Shearography is an optical measurement technique based on the interference between two laterally displaced images of the region of interest. Due to its quasi-equal-path configuration, shearography is robust enough for applications outside the laboratory. In fact, shearography has been successfully used for nondestructive testing in composite materials mainly in the aerospace, boating and, more recently, in the oil and gas industries. There are several demands in the oil and gas industry for nondestructive testing on subsea metallic elements coated with composite materials to prevent or mitigate the effects of corrosion. The present work deals with the underwater shearography for nondestructive testing in metal parts coated with composite materials. The paper begins with considerations about the propagation of light in underwater environments and a discussion on the elements that disturb the measurements with shearography such as changes in wavelength, absorption, dispersion, distortions caused by refraction in the air-glass-water interfaces among others. The different loading possibilities to reveal defects in the tested elements are analyzed: by pressure loading, thermal loading by radiation and thermal loading by induction. Exploratory experiments carried out inside water tanks involving the three loading techniques are presented and discussed. The results are compared and the combinations that led to the best results are emphasized. Finally, future works that involve a fully immersed system for underwater shearogaphy are briefly announced.
Speckle shear interferometry, or shearography, has been more and more frequently used in the aerospace and oil and gas industry for in-field nondestructive inspections of flaws in composite materials. Nowadays new applications have emerged demanding the ability to operate in harsher environments, requiring more robust systems to meet this type of application. A recent modified shearography device allows multiple and simultaneous measurements with different shearing directions on a single grabbed image. This work proposes a robust integration algorithm by error minimization to obtain full-field displacement measurement. Simulated images are used to validate the effectiveness of the integration algorithm. Further, experiments are performed on a clamped circular plate with uniform loading. The proposed algorithm leads to a more accurate estimate of defect size measurement in composite materials.
Pipelines usually transport hydrocarbons in the oil and gas industry. Small amounts of salt water may be present, which can cause corrosion from the inside to outside. Severe corrosion may produce through holes in the pipe wall. External protective layers of fiber reinforced plastics has been applied to postpone the need to stop production to perform a definitive repair, but do not stop the internal corrosion. It is very important to monitor the dimensions of through holes hidden by the protective layers of composite materials, which cannot exceed critical size and compromise pipe safety. Classical methods, such as ultrasound, do not give very reliable answers when composite materials and steel are combined. There is a great demand for a reliable and practical in field non-destructive inspection method. This work brings a hybrid and reliable solution, which combines a new configuration of a portable one-shot shearography system with finite element methods, resulting in a portable and easy to apply solution. The paper describes the principle of the portable one-shot shearography system that is able of simultaneously measuring in three shearing directions[1]. Considerations and modeling of finite element to determine the response of the composite material to the variation of the internal pressure of the duct are discussed. The report details how the experimental data and numerical model results are combined through an iterative process to determine the diameter of the hidden hole. Instead of integrating the experimental results of shearography, the authors differentiate the results of finite element in the same directions as the one-shot shearography system does simultaneously. The minimum of the square error results in an estimation of the hidden hole diameter. Experiments were performed with 150 mm diameter tubes with 20 to 50 mm diameter holes hidden by 6 to 24 mm thick glass fiber reinforced plastic protective layers. Smaller thickness and bigger diameters led to better results. Mean deviations of the order of 10% were found for the whole set of tests. Although the authors consider it is possible to improve the results, deviations of the order of up to 10% are very acceptable results and represent a significant improvement over the classic methods used today. The robustness of the optical system, the ease of use of the algorithm and the level of uncertainty already achieved make the authors believe that the techniques developed here achieve the requirements and will be very useful for the oil and gas industry demands.
