F. Salazar

A procedure for calculating through laser speckle interferometry the elastic constants of isotropic materials

The displacement suffered by the points located on one face of a parallelepipedic specimen made of isotropic elastic material is studied, while subject to simple compression, which generates not only strain but also both a translational and a rotation effect. The displacement is measured by double-exposure speckle photography. The elastic constants of a material are computed by measuring the in-plane displacement of three points. The effect of the in-plane and out-of-plane displacements due to translation and rotation are analyzed. Extreme values for the tilt angle are estimated, as well as the maximum diameter of the read-out beam. The systematic uncertainty of the method is also studied. Both Youngs modulus and Poissons ratio are experimentally measured in aluminum.

Thermal expansion coefficient measurement by double-exposure speckle photography

Abstract Described is a method for the measurement of the lineal thermal expansion coefficient of a sample in the form of a plate of an isotropic material. The speckle of the sample is registered on a photographic plate before and after an increase of the temperature of the specimen. Then, two points or more on the photographic plate are illuminated by an unexpanded laser beam, and the analysis of the Young fringes produced by these points allows for the measurement of the translation, rotation and thermal expansion of the sample. The maximum diameter of the exploring beam is analyzed and experimentally checked.

Consumed Fatigue Life Assessment of Composite Material Structures by Optical Surface Roughness Inspection

A strong knowledge of the fatigue state of highly advanced carbon fiber reinforced polymer composite (CFRP) structures is essential to predict the residual life and optimize intervals of structural inspection, repairs, and/or replacements. Current techniques are based mostly in measurement of structural loads throughout the service life by electric strain gauge sensors. These sensors are affected by extreme environmental conditions and by fatigue loads in such a way that the sensors and their systems require exhaustive maintenance throughout system life.This work is focused on providing a new technique to evaluate the fatigue state of CFRP structures by means of evaluating the surface roughness variation due to fatigue damage. The surface roughness is a property that can be measured in the field by optical techniques such as speckle and could be a useful tool for structural health monitoring. The relation between surface roughness and fatigue life has been assessed on CFRP test specimens. A tensile fatigue load with an R=0.1 (T-T) and a maximum load of 60% of the material ultimate strength has been applied. The surface roughness of the specimens has been determined from the surface topography measured by a high precision confocal microscope. Results show that the surface roughness of the specimens increases with the accumulation of fatigue cycles in such a way that the roughness could be taken into account as a fatigue damage metrics for CFRP.

Surface Roughness Measurement on a Wing Aircraft by Speckle Correlation

The study of the damage of aeronautical materials is important because it may change the microscopic surface structure profiles. The modification of geometrical surface properties can cause small instabilities and then a displacement of the boundary layer. One of the irregularities we can often find is surface roughness. Due to an increase of roughness and other effects, there may be extra momentum losses in the boundary layer and a modification in the parasite drag. In this paper we present a speckle method for measuring the surface roughness on an actual unmanned aircraft wing. The results show an inhomogeneous roughness distribution on the wing, as expected according to the anisotropic influence of the winds over the entire wing geometry. A calculation of the uncertainty of the technique is given.

Double-exposure speckle photography with three slabs for thermal expansion tensor measurement

Three mutually perpendicular slabs are cut from a block of material which is anisotropic from a thermal expansion standpoint. The three slabs are placed in a thermostatic bath and illuminated with HeNe laser light, and their speckle is registered. The set of slabs is subjected to a temperature increase, and a second speckle on the same photographic plate is exposed. Observation of the Young fringes at three points of each slab with a narrow laser beam provides ample possibilities for calculating the materials thermal expansion tensor components. The method provides the tensor calculation with one photographic plate only.

Optical Sensing of the Fatigue Damage State of CFRP under Realistic Aeronautical Load Sequences

We present an optical sensing methodology to estimate the fatigue damage state of structures made of carbon fiber reinforced polymer (CFRP), by measuring variations on the surface roughness. Variable amplitude loads (VAL), which represent realistic loads during aeronautical missions of fighter aircraft (FALSTAFF) have been applied to coupons until failure. Stiffness degradation and surface roughness variations have been measured during the life of the coupons obtaining a Pearson correlation of 0.75 between both variables. The data were compared with a previous study for Constant Amplitude Load (CAL) obtaining similar results. Conclusions suggest that the surface roughness measured in strategic zones is a useful technique for structural health monitoring of CFRP structures, and that it is independent of the type of load applied. Surface roughness can be measured in the field by optical techniques such as speckle, confocal perfilometers and interferometry, among others.

