Francesco Ginesu

On the performance of some unwrapping algorithms

This work attempts an objective comparison in terms of performance and execution speed of some of the best known phase unwrapping algorithms. First the algorithms chosen, grouped into 4 classes (sequential methods, residues methods, global least square integration methods, others) are described. Then the influence of the weighting function on each of them is investigated. So as to obtain quantitative results it is necessary to use synthetic images, so that the exact solution is known, the technique for generating these images is also illustrated. Lastly, the algorithms performance in terms of influence of the weight functions, robustness and execution time are discussed.

Ultrasonic measurements of nominal contact area and contact pressure in a wheel-rail system:

Abstract For more than a century, the contact between wheel and rail has been the subject of careful investigation by scientists and railway technicians. Many efforts have been made to understand a series of phenomena (wear, pitting, fatigue, failures, etc.) that are closely related to processes occurring in such a small contact patch. A huge amount of numerical and theoretical research work (starting from Hertzs theory) has been carried out, but there is a noteworthy lack of experimental tests, which is probably due mainly to difficulties in accessing the contact area with conventional techniques. In the present paper, an experimental investigation dealing with the evaluation of the contact area and stresses transmitted across it is proposed: the technique employed is based on a non-invasive ultrasonic method that analyses the reflection of ultrasonic waves by the contact interface. By using a set-up commonly employed in many non-destructive testing (NDT) analyses it was possible to obtain graphic maps of static contact conditions, and several tests were carried out to evaluate changes with increases in external load. Results show reasonable agreement with Hertzs theoretical predictions and suggest a possible future use of the ultrasonic method to investigate contact during motion.

Vibration analysis of polar orthotropic annular discs

Abstract With the aim of designing structural elements of filament wound composite materials, a first analysis of the vibrating modes of an annular orthotropic disc was performed. To check the reliability of both the analytical and experimental approach, a uniform steel disc was previously tested. Natural frequencies were computed by means of a finite element program, while the experimental analysis was based on real time and time average holographic interferometry. For orthotropic discs, whose material properties are generally difficult to determine, the holographic analysis of vibrations can be used as a complementary instrument for testing the material and detecting defects.

Experimental investigation on contact between cylindrical conformal surfaces

The application of an experimental technique based on ultrasonic waves to investigate contact parameters for a cylinder versus cylinder system in conformal configuration is proposed. The method analyses the ultrasonic reflection from the contact interface (or the transmission through it) and yields direct information about the size and shape of the nominal contact area; it also gives a qualitative indication of the contact pressure value, which is a parameter not explicitly related to the characteristics of the reflected wave. In order to assess the reliability of the technique as regards its ability to give a good reproduction of contact areas, the processed ultrasonic data, which can be easily represented as ‘contact maps’, were compared with the results of the application of a commercial pressure-sensitive film (Fuji Prescale HS). At the same time, a finite element model of the tested contact configuration supplied a further comparison term useful in assessing the effectiveness of the ultrasonic technique as a tool in estimating contact pressure distribution.

Damage in Composite Materials: Experiment Vs a Computational Chaotic Model

We present the results of a combined experimental and theoretical study performed using non-linear mechanics schemes to investigate the structural behaviour of a composite macroscopic material. A simple model is considered to define the order of the complexity of the real system represented by a graphite peek polymer under static and dynamic load In particular a relationship has been found between the critical points in the energy-time diagram and in the bifurcation plot.

Moiré-holographic Analysis of Reinforced Plate Models

This paper deals with an extension of moiré holography for structural analysis on reinforced plate models. It is shown that unequal optical paths, caused by lack of uniform thickness, are compensated for and do not alter the moiré fringe patterns. Moreover, a simple technique is described for replicating high-sensitivity moiré gratings on Plexiglas models. The validity of the method has been satisfactorily tested on plate models with different geometries.

Semiautomatic approach of grating techniques

Abstract After a brief description of the experimental set-up used for the strain analysis of composite laminates with graphite fibres and thermoplastic matrix, the results obtained using a software code suitable for reading and analysing fringe patterns are presented. In particular the program uses two different approaches: the first semi-automatic with algorithms proper of fringe tracking and the second based on Fourier transform.

Numerical analysis of fringe patterns for structural engineering problems

Different software codes useful in fringe pattern analysis are described. The problems of achieving, digitizing and analyzing are briefly discussed with reference to a new software code, completely written in our laboratories. Some applications of numerical processing to holographic interferometric fringe patterns are described.

Phase Unwrapping Algorithms: A Comparison

Over the last few years a great deal of work has been done to improve the reliability of phase unwrapping algorithms. Many articles are to be found in the technical literature claiming the robustness and noise immunity of the proposed algorithm.

An Application of White Light Profilometry Using Geometric Phase Shifting

Laser surface action profilometry is a well known technique for measuring the statistical parameters of a profile. However it is unable to deal with surfaces having discontinuities larger than half the laser wavelength. This limitation can be overcome using either two laser sources having very close wavelength or a single white light source. Both techniques present some problems: the first requires two laser sources or a variable wavelength laser and involves a numerical only solution. On the other hand, the white light system is based on a physical property (the short coherency length of the source) but requires an achromatic shifter. We have evaluated the accuracy of the white light system taking into account the results of the two-laser reference configuration. In order to achieve this objective an experimental set-up has been devised that can acquire data on the same area. Phase shifting has been performed using an achromatic geometric shifter in both cases, while two different procedures have been followed to process the fringe images. In conclusion, the results obtained with white light are very close to those obtained with the two-laser system. The main drawback is the very large amount of data needed to perform image processing.