Shearography is a quite robust optical measurement technique frequently used for in-field testing. Defect inspections with shearography on fiber-reinforced plastics parts, widely used in aircrafts, ships, chemical and oil and gas industries, are often performed outside of the lab. However, environmental disturbances can make this task difficult or even impossible. Mechanical vibrations, air and thermal instabilities are among the most disturbing agents in harsh environments that can destroy the interference signal or, at least, dramatically reduce the interference signal quality, what raises the measurement uncertainty to unacceptable levels. For a successful testing, it is crucial to neutralize the influence of those agents. Compactness, high stiffness, robust mechanical design and an effective clamping system are very important design considerations to minimize the influence of those agents. However, the most effective solution is using a single-shot measurement technique; namely, acquiring a single image instead of a series of images. The exposure time is kept short enough to “freeze” any relative motion between the parts or drift of the measurement signal. This paper presents, describes and explores two configurations of multiple aperture single-shot shearography used by the authors for testing fiber-reinforced plastics used in the oil and gas industry outside the lab. The first one is a two-aperture setup variation based in an already existing configuration that produces carrier fringes on the speckle pattern and allows fringe processing in the 2D Fourier plane. The second one is a new three-aperture configuration that allows acquiring three fringe patterns simultaneously form three different shearing directions for each given loading state. A set of three wedges are used in combination with the three apertures to produce simultaneous shearing interferograms in three different orientations. One single shot image is acquired using a single high-resolution camera before a loading is applied to the specimen and another single shot image for each different loading levels. Since the resulting fringe patterns have carrier fringes in different orientations they are easily separated in the Fourier plane. Once the device has no movable parts and a single shot image is acquired, it is very compact, robust and can be successfully used outside of the optical bench. The paper presents applications of both devices. They have a great potential to expand the use of shearography for testing in harsh environments.
Mechanical vibrations, air and thermal instabilities are among the most disturbing agents that make difficult or impossible a successful speckle interferometry measurement in harsh environments. Usually, they destroy the interference signal or, at least, dramatically reduce the interference signal quality, what raises the measurement uncertainty to unacceptable levels. Understanding the effects of disturbing agents on speckle interferometry is a first step towards finding strategies to perform effective measurements in harsh environments. There are three main strategies to successfully measure under unfavorable conditions: isolation, robustness and both. By isolation, we mean ways to avoid the action of the disturbing agents on the speckle interferometer and on the measurand. By robustness, we mean using a robust mechanical design, a robust optical technique, a robust data reduction algorithm and/or a robust configuration that is not much disturbed by mechanical, thermal or air instabilities. Compactness, high stiffness, robust mechanical design and an effective clamping system are very important considerations to minimize the influence of mechanical vibrations. One-shot measurement and averaging are also robust strategies to minimize the negative effects of mechanical vibrations as well as air and thermal instabilities. Protective enclosures are useful solutions for reducing air instabilities effects, but sometimes ineffective for achieving thermal stability outside the lab. Robust optical techniques are perhaps the most effective way to reduce the effects of thermal dilatation. The paper describes these concepts and discusses four speckle interferometry systems developed and successfully used by the authors in harsh environments: An achromatic speckle interferometer, using a diffractive optical element, was developed and has been applied to in-situ measure of residual stresses in pipelines. The second and third systems are compact and attachable shearography systems for in-field testing of the adhesion of joints of composite material pipes. Finally, the fourth system is a configuration of a shearography system using two apertures to produce carrier fringes for the measurement from a single image for each loading stage.
The increasing industry demand for specialized materials promotes a breakthrough in material engineering. In this context, composite materials have gained recognition and have been increasingly employed in the most diverse segments of the industry. When applied to equipment with structural requirements, it is fundamentally important to periodically inspect these materials to ensure their integrity and safety. Often inspections are performed in the field, under unstable conditions generated by operational and environmental factors. Shearography is a valuable method of nondestructive testing (NDT) for industrial applications. This optical technique is less sensitive to environmental disturbances when compared to other interferometric techniques. This paper presents a new shearography configuration based on Diffractive Optical Elements (DOE). In the proposed configuration, a diffraction grating is positioned between the camera sensor and the imaging lens. Therefore, a more compact and robust system is obtained. Another advantage of the proposed system is related to phase shifting, which is generated by the lateral movement of the diffraction grating. Thus, phase shifting is relative to the diffraction grating period and not the laser wavelength as in traditional interferometers. This feature makes the system insensitive to variations in laser wavelengths. Since the period of the diffraction grating used is about 60 times greater than the laser wavelength applied, the shearography system using diffraction grating becomes much more robust for external influences compared to other configurations based on the Michelson interferometer. This paper also presents the evaluation of the proposed shearography system as well as prospective steps.
This work presents the development of a special shearography system with radial sensitivity and explores its applicability
for detecting adhesion flaws on internal surfaces of joints of composite material pipes. The system uses two conical mirrors
to achieve radial sensitivity. A primary 45° conical mirror is responsible for promoting the inspection of the internal surface
all way around 360°. A special Michelson-like interferometer is formed replacing one of the plane mirrors by a conical
mirror. The image reflected by this conical mirror is shifted away from the image center in a radial way and a radial shear
is produced on the images. The concept was developed and tested. Two tubular steel specimens internally coated with
composite materials and having known artificial defects were analyzed to test the ability of the system to detect the flaws.
