KR840001814B1 - Semi-nondestructive residual stress measurement - Google Patents

Abstract

Residual stress measurements are made using a strain gauge and stress relief achieved by local melting of a region of the object being tested by a laser beam or other finely directed heat source. Surface strain is measured dynamically before the heat of the molten region diffuses under the gauge and results in thermal stress. The method is semi-nondestructive because the molten region recasts with little loss of material is rapid, and can be utilized on difficult geometries such as the interior surface of reactor pipe.

Description

Destructive residual stress measurement

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

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

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

4 is a time versus strain curve diagram for one instrument element.

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 of measuring residual stress by a nearly non-destructive mechanical method, that is, quasi-non-destructive method, so that the damage to the test object is extremely small so that there is no problem in continuing to use the test object.

Conventionally, destructive techniques have been used to measure residual or internal stresses. All measurement methods require stress relief of the sample, which necessarily means cutting the sample by hole drilling, electrical discharge machining (EDM), sand blacking, or tripping. Although the hole drilling operation is the most common, the drill has the undesirable feature of machining the surface to accumulate its own stress. EDM methods are suitable but difficult to implement on site. Efforts to non-destructively measure residual stress have rarely succeeded except in the case of X-rays. However, X-rays have the disadvantage of being susceptible to surface turbulence, and furthermore, X-rays are only suitable for bulk residual stress measurement because only a few atomic layers penetrate.

Residual stress is measured using a strain gauge, and also by removing the stress by locally and temporarily melting the area of the test object in the vicinity of the strain gauge. Local melting is performed by lasers, electron beams or other thin linear heating sources, and short wavelength pulses are used to limit the melting area and to protect the strain meter. The stress relief process proceeds at sonic velocity, but the diffusion of heat from the melt zone occurs more slowly. The change in strain due to stress relaxation is measured mechanically before heat diffuses down the strain meter, resulting in thermal stresses that change the measurement. Therefore, the stress relief state is achieved and measured dynamically. The present invention is referred to as almost nondestructive, i.e., quasi-nondestructive method, since the melted zone reheats within a few milliseconds before a significant amount of material is damaged.

The present invention is suitably implemented using a three-element rosette resistive strain gauge and temporarily melts a hole using a pulsed focused laser beam having a pulse wavelength of about 1 to 10 milliseconds in the center of the strain gauge. . The change in resistance, or strain, is measured before the melt zone is recast and before the heat diffuses down the strain gauge. Calibration is done by melting to remove stress. Residual stress is easily calculated as it is proportional to the measurement of the surface strain obtained as a result of the present invention. As a variant of the invention, instead of melting the hole, a near complete ring shaped region is melted around the meter.

This method of measuring bulk residual stress is fast, suitable for field measurements, and suitable for measuring residual stresses in geometrically difficult-to-measure parts, such as measuring residual stresses in the inner diameter of reactor pipes that are susceptible to stress corrosion. . The laser beam is deflected by the optical component and impinges on the inner surface of the object.

The measurement of all residual stresses requires determining the reference stress state, and the method using a spray gauge removes the stress on the sample or object to achieve the reference stress state. Residual stress, also called internal stress, is defined as a stress system in a solid that is independent of external forces. In this residual stress measurement method, the reference condition is determined by eliminating the stress in the part to be tested, but the stress is removed by temporarily melting the part adjacent to the strain gauge. The excessive or temporary melting is intended to allow the strain measurement to be completed before heat spreads from the melting zone to the strain gauge and affects the reading of the meter.

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

, 45

및 90

위치에 배치된다. 스트레인 계기(10)의 중심에는 정합패턴으로 작용하는 환형 링(14)이 스트레인 계기의 원과 동심으로 설치되어 있고, 그 안에서, 적당한가는 직진 열원에 의해 홀(15)이 일시적으로 용융된다. 아코디언 모양으로 주름잡혀 있는 계기소자(11),(12) 및 (13)와 정합패턴으로 작용하는 환형 링(14)은 플라스틱 받침대(16) 상에 식각된 박부품들이며, 이들은 프린트 제조기술로 제조된다. 계기소자의 양단에는 전지 접속을 위해 확장된 접촉 패드(17)가 있다. 로제트 스트레인계기(10)는 금속 저항계기로서, 박부품의 길이가 기계적으로 신장될때, 도체의 폭은 줄어들고 길이는 늘어나서 통상 전기 저항이 증가하도록 작용한다. 어떤 길이의 저항 소자가 그것도 역시 스트레인되는 방법으로 스트레인된 부분에 밀착되면, 측정된 저항치의 변화는 스트레인의 항으로 교정될 수 있다. 그밖에 여러가지 형태의 스트레인 계기가 공지되어 있으며, 용도에 따라 로제트 스트레인계기 대신 사용할 수 있다.Rosette strain gauge 10 consisting of three elements as shown in FIG. 1 is typically selected when the principal stress and its direction are to be determined. Three instrument elements (11), (12) and (13) are zero on the strain gauge.

, 45

And 90

Is placed in position. At the center of the strain gauge 10, an annular ring 14 serving as a matching pattern is provided concentrically with the circle of the strain gauge, in which the hole 15 is temporarily melted by a straight heat source. The annular ring 14, which acts as a matching pattern with the instrument elements 11, 12, and 13 corrugated in an accordion shape, is thin parts etched on the plastic pedestal 16, which are manufactured by a print manufacturing technique. do. At both ends of the instrument element there is an extended contact pad 17 for battery connection. Rosette strain meter 10 is a metal resistance meter, when the length of the thin part is mechanically stretched, the conductor width is reduced and the length is increased so that usually the electrical resistance increases. If a resistive element of any length is in close contact with the strained part in which it is also strained, the change in the measured resistance value can be corrected in terms of strain. Various types of strain gauges are known and may be used in place of rosette strain gauges depending on the application.

