Submitted to and accepted by Strain Journal on April 2016

Recent Developments in Measuring Creep Strain in High Temperature Plant Components

Authored by Aditya Narayanan1, Chris Maharaj2, Mark Kelly1, Andy Morris1,3, Catrin

M. Davies1, and John P. Dear1*1Department of Mechanical Engineering, Imperial College London, United Kingdom2Department of Mechanical and Manufacturing Engineering, The University of the West Indies,

St. Augustine, Trinidad and Tobago3Cottam and West Burton Power Stations, EDF Energy, United Kingdom

*Corresponding author

Abstract

Accurate measurements of creep strain are necessary to evaluate the condition and predict the

remaining life of power plant constituent materials. Optical techniques are appropriate for this

purpose as they are a non-contact method and can therefore be used to measure strain without

requiring direct access to the surface. Within this class of techniques, the Auto-Reference Creep

Management And Control (ARCMAC) camera system can be used to calculate the strain between

two points using a series of silicon nitride (SiN) target spheres (the ARCMAC gauge). There are two

iterations in system design, the Conventional ARCMAC and Digital Single-Lens Reflex (DSLR)

ARCMAC.

Experiments are conducted to determine the absolute limit of accuracy of the systems in

comparison to a strain gauge, and the relative accuracy across several orders of magnitude until

specimen failure. In addition, tests have been performed using the ARCMAC gauge at elevated

temperatures to evaluate the effect of temperature on the gauges and to investigate whether its

accuracy diminishes in creep conditions.

It was found that both conventional and DSLR ARCMAC systems can be accurate to 60 µε or less. In

accelerated creep tests, the ARCMAC gauge produced similar agreement to a linear variable

displacement transducer when used to measure creep strain. Strain variations (under 500 µε) were

noted on a steel plate subjected only to operational temperature and no stress. This error is very

reasonable compared to a critical strain value of 93,000 µε in a given high temperature-service

material. Digital Image Correlation (DIC) results using the DSLR ARCMAC system show approximately

4% error in measurement for plastic strains in the specimen. The two measures of strain

measurement (using ARCMAC and DIC) can serve to complement each other.

Keywords: Creep, DIC, optical techniques, strain measurement

Introduction

The maintenance of energy supply of a nation is a critical part of its infrastructure, without which it

cannot further its own development. At present, the vast majority of energy generated in the world

is obtained from non-renewable fuel sources, comprising both fossil fuels and nuclear material. At

the elevated temperatures present in a power plant, creep becomes the primary failure mechanism.

Accumulation of creep strain and damage causes defects to develop in component material and left

unchecked can have fatal consequences. Therefore it is important to have a well-developed

monitoring regime in order to assess accurately the condition of the plant.

In the UK, the approach to managing the integrity of the high pressure and temperature pipework

systems and components is based primarily on an Inspection Based Assessment (IBA) approach. The

IBA approach is designed to complement the statutory inspection periodicity of 4 years.

Consequently, the current inspection and assessment methods are not optimised to provide reliable

predictions of component degradation rates and hence optimal times for future

re-inspection/repair/replacement.

A range of non-destructive inspection based assessment techniques and surveys are routinely used

during a statutory plant shutdown to support an evaluation of the creep damage accumulated in a

component operating at high temperature. These techniques do not provide a direct measure of

creep damage, instead providing data that can support the subsequent residual life assessment.

However, these assessments are heavily influenced by experience of similar components/systems

and rates of damage accumulation obtained from metallurgical examination. In essence, the current

assessment approach relies upon data mining and review of large quantities of site metallurgical

examination data, from a number of different Power Stations and at different times in the life cycle.

Commonly used surveying techniques include replicas, hardness, diametral strain measurement,

extraction of material specimens and various forms of non-destructive testing (NDT) for macro

defects. Creep strain measurement offers the greatest potential to the utility, from the perspective

of being able to use the information to pro-actively manage the integrity of plant throughout the

whole life cycle of the plant. The ultimate aim is to use the creep strain data iteratively, for example

in pipework computational models that utilise appropriate creep strain material models, on-load

temperature/pressure data and pipe system loads obtained from hot and cold operation surveys of

pipe support hangers. The ideal process would then compare measured creep strain rates at key

locations with computational predictions, and thereby support the identification of the location and

extent of future inspection campaigns and/or modification to plant operation to moderate rates of

creep damage accumulation. The most popular method of creep strain measurement during

statutory outages rely on diametral strain measurements [1], either over pipe creep pips mounted

on the pipe or over the pipe surface itself (as shown in Figure 1), with suitable corrections for surface

oxidation rates. Evidence based on reviews of large datasets from periodic UK fossil fired plant

diametral surveys, showed much greater than expected variability [1]. High temperature electrical

resistance and capacitance strain gauges have also been used occasionally on high temperature

piping, but they are expensive (especially where multiple locations require monitoring) and can be

subject to interference.

