The accurate determination of the contact area between an instrumented indenter and the material under testing is important for material property measurements. Usually, it is masked by the pile-up or sink-in phenomenon. Due to the wide range of materials available in the industry, different amounts of pile-up would be generated depending on the grade of fragility of the material. This paper presents the sensitivity evaluation of dual-wavelength image-plane digital holography in order to identify these different degrees of pile-up. It was observed that the interferometer was able to identify pile-up with a height of 3μm. Additionally, this result showed a good concordance with the measurement of the same part by using a Focus-Variation Microscope (FVM). Considering the application for in-situ measurements, corrections to deal with the misalignment between the interferometer and the evaluated surface was proposed and tested. Results showed that the setup is able to work with misalignments, which will produce up to 13 fringes in the measured phase.
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.
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°.
The authors developed an achromatic speckle pattern interferometer able to measure in-plane displacements in polar coordinates. It has been used to measure combined stresses resulting from the superposition of mechanical loading and residual stresses. Relaxation methods have been applied to produce on the surface of the specimen a displacement field that can be used to determine the amount of combined stresses. Two relaxation methods are explored in this work: blind hole-drilling and indentation. The first one results from a blind hole drilled with a high-speed drilling unit in the area of interest. The measured displacement data is fitted in an appropriate model to quantify the stress level using an indirect approach based on a set of finite element coefficients. The second approach uses indentation, where a hard spherical tip is firmly pressed against the surface to be measured with a predetermined indentation load. A plastic flow occurs around the indentation mark producing a radial in-plane displacement field that is related to the amount of combined stresses. Also in this case, displacements are measured by the radial interferometer and used to determine the stresses by least square fitting it to a displacement field determined by calibration. Both approaches are used to quantify the amount of bending stresses and moment in eight sections of a 12 m long 200 mm diameter steel pipe submitted to a known transverse loading. Reference values of bending stresses are also determined by strain gauges. The comparison between the four results is discussed in the paper.
This paper presents a modular device based on digital speckle pattern interferometry (DSPI) combined with an instrumented indenter. The system is divided in two modules, the interferometric and the indentation module. The former uses a diffractive optical element (DOE) to obtain radial in-plane sensitivity. This module measures the whole shallow displacement field generated by the indentation print on the surface of the material under testing. The latter module is sized suitably with the interferometric module. The indentation module uses a mechanical/hydraulic scheme to provide the system a high loading capability. A piezoelectric loading cell and an inductive transducer are used to simultaneously measure the load applied on the ball indenter and its penetration on the material. For the experimental tests, a bench capable to apply in a specific pipe a very well-known bending moment was used. This bench is mounted with two 12- meters pipes disposed horizontally. A transverse load is applied in the central cross-section of both pipes. The load application is made by a hydraulic actuator and measured with a load cell. Strain-gages are also used in a half-bridge configuration to measure the strain in the 8 cross-sections distributed along the pipe length. Each cross-section was measured by the proposed instrumented indentation system and compared with the strain-gages and load cell measurements. The results obtained show an uncertainty level around 20-30% of the measured bending stress. Good agreement was found between the computed bending stress using the strain-gages, load cell and the proposed method using the instrumented indentation system.
Optical techniques are usually related to laboratory rooms, which are places equipped with temperature, humidity and vibration control. These techniques are very suitable for fast measurements due to their non-contact nature and full-field capability. Among them, optical methods based on the speckle phenomenon have had a great development during the last two decades because of the development of digital image processing, cameras, computers, lasers and optical components. Speckle techniques have the advantages cited for optical methods. Additionally, they are adequate for the evaluation of real components without further preparation of the surface and without high time consuming to be analyzed. This paper supplies tools, tips and reference parameters to develop interferometers based on the speckle phenomenon to be used outside the laboratory room. Finally, applications outside the lab for the measurement of mechanical and residual stresses are presented. These examples show the high potential of speckle metrology as an auxiliary tool for structure integrity assessment.
Optical techniques are usually applied inside laboratories equipped with temperature, humidity and vibration control. These techniques are very suitable for fast measurements due to their noncontact nature and their capability to measure on surfaces without special, time-consuming preparation. Among them, optical methods based on the speckle phenomenon have developed substantially over the last two decades due to the development of digital image processing, digital cameras, computers, lasers, and optical components.
