GB2528771A

GB2528771A - Creep strain measurement

Info

Publication number: GB2528771A
Application number: GB1510112.4A
Authority: GB
Inventors: Jianxin Gao
Original assignee: Welding Institute England
Current assignee: Welding Institute England
Priority date: 2014-06-25
Filing date: 2015-06-10
Publication date: 2016-02-03
Anticipated expiration: 2035-06-10
Also published as: GB201411277D0; GB2528771B; GB201510112D0

Abstract

The present invention relates to an improved method suitable for long term creep strain measurement of the steam pipes and other high temperature components during operation in a power plant or other engineering facilities using digital image correlation (DIC). The inspection surface is prepared and at least two micro indents are created thereupon. An image of the surface is acquired and further images acquired at later times. The images are compared and using DIC techniques the creep deformation is determined. The inspection area is maintained in an inert gas environment (argon gas filled cavity 4) via a sealed protective case (in device 1) so as to prevent oxidations and other contaminations of said area due to high temperature exposure over an extended period of time. An embodiment of the protective case is also proposed that contains an optically transparent element (window 6) enabling image acquisition without loss of the inert gas.

Description

CREEP STRAIN MEASUREMENT

Field of the Invention

The present invention relates to methods and devices suitable for long term creep strain measurement in high temperature environments using digital image correlation (DIG). Such environments include the steam pipes and other high temperature components during operation of a power plant or other engineering facilities over many months, years or even decades.

Background to the Invention

The measurement of long term deformation, or creep, in industry is of crucial importance to the safe operation of plant and equipment. Creep deformation eventually leads to failure of components where the component has been exposed to high levels of stress which are below the yield strength of the materials. Materials that are subject to heat for long periods are more prone to creep as creep increases as a function of temperature. Thus measuring creep in high temperature environments, such as power plants, is essential to prevent their failure which can have catastrophic consequences.

In high temperature environments, replica metallography is one of the most commonly used methods for creep damage detection. The procedure is to polish and etch the surface to be inspected and then apply a film which has been softened with an appropriate solvent. This process creates a mirror image of the tested surface which can be examined with a microscope to identify flaws such as voids and microcracks. Quantitative data can be obtained and voids of I pm or less can be easily distinguished.

Other techniques which can be potentially used for creep monitoring include strain gauges, electrical potential drop techniques, positron annihilation, X-ray diffraction and small angle neutron scattering, hardness measurement, ultrasonics and magnetic methods. However these are either unreliable or difficult to apply in high temperature environments.

Digital Image Correlation (DIC) is an optical technique for mapping the two-dimensional strain field over an area rather than the average strain between two fixed points. The principle is to create a random speckle pattern on the test piece (typically using a paint/surface coating) and then capture images with a digital camera. A correlation algorithm then computes the motion of each image point by comparing the digital images of the test object surface in different states.

Accurate strain measurements can be obtained. The technique is described in more detail in US 5,757,473 Optical strain sensor for the measurement of microdeformations of surfaces'.

One approach when using DIC is the use of a coating over an indented inspection area on a foil (see Hulshof, HJM et al, Creep Strain measurements for Risk based Monitoring of Steam Pipes and Headers', pages 213-518, Creep & Fracture in high temperature components -design and life assessment issues' -Proceedings of the ECCC Creep Conference, 12-14 september, London, UK, December 2005, ISBN: 1932078495) to enable better visual images to be acquired. Herein the foil is coated in order to protect the metal surface from oxidation and environmental links. However, in addition to ensuring adequate attachment of the coating to the surface to enable effective imaging, the main drawback of the technique is the ability of the coating to withstand and survive temperature cycling due to thermal mismatch between the coating and foil. Another critical drawback is that the deformation of the foil may not reflect the actual deformation of the parent material underneath, as the foil can only be welded to the parent material in limited areas e.g. along the boundaries.