For on-site inspections both accuracy as well as practicality must be considered before implementing

a particular monitoring strategy. Figure 2 shows some examples of locations where the more

elaborate methods may be difficult to apply due to the positioning of the pipe network. Therefore

optical techniques are viable for site usage as they utilise a camera system to take images of the

surface of a component where a visual indicator of strain has been applied. The accuracy of optical

methods is dependent on the camera and the surface indicator, while the results are present as

images and require relatively little expertise and equipment to process.

There is therefore a range of existing techniques available to the utility, whose use is governed by a

host of considerations such as perceived risk, familiarity and confidence with the techniques, plant

access, accuracy, reliability, regulator preference and tradition. Emerging techniques are currently

being researched and applied to address the limitations of the existing techniques. This recent work

is covered in the following Section.

Recent developments in methods to measure creep deformation

Following the review work of Maharaj et al. [2] and Sposito et al. [3], there have been some recent

developments with respect to methods used to measure creep deformation. These include using

magnetic field properties [4] that measure Barkhausen noise or magnetic hysteresis, eddy currents

that are disrupted by inhomogeneities [5], and replicas where computer-based image analysis has

shown some promise [6]. Potential drop (PD) methods have been used to monitor crack growth for

many years in a variety of different materials using both alternating and direct current [7], with

associated standards governing how tests must be set up [8]. Research at Imperial has been

performed with Alternating Current PD methods using a square probe arrangement to measure

strain as well as creep damage and this has been successfully applied in trials on UK power stations

[9–12].

A non-contact method for creep measurements of Niobium at 2300°C was presented [13] using a

camera system to image a spherical sample that was subject to rotation by electrostatic levitation.

Optical images were analysed to measure creep deformation and the experiment was compared to

numerical analysis, showing 31.7% agreement between the two sets of results.

An optical fibre sensor for simultaneous measurement of strain and temperature was proposed

using a pair of regenerated gratings at a temperature of 900°C [14]. A 3 by 3 matrix was employed to

characterize the sensor for discrimination between strain and temperature in a temperature

environment. The strain and temperature test results show good agreement with a characterized

model with a deviation in measured strain and temperature of 28.3 µε and 4.1°C at the ranges of 0-

1000 µε and 25-900°C, respectively.

Digital Image Correlation (DIC) is a method of strain measurement that enables the evaluation of the

strain experienced by an entire region. A stochastic pattern (usually paint) is applied to an area of

interest and successive images of the pattern are captured as it deforms along with the substrate

material. The images are processed using commercial software to determine the strain field over the

entire area. As far as DIC is concerned, black body radiation has been a significant problem at

elevated temperatures. Methods have been suggested using various configurations of optical filters

and blue lighting to prevent the radiation from reaching the sensor [15], [16]. Using only relatively

low levels of illumination, accurate DIC measurements up to 1100°C were obtained with the

potential to monitor strains up to 1400°C [15], while the thermal expansion coefficient of an

austenitic stainless steel has also been determined using a similar setup [16].

A DIC system which enables a high temperature displacement and strain measurement up to 1100 °C

was presented and demonstrated by experimental measurements [17]. A high performance heat

resistant coating material suitable for large deformations was formulated consisting of amorphous

precipitated silica and titanium dioxide. However, the coating needed to be cured to obtain its

maximum heat resistance to prevent it degrading under the influence of heat. Surface radiation is

suppressed through the application of monochromatic illumination. It was found that image

saturation was almost eliminated and the DIC evaluation showed good results in all directions.

A related optical system for microscopic observation and strain measurement at high temperature

using DIC principles was developed [18]. The system consists of a ultra-violet (UV) CCD camera, UV

illumination, filter and long-distance lens. It was found that reducing thermal radiation and

illuminating the sample by UV light was effective for avoiding the disturbance of thermal radiation at

high temperatures. The contrast and spatial resolution required for DIC analysis at the microscopic

level were maintained up to 1400°C. The measured average strains in the x and y directions (εxx and

εyy, respectively) were 11300 µε and 11700 µε with the strain distribution fluctuating from 5200 µε

to 16000 µε. The non-uniform strain map was attributed to the system error of electronic noise

during the observation process, the heat haze during the heating process, and the mechanical shift

in experimental set-up.