However, applying speckle methods outside of the laboratory becomes a challenging task. This book presents techniques and tools that will enable the development of robust measurement instruments to be used outside the laboratory for nondestructive structural-integrity-evaluation devices. Additionally, several technical solutions that combine mechanical systems to solve industrial measurement demands are described.
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.
This paper shows a portable device to measure mainly residual stress fields outside the optical bench. This system
combines the traditional hole drilling technique with Digital Speckle Pattern Interferometry. The novel feature of this
device is the high degree of compaction since only one base supports simultaneously the measurement module and the
hole-drilling device. The portable device allows the measurement of non-uniform residual stresses in accordance with the
ASTM standard. In oil and gas offshore industries, alternative welding procedures among them, the friction hydro pillar
processing (FHPP) is highlighted and nowadays is an important maintenance tool since it has the capability to produce
structure repairs without risk of explosions. In this process a hole is drilled and filled with a consumable rod of the same
material. The rod, which could be cylindrical or conical, is rotated and pressed against the hole, leading to frictional
heating. In order to assess features about the residual stress distribution generated by the weld into the rod as well as into
the base material around the rod, welded samples were evaluated by neutron diffraction and by the hole drilling
technique having a comparison between them. For the hole drilling technique some layers were removed by using
electrical discharge machining (EDM) after diffraction measurements in order to assess the bulk stress distribution.
Results have shown a good agreement between techniques.
A portable device to essentially measure residual stress fields outside an optical bench is presented. This system combines the hole-drilling technique with digital speckle pattern interferometry. A novel feature of this device is its high degree of compaction since only one base supports simultaneously the measurement module and the hole-drilling device. A new version of the American society for testing and materials standard E837 for the measurement of residual stresses has been improved including a computation method for nonuniform residual stresses. According to this standard, a hole with a maximum depth of 1.0 mm should be introduced into the material to assess the stress distribution along the hole’s depth. The discretization of the stress distribution is performed in 20 equal steps of 0.05 mm, getting the deformations generated for stress relief in every drilling step. A description of the compact device showing the solution for a fast and easy interchanging process between modules is also presented. The proposed system was compared with a traditional method using strain gages, and a good agreement was shown between stress distributions measured with both methods. Finally, the portable device was used to evaluate the residual stress distribution in a sample with a rod welded by friction hydro pillar processing.
The authors develop a special configuration of a digital speckle pattern interferometer using a custom diffractive optical
element. The resulting interferometer is sensitive only to the in-plane radial displacement component. The area of
interest is double illuminated by light diffracted from a special diffractive optical element in such a way that the
interference fringe is not wavelength dependent. The authors present in this paper different strategies for improving
image quality of the fringe patterns exploring wavelength and coherence properties. The results of the different
approaches are presented and compared.
This paper presents a portable device to essentially measure residual stress fields outside the optical bench. This system
combines the hole drilling technique with Digital Speckle Pattern Interferometry (DSPI). The novel feature of this device
is its high degree of compaction since only one base supports simultaneously the measurement module and the holedrilling
device. A new version of the ASTM E837 standard for the measurement of residual stresses has been improved
including the computation method for non-uniform residual stresses. According to this standard, a hole with depth of
1.0 mm should be introduced into the material to assess the stress distribution. The discretization of the stress distribution
along the hole depth is performed in 20 equal steps of 0.05 mm, getting the deformations generated for stress relief in
every drilling step. A description of the compact device showing the solution for a fast and easy interchanging process
between modules is performed in this paper. The proposed system is used to evaluate the residual stress distribution into
a sample with a rod welded by friction hydro pillar processing (FHPP).
A measurement prototype based on digital holography for the simultaneous measurement of out-of-plane and radial in-plane
displacement fields is shown. This prototype enables recording two holograms at the same time with a single
image taken by a digital camera and evaluating separately in-plane and out-of-plane displacement components. An
axicon-type diffractive optical element (DOE) is used for the illumination of the object, which causes radial sensitivity
vectors. Blind holes as well as spherical indentations were preformed over a welded steel plate (containing residual
stresses). By using the digital holography setup, typical out-of-plane and in-plane displacement fields, generated when
the hole was introduced into the stresses material, were measured and compared with theoretical ones. Good agreement
was found between them. In addition, a mature digital speckle pattern interferometry (DSPI) setup was used to measure
only the in-plane component around the hole. Good agreement between both systems was also found. Finally,
displacements fields were measured around indentation marks. In this case, preliminary results show that out-of-plane
displacements are larger than in-plane ones, enabling its use for residual stress computation or maybe material properties
determination.
