Application of the acousto elastic effect
The instrumentation use of the acousto elastic effect makes qualitatively and quantitatively new measuring and safeguards methods possible in buildings and in the mountains. As example of possible applications the monitoring of local tensile states in buildings and under take can be enough over Period to be called. Further new and at the same time economical applications result, like on-line monitoring of the answer spectrum of buildings of any kind in seismically active areas. Already during the building phase measurements are possible in foundations and stakes. Also tunnel-build with high overlays and/or large ground pressure features can so on-line be supervised. The measuring bodies can be brought in already in the phase of the building production, i.e. concreting. Also an additional installation into boreholes (method of the hard inclusion) or the stress measurement in expansion joints of buildings is possible.
Instrumentation bases of in situ measurement
Contrary to the stress analysis of construction units, where generally the change of speed of the transversals and longitudinal waves is seized and evaluated, in line stress measurement regarded here uses only the change of the speed of the longitudinal waves within the thickness of a measuring body. Past direct measurements of the speed of sound in rocks or concrete are unsuitable for regulations of the stress ratios. Rock anisotropies, tears etc. affect saliently these measurements. Particularly different contents of pore waters make such measurements with difficulty comparable and unsuitable for a monitoring. The instrumentation influence of changing porositys and/or dampness contents can lie the far over stress-dependent portion of the measuring effect. These new applications of the acousto elastic effect for the interests of the geotechnology are described by several relevant patent specifications [Jäger, 2005,2006,2007, 2008,2009]. The force application changes also the mechanical stress in the measuring body. Since this mechanical stress is not directly measurable, one must select either the detour over a mechanical size or over further directly dependent variables. The ultrasonic speed is like that one, from the mechanical stress, dependent variable. The acousto elastic effect can take place both via the measurement of the longitudinal (thrust) wave and via the measurement of the transversals (shearing) wave or via evaluation of the change of both waves.
It is valid the reversibility between expansion and compression.
The Hook law is valid only for the elastic range.
The change of the speed of sound depends apart from the dependence on the influencing mechanical stress also on the temperature. In practice the temperature equalizing between measuring bodies and surrounding building adjusts itself sufficiently fast. Larger variations in temperature are concrete in the stationary installation in the mountains or in tunnels, in the annular space between Tübbing and mountains not to expect. With applications, where on a changing ambient temperature is to be counted, temperature measurements are capable of being implemented for compensation conceivably and easily in the measuring body. By the elastic behavior of the measuring section between the ultrasonic sensors also the length of the measuring section is changed. There it admits is that for example the change of the speed of sound is substantially larger by the effect of a mechanical stress (upsetting measure-strains), how the influence of the pure length variation by this tension or application of force on measure-strains, can via the measurement of the speed of sound a sufficiently exact determination of the tension within the measuring body take place. The acousto elastic coefficients stated above are very small in relation to the absolute speed of sound. The direct instrumentation evaluation by a usual running time is too inaccurate, since the dissolution is not sufficient here. A direct frequency counting over microprocessors separates, there the cycle time (computing clock) around the factor 1000 to 10000 is larger than the demanded usable dissolution. Metal plates of few centimeters result i in the case of only one reflection running times of the ultrasonic impulse smaller 10µs.