KR840001813B1 - Residual Stress Measurement Method - Google Patents

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Description

Residual Stress Measurement Method

1 is a plan view of a rosette strain gauge consisting of three elements for melting holes.

2 is a time versus strain curve diagram of an object having residual stress and a calibration sample annealed.

3 is a cross-sectional view of a reactor pipe showing a laser and optical system for performing local melting to measure residual stress on the inner circumferential surface of the reactor pipe.

4 is a schematic block diagram of a strain measurement system using a laser.

5 is a state diagram of a molten region in a nearly complete ring form for stress relief.

The present invention relates to a method for measuring bulk residual stress in a non-destructive method, i.e., a quasi-non-destructive method.

Residual stress, also called internal stress, is defined as a stress system inside a solid that is independent of external forces. Two important residual stress measurement methods commonly used today are the stress relief method and the X-ray method. In the case of conventional stress relief methods, this method is destructive as if a hole had to be drilled or machined to remove the stress.

Efforts to measure residual stress in a non-destructive manner have rarely succeeded except by X-ray methods. However, the X-ray method can also be applied only within the range of 0.5 mils from the surface and is inadequate for measuring residual stresses on the inner surface of pipes or other objects.

In the application associated with this application, in the case of the "quasi non-destructive residual stress measurement" of John Ruediger Meder Vitrol and Marshall Goddle Jones, assigned simultaneously to the same assignee, the heat in the locally melted area is below the strain gauge. As a result, the surface strain is measured before thermal stress occurs. Quasi-non-destructive means that the damage in the tested area is small enough that it is not harmful for future use.

Residual stresses are determined by measuring the surface strain produced when stresses are removed in the locally melted region near the strain gauge in the object under test. Local melting is performed by laser beams or other thin straight heat sources, and short wavelength heat pulses are used to limit the range of the molten area and to protect the strain meter. After the short wavelength heat pulses are blocked, the molten region begins to resolidify. As the temperature slows and the recast stress becomes significant, the thermal stress disappears, and the strain change caused by the weakening of the stress can now be measured using a simple network. The effects of thermal and recast stresses are eliminated by subtracting the calibration measurements of the annealed sample of the same material as the test object from the measurements of the object to be tested. As a result, the removed strain is used to calculate the residual stress.

The method is suitably carried out by using a rosette strain gauge consisting of three elements, and a hole is temporarily melted in the center of the rosette by a laser driven in a pulsed manner. The measurement of the calibration strain on the annealed sample is carried out in the same manner by the test of the test object in the same manner, after a sufficient time (10 seconds to 30 seconds) after the hole has resolidified to stabilize the thermal stress or recast stress. Is executed. Instead of melting the holes, when a more accurate and effective method is needed, a nearly complete ring is melted around the strain gauge to eliminate the stress.

This method of measuring bulk residual stress is a non-destructive method that is almost non-destructive because almost no object is broken, and it is possible to measure the residual stress on the inner surface of the pipe or other parts by deflecting the laser beam using optical components. This method is also more expensive than the prior art.

This residual stress measurement method is based on a known method of drilling holes to measure residual stress. The residual stress is evaluated by measuring the surface strain resulting from the stress removal from the hole. In the case of this method, the hole is temporarily melted in the test object in the vicinity of the strain gauge to remove the stress. In some cases, the method of melting a hole is insufficient in sensitivity or accuracy, so a trepanning technique is used when a more sensitive and accurate method is needed. In this case, instead of melting the hole in the center of the strain gauge, a strained ring or an almost complete ring around the strain gauge is melted. This quasi-non-destructive method can be applied to not only the tripping method but also the hole melting method or other locally melted areas.

Hereinafter, with reference to the accompanying drawings will be described in more detail.

