Stress measurement calibration

Very shortly, the first one is based on the stress measurement performed using a rosetta strain gauge located in an area of sufficiently uniform stress distribution. In this case, the calibration factor Cr can be easily obtained by the following equation ... 

An experimental activity on the stress measurement of a pressure vessel using the SPATE technique was carried out. It was demontrated that this approach allows to define the distribution of stress level on the vessel surface with a quite good accuracy. The most significant advantage in using this technique rather than others is to provide a true fine map of stresses in a short time even if a preliminary meticolous calibration of the equipment has to be performed. 

Photoelastic measurement is a very useful method for identifying stress in transparent plastics. Quantitative stress measurement is possible with a polarimeter equipped with a calibrated compensator. It makes stresses visible (Fig. 5-2). The optical property of the index of refraction will change with the level of stress (or strain). When the photoelastic... 

It remains an objective to develop potentiometric sensors with longer lifetimes, greater reproducibility and greater stability. The importance of an appropriate statistical treatment of the results in order to determine their precision is stressed. Frequent calibration is necessary, at least at the beginning of each measurement session and in a medium as similar as possible to that where the sensors are to be employed, in order to ensure the accuracy of the analytical determinations. 

Viscometric measurements were made with a Haake Rotovisco MV 1 system with a rotor of diameter 40.08 mm, and length of 60 mm and a cup of 42.00 mm inside diameter. The shear stress was detected using a Head 50. Before measurements, calibrations were made with standard oils to obtain the instrument constant to convert stress readings to shear stress. Shear rate could be varied over the range of 50.74 to... 

In this chapter we will be concerned with the line shift due to uniform strain. From this shift the strain may be calculated and, knowing the strain, we can determine the stress present, either by a calculation involving the mechanically measured elastic constants of the material, or by a calibration procedure involving measurement of the strains produced by known stresses. X-ray diffraction can therefore be used as a method of stress measurement. Note, however, that stress is not measured directly by the x-ray method or, for that matter, by any other method of stress measurement. It is always strain that is measured the stress is determined indirectly, by calculation or calibration. 

Methods have been proposed for calculating the proper values of and v to use with x-ray stress measurements from values measured in various directions in single crystals. Such calculations usually rest on assumptions of dubious validity, and the results are not in good agreement with experiment. The safest procedure is to measure on a specimen subjected to known stresses, and specific examples of calibration procedures may be found in [16.3, 16.19, 16.20, 16.21, 16.22, 16.24]. 

The A e part of any stress constant is the important part, because it is determined only by the properties of the material, and it is the quantity that should be reported as the result of a calibration experiment. Similarly, any report of x-ray stress measurements should include the value of Kg used in the calculations. 

The shear stress measurement range of the YR-1 (in Pascals) is determined by the size and shape of the vane spindle and the full scale torque range of the calibrated spring. 

A finite-element mesh was constructed as shown in Figure 5. The hydrological properties of the different units within the finite-element model was first estimated from local geohydrological field data and then calibrated to match observed head distribution and inflow into the open drift. An initial stress was assigned according to = 10 MPa, Oh = 15 MPa, and On = 30 MPa, where Oh is oriented 45° from the axis of the FEBEX tunnel (Figure 5b). These values are within the range of stress measurements in the GTS area (Pahl el al.. 1989). 

Constant stress measurements are usually made using a jig incorporating a calibrated spring to apply the specified stress. 

A Renishaw inVia Raman Microscope was utilized for scratch induced residual mechanical stress measurements. Using a Si-laser (532 nm), Raman spectra were collected from several SiC particles present within the scratch grooves. The laser spot size used was around I pm. For comparison purposes, Raman spectra were also collected from several SiC particles residing outside the scratch grooves. All the Raman measurements were performed at room temperature. The Raman spectrometer was calibrated with a Si standard using a Si band position at 520.3 cm". ... 

A simpler approach is based on empirical correlations between NMR parameters and sample properties, which often can be established by calibration of an NMR parameter against a material parameter (155,156). An example of stress-strain calibration is given in Figure 26 by calibration of T2 of filled poly(dimethyl siloxane) rubber against strain (153), (cf Fig. 26a). By use of the stress-strain curve (Fig. 26b) from mechanical measurements, T2 can be calibrated against stress (see Fig. 26c). 

The mechanical properties of hydraulic mortars are closely associated with the setting process which takes place when such mortars are mixed with water. The progress of this process can be monitored by time-dependent measurements of various properties including mechanical parameters such as tensile stress. Frequently, calibrations are required. Moreover, complications are encountered by invasive procedures. On the contrary, optical methods generally avoid such interferences since mechanical contacts between object and analytical tool do not take place. Measurements which are recorded in absolute units provide a further very important advantage. These conditions are met by LRC. 

The true velocity in the near-wall region determined from the hot wire calibrated in the free stream is affected due to the heat conduction in the near-wall region. Therefore, the velocity obtained from the hot wire needs to he corrected based on the empirical relationship. The substitution of this corrected velocity in Eq. (9) provides the waU shear stress. The sensitivity of this approach depends on the wall distance, i. e., there is an optimal wall distance at which the hot wire sensitivity is optimal. In addition to that, the hot wire should be located inside the inertial sublayer to satisfy the basic assumption essential for the shear stress measurement. 

In experimental aerodynamics, surface hot-wire probe has proved to be the most successful standard measurement technique to determine the laminar to turbulent flow transition, local separation, and shear stress fluctuations. The flush-mounted thermal shear stress sensor is one of the most successful techniques for shear stress measurement and is available in various forms, that is, sensor skin and so on (Xu et al, 2003), due to rapid development of MEMS manufacturing processes. The quantitative determination of shear stress depends on the proper calibration with a reference method. The calibration issues have been described in a later section. 

In particular, the known stress calibration method was chosen, therefore 6 rosetta strain gauges (R1-R6) on the shell and 7 (R7-R13) on the the head were applied. Their distances measured from the head centre are listed in table 1. R3 and R4 were applied only to check a uniform stress level on the shell surface. 

The calibration curve of each rosetta strain gauge was so obtained and ftg.5 shows the sum of the principal stresses at the measuring points versus pressure inside the vessel. Further tests were carried out to obtain the calibration factor and to check that it remained constant on the whole scan area of the test surface. This was achieved through additional measurements using the SPATE system on fixed points on the surface located very close to the applied rosetta strain gauges. This procedure gave the following results ... 

The equations and methods for determining viscosity vary greatly with the type of instmment, but in many cases calculations may be greatly simplified by calibration of the viscometer with a standard fluid, the viscosity of which is known for the conditions involved. General procedures for calibration measurement are given in ASTM D2196. The constant thus obtained is used with stress and shear rate terms to determine viscosity by equation 25, where the stress term may be torque, load, or deflection, and the shear rate may be in rpm, revolutions per second (rps), or s F... 

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... 

Stress in crystalline solids produces small shifts, typically a few wavenumbers, in the Raman lines that sometimes are accompanied by a small amount of line broadening. Measurement of a series of Raman spectra in high-pressure equipment under static or uniaxial pressure allows the line shifts to be calibrated in terms of stress level. This information can be used to characterize built-in stress in thin films, along grain boundaries, and in thermally stressed materials. Microfocus spectra can be obtained from crack tips in ceramic material and by a careful spatial mapping along and across the crack estimates can be obtained of the stress fields around the crack. ... 

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