The system presented very good results on all inspected specimens. The experimental results obtained in this work are
promising and open a new front for inspections of inner surfaces of composite pipes with shearography.
Speckle shear interferometry, or shearography, has been more and more frequently used in the industry for in-field
nondestructive inspections of flaws in composite materials used in the aerospace and oil and gas industry. Nowadays
new applications has emerged demanding the ability to operate in harsher environments. Bringing interferometric
systems to harsh environments is not an easy task since they are very sensitive to many harsh environmental factors. Due
to the quasi-equal-path property, shearography is an intrinsically robust interferometric technique that has been
successfully used in the field, but there are still limits to overcome. Mechanical vibrations are probably the most
challenging factor to cope in the field measurements. This work presents a potentially robust shear interferometer
configuration. It uses a Wollaston prism as the shearing element rather than a traditional Michelson interferometer and
polarizers to achieve the phase shift. The use of the Wollaston prism makes the optical setup more compact and robust,
given that a rotating polarizer is the only movable part of the interferometer.
Piezoelectric translators (PZTs) are very often used as phase shifters in interferometry. However, they typically present a
non-linear behavior and strong hysteresis. The use of an additional resistive or capacitive sensor make possible to
linearize the response of the PZT by feedback control. This approach works well, but makes the device more complex
and expensive. A less expensive approach uses a non-linear calibration. In this paper, the authors used data from at least
five interferograms to form N-dimensional Lissajous figures to establish the actual relationship between the applied
voltages and the resulting phase shifts [1]. N-dimensional Lissajous figures are formed when N sinusoidal signals are
combined in an N-dimensional space, where one signal is assigned to each axis. It can be verified that the resulting Ndimensional
ellipsis lays in a 2D plane. By fitting an ellipsis equation to the resulting 2D ellipsis it is possible to
accurately compute the resulting phase value for each interferogram. In this paper, the relationship between the resulting
phase shift and the applied voltage is simultaneously established for a set of 12 increments by a fourth degree
polynomial. The results in speckle interferometry show that, after two or three interactions, the calibration error is
usually smaller than 1°.
Phase retrieval is a basic step for most interferometric techniques. Both spatial and temporal approaches are frequently
used. Temporal methods require a sequence of images with very well know phase shifting increments to produce
accurate results. Sometimes environmental disturbances can add random phase values that can result in a sequence of
images with virtually unknown phase shift increments. This paper presents and evaluates an approach to retrieve phase
values from a sequence of five or more phase shifted images by unknown quantities. The phase shifting increments are
determined from Lissajous ellipsis. This paper introduces the use of N-dimensional Lissajous figures to determine phase
shifting increments. The use of additional dimensions makes the phase shifting increment determination more robust and
less dependent of the pixel choices. The mathematical background is detailed and discussed. The paper presents and
evaluates simulations and real world examples using fringe projection and speckle interferometry.
Shearography is an optical and nondestructive technique that has been largely used for damage detection in layered
composite materials where delaminations and debondings are found among the most common flaws. Shearography is a
relative measurement on which two images are recorded for different loading conditions of the sample. The applied
loading induces some deformations into the sample generating a displacement field on its surface. The absolute
difference between two phase maps recorded at two different loading instances produces an interference fringe pattern
which is directly correlated to the displacements produced on the material surface. In some cases, depending on the
loading level and mainly on the sample geometry, interference patterns will contain fringes resulting from geometry
changes. This will mask those fringes correlated to flaws presented into the material, resulting in an image
misinterpretation. This phenomenon takes place mainly when the sample has curved geometries, as for example pipe or
vessel surfaces. This paper presents an algorithm which uses a mathematical processing to improve the visualization of
flaws in shearographic images. The mathematical processing is based on divergent calculation. This algorithm highlights
defected regions and eliminates fringes caused by geometry changes, providing an easier interpretation for complex
shearographic images. This paper also shows the principle and the algorithm used for the processing. Results, advantages and difficulties of the method are presented and discussed by using simulated fringe maps as well as real ones.
Shearography is an optical and nondestructive technique that has been used for damage detection in layered composite
materials. In order to facilitate and speed the process of marking of defects during a shearographic inspection of large
structures of composite materials, a multimedia projector has been employed. The phase map of the geometry measured
by fringe projection is used to mapping the coordinates of the shearographic image acquired during the inspection
process with their respective pairs in the projection image. Animations of the shearographic result can be projected on
the real structure during the inspection, facilitating the identification and marking of defects by the inspector. This paper
shows the principle and algorithms used for the projection of detected defects.