,90

및 225

위치에 있다. 계기(10)의 중심에는 정합 패턴으로 작용하는 환형 링(14)이 계기상의 원과 동심으로 설치되어 있으며, 그 안에서 적당한 가는 직진 열원에 의해 홀(15)이 일시적으로 용융된다. 주름잡혀 있는 계기소자(11),(12),(13) 및 정합패턴의 환형링(14)은 플라스틱 받침대(16) 상에 부식시킨 박부품이며, 이들은 통상 프린트 배선기술에 의해 제조된다. 계기소자의 양단에는 전기접속을 위해 확장된 접촉패드(17)가 형성되어 있다.The rosette strain gauge 10 consisting of three elements shown in FIG. 1 is usually selected when the principal stress and one circumferential direction have to be determined. Three instrument elements 11, 12 and 13 are arranged around the circumference of the strain gauge 10, each 0

, 90

And 225

In position. In the center of the meter 10, an annular ring 14 serving as a matching pattern is provided concentrically with the circle on the meter, in which the hole 15 is temporarily melted by a suitable thin straight heat source. The corrugated instrument elements 11, 12, 13 and the annular ring 14 of the matching pattern are thin parts which are corroded on the plastic pedestal 16, which are usually manufactured by a printed wiring technique. On both ends of the instrument element there is formed an extended contact pad 17 for electrical connection.

Rosette strain gauge 10 is a metal resistance gauge, and when the length of a thin part is mechanically elongated, the width of the conductor is reduced and the length is increased so that the electrical resistance is usually increased. If the resistive element of any length is brought into close contact with the strained part in the same way as the strain is applied thereto, the change in the measured resistance can be corrected as strain. Several types of strain gauges are known and may be used in place of rosette strain gauges 10 depending on the application.

For stress relief, the hole or region 15 is locally melted in the center of the instrument 10 by a focused laser beam or electron beam or other thin straight heat source. Among these heat sources, a pulsed laser having a pulse wavelength of about 1 to 10 milliseconds is preferable.

This short wavelength heat source limits the melting zone and protects the strain gauge 10, while it is important to melt the metal or plastic object to be tested without trace. When melting is properly controlled, these holes are almost closed upon solidification. The quasi-non-destructive nature in this method is assured because the melt zone is recast within a few milliseconds before a significant amount of material is lost.

Hereinafter, the process of measuring the residual stress by local melting will be described. First, the laser injects energy into the object for a certain time to form a melting zone, and the residual stress is removed in the vicinity. At the same time, thermal stress is also provided because of the uneven temperature distribution. After the laser pulse is blocked, the molten region begins to solidify. Thermal stress disappears as temperature decreases and recast stress becomes more pronounced.

The change in strain due to stress warming is measured after remelting of the melt zone and after a sufficient time of about 10 to 30 seconds has elapsed to stabilize the thermal stress and recast stress. Referring to the curves for the objects with residual stress in the time-strain strain curve of FIG. 2, the maximum of the strain occurs while the region is molten, and after a while the strain is generally constant. The effects of thermal stress and recast stress are eliminated by subtracting the calibration measurements of the annealed sample of the same material as the test object from the measurements of the object to be tested.

The annealed sample, such as stainless steel, must be of the same material as the test object, but it does not have to be the same shape (ie one can be a pipe and the other can be a flat plate) and the annealed piece does not have a defined residual stress. In addition, the stress is taken because it can be measured freely. Calibration strain measurements are made by overlapping the test made on the object with approximately the same test on the annealed sample. Referring to the curve for the sample annealed in the time-strain strain curve of FIG. 2, the surface strain and the like are measured after the molten region solidifies and the influence of thermal stress and recast stress is stabilized. The removal strain Δε is obtained by subtracting the strain for the annealed sample from the strain for the object with residual stress, which is used to calculate the residual stress. In the stress field of the axis, by replacing three instrument elements in different directions, three unknowns, two main stresses, and one main direction can be solved.

By using linear heat sources that are far apart, residual stress can be measured in parts that are difficult to measure, such as the inner surface of any part. Referring to FIG. 3, take the example of measuring the residual stress on the inner diameter (ID) of the reactor pipe (18). In nuclear reactor piping, residual stress by bulk welding is the main cause of the problem of cracking due to stress corrosion between particles, and since such cracking occurs on the inner surface of the pipe, residual stress measurement on the inner surface is necessary.

