1,721,142 research outputs found
On the Gyroscopic Stabilisation of a Single-Track Vehicle: Experimental Results
No full textThis paper analyses the experimental behaviour of a 2 degrees of freedom vehicle model stabilised by
a gyroscopic device. After a brief review of the system and the governing equations of motion, the
experimental device is presented in detail and its performance discussed. The response in the cases of
absence of control, constant external moment and sinusoidal external solicitations is examined,
showing the agreement between experimental behaviour and numerical simulation
Two-dimensional phase unwrapping by quad-tree decomposition
No full textOne 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 2 Pi. 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 jump͒ 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
Using Optical Interferometry to Restart the Ring-Core Method
No full textResidual stresses are a well-known technical problem because they add to the stress field induced by external loads, thus causing mechanical components to fail at a load level significantly lower than expected. Of the various techniques developed to measure them, the ring-core method is one of the few which in principle can be restarted (by removing the core and re-installing the strain gauge rosette). Thus, it is theoretically able to measure residual stress at significantly greater depth than other methods. Although the idea is interesting, its practical implementation is quite difficult: in particular, re-installing the rosette and re-wiring is almost impossible when depth becomes significant, thus the incremental measurement is more a theoretical possibility than a real experimental approach. In this work we propose to replace the strain gauge rosette with an optical (interferometric) technique. In this way the incremental approach becomes viable, although, depending on the optical technique used, some practical problems have to be addressed
Full field methods and residual stress analysis in orthotropic material. II: Nonlinear approach
No full textThis paper is the second part of a work that analyses the problem of residual stress determination in an orthotropic material by the hole drilling technique combined with full field optical methods. Due to the complex behaviour of the mate- rial, two theoretical formulations capable of describing the displacement field in an infinite plate in tension with a hole have been proposed: a simplified one, which can treat only a subset of all orthotropic materials, and a general, much more com- plex one. In the first part of this work, it has been shown that by using the simplified formulation, it is possible to develop a linear least square approach to the residual stress identification problem capable of treating a large class of orthotropic materials. This second part shows that the same approach can be extended to the general case providing that a somewhat more complex, nonlinear algorithm, is acceptable
Residual Stress Measurement Using Hole Drilling and Integrated Digital Image Correlation Techniques
No full textThe effectiveness of optical (mostly interferometric) methods for the measurement of residual stresses is largely demonstrated in literature. Nevertheless, these techniques are still confined to optical laboratories due to their high sensitivity to vibrations which makes it very difficult to perform the measurement in an industrial environment.
Digital Image Correlation (DIC) has recently been proposed as a possible solution to this problem: this
non-interferometric technique is much less affected by vibrations, but its sensitivity is relatively low, thus negatively affecting the accuracy of results.
This work proposes to use a variant of Digital Image Correlation, known as Integrated DIC (iDIC), in combination with the hole drilling technique. Since iDIC directly incorporates in its formulation the displacement field related to hole drilling, it overcomes most of the problems of standard DIC; in this way it is possible to obtain accurate results without using interferometric techniques
On the Gyroscopic Stabilisation of a Single Track Vehicle: Free Control Response when Running Straight Ahead
No full textThis paper analyses the straight-ahead stability of a single track, free control vehicle, using a
gyroscopic rolling stabilisation device. Studying the eigensolutions of a 8 degrees of freedom model
(six used by the vehicle itself, one to model the driver and one for the stabilisation facility) makes clear
how the control system may be effective in damping the oscillations around the vehicle longitudinal
axis: at the price of a small perturbation of Yaw mode, the Cap Size mode becomes negligible, while
the Weave mode gets stable in the whole speed-field
Far-Field Boundary Conditions for Calculation of Hole-Drilling Residual Stress Calibration Coefficients
Full text linkThe Hole-Drilling method for residual stress measurement, both in its standard version based on strain gauge rosettes (ASTM E837-08e1 2008) and its derivative using optical methods for estimating the displacement field around the hole (Baldi (2005) J Eng Mater Technol 127(2):165–169; Schajer and Steinzig (2005) Exp Mech 45(6):526–532; Schajer (2010) Exp Mech 50(2):159–168), relies on numerical calibrated coefficients (A and B) to correlate the experimentally acquired strains (displacements) with residual stress components. To estimate the A and B coefficients, two FEM (Finite Element Method) computations are required, the former related to a hydrostatic stress state, the latter to a pure shear case. Both can be implemented using either a semi-analytical approach (i.e. an axis-symmetric mesh expanded in the tangential direction using a Fourier series) or a tri-dimensional mesh, usually exploiting the double symmetry of the problem. Whatever the approach selected, the definition of constraints to be applied to the outer boundary is critical because the hole-drilling method assumes an infinite plate, thus both the usual solutions—fully constrained or free boundaries—are unable to correctly describe the theoretical situation. In the following, the problem of correct simulation of the infinite domain will be discussed and two simple and effective solutions will be proposed
Residual Stress Analysis of Orthotropic Materials Using Integrated Digital Image Correlation
No full textMeasuring residual stress in an orthotropic material is a difficult task due to the complex behavior of the material. Recently, two different approaches based on Smith's simplified real value formulation and the general solution developed by Lekhnitskii have been proposed. Both solutions assume the measurement of the displacement field via interferometric optical methods and estimate stress values through solving an inverse problem. However, the high sensitivity to vibrations of interferometric techniques makes their use difficult outside optical laboratories; standard Digital Image Correlation could be used, but its low sensitivity and relatively high standard deviation of displacements severely affect the reliability of estimates.
In this work we propose to integrate the residual stress displacement functions related to orthotropic materials into the shape functions of Digital Image Correlation. This makes it possible overcome most of the problems related to low sensitivity and large standard deviation because a single large patch can be used for the measurement, thus providing an accurate and reliable algoritm for the measurement of residual stress
Phase unwrapping by region growing
No full textThe phase unwrapping problem consists in singling out an integer field whose values make the original
wrapped phase field continuous. Even if in principle the problem is very simple—a direct integration of
the wrapped phase field suffices—in the presence of noise and͞or undersampling, the solution is no longer
unique and the direct integration methods usually fail to find an acceptable solution. This work presents
what is to my knowledge a new unwrapping algorithm that attempts to find the solution by iteratively
merging and shifting the continuous areas until a single region is built or no further moves are possible.
Unlike the tile methods, the regions can have arbitrary shape and need not be single-connected so that,
by removing the predefined size and shape constraint, the algorithm is very robust. The greater freedom
of the regions’ shape makes their handling more problematic, so that certain implementation aspects,
critical to algorithm performance, are presented here. Some unwrapping examples are also presented
and memory requirements are discussed
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