The ARCMAC gauge is a point-to-point optical strain gauge that has already been implemented in

many UK power stations at selected positions of interest, as seen in Figures 2 and 3, and has been

the subject of ongoing development [19], [20]. The gauge provides displacement data between two

points selected on a test piece. This is performed by the use of a pair of Inconel gauge plates with an

arrangement of three reflective, Silicon Nitride (SiN) spheres located on the two plates as shown in

Figure 4. A collimated light source is used to create the appearance of a light spot on the centre-

points of each of the spheres, to locate their relative positions. The ‘paired’ spheres are fixed at a

distance of 3 mm apart and are used as the reference distance. A camera then takes an image of the

arrangement at progressive points during the component’s deformation. As the strain increases, the

plates move further apart - as do the spheres and their lights spots in subsequent captured images.

Next, a series of image manipulating techniques (resizing and thresholding) is performed so as to

measure the distance between the gauges, in pixels. Then, strain is calculated using Equation 1,

where G is the distance between the weld points of the two gauge tablets, Aactual is the reference

distance (3 mm) and B and A are the distances marked on Figure 4.

ε=

B tAt

−B0A0

GAactual

(1)

Images are taken using one of the two generations of ARCMAC camera system, either the

conventional ARCMAC (a 0.8 megapixel, handheld charged-coupled device (CCD) camera with a

telecentric lens and in-built lighting unit, controlled and triggered using a connected laptop

computer) or the DSLR ARCMAC (a commercially available, 10 megapixel single-lens reflex camera

with a macro lens and on-board memory card). The purpose of developing this newer system has

been twofold: providing a less bulky camera unit and therefore making the system easier to use for

on-site inspections, and giving a greater field of vision (with enhanced image resolution) so as to

allow digital image correlation to be performed. Schematic diagrams of both camera systems are

shown in Figure 5. The system, which contains no moving parts, ensures that an image is captured at

a constant distance away from the gauge tablets.

This paper documents experimental investigations into the accuracy of two methods that use optical

principles to measure strain. With regard to the ARCMAC system, the levels of accuracy achievable

using the ARCMAC gauge on a strained component are investigated at both room and service

temperatures.

Experimental Methods

Experiments at room temperature were performed to gauge the suitability of the ARCMAC system in

general as a strain monitoring method. The purpose of testing at low strain was to establish an

absolute value of accuracy obtained using the Conventional ARCMAC and DSLR ARCMAC systems.

The DSLR ARCMAC system offers enhanced image resolution, greater field of vision, and a significant

improvement to its portability. The enhanced image resolution and greater field of vision can also

serve to evaluate a strain field map via DIC. The Conventional ARCMAC and DSLR ARCMAC results

were compared a strain gauge adhered to the specimen surface. Testing at high strains was

conducted in order to examine the accuracy of the optical techniques through a range of different

strain magnitudes. This is more relevant to creep, as creep failure strains reach orders of magnitude

more than 1000 times of those achieved within the elastic limit of a material.

Room Temperature Assessment Methods

Comparisons at Low Strain

Mild Steel specimens were loaded in a tensile testing machine to a maximum stress of 115 MPa (45%

of the yield strength), allowing repeatable measurements to be made without significant alterations

in material behaviour. The camera system in use was clamped and mounted on a stand, made to

focus on the ARCMAC gauge or speckle pattern depending on which was the subject of that

particular test. Images were photographed at increments of 25 µε as measured using a strain gauge

connected to a Vishay P3 Strain Indicator and Recorder. Three sets of measurements were made

using each technique, and the accuracy of the system was calculated as the root mean square

(R.M.S.) values of the difference between the strain gauge measurement and the corresponding

optical measurement expressed in µε.

Comparisons at High Strain

Tensile tests to large values of strain were conducted on specimens of stainless steel. An Instron

2620-601 clip-on extensometer was used in order to obtain measurements for comparison with

those obtained using the optical techniques. Images were captured at increments of 0.01 mm

initially, to obtain more data points while loading was in the elastic region. Once plasticity had been

reached, the imaging rate was increased to increments of 0.25 mm of specimen displacement. In

addition to tests using the ARCMAC gauge, a series of similar experiments were performed with the

DSLR ARCMAC system to investigate whether it could be used to obtain accurate data for DIC.