This paper presents a novel technique to investigate coating adhesion using a radial speckle interferometer and a
microindentation test. The proposed technique is based on the measurement of the radial in-plane displacement field
produced by a microindentation introduced on the coated surface of the specimen. The advantages and limitations of the
proposed technique are shown.
This paper presents the optical setup of a radial in-plane digital speckle pattern interferometer (DSPI) which uses an axissymmetrical
diffractive optical element to obtain double illumination. The application of the DOE gives true in-plane
sensitivity that is independent on the wavelength of the laser used as illumination source. Furthermore, it only depends
on the grating period of the DOE. A new optical layout was introduced in order to obtain a circular measurement area of
about 5 mm in diameter. A brief description of the DOE and the portable strain sensor are presented. A detailed
explanation of the clamping system is presented showing its ability to deal with rigid body displacements. Finally, some
experimental results are shown enlightening that it is able to measure mechanical stress fields from only one difference
phase map.
We present a measurement system based on digital holography and two light sources, which is suitable for high speed
three-dimensional deformation measurements. Circular in-plane sensitivity is achieved using a diffractive optical
element (DOE) for illumination in combination with two laser sources creating two holograms on one camera frame.
Both holograms are separated using two different spatial carrier frequencies. As the two lasers are illuminating the object
under a different angle it is possible to calculate in-plane and out-of-plane deformation out of two camera frames and the
resulting four holograms. The system may be applied e.g. for high speed deformation measurements or the measurement
of residual stress. Besides first measurement results obtained with the holographic device and a comparison to results of
an already existing measurement system based on digital speckle pattern interferometry (DSPI) are presented.
A digital speckle pattern (DSP) interferometer using a special diffractive optical element (DOE) was developed by the
authors. A collimated laser beam is diffracted by the DOE in such a way that the first diffraction orders produce a
circular double illuminated measurement area. Due to natural symmetry of the illumination scheme, the interferometer
reaches pure radial in-plane sensitivity. It is demonstrated and verified that the resulting interferometer is not sensitive to
laser wavelength variations at all. Its configuration is presented as well as its performance evaluation for residual stress
measurements using the blind hole-drilling method.
Digital speckle pattern interferometry is potentially capable to solve a large variety of measurement and inspection
demands in industrial applications. However, it is not so widely used in industry due to some special requirements that
are not easily fulfilled on the shop floor. This paper presents some reflections about what would be necessary for an
interferometer to be successfully applied in industrial environments: it must be robust, flexible, compact, stable, friendly
and cooperative. Next, a case study, that fulfills those requirements, is presented in details. It involves a digital speckle
interferometer designed to measure residual stresses in-field. It was developed using an axis-symmetrical diffractive
optical element in such a way that it is not sensitive to the laser wavelength at all. It produces radial in-plane sensitivity
on a circular region. The interferometer was accommodated in a compact construction that made it robust enough for infield
applications. A magnetic kinematic mounting base is used to firmly attach the interferometer to the surface where
residual stresses have to be measured. The same kinematic base is used for positioning an ultra-high speed pneumatic
drilling unit. In order to measure residual stresses, a reference phase pattern is first acquired from a sequence of four 90°
phase-shifted images. After that, a small blind hole is drilled on the surface to be measured. The residual stresses are then
relieved at the borders of the blind hole, what produces a local displacement filed. A second phase pattern is then
acquired. The radial displacement filed is computed from the phase difference and it is fitted to a mathematical model.
The principal residual stresses are then determined. The interferometer was used outside of the laboratory for residual
stresses measurement in a gas pipeline in a risky area. The goal was to investigate the effectiveness of a repair.
This paper presents a new configuration of a digital speckle pattern interferometer that uses a binary diffractive optical
element (DOE) to achieve radial in-plane sensitivity. The use of the DOE ensures constant sensitivity to the
interferometer since it only depends on the grating period and does not depend on the wavelength of the illumination
source. The paper describes the principles as well as the concepts of a portable device that was integrated to a drilling
module to apply the hole drilling method for residual stresses measurement. Comparative results showed that the
combined system can measure residual stress fields with uncertainty comparable with the classical strain gage based hole
drilling method, but at least four times faster. A practical application of residual stresses measurement outside the
laboratory is briefly presented.
