RU2670217C1 - Method of measuring stress-strain state of metal constructions without static unloading - Google Patents

Abstract

FIELD: technological processes; measurement technology.SUBSTANCE: invention relates to the detection and control of the stress-strain state of a metal structure (object) under load and can be used to evaluate its strength and predict the load-bearing capacity. At controlled points on a structure in a deformed stressed state, measurements of surface deformations ε, the controlled points are selected in such a way that they have the possibility of additional loading regardless of the structure. At controlled points, additional deformations are created, using the known external force P, increase the strain by Δε in stepwise manner, detect change in the external force ΔPuntilincrease is more than some control value. Two cycles of measurements are carried out, in one of which additional deformations coincide in direction with the measured ones, while in the other they are opposite in direction, wherein the reference value is selected to be the same for both measurement cycles, after which the deformation of the structure is determined as half the difference in the measured additional deformations.EFFECT: extending the range of application of the method and increasing the accuracy of measurements.1 cl, 4 dwg

Description

The invention relates to the field of determination and control of the stress-strain state of a metal structure (object) under load, and can be used to assess its strength and predict bearing capacity. In this case, the elastic modulus E must be known for the material of construction.

The method can be used in monitoring the bearing capacity of the construction of industrial-civil buildings, special structures (metro, bridges, nuclear power plants, etc.).

The known method [RF Patent No. 2302610] consisting in the fact that on the surface of the structure, which is in a stress-strain state, strain gages are fixed and surface deformations are measured, which are taken as final. Then, the material is cut out around the strain gauges to a depth corresponding to the removal of the stress state of the structure at the strain measurement points, and the surface deformations of the structure, which are taken as the initial ones, are measured. Based on these initial and final deformations, surface stresses are determined under load.

However, in the prototype I have my drawbacks, namely:

- cutting material in the test structure around the measuring strain gauge violates the integrity of the test structure. Since the study of the stress-strain state of the structure, as a rule, is carried out in the most loaded places, this reduces the safety of the structure during the study;

- cutting material in the studied design around the measuring strain gauge is a complex technological process. If the number of measurement sites is large, then this complicates the measurement process.

не увеличится более значения, соответствующего нормированному отклонению от закона Гука механической характеристики материала конструкции. После этого нагружение прекращают, а деформацию конструкции определяют, вычитая из известного значения деформации для заранее известной механической характеристики материала конструкции измеренную дополнительную деформацию.For the prototype adopted the method [RF Patent No. 2550826] consisting in the fact that at controlled points on a structure in a stress-strain state, measurements of surface strains ε are made. Moreover, the controlled points are selected in such a way that they have the possibility of additional loading, regardless of design. At controlled points, additional deformations, which coincide in the direction with the measured ones, are created using the known external force P, stepwise increase the deformation by Δε, and the change in the external force ΔP _{i is} measured. The load is increased until

will not increase more than the value corresponding to the normalized deviation from Hooke's law of the mechanical characteristics of the material of the structure. After this, the loading is stopped, and the deformation of the structure is determined by subtracting the measured additional deformation from the known deformation value for the previously known mechanical characteristics of the material of the structure.

The disadvantage of the prototype is that this characteristic is not known for all grades of steel and various alloys.

The technical result of the invention consists in expanding the range of application of the method and improving the accuracy of measurements.

не увеличится более некоторого контрольного значения, характеризующего отклонение от закона Гука механической характеристики материала конструкции, причем это значение выбирают одинаковым для обоих направлений дополнительной деформации. Проводят два цикла измерений для обоих направлений дополнительной деформации, после чего, поскольку металлы имеют одинаковую упругую характеристику на сжатие и растяжение, деформацию конструкции определяют как половину разности измеренных дополнительных деформаций.The essence of the method consists in the fact that the surface strain ε is measured at a controlled point on a structure in a stress-strain state. Using a known external force P, additional strains are created at a controlled point, first coinciding in direction with the measured ones, and then opposite in direction. In this case, the deformation is increased stepwise by Δε, the change in the external force ΔP _{i is} measured. The load is increased until

no more than a certain control value characterizes the deviation from the Hooke law of the mechanical characteristics of the material of construction, and this value is chosen the same for both directions of additional deformation. Two measurement cycles are carried out for both directions of additional deformation, after which, since metals have the same elastic characteristics for compression and tension, the structural deformation is defined as half the difference between the measured additional deformations.

As a control value, it is most convenient to take a value of 0.002, corresponding to the determination of the elastic limit, since at large values, the appearance of residual deformations and a violation of the symmetry of the loading curve, and at lower values, the requirements for the accuracy of measuring instruments increase.

The proposed method for determining the stress-strain state of structures without removing static loads is illustrated by drawings, where:

- figure 1 is a diagram of a change in the additional external force P at some controlled point of the loaded structure from the additional strain ε;

- figure 2 is a diagram of the metal box span of the bridge and its cross section;

- figure 3 is an example of a device for additional loading, coinciding in the direction with the measured (tensile);

- figure 4 is an example of a device for additional loading, opposite in direction to the measured (compression).

The figure 1 presents a diagram of loading material of a structure in a stress-strain state from its own weight. Point 1 corresponds to the initial determined value of the strain ε _to . With the help of an external force P, additional strains are created stepwise by Δε. In this case, the increment of the external force ΔP _{i is} also measured. The smaller Δε, the more accurate the measurements.

