CZ34208U1 - Test specimen with precise crack shapes for non-destructive testing - Google Patents

Description

Test specimen with precise crack shapes for non-destructive testing

Field of technology

The technical solution concerns a test specimen used in the field of non-destructive testing (NDT) for verification of the inspection procedure, for verification and training of NDT equipment operators, as well as the whole system of implementation of non-destructive inspections. The test specimens make it possible to verify that the test procedure used by the proposed device is able to find, indicate and, in the required quality, also evaluate the imperfections which are present in the inspected material. This significantly increases the reliability of the results of non-destructive inspections performed in operation and increases the safety of operation of the tested equipment.

Demands for safe operation are required especially in the field of nuclear energy, but also in the energy, chemical and other industries, where the emphasis is on operational reliability. The consequences of equipment failures are mainly economic, when even a short-term cessation of production often has significant economic consequences. Currently, the demand for product quality is growing significantly, which is verified by automated control. Even in these cases, test specimens with manufacturing defects are used to set up non-destructive inspection systems.

Prior art

For the needs of verification and qualification of non-destructive testing, personnel, as well as the whole system, test specimens with artificial defects are used. The test specimens are designed according to the design of the inspected equipment, the test method and the expected defects, the occurrence of which is expected in the tested area. Ultrasonic testing, irradiation test, penetration test, magnetic pouring test, measurement of acoustic emission, etc. are used as basic non-destructive methods, and the test specimens meet the requirements for these individual methods.

The test specimens used to verify the proposed non-destructive testing procedures shall correspond to the inspected parts of the equipment or are a simplified model thereof. Both simple sheets and profiles as well as very complicated machine parts or welded structural units made of various materials, including composites, can be used as test specimens.

Defects are produced in the body, the effort of which is that the shape and geometry correspond as much as possible to the defects, the occurrence of which was detected in the inspected object or is assumed. The number, type, dimensions and location of defects in the test specimen are determined by a set of requirements for the level of the inspection procedure. The design must be comprehensive so as to enable the functionality of the control system to be verified by meeting the qualification criteria or be applicable to the verification of non-destructive inspection (NDT) personnel. At the same time, however, the price of test specimens is also taken into account, as the production of complicated specimens with a welded joint according to the original technology can be in the order of millions of CZK per test specimen.

Test specimens for ultrasonic testing are designed with basic types of defects, such as flat-bottomed bores, side bores, grooves, etc. These test specimens are used to verify and adjust the sensitivity of measuring instruments. In most cases, universal gauges can be used, taking into account the material being tested.

For the needs of qualification of non-destructive testing, training of operators and calibration of devices, test specimens with artificial defects are used. These man-made defects have a simple and precisely defined geometry and are located in the required places of the body. Using a group of defects with different size, position and inclination, the ability of the NDT system (personnel, measuring equipment and inspection procedure) to detect and evaluate them is verified.

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Test specimens with artificial defects, usually containing electroerosively produced imperfections of the specified profile, which is a simplified model of "real" defects. In most cases, it is a semi-elliptical defect replacing a crack or a rectangular defect representing insufficient penetration. These defects are used to verify the proposed inspection procedure, to detect defects in the selected area and to verify the accuracy with which they can be evaluated.

There are also test specimens containing realistic defects, which by their nature correspond to the actual defects caused during operation. They are created by similar or identical processes that create operational defects. In contrast to defects in operation, they are formed under higher loads and more intense degradation conditions than the material is loaded during operation, and therefore imperfections arise significantly faster. From the point of view of non-destructive testing, these realistic defects behave almost identically to operational defects. At present, their production is very time and technologically demanding, and therefore expensive.

In some cases, such a defect cannot be produced in the required position of the test specimen. At the same time, the number of defects is technologically limited, as well as the impossibility of defining exactly their geometry, shape, direction of growth and size in the test specimen.

It was therefore necessary for the test specimens (ZT) to be provided with a different type of defect, which would be cheaper to acquire and would correspond more to real defects than artificial defects. This was achieved by producing defects by wire cutting.

The origin of electrospark machining technology dates back to the middle of the 20th century. Industrial introduction into production took place in the 1970s and 1980s. With the development of technology and CNC (computer controlled machine tool) control systems, there is an increase in performance and cutting accuracy. Performance and accuracy are determined by the accuracy of the machine tool, the properties of the generator generating electrical pulses, the shape and material properties of the electrode (standard wire size is 0.2 mm, special tungsten wires for micromachining can be up to 0.03 mm in diameter) and dialectic properties. the nature of the discharge.

The essence of the technical solution

These shortcomings are eliminated by a test specimen with precise crack shapes for non-destructive testing (hereinafter referred to as "ZT wire cutter"), according to this technical solution, the essence of which is that the test specimen is provided with an artificial defect (s) made by electroerosive wire cutting. Defects in the test specimen are precisely determined in size, shape and width. In profile, these defects represent a model of a real probable crack occurring in the controlled part of the device

The essence of the new technical solution are test specimens for non-destructive testing with precise shapes and dimensions of cracks, incl. guaranteed crack widths. Thanks to the knowledge of the exact shape and dimensions of the crack, these test specimens can be used to verify the functionality of the NDT system and the ability to detect and evaluate the indications found during ultrasonic testing. The test specimens are also useful for verifying the sensitivity of the ultrasonic method to a specific crack shape and size and are very suitable for the validation of programs modeling NDT methods (for detecting real and SW (software) simulated responses to a specific crack).

