Full-field Measurement Techniques

2.2.3.1 Laser Speckle Interferometry

W hen a rough surface is irradiated w ith a coherent light beam, such as produced by a laser, each point of the rough surface reflects the light in a different direction, creating a 'speckle p attern' on the image plane. This speckle p attern is unique for a surface; if the surface deforms, the speckle p attern will change its form. The speckle interferom etry exploits this feature and captures the change in the speckle pattern, which is related to the surface deformation. The change in speckle patterns can be determ ined by taking the difference betw een them. This can sim ply be obtained by optically interfering the speckle patterns obtained from the initial and the deform ed surface states. ESPI (Electronic Speckle Pattern

Interferometry) [52] uses a CCD camera for taking the difference betw een the initial speckle image and the deform ed speckle image. Speckle interferom etry is a non-contact optical m ethod and as it can be observed in real time, the w hole process of a tensile test can be m easured including plastic deform ation until specimen rupture. There are not m any examples w here it has been applied to high tem perature creep measurements, apart from m easurem ent of surface deform ation in a short term (30 min) creep test [53]. ESPI is a pow erful full-field optical technique for the analysis of the strain field at the surface of

inhomogeneous m aterial under load. However, this sensitive electric

m easurem ent system is not suitable for long term high tem perature creep tests.

2.2.3.2 Moire Interferometry

Moire is the generic term for full field m easurem ent techniques, which utilise the interference effect betw een some form of specimen grating and

reference grating to magnify the surface deformations and create a contour m ap which is related to surface displacement - a moire fringe pattern or interferogram [54]. The m oire pattern is a full field representation of the relative displacement betw een the gratings. This property of moire makes it an excellent tool for observing and quantifying the gradients in localized deformation. In practice, a grating is attached on the surface of the test piece. The grating deforms together w ith the test piece and w hen an undeform ed (reference) grating superim posed onto it, a moire pattern depicting the nature and the m agnitude of the deform ation field is obtained. Each moire fringe represents a line of constant displacement in the direction perpendicular to the direction of the reference grating. The moire effect described above is term ed mechanical moire because the fringes are formed from the mechanical crossing of the two gratings. The mechanical moire effect is lim ited by the frequency (inverse of pitch) of the grating employed. Optical moire, commonly know n as moire interferometry, provides higher sensitivity by

employing the principles of light interference and diffraction. Figure 2.34 shows a basic moire interferom eter w ith a specimen grating exposed to a two-beam

interference system. The diffraction effect split the incoming light beam s into m ultiple preferred rays on the grating. W hen the grating is undeform ed, the -1

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and +1 diffraction order from the beam s emerge perpendicular to the grating w ithout any interference. W hen the specimen is deformed, the -1 and +1 orders will exit the specimen grating w arped and interfere w ith each other. The result is a moire fringe pattern of the in-plane displacements.

Several researchers have studied deform ation at high tem perature using m oire interferom etry [55],[56],[57],[58]/[59]. A difficult requirem ent of high tem perature moire w ork is to apply a specimen grating which has to rem ain highly visible through a test at high tem perature. A few different types of grating can be applied at high tem perature (e.g. deposited metal-film grating, etching grating, refractory grating). For example, Cloud et al [55] have developed high resolution grating photography using refractory grating. The surface strain was m easured using a sample fringe pattern after 1020 hours at 600 °C. H uim in et al [57] and H yde et al [59] also achieved around 1000 hours creep strain

m easurem ent at elevated tem perature. However, strain w as derived from tw o images, initial and after deformation, and therefore did not provide continuous evolution data. Similarly, H ongo et al [60] developed a system based on a m oire interferom etry technique to m easure the creep strain distribution in a m ulti-pass w elded joint. Although their results show the strain distribution across the w eld, HAZ and parent for a long term creep test, they reported that the m ethod requires tedious specimen preparation since a durable diffraction grating needs to be produced on the specimen surface and frequent interruptions w ere necessary during the creep test for the test piece to be taken out of the furnace to perform the m easurem ents and this was not ideal for reliable results. Thus the technique w as

not suitable for continuous "in-situ" tim e-dependent m easurem ent of creep strains.

2.2.3.3 Digital Image Correlation

The digital image correlation (DIC) technique, w hich provides full field m easurem ent of surface deformations, has been successfully employed by m any researchers to m ap the strain variation spanning cross-weld specimens during room tem perature tensile tests [61], [62], [63], [64], [65]. The working principle of DIC is based on sophisticated computational algorithms that track the grey value patterns in digital images of the test surfaces, taken before and after a loading event that produces surface deformations. The principles of DIC m ethod will be discussed in detail in the next section, Here, an introduction is m ade for the use of DIC at elevated tem peratures.

Lyons et al [66] w ere the first to demonstrate the capability of DIC to

m easure strains at high tem peratures. They m easured free therm al expansion and strains due to tensile loads on Inconel 718 superalloy specimens at tem peratures u p to 650°C [67]. They used LSI boron nitride and alum inium oxide-based ceramic coating on the specimen surface to create the speckle pattern for DIC image

m atching and to prevent oxidation at high tem perature. They also used a small fan near the furnace w indow to stop heat current causing therm al variation on

displacement data. The results indicated accurate strain m easurem ent of therm al expansion and during a tensile test. Using the same system, Liu et al [68] studied the strain field on a single edge notched specimen at 650°C and 704°C. The test duration was 146 hours and the results were verified w ith FEM.

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Maharaj, al [69][70][71]have developed an optical creep strain m onitoring system for life assessment in pow er generation plants in UK. This m onitoring system combined DIC and ARCMAC, Auto-Reference Creep M anagem ent and Control, optical strain gauge system. The system used a pair of optical gauges for point-to-point monitoring of changes in strain across steam pipe weld. The strain gauge consists of two Inconel gauge plates w ith fixed silicon nitride spheres, DIC w as used to determ ine the separation of the centres of the circular points betw een the gauge plates. They evaluated on site strain variation in their experiment.

How ever, this technique is unable to provide full-field creep deform ation

m easurem ent, since only the strain betw een point-to-point m arker is determ ined.