RU59826U1 - SAMPLE FOR TESTING FOR MECHANICAL-CORROSION STRENGTH OF LARGE DIAMETER PIPES - Google Patents

Abstract

The utility model relates to the field of testing materials for strength, in particular for testing large diameter pipes for mechano-corrosion strength. A test specimen for testing the mechanical-corrosion strength of large-diameter pipes consists of a segment of the test pipe, the ends of which are designed for loading by forces directed along one axis. It is additionally equipped with a beam connected to the segment of the test pipe, the lower part of which repeats the curvature of the test pipe, while a groove is made on the convex surface in the central part of the beam adjacent to the test pipe, the length of which is determined by the formula:

Where

C ₁ - const = 0.0225;

R is the radius of the pipe of large diameter, cm

D is the cylindrical stiffness of the test pipe kg · cm,

M _{of additional} - the permissible bending moment for the tested pipe steel kg · cm, and the ratio of the height of the beam to the thickness of the segment of the tested pipe is 4 ÷ 6. This design will allow, when mechanical effort is applied, to create stresses in it that correspond to the stresses of a real pipeline, which increases the accuracy of the test

pipes for mechanical corrosion resistance. 1 s.p. f-ly, 2 ill.

Description

The utility model relates to the field of testing materials for strength, and in particular for testing large diameter pipes for mechano-corrosion strength.

A known sample for testing metal pipes in a biaxial stress state, which is a split ring with a recess located on the outer surface of the sample. The recess is made in the form of an annular groove symmetrically with respect to the width of the sample, the depth and width of which is selected from the conditions for ensuring a stress ratio in the central part of the sample corresponding to the stress state of the pipe (see RF patent No. 2073842, IPC ⁶ G 01 No. 3/08 published by BI No. 5 , 1997).

A disadvantage of the known sample is that it is impossible to create a uniform stress field over the entire thickness of the test sample, which reduces the accuracy of estimating the long-term strength of the pipe metal, since in real conditions the bending voltage scheme does not correspond to the actual operation of oil and gas pipelines.

Another significant drawback of the known sample is that to create biaxiality in the central part of the sample, a groove is made, the value of which is selected depending on the required state of the main stresses, which leads to a violation or change in the state of the surface of the pipe layers and, as a consequence, reduces the accuracy of the tests when determining the strength metal pipe.

Thus, the known sample does not provide sufficient accuracy for assessing the long-term strength of the pipe metal, due to the discrepancy between the actual operation of the pipe.

Closest to the claimed technical solution is a sample consisting of a segment of the test pipe, the ends of which are designed for loading by forces directed along one axis (see RF patent No. 2174225, IPC ⁷ G 01 N 3/08, publ. September 27, 2001 .).

The disadvantage of the prototype is that it is impossible to create a uniform field of tensile stresses in the entire working thickness of the sample in the working area, which reduces the accuracy of estimating the long-term strength of the pipe metal, since in real conditions the trapezoidal stress diagram along the thickness of the sample does not correspond to the work of oil and gas pipelines.

The objective of the proposed technical solution is to bring the testing conditions of the large-diameter pipe segment as close as possible to the real ones and to increase the accuracy of testing the pipe metal for mechanical-corrosion strength.

The solution to the technical problem lies in the fact that in the known sample for testing the mechano-corrosion strength of large-diameter pipes, consisting of a segment of the test pipe, the ends of which are designed for loading by forces directed along one axis, according to a utility model, it is additionally equipped with a beam connected with a segment of the test pipe, the lower part of which repeats the curvature of the test pipe, a groove is made on the convex surface in the central part of the beam adjacent to the test pipe, length l which is determined by the formula:

Where

C ₁ - const = 0.0225;

R is the radius of the pipe of large diameter, cm

D is the cylindrical stiffness of the test pipe kg · cm,

M _Iz.dop - permissible bending moment for the tested pipe steel kg · cm,

and the ratio of the height of the beam to the thickness of the segment of the test pipe is 4 ÷ 6.

This design of the sample will allow, when mechanical effort is applied, to create stresses in it that correspond to the stresses of a real pipeline, which increases the accuracy of testing large diameter pipes for mechanical corrosion resistance.