High Temperature Assessment Methods

Use of ARCMAC Gauges to Measure Creep Strain

Creep tests were performed on specimens each with an ARCMAC gauge attached, being interrupted

as various points before failure. Specimens 1, 2, 3 were at 650oC in air and lasted 97, 102 and 219 h

respectively. Specimen 4 was at 550°C in air and lasted 1583 h. The gauge was photographed using

the DSLR ARCMAC system before commencing the test and after its completion i.e. when unloaded

and cooled. A Linear Variable Differential Transducer (LVDT) was used to measure the specimen

strain during the test, with the value measured on unloading compared to the value calculated using

the ARCMAC gauge. The purpose of this was to establish whether exposure to a high temperature

test affects the effectiveness of the sensor.

Oven testing of ARCMAC gauge pairs

Four gauge pairs were installed on a CMV plate material (machined out of pipe material made of

0.5Cr-0.5Mo-0.25V low-alloy steel) as shown in Figure 6. The plate material was not subjected to any

stress, only temperature. The plate and gauges were placed in an oven and initially heated in air to a

temperature of 580oC and then increased to 600oC closer to the end of the 389 day (~ 9300 hr high

temperature exposure) test. 580oC is the operating temperature of power station steam piping. At

specified time intervals, the plate was removed from the oven and allowed to cool in air to room

temperature. Three repeat images for each gauge were acquired with the ARCMAC camera and

subsequently processed to determine the strain. The uncertainty (Um¿¿ in the strain values for the

repeat images were calculated using Equation 2.

Um=εmax−εminN i

(2)

εmax is the maximum strain obtained with the 3 images, εmin is the min, and N i is the number of

repeat images (three).

Clarity of ARCMAC Target Spheres with continued exposure to temperature

Two specimens with ARCMAC gauge (Gauge A and Gauge B) attached were subjected to a prolonged

period exposure in air (beyond 10,000 hrs) at a temperature of 650oC. At different points during the

test they were allowed to cool and photographed using the Conventional ARCMAC system using four

different camera shutter times before being returned to temperature. The images captured were

then subjected to the aforesaid processing method. After performing the threshold however, the

greyscale distribution of the image was analysed to determine the size of the light spot in pixels (px)

present in each image. This was in order to investigate whether the temperature caused any

discolouration or deterioration in the quality of light spot produced by the light source within the

ARCMAC system as well as whether the shutter speed affected the quality of image.

Results

Room Temperature Assessment Methods

Results obtained for the tests at low strain are presented within this section. In all cases, the data

from a strain gauge is compared with the camera system specified. Strain gauge data has been

plotted along with a line of best fit to illustrate the trend clearly, and the graphs are plotted on the

same scale of axes to better enable comparisons. Results from the camera systems have been

filtered for noisy data by removing any non-positive measurements, as these are indicative of

compressive strains, something a tensile test will not produce. Errors are expressed as the root

mean squared (r.m.s) mean of the difference between the strain gauge results and that of the

camera system tested.

Figure 7 shows the results obtained for tensile tests performed using the conventional ARCMAC

system to measure strain within the elastic region. The errors of the sets of results are 49 µε, 50 µε

and 54 µε with standard deviations of 32 µε, 28 µε and 28 µε respectively. The results for three tests

using the DSLR ARCMAC system are displayed in Figure 8. Here, the average error has been

calculated as 61 µε, 50 µε and 78 µε and standard deviations 40 µε, 46 µε and 77 µε respectively.

Tests to failure have been performed for both aforementioned configurations, as well as DIC using

the DSLR system. As these tests encompass several orders of magnitude, errors described here will

be expressed as a percentage of the magnitude of extensometer measurement. Errors incurred

during the elastic region have not been considered as they unduly inflate the average (for example, a

40 µε error at 100 µε gives an error of 40% despite being considered the upper limit of accuracy by

commercial DIC software manufacturers [21])

Data gained from using the conventional ARCMAC in high strain tests is presented graphically in

Figure 9. The mean errors in measurement are 4.8%, 11.2% and 1.7% with respective standard

deviation of 5.5%, 5.9% and 1.4%. Tensile tests performed using the DSLR ARCMAC are shown in

Figure 10. Here, the errors in measurement are 10.1%, 10.8% and 9.8% with standard deviations of

1.1%, 6.7% and 5.7% respectively.

