstone bridges. However, even with such enhancements the signal of the strain gauge remains low and is susceptible to environmental effects and other limiting effects. In some applications, the effects of changes in temperature have been compensated to some extent by selecting a sensor material and a backing material having a thermal expansion coefficient similar to that of the reference material of the object being monitored for strain. This technique reduces the effect of temperature but does not eliminate the effect. In a downhole drilling application, for example, the temperature on a drilling collar can change 150° C. which causes an expansion of the collar about 25 times greater than the strain induced due to drilling loads. This means that if the error in temperature measurement is 1%, the error in strain measurement can readily reach 20%.
Other methods employed to compensate for temperature changes include placement of temperature compensating measuring devices in a Wheatstone bridge. Look-up tables or polynomial fitting also can be employed to model the effect of temperature on the strain measurements, and sometimes temperature effects can be compensated via software. However, existing approaches are not able to sufficiently compensate for the many environmental factors and other effects encountered in relatively extreme applications to provide accurate and consistent strain measurements.
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