In cases where the ZT result is not significantly affected by the curvature of the surface of the inspected area, the results of ultrasonic testing on ZT produced by wire cutting correspond significantly more to inspections of real crack material than is achieved by test specimens with artificial defects.

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ZT wire cutters have several manufacturing limitations, including in particular:

• production only on planar bodies, • the defect profile is the same over the entire width of the test body, • the top of the defect is terminated by a radius that corresponds to the width of the sparked defect (see ZT wire cutter production technology).

Advantages of defects in ZT wire cutter:

• possibility to configure the defect profile exactly according to the expected (detected) crack growth, • possibility to define specific defect geometries, eg branched, convex / concave shape, with different inclination, with different height, etc., • possibility to place defect in base material, thermally affected area, in the welded joint • exactly known profile (for real defects detected only on the basis of destructive inspection, when

ZT destroyed), • fast and cheap production, • significantly help to increase the level and scope of performed qualifications of personnel, which will lead to an increase in the level of operational inspections, and thus to increase safety of NPP (nuclear power plant) operation.

Technology of production of ZT wire cutter

The production of ZT wire cutter is realized by electroerosive machining, in which the material is removed by small electric discharges (sparks) between the workpiece (test body) and the electrode (tool - wire) in a liquid dielectric. During machining, the workpiece is eroded preferentially, but there is also little wear on the electrode. A moving wire is used to make the defect. During excavation, the electrode (wire) moves towards the test specimen until the electric discharge jumps. Each discharge separates a microscopic piece of material that is washed away by a dielectric. In this way, the wire is gradually "cut" through the material according to a precisely determined trajectory.

The production of a ZT wire cutter consists in the production of a flat plate or weldment from the required material of a specified thickness and, in the case of a weldment, a welded joint produced in accordance with the welding procedure (WPS) used on real technology. This board is cut into ZT semi-finished products, in which defects will be produced. The width of the semi-finished products is given by a compromise between the requirements of the ultrasonic method and the thickness of the material, in which the selected equipment and wire are able to efficiently and accurately spark defects. Subsequently, the semi-finished products are fixed in a wire cutter and the production of defects is realized. Wires with a thickness of 0.1 mm are used in the production. KZT wire cutter is prepared drawing documentation with verification of the actual geometry of the defect made in the body.

The main limitation of the whole process is the possibility of producing defects only in planar bodies. The advantage of ZT wire cutter is the easy production of defects with precise geometry (profile). This makes it possible to model the shape of cracks, the occurrence of which is detected or suspected in the inspected device.

Explaining the benefits of ZT wire cutter

ZT wire cutter is a new intermediate stage of test specimens between ZT with real defects and ZT with artificial defects, where it is possible to obtain a very accurate idea of the nature of the response from real cracks during ultrasonic inspection of these ZT. Almost any profile and the size of the defect can be produced very precisely via CNC control of the machine tool. Thanks to the exact knowledge of the defect profile, it is also possible to perform verification (validation) of computer software modeling ultrasonic testing.

Compared to test specimens with realistic defects, their production is significantly simpler and cheaper. At the same time, the production principle limits the shape and size of the ZT wire cutter to a planar plate only.

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Explanation of drawings

The ZT wire cutter will be explained in more detail with the aid of the drawing, in which Fig. 1 represents a standard sparked flame defect in the test specimen and Fig. 2 represents a test specimen with a defect obtained by means of a wire cutter.

Examples of technical solution

To verify the applicability of ZT wire cutter in the field of ultrasonic testing, about 15 test specimens were made of carbon steel class. 12 and austenitic chrome-nickel steels with thicknesses of 12 and 35 mm. In these ZT, crack models of various sizes and shapes were produced according to real cracks found on NPP equipment. The nature of the responses was evaluated on these bodies and compared with the responses obtained on standard ZT with artificial defects. At the same time, simulations were performed in the ČIVA SW program to verify the agreement between the measurement results and the performed simulations.

To evaluate the nature of the response from wire cutter defects, test specimens were made in which defects were made, where one defect represents a standard sparked flame defect (Fig. 1) and the other represents a defect made by a wire cutter with a rugged surface (Fig. 2).

Industrial applicability

Test specimen in the field of non-destructive testing with precise crack shapes, according to this technical solution is another group of test specimens with which it is possible to perform qualification of ultrasonic testing and personnel and these test specimens are also suitable for validation of simulation SW. ZT wire cutter will significantly help to increase the level and scope of performed qualifications of personnel, which will lead to an increase in the level of operational inspections, and thus to an increase in the safety of NPP operation.

These test specimens are an intermediate stage between test specimens with artificial and realistic defects.

Claims (1)

CLAIMS FOR PROTECTION 1. A test specimen in the field of non-destructive testing with precise crack shapes, characterized in that the test specimen for non-destructive testing is provided with an artificial defect by electroerosive wire cutting, said defect in the test specimen being precisely determined by size, geometry and width.