Alternatively, DIC can be used to determine global strain rather than ARCMAC point to point

measurement. Results of strain measured by DIC using the DSLR system have been calculated by

using a global average over the entire region of interest visible due to the speckle pattern and can be

seen in Figure 11, where the mean errors were 4.3%, 4.4% and 3.9%, with standard deviations of

2.5%, 3.4% and 3.0% respectively.

High Temperature Assessment Methods

Results comparing creep strain measured using the ARCMAC gauge with corresponding

measurements made using a LVDT are in Table 1. As can be seen, the ARCMAC gauge shows a

reasonable level of accuracy with an average error of 12.2% on average (a standard deviation of

5.5%), which is to be expected considering the room temperature results. This suggests that results

obtained using the ARCMAC gauge at elevated temperatures can produce results of a similar

accuracy as those at room temperature.

Following the determination of the accuracy through comparison with the LVDT at time and

temperature, a test was done with four gauge-pairs attached to a CMV plate at steam piping

operational temperature (up to 600oC) but no stress. At these temperatures and at no stress loading,

the strain should ideally be zero with time. Figure 12 shows the gauges after exposure to

temperature for four weeks. Figure 13 shows the strain results for the 4 gauge pairs. The strain

variations observed could be due to slight changes in the reflection characteristics of the gauge

spheres due to temperature and time. This can be substantiated by examination of Figure 14 that

reveals slight changes in the shape of the reflection of the centre sphere. An example is shown with

change in the shape between Day 172 and Day 273 images as identified with the blue arrows. The

strain variations (under 500 µε) noted though are small compared to the magnitude of the strain

that is measured before the component must be removed from service. For some high temperature

service materials, the onset of tertiary creep occurs at a strain of 93000 µε [22]. The maximum strain

uncertainty value obtained in the experiment was 40 µε.

Results examining the clarity of target spheres after exposure to prolonged time beyond the

previously mentioned test and at 650oC are shown in Figure 15. It is clear that, as expected, a

reduced shutter time decreases the size of the light spot present within the image as there is less

time for the reflected light to be detected by the CCD camera. Noticeably, there is no discernible

correlation between exposure time at elevated temperature and the size of the light spot in pixels.

Gauge A and Gauge B were two separate gauge plates to explore variability. Although for Gauge A

there appears to be a great amount of variation in the size of the light spot, for Gauge B this is

relatively constant. This test serves as a comparative measure alone with the first data point to the

left being considered the reference.

Discussion

The purpose of ARCMAC testing at lower strain values was to establish an absolute value of the

accuracy of either system including the ability of the DSLR ARCMAC to perform DIC. At low strain,

both ARCMAC techniques showed roughly equivalent accuracy. The results provided by the

conventional ARCMAC system over three experimental trials showed a mean error between 49 and

54 µε. The DSLR ARCMAC had slightly more error (50-78 µε) although not significant enough to

suggest that this is out of the realm of the natural variation in results. In fact, if an overall accuracy is

expressed over all three tests for each system, the result is 51 µε for the conventional ARCMAC and

60 µε for the DSLR ARCMAC, a difference which is minute in comparison with the increased

practicality of using the DSLR ARCMAC system.

From this it may be inferred that the ARCMAC technique can provide equivalent accuracy to other,

widely used optical techniques. This evidence firstly shows that the ARCMAC system in general is a

robust strain monitoring technique and secondly, that it may be applicable to situations other than

what it was designed for i.e. where lower values of strain are desired to be monitored.

Experimentation was performed to higher strain values in order to determine whether they

represented a viable option at strains that approached or exceeded creep failure strains seen in

power station components. Measurements at high strain show the conventional ARCMAC giving

superior accuracy to the DSLR ARCMAC system overall in the plastic region of the specimen. This is at

least partially attributable to lens distortion, although it should be noted that as one test using the

conventional ARCMAC system has poorer accuracy than all those of the DSLR ARCMAC, it is likely to

be less of a concern than initially thought. The performance of the conventional ARCMAC system

suggests that its use on plant so far is likely to have yielded reliable data.

DIC results using the DSLR system consistently show approximately 4% error in measurement for

plastic strains in the specimen, which is comparable to the results of the conventional ARCMAC

system, suggesting that DIC can be performed accurately for on-site inspections. If an average error

over all the three sets (Conventional ARCMAC, DSLR ARCMAC, and DIC) is taken, the values are 5.9%,

10.2% and 4.2% respectively. It is interesting to note that at higher strains, the performance of the

DSLR system for DIC is overall more accurate and consistent than both ARCMAC systems. This is

could be because at higher strains taking a global average over the surface of the sample suppresses

the effect of strain localisations or fluctuations in strain measurement. With both ARCMAC systems,

the calculated strain is dependent on one point-to-point measurement, meaning an average value is

not computed and that any local fluctuations in strain can impact the measurement.