Hulshof alternatively proposes in W000/42382, Method and apparatus for measuring of displacements within a plane', the use of a thin inert metal such as gold placed on the material to be measured and covered by a device that is permanently mounted on the material. The deformation of this inert metal part is measured representing the creep deformation that the material is experiencing.

As with other similar techniques, drawbacks include the secure attachment of the metal to the underlying material thus affecting deformation and hence measurements. Again the deformation of the inert metal may not reflect the actual deformation of the parent material underneath, as the inert metal can only be welded to the parent material in limited areas e.g. along the boundaries.

The use of speckle pattern interferometry, where creep measurement is calculated by comparing specked images of reflected or diffused coherent light from the surface to be measured, is explored in CN201096733, A measuring device for coated layer high-temperature creep distortion'. Xuan et al. disclose a creep deformation measurement apparatus comprising a high-temperature heating furnace which is connected with a loading mechanism; therein a coated-layer specimen is arranged. An inert gas bottle is connected with the heating furnace. An incandescent light source and a COD video camera with a 3D translational tripod are arranged at the front of the opening of the heating furnace where the 3D translational tripod is adjusted, so that the normal of the coated-layer specimen observed from the opening of the heating furnace is perpendicular to the axial line of the COD video camera. The video camera is connected with a data acquisition card of a digital speckle correlation calculating system. The coated-layer specimen is directly connected with a metal substrate; therefore, the measuring device can obtain the full-area and micro-area creep deformation of the coated layer in a high-temperature environment. Inert gas is utilised to prevent oxidation of the specimen surface. One of the drawbacks of the technique is that it only works on the coating, not the parent material itself.

As mentioned above, it is unlikely that any coating can withstand and survive high temperature cycling due to thermal mismatch between the coating and underlying material. Other drawbacks of this technique include the requirement to have a specific furnace to house the specimen under test, and the need for a continuous inert gas supply for the duration of testing and again the coating of the specimen.

Summary of the Invention

According to the invention there is provided a method of using a creep strain device to monitor creep strain in an inspection surface of a component, the creep strain device comprising a body for mounting against the component, the body comprising: a chamber having an aperture for receiving the inspection surface of the component and further arranged to provide a gas-tight seal to said component such that, when the body is mounted against the component, the component seals the aperture so as to enable the internal environment within the chamber to be sealed from the external environment outside of the chamber, wherein the chamber has a window through which the inspection surface may be imaged by an imaging apparatus; the method comprising: a. Providing two or more markings on the inspection surface of the component; b. Mounting the body to the component so as to place the inspection surface within the internal environment of the chamber; c. Applying each of a stress and an elevated temperature to the inspection surface for a time period; and d. Monitoring the change in geometry of the two or more markings in accordance with the time period using the imaging apparatus.

The invention therefore enables a DIG or related technique for the measurement of creep that overcomes the problems associated with the use of foils or thin metal plates. In a preferred example the invention provides for a technique that enables the use of micro indented surfaces for DIG measurements at high temperatures enabling accurate visual readings with minimal surface deterioration. The invention is preferably used at temperatures within the range of ambient (for example 25°G) to 1000°G. In some applications the temperature range is ambient to 650°G. Typical applications include monitoring of chemical plants, oil refineries and power generation plants.

Thus, a technique is proposed which is based on localised sealing of an inspection area (surface) so as to place the inspection area in an internal environment of the chamber which is isolated from the external environment.

This enables an inert gas to be used to protect the original material with the inspection area from oxidation at high temperature for an extended period of time (up to many years). This sealing will sustain numerous cyclic temperature changes from ambient temperature to over 600°G, so that long term creep strain during operation of steam pipes and other high temperature components in power plants or other engineering facilities can be measured over many months, years, or even decades. A DIC approach is typically employed to process the digital images of the inspection area wherein the surface is prepared and at least two micro dents are produced thereon. This may be achieved by locally deforming the inspection surface. An image of the treated surface is acquired and subsequent images of the surface are taken at later elapsed times. An initial image may be taken prior to the said time period during which the elevated temperature and applied stress are applied. The images may be compared with one another using DIC algorithm to calculate the strain fields between them.