This paper describes an optical device that uses a new configuration of a technique known as deflectometry
applied to ballistic identification. The main novelty is characterized by the use of a 45° conical mirror to measure the
near cylindrical surface of the bullet. deflectometry is an optical technique sensitive to variations in topography and
unevenness of a surface. This technique allows to identify and to measure the geometry of objects based on the
distortions observed in a sequence of image patterns reflected on the surface of interest. The measurement by
deflectometry is very sensitive to the surface local gradients and curvatures. In this paper it is applied to forensic ballistic in order to verify if a given bullet could be fired by a suspect weapon. Comparisons between images of bullets fired
by the same weapon were made.
Composite materials tubes are being used in various industrial segments, including the oil and gas industry1. The union
between adjacent composite tubes is often accomplished through adhesives, and thus the inspection for flaws in
adhesive-bonded joints becomes crucial. In this context, tubes and elbows made of epoxy resin reinforced with glass
fiber were assembled with adhesive in the Quick-Lock® 1 configuration forming loops (spools). During the assemblage of
these loops, artificial defects (areas without adhesive or disbondings) were inserted in its joints to evaluate the capability
of failure detection by shearography. This paper presents and discusses results obtained with shearography.
Shearography demonstrated great potential for application in the adhesive-bonded joints inspection, as it detected all
defects artificially inserted and also real defects present in the loops joints.
This paper presents a new optical system to measure internal cylindrical surfaces combining photogrammetry and fringe
projection. The device uses two identical cameras, equipped with spherical and conical lenses, facing each other and
aligned with the optical axis. A 360° helical fringe projector is used to project a sequence of phase shifted helical fringe
pattern in the inner surface to be measured. The phase patterns are used to identify corresponding points and to
reconstruct the surface in a regular cylindrical mesh using an alternative approach. A prototype was built, calibrated and
tested. The paper presents the results of an application where two welded joints were measured in a 150 mm (6")
diameter pipe. The goal was to inspect for defects in the internal part of the welding seams and to measure the transversal
misalignment between the jointed parts.
This paper presents an endoscopic optical system to measure 3D shapes of inner cylindrical surfaces that combines
photogrammetry and fringe projection. The device has two identical cameras aligned with the optical axis and facing
each other, two conical lenses and one 360° helical fringe projector. The helical fringe pattern is projected in the inner
surface to be measured, phase shifted and acquired by both cameras. The phase patterns are used to identify
corresponding points and to reconstruct the surface in a regular cylindrical mesh using an alternative approach. A
prototype was built, calibrated and tested. The paper presents the results and an application to inspect internal welding
seams in 150 mm (6") diameter pipelines.
KEYWORDS: Cameras, Clouds, Calibration, 3D metrology, 3D acquisition, 3D image processing, Image processing, Fringe analysis, Optical spheres, 3D modeling
This paper presents a very simple and effective procedure to combine data from two cameras, and different positions, to
produce clouds of points in regular meshes. The main idea starts by setting two independent coordinates for a node from
a regular mesh. The third coordinate is found by scanning the dependent coordinate across the measurement volume until
the phase values of the fringe patterns, acquired by the cameras, reach the same common value. This approach naturally
produces structured clouds of points independently of the number of cameras used. To measure large or complex
volumes, some marks are distributed by the geometry. Two geometry parts with common marks are measured in
different positions and stitched. Many parts can be measured and stitched, until the complete measurement of geometry.
The final result is a regular mesh of the cloud of points.
This paper presents an algorithm based on Least Absolute Method to align and stitch multiple adjacent cylindrical clouds
of points measured by white light interferometry using conical mirrors. The evaluation of the aligning and stitching
algorithm was initially performed by using several numerically simulated clouds of points (COP) of cylindrical surfaces
with small shape errors and quite rough surfaces. In order to evaluate the algorithm, each numerically generated COP
was split into two parts but always keeping an overlapping area. Numerical translations and rotations were applied in one
part to simulate real misalignments. After this, the algorithm was applied to align each adjacent COP pair and to obtain a
stitched COP, and the result was compared with the original one. In this way, the performance of the presented algorithm
was evaluated and analyzed for several overlapped areas. Excellent results were obtained with an overlapping area of
25% of the total measured length. The differences between the stitched and original cloud of points were always far
below the roughness level of the measured surface. A brief description of a modified white light interferometer to
measure in cylindrical coordinates as well as early applications of the algorithm in real measurements is also presented.
