A strain gauge is a device that measures stress and strain within a given area. Thanks to the data obtained, it is possible to find out the level of tension in an object, which allows improving its structure and avoiding dangerous situations. The strain tensor is used to calculate the best tension in various construction tools, as well as when creating reinforced concrete structures. Also, these devices have found their application in mechanical engineering and textile industry.
Description and operation
Mortgage string strain gauges TB-200 VNIIG.Z are designed for remote measurement of relative tensile and compression strains in massive concrete structures (mainly dams), during long-term field observations of their condition without access to them for repair and correction , and also for remote measurement of the temperature of the medium at the installation sites of strain gauges. Overhead string strain gauges TB-200 VNIIG.N are used to measure forces in metal elements (pipelines, metal finishes of tunnels, bridges, etc.).
Power parameters of tensometers:
Strain Gauge Output Parameters:
s = AT 2 + BT 1 + C
where s is the value of the measured relative strain of the strain gauges, million ‘ 1 ;
T is the period of the output signal of tensometers, ms;
A = 802.63 ms 2 , B = – 85.49 ms, C = – 659.72 million ‘ 1 – the nominal values of the coefficients of the calibration characteristics of the strain gauges, determined during the graduation of a representative sample of strain gauges.
Note When using strain gauges to determine the relative deformation of large structures, for example, dams, it is allowed to use a special unit of measurement – 10 ′ 5, the proportion of relative deformation equal to 10 million ‘ 1 .
The upper and lower limits of the period of the output signal of the strain gauge when determining the individual calibration characteristics must correspond to table. 1.
|Elongation of the string, microns||Relative deformation tensiometer, mn ‘ 1||T lower, μs||T upper, ISS||T average, ISS (nominal)|
The calibration characteristic of strain gauges by temperature has the form:
T = (R / R o -l) / a T
where T is the value measured by tensometers, ° C;
R is the resistance of the strain gauge, measured at a temperature of T ° C, Ohm;
Ro is the resistance of the strain gauge, measured at a temperature of 0 ° C, Ohm;
at = 0.00428 – temperature coefficient of the copper wire from which the coil of the electromagnetic head of the strain gauge is made, 1 / deg.
Overall dimensions of strain gauges:
for tensometers to be inserted into an object array
for strain gauges superimposed on the surface of the object
The mass of the strain gauge, including the connecting cable, is not more than 1.5 kg.
The scope of supply for the TB 200-VNIIG tensometer is shown in Table. 2.
|TB.200-01.00.00 RE||Strain gage TB 200-VNIIG.KH Label with calibration data Operation manual (one per lot of strain gages)||String strain gauge TB 200-VNIIGTU 42 73-200-00129716-04|
The strain gauge device TB 200-VNIIG is shown in Fig. 1.
The strain gauge consists of a continuous cylindrical body 1 and two anchors 2, in which a tensioned string 3 and an electromagnetic head 4 are mounted to excite the string and create a variable EMF from the natural vibrations of the string.
The force from the deformable concrete is transmitted through the anchors to the strain gauge body, and the rigidity of the strain gauge body is designed so that it follows the deformation of the concrete, practically without resisting it. The approach or removal of the anchors 2 of the strain gauge causes a change in the length of the string 3 and the associated change in its tension. This, ceteris paribus, uniquely determines the change in the frequency of natural vibrations of the string.
The string is removed from the state of rest by a short-term pulse of electric current supplied to the winding of the coil 4 of the electromagnetic head 5, by which its “pinch” is carried out, followed by damped oscillations. The string oscillating in the gap of the magnetic circuit of the electromagnetic head 5 induces a variable EMF in the turns of the coil. The frequency of the EMF induced in the coil 4 of the strain gauge is equal to the frequency of oscillations of the string. The measurement of the frequency of string vibrations is carried out using a periodometer.
The calibration characteristic of each strain gauge is individual. This dependence is obtained by calibrating the strain gauge string on a special installation that reproduces a given deformation. Based on the measurement results, a calibration curve is constructed that graphically depicts the response of the strain gauge to a change in the distance between the anchors (Fig. 4).
Using the obtained oscillation frequency of the string, using the calibration curve of the tensometer, find the value of the axial relative deformations of the base of the tensometer, and, consequently, the concrete mass within this base. Knowing the relative deformation of concrete within the base of the tensometer, it is possible to determine the stresses in the concrete mass under study, if the elastic modulus of this concrete is known, by the equation:
o = e * Ei
where o is the stress in the concrete mass under study, MPa;
e is the obtained value of the measured relative strain of the base of the strain gauge;
Ei is the elastic modulus of the material of the studied concrete mass, MPa.
Under the influence of the temperature of the investigated medium (concrete or rock), the resistance of the electromagnetic head also changes. By changing the resistance of the electromagnetic head relative to its resistance at zero temperature, using the typical dependence of the relative resistance on temperature, determine the temperature of the strain gauge and the area of the medium under study adjacent to the surface of the strain gauge. An individual calibration characteristic of the strain gauge by temperature is obtained by measuring the resistance of the electromagnetic head of the strain gauge Ro in a zero thermostat.
The manufacturer guarantees compliance of the produced tensometers with all the requirements of TU 42 73-200-00129716-15 subject to the conditions of operation, maintenance, storage, transportation established by the operational documentation.
The warranty shelf life of the strain gauge is 24 months from the date of manufacture. The warranty period of operation of the strain gauge is 5 years from the date of commissioning.
Warranty obligations expire:
Warranty and post-warranty repair of the strain gauge is made at the manufacturer.
Due to the large number of applications, there are several types of strain gauges used in a variety of situations. They differ from each other in appearance and way of working. Globally, devices are divided into mechanical and electrical options. At the same time, there are 4 more types of the second type of devices:
The first of the variety of devices appeared mechanical strain gauges, which were required for the application of new mathematical methods for the study of materials. Subsequently, with the development of technology, electronic options began to be developed, which are used in modern industry.
To understand the principle of operation of these devices, it is necessary to consider precisely mechanical tensometers. Calculations are made here by revealing the numerical value of the dependence of the elongation of an object on its internal stress as a result of the action of a deforming load. The force with which the strain gauge acts on the sample is determined by the ratio of the lengths of the arms of the device. Often, the coefficient is in the range from 1 to 12 thousand.
Our company is engaged in the sale of this type of devices. A universal multi-strain gauge operates with a small piece of steel wire. It is attached to the inside of the tube to the ends serving as stoppers. The product is held in place by mounting blocks. The device calculates the strain level by calculating the dependence of the oscillation frequency on the wire tension.
The sensor is mounted on the outside of the body of the tested item by fixing the template. This is done using bolts or glue. In this case, the attached device can be used several times. Data is obtained by attaching a cable.
The price of strain gauges depends on its type, purpose, as well as the popularity of the manufacturer. The cost of simple mechanical devices starts from a thousand rubles, but electronic options can cost the buyer tens of times more. This difference is due to much higher measurement accuracy and increased reliability.
When choosing a device, you must pay attention to the quality of its manufacture. The accuracy of its measurements will depend on this parameter, which can seriously affect the success of your work. We, as a manufacturer of strain gauges, guarantee the highest quality of the products provided. During its production, all standards established by applicable law are observed, which is confirmed by the availability of relevant certificates. To buy a strain gauge, just add the product to the basket, and place an order. Delivery is carried out by mail, pickup is also possible.