станет больше 0,002. Запоминают величину деформации ε_пр. Конструкцию разгружают и начинают новое нагружение в противоположном направлении тем же методом. Запоминают величину ε_обр. Так как упругие свойства металлов одинаковы на растяжение и сжатие, точки 2 и 3 симметричны относительно начала координат точкам 2' и 3' и, соответственно, ε_к=0,5(ε_обр.-ε_пр.).After the material of the structure reaches, for example, the elastic limit at point 2, in the next step at point 3, the value

will become more than 0.002. The strain ε ε is _stored. The structure is unloaded and a new loading is started in the opposite direction by the same method. Remember the value of ε _arr. Since the elastic properties of metals are the same in tension and compression, points 2 and 3 are symmetrical with respect to the origin of points 2 'and 3' and, accordingly, ε _к = 0.5 (ε _arr. -Ε sp _. ).

Figure 2 shows a drawing of a typical metal box-shaped span 1 of the bridge and its cross section. The controlled point is selected on the surface of the lower plate at the edge of the side flange 2. At the initial moment, the tensile strains at the controlled point are equal to the deformations of all the points of the tender plate of section 1-1 of their own weight, however, the side flange 2 can be additionally deformed regardless of the points of the middle section.

The method can be implemented, for example, using the following devices.

In the direction of stretching, you can apply a mechanical device (figure 3). A jack 3 with an oil pressure sensor 4 is installed between the shelf 2 and the beam 5 with hooks 6 hooked to the lower surface of the shelf 2 on which the strain gauge 7 is installed. The outputs of both sensors are connected to the inputs of the computer 8, the control output of which is connected to the controlled pump station 9 (computer 8 and station 9 are shown schematically). Under the action of the force from the jack 3, the portion of the shelf 2 between the hooks 6 is bent, and tensile stresses arise on its lower surface, additional to those already acting from the bend of the entire span 1 of the bridge.

по показаниям датчика 4 давления масла в домкрате 3. Процесс повторяется до достижения материалом полки 4 точки 2 - предела упругости.Measurements are carried out as follows. The pump station 11, at the command of the calculator 8, supplies oil to the jack 3 until all the gaps in the structure are sampled, which is determined by the change in the readings of the strain gauge 7. Then, the pressure in the jack 5 increases until the first stage of deformation increment Δε is reached. After that, the calculator determines the value

according to the readings of the sensor 4 of the oil pressure in the jack 3. The process is repeated until the material of the shelf 4 reaches point 2 - the elastic limit.

In the compression direction, measurements can be carried out, for example, as follows (figure 4). It is impossible to bend the shelf in the other direction, since tensile stress also increases during bending and the bridge shelf flows. However, you can take advantage of the thermal expansion of a small metal zone: the rest of the structure will not allow the small zone to deform and tensile stresses will first be removed in it, and then compression stresses will arise. If they exceed the elastic limit, then after cooling in the heating zone, residual deformations will appear.

To take measurements in the same place of the test shelf, two holes 10, which are marks of the strainmeter 11, were punched with a core. Around the holes 10, the heating zone 12 was marked.

First, an initial (zero) readout of the readings of the strainmeter 11 is made, placing its legs in the wells 10. Then, using a high-frequency current inductor, an infrared emitter, a gas burner, etc. (not shown) produce the most rapid surface (approximately to the depth of the holes 10) heating of the metal in zone 12 to a temperature that is previously calculated by the formula

,

,

where: σ _0,2 is the approximate value of the elastic limit of a given metal,

1.8 - coefficient providing 20% margin for inaccuracy of σ value _0.2

α is the coefficient of thermal expansion of the metal.

As a rule, the latter coefficient is either known with sufficient accuracy, or it is not difficult to measure it beforehand. For steel, for example, the heating will not exceed 300 °. The surface temperature in zone 12 is measured by any known device, for example, a pyrometer. After that, the metal is allowed to cool to the initial temperature and the presence of residual deformations is checked with a deformometer 11. If they are not, spend the next heating cycle, increasing the temperature by 25-50 degrees. After the appearance of residual deformations, the temperature of the previous cycle is stored and the value ε _arr = (TT- _start ) α is calculated.

The positive effect of the application of the proposed method for determining the stress-strain state of metal structures without removing static loads is that measurements can be performed for metals whose proportionality limits are unknown or inaccurately determined. The limiting value under additional loading is chosen arbitrarily and sufficiently large, which allows to increase the accuracy of measurements.

Claims (2)

1. The method of determining the stress-strain state of a metal structure without removing static loads, which consists in the fact that at controlled points on a structure in a deformed stressed state, surface strains are measured ε, the controlled points are selected so that they have the possibility of additional loading irrespective of the design, at controlled points create additional deformations with the help of a known external force P, increase the deformation stepwise on Δε, measure the change in external force ΔP _i until

it does not increase more than a certain control value, characterized in that two measurement cycles are carried out, in one of which additional strains coincide with the measured ones, and in the other they are opposite in direction, and the control value is chosen the same for both measurement cycles, after which the structural deformation defined as half the difference of the measured additional strains. 2. A method for determining the stress-strain state of a metal structure without removing static loads according to claim 1, characterized in that the control value is 0.002.

Images