When subsequent images are compared to the original image, accumulated strain can be obtained. When successive images are compared, incremental strain can be obtained.

The inspection area may be sealed with an inert gas post indentation by way of a body in the form of a case that encapsulates the area so as to create and maintain a protective environment to the treated surface of the original material thereby protecting the inspection area from oxidation. Since usually the key issue to address is oxidation, using the present method air can be expelled for the whole duration of the successive measurements. Alternatively a vacuum could be used (following inert gas flushing) but this is difficult to achieve, so inert gas replacement is optimal.

We also provide a creep strain device for use in the method, the creep strain device comprising a body for mounting against the component, the body comprising: a chamber having an aperture for receiving the inspection surface of the component and further arranged to provide a gas-tight seal to said component such that when the body is mounted against the component, the component seals the aperture so as to enable the internal environment within the chamber to be sealed from the external environment outside of the chamber, the chamber having a window through which the inspection surface may be imaged by an imaging apparatus; a sealable gas port for providing selective gaseous communication with the internal environment of the chamber; and, an attachment device for mounting the body to the component.

Furthermore a system is also provided for use with the method, said system including the creep strain device and an imaging apparatus adapted to image the inspection surface when in use.

The invention is advantageous over the techniques described in the prior art, specifically CN201096733, wherein in-situ in line assessment of creep of the material itself, and not a specimen, thereof is undertaken thus reflecting actual deformation of the material in the production environment. Additionally specialist equipment, notably an oven and the storage and supply of continuous inert gas, is not required.

Coating of the inspection area is also not required, with changes in micro-indentation of the material surface itself utilised for creep measurement. Thus issues concerning adherence of the coating on the surface (inspection area) and degradation of the coating and / or surface are overcome.

The body may be mounted to the component surface using techniques known in the art, preferably those that minimise the loss of the inert gas being favoured e.g. mechanical means or by welding. An attachment device may be provided for mounting the body to the component in step (b). The attachment device may be arranged for single use, to provide permanent attachment or the attachment device may be reusable.

In a preferred device a window is provided in the case through which inspection is undertaken (positioned in the top of the case for example). The window may be optically transparent to enable inspection of the surface whilst retaining the protective environment. The optical quality of this optically transparent window has a strong influence on the accuracy of the measured displacement field thus for example high temperature glass should be considered. Furthermore, the glass should be strong enough to withstand the internal pressure which will be built up at high temperature. A theoretical analysis showed that the internal pressure would approximately reach 3 times atmospheric pressure when an air-tight protective case is heated up to 600 CC from room temperature, if initially filled with a gas at atmospheric pressure.

The protective body may be designed to couple detachably with the image taking device wherein an element of the device is removed enabling connection to the device. Inert gas may need to be injected into the protective case during and / or post image acquisition to maintain the protective environment over the inspection area. A port may be provided to enable selective gas communication with the chamber interior so as to achieve this purpose.

The inspection area is typically covered in two or more micro dents, such as those produced using a micro-indentation device. The pattern may be predetermined or random. At least two micro-indents are needed for this implementation but preferably a dense array is provided, for example using tens or hundreds of micro-indents, which improves the accuracy of the measurements. The provision of indentations in two dimensions allows two-dimensional creep strain information to be obtained.

The imaging apparatus preferably outputs data to enable further anaylsis.

Typically the method therefore further comprises a step (e) of: producing output data representing the strain within the inspection surface in accordance with the monitoring. The invention also includes a computer program product comprising program code for performing the method when said computer program product is executed upon a computer system.

The inspection area is preferably prepared by grinding or by using any other technique that produces a smooth finish on a surface. The quality of the surface finish is desired to be sufficient for the indentations to be monitored by the DIC apparatus.

