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Materials Engineering Group
 


Materials Engineering

Departmental Staff - Salih Gungor

Digital Image Correlation (DIC)

 

Digital image correlation has recently 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 (Figure 1).

 

Fig 1 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.

Examples
Strain mapping of a model lap-joint using DIC

Figure 2 Determination of full field of displacement vector and shear strain in a model lap joint using DIC.

Determination of Weld Metal Mechanical Properties using DIC

 

The aim of this study was to develop a method of extracting local mechanical properties from weld metal by strain mapping using the digital image correlation technique. The feasibility of determining local stress-strain behaviour in the weld zone of a 316H stainless steel pipe with a girth weld was investigated 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. To check the reliability of the technique, a set of micro tensile samples, with gauge length of 3.7mm 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 ).

Figure 3 Strain distribution within the gauge of a cross-weld tensile specimen at an
applied stress of 540 MPa.

Link to page about Moiré Interferometry for strain measurement

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