The authors have been working on the development of a white light interferometer to measure both inner and external
cylindrical surfaces. High precision diamond turned conical mirrors were used to shape the light beam appropriately.
Collimated light from a Michelson-like white light interferometer was directed to the conical mirror. The reflected light
was radially directed to the cylindrical part to be measured. A telecentric lens system captured the image of the measured
cylindrical part reflected on the conical mirror surface. Scanning of the measured volume was performed in cylindrical
coordinates by linear displacement of a flat mirror in the reference arm of the interferometer. A cloud of points was
directly built in cylindrical coordinates. The present paper focus on two main topics of recent development: wear
measurement of cylinders and cylindrical part alignment. An appropriate mathematical model was developed to fit the
cylindrical cloud of points to the experimental data. The use of the least absolute method instead of the least squares
method for fitting the model is presented and discussed. This model is used to align two measured cloud of points to a
same reference coordinate system, one acquired before and the other after a wearing test. Direct comparison between
both clouds of points allowed computing the volume of the removed material by wearing. Results are presented and
discussed for both cases.
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.
This paper introduces a new design of a white light interferometer, suitable for measurement of inner cylindrical or quasi-cylindrical parts. A high precision 45° conical mirror is used to direct collimated light in the radial direction to the surface to be measured. The image of the measured surface, distorted by the conical mirror, is formed in the sensor plane of a digital camera. A mapping algorithm is used to reconstruct the cylindrical geometry from the distorted image. The remaining of the interferometer is quite similar to a conventional white light interferometer, where a flat reference mirror is scanned through the measurement range while an algorithm is searching for the maximum contrast position of the interference pattern. The alignment and calibration of the interferometer using a reference master piece are described and discussed. The performance evaluation of the interferometer as well as practical applications are also presented and discussed. This new configuration makes possible to measure inner surfaces in true cylindrical coordinates.
We present the application of a portable radial in-plane speckle interferometer to the measurement of residual stresses when local heating is used as a stress relaxation technique. We also present a numerical method to calculate the residual stresses from the radial in-plane displacement field that is measured subsequent to the heating and cooling of the interrogated spot. A finite element analysis of the thermomechanical response of the specimen under test is performed to adjust the model used for residual stress determination. To evaluate the performance of this technique, results obtained using several preloaded specimens are compared with the stress introduced in them. It is shown that the speckle interferometer and local heating combined system allows the measurement of residual stresses with an average deviation of approximately 7% for a range of uniaxial stresses from 25 up to 75% of the yield stress of the material.
This paper presents an application of a radial in-plane digital speckle pattern interferometer and a local heating combined system to measure residual stresses in ductile materials. The system allows the local heating of a small spot and the measurement of the radial in-plane displacement field generated around it. In order to quantify
the residual stresses from the measured displacement field, the thermomechancial problem is simulated using the finite element method and a modified hole-drilling model is applied. The performance of the system is evaluated by measuring the stress generated by a preloaded specimen. It is shown that the combined system can measure residual stresses with a typical error of approximately 5%.
This paper presents a combined system based on DSPI and local heating, which alows the evaluation of residual stresses. To determine the residual stress values, a finite element analysis (FEA) of the thermomechanical process is carried out. The feasibility of this approach is demonstrated by comparing the results obtained from measurements in a preloaded specimen and the stress values introduced in it.
This paper presents a temporal speckle pattern interferometry
(TSPI) system which uses thermal waves to detect internal defects.
The system allows the measurement of out-of-plane thermoelastic
displacements generated when a specimen is locally heated with a
temporally modulated CO2 laser. The defects can be detected by observing the perturbations which appear in the induced
displacement field. Displacements are determined from the
calculation of the optical phase distribution using a temporal
phase shifting method and temporal phase unwrapping. The
description of the TSPI system is followed by the presentation of
experimental results that demonstrate that the detectability of
certain type of flaws is improved by the use of thermal waves.
This paper presents a combined system based on digital speckle pattern interferometry and laser annealing to determine residual stresses in ductile materials. The system allows the measurement of the radial in-plane displacement field generated when a specimen subjected to residual stresses is locally heated. This novel approach for measuring residual stresses has the advantage of being nondestructive and can be easily applied to in situ tests. The description of the combined system is followed by the presentation of preliminary results which illustrate the ability of both techniques to identify residual stress fields and also to determine their principal directions.
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