The inert gas may be any suitable gas that would diminish or eliminate the deterioration, specifically oxidation, of the inspection surface that is the area containing a pattern of indentations whose deformation is being measured in order to determine creep strain. Argon is a useful inert gas for this purpose.

Brief Description of the Drawings

The invention will now be described with reference to the accompanying drawings, in which: Figure 1 illustrates a first example device in accordance with the invention; Figure 2 depicts the attachment of the first example device to a pipe on which creep measurement is to be undertaken; Figure 3 shows the surface appearance of the material sample before it is subjected to elevated temperature; Figure 4 shows the same sample after exposure to elevated temperature after 7 days where the samples have been maintained in an inert gas environment during the period of temperature exposure; Figure 5 shows the same sample after exposure to similar conditions after 14 days; and, Figure 6 shows the strain between the images in Figure 4 and Figure 5 calculated using DIC algorithms.

Description of Example

In a preferred embodiment of the methodology, a device (1) graphically represented in Figure 1 is positioned over the inspection area (2) and clamped to the surface (3). A soft copper sealing liner is preferentially used to maintain a continuous seal around the device so as to minimise the loss of inert gas from inside the device. The device is manufactured from mild carbon steel or other suitable metals which can sustain the high temperature that the underlying component is operated. An internal cavity (4) is provided into which an inert gas such as argon is introduced via a gas input component (5). A high temperature gas window (6) is incorporated positioned opposite the inspection area so as to provide an appropriate and consistent visual inspection point. The DIC apparatus is offered up to the window (6) and an image acquired of the material surface inspection area (2). Sealing rings (7) are employed to provide a good seal between the device and the window primarily to maintain the seal of the device and thus prevent inert gas from escaping from the cavity (4). In addition a retaining ring (8) is incorporated maintain the position of the window. Features can be incorporated into the device to enable the DIC apparatus to be positioned consistently on the device; a camera rod location (9) for this purpose is incorporated into the device shown in Figure 1. To prevent dust and other particulate exposure of the glass window, a dust cover (10) is positioned on a portion of the external surface of the device.

The device may be attached to the inspection area surface by a variety of means. Figure 2 depicts the attachment of the device (1) to a pipe (11) on which creep measurement is to be undertaken. A mechanical strap (12) is used to attach the device to the pipe. Alternatives could be by means of welding, adhesive bonding, other mechanical means or a combination of these. A mechanical flexible strap is a preferred embodiment as it does not damage the original component as welding does. A strap can also withstand the temperatures, and temperature cycling, that the material is exposed to and thus minimize the loss of inert gas and additionally minimizes any movement of the device over the material surface which would diminish the consistency of the images acquired of the inspection area.

An example application of the apparatus shown in Figures 1 and 2 is now discussed. An inert gas was used to limit the oxidation of a chromium-molybdenum plate, specifically 9 CrMoV [P(T)91], which is representative of that used in high temperature applications.

A P91 sample was indented with a speckle pattern using a 0.6mm drill. The resulting initial appearance of the sample is shown in Figure 3. The dents can be clearly seen. The speckle pattern comprises in excess of one hundred indents distributed in an irregular two-dimensional pattern.

The sample was maintained in an argon atmosphere during further heating via a steel cylindrical chamber so that the internal pressure which would be built up at high temperature can be withstood. Theoretical calculation indicates that the internal pressure will reach approximately 3 times atmospheric pressure when the air-tight chamber is heated at 600 00. The sample was heated within the chamber for 7 days at 600°C. An image of the sample, Figure 4, was then acquired. The dents can be clearly observed, with the only change to the overall appearance being a change in colour from the initial silver colour to grey. The sample was then heated for a further 7 days at 60000 where thereafter an image was acquired, shown in Figure 5. It can again be seen that the dents are clearly visible. Oxidation of the sample is not evident although glittery dots were observed, remnants of lubrication oil that the sample was exposed to after the first elevated temperature trial.