Surface irregularity factor as a parameter to evaluate the fatigue damage state of CFRP

This work presents an optical non-contact technique to evaluate the fatigue damage state of CFRP structures measuring the irregularity factor of the surface. This factor includes information about surface topology and can be measured easily on field, by techniques such as optical perfilometers. The surface irregularity factor has been correlated with stiffness degradation, which is a well-accepted parameter for the evaluation of the fatigue damage state of composite materials. Constant amplitude fatigue loads (CAL) and realistic variable amplitude loads (VAL), representative of real in- flight conditions, have been applied to “dog bone” shaped tensile specimens. It has been shown that the measurement of the surface irregularity parameters can be applied to evaluate the damage state of a structure, and that it is independent of the type of fatigue load that has caused the damage. As a result, this measurement technique is applicable for a wide range of inspections of composite material structures, from pressurized tanks with constant amplitude loads, to variable amplitude loaded aeronautical structures such as wings and empennages, up to automotive and other industrial applications.

Simulation of Rough Surfaces and Analysis of Roughness by MATLAB

The simulation of physical phenomena in science and engineering has become an important tool because it allows studying a wide range of real problems. On the other hand, it allows resolving problems that, because of its difficulty, it would be not possible to solve by analytical methods. Moreover, simulation is fast and versatile since it permits to vary parameters of the problem easily, allowing analyzing the effect of the modification of them in the response of the system examined. Simulation requires programming, for which there are many different languages. Each of them has a particular internal structure that distinguishes it from others. Therefore, depending on the problem to be study, it may be advisable to use a specific programming language. In the scientific-technical context MATLAB has been increasingly used by the great advantages that it offers. For example, the instructions are interpreted and not compiled, the user to enter commands interactively. The data processing is flexible. They can be read and stored in two different formats, ASCII and MATLAB format. ASCII has the advantage that the data and results may be used for other programs. However, MATLAB format may be faster. On the other hand, many functions and libraries of MATLAB are written MATLAB language, enabling the user access to the source files. It is possible to execute instructions of the operating system without exiting the program. Moreover, this language is portable in platforms as Windows or Apple, commonly employed by the researcher. From the point of view of numerical calculation, the use of matrices as basic elements makes it efficient and easy to employ, being also possible to perform graphics of curves and surfaces. Finally, the operations can be performed with simple and intuitive expressions similar to those used in science and engineering. MATLAB has been used for many applications in general physics, mathematics, optics, electronics, chemistry, biology, medicine, and artificial intelligence, among others. Now we want to employ MATLAB to simulate an optical procedure to measure surface roughness. Thus, the aim of this paper is the determination of the roughness of a surface from the analysis of the speckle pattern obtained in the far field, when the object is illuminated with a monochromatic beam perpendicularly to its surface.

Maximum diameter of the read-out laser beam in double-exposure speckle photography

A simple model is proposed for a speckle pattern produced by a diffuse surface and intercepted by the plane of a photographic plate. In this model, all the spots have the same form but are randomly distributed. During the translation, rotation, and strain of the diffuse surface, it is assumed that the shape of each spot remains the same. The transmittance of the processed plate that would be produced when exposed before and after the displacement of the diffuse surface, is calculated. An equation is obtained for the intensity of the diffracted light when the plate is illuminated with a circular interrogation beam. The lower limit that the read-out laser beam diameter should have so that the Young fringes are visible is calculated and a criterion is proposed. The theory is checked with laboratory experiments.

Magnetic field homogeneity of a conical coaxial coil pair

An analytical study of the magnetic field created by a double-conical conducting sheet is presented. The analysis is based on the expansion of the magnetic field in terms of Legendre polynomials. It is demonstrated analytically that the angle of the conical surface that produces a nearly homogeneous magnetic field coincides with that of a pair of loops that fulfills the Helmholtz condition. From the results obtained, we propose an electric circuit formed by pairs of isolated conducting loops tightly wound around a pair of conical surfaces, calculating numerically the magnetic field produced by this system and its heterogeneity. An experimental setup of the proposed circuit was constructed and its magnetic field was measured. The results were compared with those obtained by numerical calculation, finding a good agreement. The numerical results demonstrate a significant improvement in homogeneity in the field of the proposed pair of conical coils compared with that achieved with a simple pair of Helmholtz loops or with a double solenoid. Moreover, a new design of a double pair of conical coils based on Braunbeks four loops is also proposed to achieve greater homogeneity. Regarding homogeneity, the rating of the analyzed configurations from best to worst is as follows: (1) double pair of conical coils, (2) pair of conical coils, (3) Braunbeks four loops, (4) Helmholtz pair, and (5) solenoid pair.