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3D contour and strain mapping for non-destructive evaluation of engineering components

Periodic Report Summary - STRAINMAP (3D contour and strain mapping for non-destructive evaluation of engineering components)

This project aims to develop a kit of optical 3D contour and strain mapping techniques for non-destructive evaluation (NDE), non-destructive testing (NDT) and structural integrity assessment of engineering components. Digital shearography (DS), projection Moiré (PM), digital image correlation (DIC) and image processing techniques will be further developed. During the two year period, extensive work has been carried out on DS, PM and DIC techniques, both hardware and software with the following achievements:

(1) Novel methods and algorithms to address the difficulties in measuring deformation of complicated microstructures were developed. Modified digital Moiré methods and digital geometric phase techniques based on micro-gratings and micro-marker identification methods were proposed, an application to measure the deformation of MEMS was made. In these methods, specimen grating and micro-marker fabricated techniques were developed. These gratings and markers were fabricated by FIB (focused ion beam) directly on the surface of the MEMS tested. Digital reference grating, Gaussian de-blurring algorithm, FFT and IFFT algorithm were used.

(2) DS has been used for strain measurement and defect detection in materials and components including metal, composites, polymeric material, and a complicated cone structure. The defect detection capabilities of DS for hole and crack type defects in polymeric and metal materials were investigated, and the optimal means of stressing was analysed. With the combination of fringe multiplication and phase extracting techniques, the defect detection capabilities have been improved.

(3) An effective and fast phase unwrapping algorithm for strain measurement using shearography was developed. A novel fast phase shifting technique/system involving the phase unwrapping algorithm and a (4, n) spatial phase-shifting technique has been proposed. It is based on the principle of polarisation phase shifting. A 3×3 or 5×5 windowed phase filtering algorithm and a least squares unwrapping algorithm were developed. This technique has been applied to quantitatively measure the 3D strain, and to carry out NDT, of components under quasi-static conditions. It has the potential for use under dynamic conditions, although this was not demonstrated in this work. Also, a remote shearography system with wireless CCD was developed for defect detection in harmful circumstances, such as under radiation.

(4) A novel digital image processing system has been developed. Apart from the conventional image processing capabilities for fringe pattern interpretation, the system has an extra function for CTE measurement using a new digital image correlation DIC method, deformation pattern based digital image correlation (DPDIC). Experiments showed that the measurement accuracy had been improved over conventional approaches.

(5) A new optical set up for projection interferometry moiré method for 3D contour mapping, was demonstrated in the laboratory for the first time. A fine moiré fringe pattern created by shearing interferometry was projected onto an object surface under test. The technique can be applied to fast 3D shape measurement and reconstruction for objects from macro to micro (micrometer level) scales. Validation tests performed included a small bottle with height of 3 cm and a small ball with a diameter of about 50 micron. In addition, a double shear interferometry system for fast NDT and residual stress analysis of transparent material was developed. Certain types of defects in glass such as bobble, scratch, impurity etc were observed by using this new system. It also has potential for analysing residual stress in transparent material. A patent application for this technique is under consideration.