The locally melted regions or holes 15 for stress relief are formed in the center of strain gauge 10 by focused laser beams or electron beams or other thin short wavelength heat sources. Of these, the laser is most suitable, which is driven in a pulsed manner and has a pulse wavelength of about 1 to 10 milliseconds. It is important that the short wavelength heat pulses limit the melting region 15 and protect the strain meter 10, and also melt the trace of the metal or ceramic object to be tested. If melting is properly controlled, the hole is almost closed upon resolidification. The quasi-non-destructive nature of the method is realized because the melt zone is recast within a few milliseconds before a significant amount of material is damaged.

Extremely melting near strain gauge 10 eliminates stress, and before the melt zone resolidifies, heat in the melt zone also diffuses below strain gauge 10, resulting in thermal stresses that change the measured value. Before, it is necessary to measure the change in strain due to stress relaxation mechanically. The stress relief process is carried out at the speed of sound in the material, and the thermal process, ie, thermal diffusion, proceeds at a slower rate. Thus, the stress relief state is reached and measured dynamically. It does not need to be calibrated, but is automatically carried out by melting giving a zero reference point.

By using distant linear heat sources, it is possible to measure residual stresses in parts that are difficult to measure geometrically, such as the inner surface of an object. 2 shows an embodiment for measuring the residual stress on the inner diameter ID of the reactor pipe 18. The residual stress due to the bulk welding of the reactor pipe 18 is a major cause of cracking due to stress corrosion between particles. Cracks due to stress corrosion occur on the inner surface of the pipe 18 and this is where it should be measured, after the power laser 19 is far away and the laser beam 20 is incident on the pipe 18, The prism 21 and the focusing lens 22 are deflected to the rosette strain gauge 10 attached to the pipe. Optical components that deflect the beam, including the laser heat source and the mirror, can be installed on the moving carriage. The quasi-non-destructive residual stress measurement method is faster, less expensive than conventional methods, and is suitable for in situ measurement of residual stress.

3 shows a block diagram of a complete strain measurement system using a laser. The laser assistance system is equipped with a matching laser 25, a power laser 26 and a focusing lens 27, which are commercially available parts. For example, the ground power matching laser 25 is a helium-neon laser and is installed on the same side as the power laser 26, while the power laser 26 is a neodymium doped glass laser having a wavelength of 1.06 microns. to be. The matching laser 25 has a different wavelength in the infrared region and ensures that the powered laser heat source pulses are located centrally in the instrument 10. Strain gauge 10 is the same as that of FIG. 1, and is a commercially available rosette drill for hole-drilling. The overall size of the strain gauge 10 is less than 1/4 inch, relatively small, and adheres to the object 28 with a suitable adhesive. After melting the hole with a laser, the strain change is measured with a simple network comprising a current source and an amplifier 29. Constant current is supplied to each of the instrument elements 11, 12 and 13, and the voltage measured at both ends of the instrument element is directly proportional to the strain, i.e., the stress. The strain is determined by multiplying the strain by the elasticity of the test material. 4 shows time-strain strain curves for objects or parts with residual stresses. What is to be measured is the maximum value of the strain, which is a zero reference state for the stress-relieved material, which occurs when the material is melted.

As the remaining parts of FIG. 3, oscilloscopes 30-32 are provided, one for each of the instrument elements 11, 12, and 13, which operate as three parallel channels, and also reproduce data. There is a camera and a multichannel tape recorder 33 to determine when to check. The trigger 34 supplies a start signal to the laser 26 and the oscilloscopes 30-32, and also starts a clock 35 for delaying the operation of the tape recorder 33. Knowing three strains for a zero-zero strain gauge, the stress calculation and the calculation of the magnitude and direction of the principal stress are by routine work. In 1966, on the pages 577 to 586 of Experimental Mechanics, Eng, Rendler and I. Wigness's paper, "Method for Measuring Residual Stresses Using Strain Gauges with Hole-Drilling" and , T.G.White Latitude and N. L. Burke's second edition, "Measurement Measurements", published by Addison-Wesley Publishing Co., 1973 (US Library Catalog Number 70-85380). Three voltages representing the strain are supplied to the microprocessor to calculate the principal stress and its direction.

In some cases, the hole melting method is insufficient in strength or accuracy, so when the more accurate and sensitive technique is required, the tree panning method is used. In this case, instead of melting the hole in the center of the meter, a nearly complete ring-shaped region 36 is removed around the meter, as shown in FIG. The narrow focused laser beam is deflected to project an almost complete ring.

Separate methods are described in "Residual Stress Measurement Methods" by Robert Allen Thompson and Xinfang, both assigned to the same assignee and filed concurrently with the present application. The measurement of strain is carried out after the locally melted area has been recast and after the thermal stress has been diffused, and the calibration measurements for annealed samples of the same material from measurements of objects with residual stresses of zero stress and recast stress It is removed by subtraction.

Claims (1)

In the quasi-non-destructive residual stress measuring method of attaching the strain gauge 10 to the surface of the test object 28 and measuring the surface strain of the object 28 to determine the residual stress, the thin linear heating source 19 and 26 are used. By locally melting the region 15 of the object 28 in the vicinity of the strain gauge 10, thereby removing the stress and achieving a zero reference state for the destressed region 15, the region of the object 28. While (15) is molten, before heat spreads under strain meter 10, resulting in a zero stress that changes the measured strain change with respect to the zero reference state, Wherein the change in strain due to stress relief in the system is measured dynamically.

Images