Fringe projection has been widely used for 3D geometry measurement in several classes of applications. The basic
system is formed by a fringe projector and a camera. A triangulation algorithm is frequently used for retrieving 3D
information from a scene. Alternatively, two cameras can be used in combination with one fringe projector. This
configuration produces a significant measurement uncertainty improvement since only phase information encoded in the
fringe pattern is used to locate homologue points in the triangulation algorithm and lack of linearity or imperfections of
the fringe projector does not induce measurement errors. However, some parts with complex geometry can not easily
been seen from both cameras in a convenient angle, what limits the applicability of this configuration. Frequently the
clouds of points acquired from such systems are non-structured and, consequently, a non-regular mesh is obtained. This
paper presents a very simple and effective procedure to combine data from multiple cameras to produce clouds of points
in a regular mesh. The main idea starts by setting two independent coordinates for a node of a regular mesh. The third
coordinate is found by scanning the dependent coordinate across the measurement volume until the phase values of the
fringe patterns, acquired for the multiple cameras, reach the same common value. That approach naturally produces
structured clouds of points independently of the number of cameras used. As an example, a 3D shape is acquired by an
ordinary multimedia projector and a set of four low cost webcams. A calibration is necessary to reference the four
webcams into the same coordinate system. For that, a reference object, composed by a set of small spheres in calibrated
positions, is used.
Shearography is an optical and nondestructive technique that has been largely used for damage detection in layered composite materials where delaminations and debondings are the most common flaws. In Shearography the sample under test is illuminated using a laser and imaged on a CCD camera. A special optical shearing element allows a coherent superposition of two laterally displaced images of the surface of the sample on the CCD plane. The images are recorded for different loading conditions of the sample. The loading should induce some deformation or alter the deformation state of the surface of the sample. In this work a thermal loading has been used. The absolute difference of two phase maps recorded at different loading situations of the sample results in an interference fringe pattern which is directly correlated to the difference in deformation state. The phase maps usually have strong noise and low contrast. For this reason, a further improvement in image quality should be obtained. Two methods, known as "difference of phases" and "phase of differences", are used with a spatial and temporal phase unwrapping algorithms, respectively, in an experiment using a steel pipe wrapped in a composite sleeve. A qualitative comparison of the methods is done and the results, advantages and difficulties are discussed.
The authors introduced in 2002 a different approach to record a digital complex hologram. A phase-shifting shearing interferometer is used in such a way that no explicit reference wave is required. Twelve different images are acquired and three phase difference patterns are calculated: (a) the phase difference between the two wave fronts with no shearing, (b) the phase difference between two neighbor pixels in the x direction and (c) the phase difference between two neighbor pixels in the y direction. This information is combined to compute the amplitude and phase components of each pixel of the digital complex hologram. A plane wave is used to remove imperfections of the optical components and misalignment errors. In this mean time a new phase propagation algorithm was developed and some advances were achieved in the experimental side. The quality of the complex digital hologram was dramatically improved as well as the reconstructed image. This paper presents the recent progress and results and analyses some possibilities to apply this technique in non-isolated environment.
With the development of high-resolution CCD cameras, digital holography was made possible and has been used in laser metrology1. A reference wave interferes with the object wave and an amplitude hologram is formed and digitally recorded on the high-resolution camera CCD. The object intensity and phase information is numerically reconstructed. In this work a different approach is introduced. A digital complex hologram of the object wave is determined without an explicit reference wave. In order to do that, a shearing device is introduced in front of the CCD of a high-resolution camera. A phase shifting device is also used to change the relative phase between each shearing pair. Twelve different images are acquired: (a) four 90 degree(s) phase-shifted images without shearing, (b) four with shearing in the x direction and (c) four with shearing in the y direction. Those images are combined and three phase difference maps are calculated: (a) the phase difference between the two wave fronts without shearing, (b) the phase difference between two neighbor pixels in the x direction and (c) the phase difference between two neighbor pixels in the y direction. To compute the complex hologram, the amplitude and phase values for each pixel must be determined. An initial arbitrary phase value is assigned to a seed point of the image. The phase values of the next neighbor pixels are propagated using the available three phase difference maps and an appropriate algorithm. The digital complex hologram is used to reconstruct the object wave. The intensity and phase patterns are numerically computed in the object plane in a way similar to conventional digital holography. This work presents the mathematical models to compute the digital complex hologram and its numerical reconstruction. In addition, a very early application of digital complex holography to record and reconstruct a point source is presented.
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