Nevertheless, the strain between these two images calculated using a DIC algorithm showed a value of 2.2x10-4 compared with the actual zero creep strain, indicating high measurement accuracy. This is shown in Figure 6. As will be understood, strain is a tensor (vector of a vector"). The image in Figure 6 shows only one strain component, i.e. normal strain along the horizontal direction (longitudinal strain). Other strain components such as normal strain along the vertical direction and the shear strain are not shown as they are less important for this longitudinal specimen.

It is an advantage of the arrangement of a permanently mounted device that the timing of inspections can be tailored to suit plant operation procedure. Thus for example inspections can be undertaken during operation or during outage. The device enables consistent measurement of the defined inspection area improving the reliability of results.

The examples and illustrations shown herein are representative of the invention and are not limited to the same.

Claims

CLAIMS1. A method of using a creep strain device to monitor creep strain in an inspection surface of a component, the creep strain device comprising a body for mounting against the component, the body comprising: a chamber having an aperture for receiving the inspection surface of the component and further arranged to provide a gas-tight seal to said component such that, when the body is mounted against the component, the component seals the aperture so as to enable the internal environment within the chamber to be sealed from the external environment outside of the chamber, wherein the chamber has a window through which the inspection surface may be imaged by an imaging apparatus; the method comprising: a. Providing two or more markings on the inspection surface of the component; b. Mounting the body to the component so as to place the inspection surface within the internal environment of the chamber; c. Applying each of a stress and an elevated temperature to the inspection surface for a time period; and d. Monitoring the change in geometry of the two or more markings in accordance with the time period using the imaging apparatus.
2. A method according to claim 1, wherein a plurality of images are obtained using the imaging apparatus at different times during step (d).
3. A method according to any of the preceding claims, wherein the monitoring is performed using a digital image correlation (DIC) method.
4. A method according to any of the preceding claims, wherein the markings in step (a) are provided by locally deforming the inspection surface of the material to form micro-indentations.
5. A method according to any of the preceding claims, wherein the body is provided with a sealable gas port for providing selective gaseous communication with the internal environment of the chamber, and wherein the method further comprises filling the internal environment of the chamber with an inert gas either during or after step (b).
6. A method according to claim 5, wherein the inert gas is argon.
7. A method according to any of the preceding claims, wherein the chamber is sealed during the performance of step (b).
8. A method according to any of the preceding claims, wherein the body further comprises an attachment device for mounting the body to the component in step (b), preferably said attachment device being reusable.
9. A method according to any of the preceding claims, wherein the said window used for said monitoring in step (d) is an optical window.
10. A method according to any of the preceding claims wherein the body is permanently mounted to the component.
11. A method according to any otthe preceding claims, further comprising: e. producing output data representing the strain within the inspection surface in accordance with the monitoring.
12. A computer program product comprising program code for performing the method according to claim 11, when said product is executed upon a computer system.
13. A creep strain device for use in the method of any of claims I to 11, the creep strain device comprising a body for mounting against the component, the body comprising: a chamber having an aperture for receiving the inspection surface of the component and further arranged to provide a gas-tight seal to said component such that when the body is mounted against the component, the component seals the aperture so as to enable the internal environment within the chamber to be sealed from the external environment outside of the chamber, the chamber having a window through which the inspection surface may be imaged by an imaging apparatus; a sealable gas port for providing selective gaseous communication with the internal environment of the chamber; and, an attachment device for mounting the body to the component.
14. A creep strain monitoring system for use in the method of any of claims I to 11, the system comprising: a creep strain device according to claim 13; and an imaging apparatus adapted to image the inspection surface when in use.
15. A method as substantially hereinbefore described with reference to the examples illustrated in the accompanying drawings.
16. A creep strain device as substantially hereinbefore described with reference to the examples illustrated in the accompanying drawings.