Tests were performed at high temperature to investigate the behaviour of the gauge under

operational conditions. Firstly, it is clear that that influence of temperature on the quality of image

produced is negligible as the light spots are relatively consistent in size, especially for Gauge B.

Variations present in results for Gauge A may be from other sources such as errors in positioning of

the camera, or dust and grit that may be present on the surface. This is further evidenced by results

from creep tests comparing the ARCMAC gauge to a LVDT, which show reasonable agreement. Strain

variations (under 500 µε) were noted on a steel plate subjected only to temperature and no stress.

This error is very reasonable compared to a critical strain value of 93,000 µε in a given high

temperature-service material. The maximum strain uncertainty value obtained in this experiment

was 40 µε.

Conclusion

Both ARCMAC camera systems have shown promise as tools for a condition monitoring regime.

Previous use of the conventional ARCMAC for on-site inspections is therefore expected to give good

measurements of strain accumulating in a component. The DSLR ARCMAC shows comparable

accuracy to its predecessor and hence can be considered a viable option for use. This is particularly

promising considering that it also produces accurate results in testing using a DIC speckle pattern for

strains at higher orders of magnitude, meaning it can perform a dual function.

All techniques have been shown to produce data that can replicate measurements made by

traditional strain measuring methods such as strain gauges and extensometers to a reasonable

extent. It can be seen that both conventional and DSLR ARCMAC systems can be accurate to 60 µε or

less, and that the ARCMAC gauge may be suitable in situations other than creep conditions, where it

is not possible to use traditional methods of measuring strain.

Finally, it is important to note that accelerated creep tests showed that the ARCMAC gauge can

produce similar agreement to a LVDT when used to measure creep strain. Along with a relatively

small strain error (under 500 µε) for the high temperature zero stress case and considering that the

visibility of the light spot remains consistent over 10,000 hours, it suggests that the gauge is reliable

for enduring the time-scales seen by a plant component and can been employed successfully on

power plants in the UK.

Acknowledgments

The authors thank Gareth Hey and Professor Scott Lockyer at E.ON for their help and advice in

development of the ARCMAC measurement system and technical support in its use. Dr Haoliang

Yang and Dr Joseph Ahn were also valuable providers of support in experimentation.

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Figures

Figure 1: Example of diametral gauge being used

Figure 2: Pipe network presenting difficulties in terms of access. At the top of the photograph, the

transverse pipe work has ARCMAC gauges with protective covers in place

Figure 3: Pipe with biaxial ARCMAC gauge attached

Figure 4: Close-up of ARCMAC Gauge

Figure 5: Schematic of a) Conventional ARCMAC (Top), b) DSLR ARCMAC (Bottom). Key; 1-Camera, 2-

Lens, 3-Light Source, 4-Beam Splitter, 5-Specimen Surface, 6-ARCMAC Gauge

Figure 6: Schematic of oven test plate specimen

Figure 7: Stress against strain comparing the conventional ARCMAC system with a strain gauge for

three samples. Tested at 25oC using mild steel

Figure 8: Stress against strain comparing the DSLR ARCMAC system with a strain gauge for three

samples. Tested at 25oC using mild steel

Figure 9: Stress against strain comparing the conventional ARCMAC system with an extensometer for

three samples. Tested at 25oC using mild steel

Figure 10: Stress against strain comparing the DSLR ARCMAC system with an extensometer for three

samples. Tested at 25oC using mild steel

Figure 11: Stress against strain comparing DIC using the DSLR system with an extensometer for three

samples. Tested at 25oC using mild steel

Figure 12: Photo of ARCMAC oven test specimen (CMV material) after exposure to 580oC for four

weeks

Figure 13: Graph of strain values versus time for gauges #1 to #4. Tested using CMV material

Figure 14: Cropped image before resizing was applied for Gauge 1

Figure 15: Evolution of light spot size in pixels with time. Tested in air at 650oC

Table

Table 1: Final strain measured in creep tests using LVDT and ARCMAC gauge

Specimen LVDT Strain (µε) ARCMAC Strain (µε)

1 145,000 149,000

2 212,000 177,000

3 195,000 223,000

4 106,000 90,000