Digital Image Correlation (DIC)
Digital Image Correlation (DIC) can be used to characterise inhomogeneous deformation when conducting cross-weld tensile/creep tests, creep crack growth and creep-fatigue tests.
How it works
Digital image correlation has become one of the most important optical techniques in the field of experimental mechanics.  It provides full field measurement of surface deformations on surfaces with no, or very little, preparation.  It may be more appropriate to call it an ‘image analysis’ technique, since its fundamental working principle is based on sophisticated computational algorithms that track the grey value patterns in digital images of test surfaces, taken before and after an event that produces surface displacements, such as heating or mechanical loading.

The basic principle of digital image correlation.
One of the main advantages of DIC is that, as the technique works on digital images of the test surface, the size of the test surface is immaterial.  The technique works, as well as on large structures, such as bridges or aeroplanes, as at the micro- and nano-scale from images taken using techniques like optical or electron microscopy.

Strain mapping of a model lap-joint using DIC

Determination of full field of displacement vector and shear strain in a model lap joint using DIC.
Variation of Mechanical Properties in Weldments
Here, we  investigated the feasibility of determining local stress-strain behaviour in the weld zone of a 316H stainless steel pipe with a girth weld by tensile tests of specimens machined from the pipe so that it contained the weld at its centre. The tensile test was recorded using a high resolution digital camera and the DIC technique was used to obtain the complete set of full field displacement maps during the tensile test.  The local strain was calculated at every sub-region of 32×32 pixels, which enabled the local stress-strain behaviour for this region to be determined.  Results from these tests show the variability of the elastic modulus, yield stress and UTS across the weld.

Strain distribution within the gauge of a cross-weld tensile specimen at an applied stress of 540 MPa.
To check the reliability of the technique, a set of micro tensile samples, with gauge length of 3.7 mm and cross-sectional area of 0.7×0.7 mm2, were machined from the various locations in and around the weld zone.  The comparison of stress-strain curves determined from micro-samples to stress-strain curves from the corresponding locations within a larger more conventional tensile specimen shows reasonably good agreement. (Click here for more information).
Comparison of local stress-strain curves obtained by standard and mini-tensile specimens.
DIC measurements at elevated temperatures
The life of heat exchanger units in power plants operating at high temperatures and pressures is largely governed by the integrity of the welded regions. The assessment of structural integrity of these structures require the time-dependent non-uniform creep deformation in and around the welded joints, as well as the variation of mechanical properties at the operating temperatures. For this purpose the experimental measurement system developed above for the cross-weld specimens, has been further developed for the measurement of the full field strain fields at weldments during creep and tensile tests above 550ºC.
Experimental set up for DIC imaging during testing at 550ºC.
The problems associated with the acquisition of images for the use of DIC at high temperatures, e.g. image distortion due to convective currents and the degradation of surface appearance due to oxidation were explored and the techniques to overcome these have been developed.
Tensile and creep tests at 550ºC and 650ºC with plain AISI Type 316H stainless steel samples were carried out to assess the applicability of DIC for the strain measurement at elevated temperatures.  The average strain values across the gauge length obtained by the DIC technique were found to agree well with those measured using an extensometer.
DIC validation tensile test at 550ºC.
DIC validation creep test at 650ºC.
Creep tests on weldments were conducted on cross weld specimens cut from an AISI Type 316H stainless steel thick section cylindrical butt weld. 

Photograph of the AISI Type 316H stainless steel thick section weldment indicating the location and geometry of the cross-weld creep test specimen
The variation of local creep curves in the welded region were obtained from creep tests conducted under various loads, giving creep lives between a few days and up to 4 months.

Measured variation in creep strain across the weldment as a function of exposure time, under an applied stress of 300 MPa at 550ºC.  The graph also shows the variation in room temperature (pre-test) hardness along the gauge length.