The power laser 19 is far apart and the laser beam 20 is incident into the pipe and then deflected to the center of the rosette strain gauge 10 which is coupled by the prism 21 and the connecting lens 22. Optical components that deflect the beam, including laser heat sources and mirrors, may be installed on the moving carriage. The quasi-non-destructive method of the present invention is fast, less expensive than conventional methods, and is suitable for field measurements of residual stress.

4 shows a block diagram of the entire strain measurement system using a laser. The laser device includes a matching laser 25, a power laser 26, a focusing lens 27, and the like, which are commercially available parts. In some laser devices, the low-energy matching laser 25 is a helium-neon laser and is provided coaxially with the power laser 26. On the other hand, the power laser is a neodymium-glass laser having a wavelength of 1.06 microns. The matching laser 25 has a different wavelength in the infrared region and ensures that the heat source pulse of the power laser is in the center of the meter 10. The meter 10 is the same as in FIG. 1 and may be a commercially available rosette strain gage for a drill hole. These instruments are relatively small, 1/4 inch or less in total, and are attached to an object or sample 28 with a suitable adhesive. The change in strain after laser melting is measured with an electrical network comprising a current source and an amplifier 29. Each instrument element is supplied with a constant current, and the voltage measured across the element is directly proportional to the strain, or stress. The stress is determined from the strain calculated by the appropriate equation including the Poisson ratio of the material and the modulus of elasticity E. If the pipe 18 is a test object and the bar 28 is a sample, the same instrument is used for both tests, and the pulse wavelength and energy input of the laser device and the laser pulses overlap. Once the calibration strain measurements for a given material have been determined, it is not necessary to repeat this process for objects of the same material each time. In addition, the voltage representing the calibration measurement of the annealed sample can be automatically subtracted by the device from the measured voltage for the object with residual stress. There are many ways to do this, but it is obvious to those skilled in the art.

Knowing the three removal strains Δε for the rosette strain gauge 10, the calculation of the magnitude and direction of the principal stress is a common task. Yen on page 577, page 586, of Experimental Mechanics, December 1966. G. Renderer and child. Vigneden's "Method for Measuring Residual Stresses Using Strain Gauges for Hole-Drilling" and Tee, published by Addison-Wesley Publishing Co., 1973. G. Beckweed and Yen. L. See Burke's "Mechanical Measurements" Second Edition (American Library Catalog Card No. 70-85380). The microprocessor is supplied with three voltages representing the removal strain to calculate the principal stress and its direction. The present invention measures the bulk residual stress, ie the mean stress at a depth of 50 mils or more from the surface.

The above-mentioned tree panning method is shown in FIG. 5 and is used when a more accurate and effective method is needed.

In this case, instead of melting the hole in the center of the meter, the almost complete ring-shaped area 30 is melted around the meter. By deflecting the thin focused laser beam, it projects a nearly complete ring, leaving room for the conducting wire to the instrument element to pass through.

Claims (1)

In the destructive residual stress measuring method of attaching the strain gauge 10 to the surfaces of the test objects 18 and 28 and measuring the surface strain to determine the residual stress, the object 18 near the strain gauge 10 may be used. Temporarily and locally melt with a straight heat source (19,26) that goes through the region (15) to remove stress, and after the molten region is resolidified, the thermal stress disappears and the recast stress After sufficient time has elapsed, the strain change due to stress relaxation is measured by the strain gauge 10, and is also temporarily and locally as a direct heat source that goes to the region of the sample 28 near the strain gauge 10. To remove the stress, and after the time has elapsed after the molten region has resolidified and sufficient time has elapsed so that the thermal stress disappears in the resolidified melt region and the recast stress is significant. Is measured by the strain gauge 10 to subtract the measurement of the calibration strain for the annealed sample 28 of the same material as the test object from the measured value of the strain on the test object 18. Residual stress measurement method, characterized in that.

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