The essence of the utility model is illustrated by the drawing, where Fig. 1 shows a sample of the test medium, and Fig. 2 is a graph of the experimentally obtained dependence of the bending strain of the pipe element in the groove area on the load.

The test specimen for the mechanical corrosion resistance of large diameter pipes consists of a segment of the test pipe 1, beam 2, which repeats the configuration of the segment of the test pipe 1 and groove 3, made on a convex surface in the central part of the beam 2 in contact with the segment of the test pipe 2.

The test sample was carried out as follows.

Obtaining tensile stresses was provided by a special beam 2, the configuration of which repeated the curvature of the segment of the test pipe 1 (see figure 1). At the same time, the height of the beam 2 contributed to the fact that the test segment of the pipe 1 over the entire thickness was in the field of tensile stresses since the neutral axis of the entire section was at a distance z = (h _Tr + H _δ ) / 2, where h _Tr - the height of the pipe, N _δ - the height of the beam.

During the test, the sample was loaded according to the scheme of pure (four-point) bending. In this case, in the beam 2 and the investigated segment of the pipe 1, the state of pure bending was realized. With a bending loading scheme, the stresses in the outer and inner surfaces of the pipe were determined using well-known material resistance formulas. The voltage across the thickness of the pipe, in this case, was heterogeneous and depended on the thickness of the beam 2 and pipe 1.

The optimal ratio according to experimental data H _δ / h _{tr is} determined within 4 ÷ 6.

To align the stress plot along the thickness of the segment of the tested pipe 1 (see figure 2) used the groove effect, while the length of the groove 3 was determined from the dependence:

where l is the length of the groove, cm; C ₁ - const = 0.0225; R is the radius of the pipe of large diameter, cm; D is the cylindrical stiffness of the test pipe, kg · cm, M from _dop is the allowable bending moment for the test pipe steel, kg · cm, which was determined by the formula:

where E is the modulus of elasticity = 2 · 10 ⁶ kg / cm ² ; h is the thickness of the pipe, cm; ν is the Poisson's ratio.

The effect of stress equalization in the groove region 3 was that during loading of the beam 2 with the segment of the tested pipe 1, a bending strain of the segment of the tested pipe 1 was directed in the depth of the groove 3. Thus, a bending strain occurred in the segment of the tested pipe 1 and, as a result, voltage equalization.

The joint work of the beam 2 and the segment of the test pipe 1 was ensured by welding the ends of the segment of the test pipe 1 to the beam 2.

To study characteristic defects in the central part of the studied pipe 1, a surface incision was made from which a fatigue crack was grown. Simultaneous loading by a pair of forces made it possible to reveal the properties of the weakest zone; in this case, bending is most preferable. During the tests, the load applied to the beam 2 and, accordingly, to the segment of the test pipe 1, was maintained constant. As a result, the total supply of elastic potential energy was maintained at a level that ensured the development of a fatigue crack. Large stock potential

energy dramatically reduces the time before cracking and increases the speed of cracking compared with tests with constant total deformation.

Using the proposed sample for testing the mechanical corrosion resistance of large diameter pipes in comparison with the prototype will allow, with the application of mechanical force, to create stresses in it that correspond to the stresses of the real pipeline and to increase the accuracy of the mechanical corrosion testing of the pipe.

Claims (2)

1. A sample for testing large diameter pipes for mechanical corrosion strength, consisting of a segment of the test pipe, the ends of which are designed for loading by forces directed along one axis, characterized in that it is additionally equipped with a beam connected to the segment of the test pipe, the lower part of which repeats the curvature of the test pipe, while on the convex surface in the Central part of the beam adjacent to the test pipe, a groove is made, the length of 1 of which is determined by the formula:

, where C ₁ -const = 0.0225; R is the radius of the pipe of large diameter, cm; D is the cylindrical stiffness of the test pipe, kg · cm; M _{of additional} - permissible bending moment for the tested pipe steel kg · cm. 2. A sample for testing pipes according to claim 1, characterized in that the ratio of the height of the beam to the thickness of the segment of the test pipe is 4-6.