Antonio Baldi

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.

Two-dimensional phase unwrapping by quad-tree decomposition

One problem to be tackled when interferometric phase-shifting techniques are used is the method in which the phase can be reconstructed. Because an inverse trigonometric function appears in the formulation, the final data are not the phase, but the phase modulo 2pi. A new phase-unwrapping algorithm based on a two-step procedure is presented. In the first step, the digital image to be analyzed is divided into continuous patches by a quad-tree-like recursive procedure; in the second step, the same level patches are joined together by an error-norm-minimizing approach to obtain larger, almost continuous ones. The basic idea of the procedure is to simplify the problem by factoring the complete image into square, variable-size, homogeneous areas (i.e., regions with no internal phase jump) so that only interfaces need to be dealt with. By hierarchically recombining the so-obtained subimages, an unwrapped phase field can be obtained. After a complete description of the algorithm, some examples of its use on synthesized digital images are illustrated. As the algorithm can be used with and without quality masks and the error-minimizing step can use different norms, a full class of unwrapping algorithms can be implemented by this approach.

Application of an ultrasonic technique to assess contact performance of bolted joints

Bolted joints are a widespread type of connection used in mechanical engineering, but their performance, in terms of contact area size and shape and pressure distribution, is not easy to verify due to the nature of closed contact, which makes it difficult to investigate them with most experimental techniques. This study proposes the application of an ultrasonic method (through immersion scans of the potential contact region between the members of a bolted joint) aimed at obtaining both graphical maps of contact conditions and, after a calibration process, actual contact pressure distributions. The analysis is carried out on a number of joints characterized by different values of plate thickness and applied load. Moreover, to evaluate the quantitative accuracy of the ultrasonic technique, the results have been compared to those of another experimental technique (pressure-sensitive film) and numerical findings obtained by a finite element model of the experimental setup.

Ultrasonic Measurements of Contact Area and Pressure Distribution of a Pneumatic Tire on a Rigid Surface3

Abstract Contact phenomena which occur at the tire-ground interface play a crucial role in most issues related to optimal performances of the vehicle, safety, comfort, and energy consumption. Thus, it is essential to have available experimental tools capable of supplying detailed information about the main contact parameters (size and shape of nominal contact area and contact pressure distribution), especially when unknown or unpredictable external conditions make it difficult to use numerical tools in assessing them. Although a number of laboratory techniques have been devised to address this problem, here we propose a novel approach that exploits the property of ultrasonic waves to be differently reflected by a contact interface depending on its stress state. This noninvasive method is capable of supplying in real-time detailed maps of contact conditions as well as quantitative information with regard to geometric features of the contact area and contact pressure distribution values after suitable postp...

A posteriori compensation of the systematic error due to polynomial interpolation in digital image correlation

Abstract. It is well known that displacement components estimated using digital image correlation are affected by a systematic error due to the polynomial interpolation required by the numerical algorithm. The magnitude of bias depends on the characteristics of the speckle pattern (i.e., the frequency content of the image), on the fractional part of displacements and on the type of polynomial used for intensity interpolation. In literature, B-Spline polynomials are pointed out as being able to introduce the smaller errors, whereas bilinear and cubic interpolants generally give the worst results. However, the small bias of B-Spline polynomials is partially counterbalanced by a somewhat larger execution time. We will try to improve the accuracy of lower order polynomials by a posteriori correcting their results so as to obtain a faster and more accurate analysis.

Design for Robustness and Cost Effectiveness: The Case of an Optical Profilometer

The paper presents the design of an optical profilometer, a device used for the reconstruction of the micro‐geometry of mechanical parts in applications where high precision is needed. The design is based on Robust Design, a major methodology for quality improvement of engineering systems. Several design solutions, namely different hardware and software setups of the device, are compared in order to select a configuration realising a desired trade‐off between performance and cost. The peculiarity of the dsgin strategy is the use of a computer m odel of the measurement process where the physical part of the process is simulated. This allows for an extensive exploration of the design space, thus opening the way to product innovation.

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.

Combining Hole-Drilling and Ring-Core Techniques

The hole-drilling and ring-core techniques are two well-known approaches to residual stress measurement. They apparently are complementary: in the former, a small hole is drilled in the specimen and residual stresses are estimated from the displacement/strain field around the hole; in the latter a small ring is milled and residual stresses are computed from the displacement/strain field inside the ring. However, owing to the different stiffness of the constraining region, their sensitivity and depth range are somewhat different.

A Low-Cost Residual Stress Measuring Instrument

The hole-drilling method is the approach most used for residual stress measurement and most of the commercially available instruments are based on this working principle. The idea is quite simple: drill a small hole on the surface of the components and measure the strain/displacement components resulting on the surface. The stress components can be estimated from these data with a reverse calibration process. The measurement of the displacement/strain field is usually performed using either strain gauges or interferometric optical methods, thus the cost of the instruments is significant.

Sensitivity Analysis of i-DIC Approach for Residual Stress Measurement in Orthotropic Materials

Measuring residual stress in orthotropic materials is significantly more difficult than in isotropic materials. Indeed, in addition to the usual technical problems, several new points have to be taken into account, e.g. the accurate identification of material principal directions and the thickness of the laminae, (assuming we are dealing with composite materials).