Digital Speckle Pattern Interferometer Research Articles

Investigation of the influence of broadband mechanical disturbances on the quality of recording interference patterns of GTE-wheel oscillations using a digital speckle pattern interferometer

Investigation of the influence of broadband mechanical disturbances on the quality of recording interference patterns of GTE-wheel oscillations using a digital speckle pattern interferometer

Study on creep characteristics of geogrid based on modelling and experimental method

The creep characteristics of geogrid have significant influence on the analysis of retaining wall's mechanical behaviour and deformation characteristics. This paper uses digital speckle pattern interferometer to observe the model test of a retaining wall with embedded geogrid, and analyses the stress and strain distribution of soil behind the retaining wall according to the observation data. The mechanical model of stress distribution of geogrid is established in this paper, and the creep model of geogrid is also established to simulate the time-related behaviour of geogrid. In order to prove the rationality and reliability of the model proposed in this paper, the PFC2D software platform is employed to verify the numerical model of the reinforced retaining wall, and the creep model of geogrid is included into the PFC2D software. The calculation results show that the displacement of the retaining wall can be significantly reduced because of the anchoring effect of the geogrid. The time-dependent displacement of the retaining wall can approximately meet the logarithmic relationship. Therefore, the creep model of geogrid proposed in this paper is effective and reliable, and the method proposed in this paper can provide valuable reference information for similar engineering practice.

Study on creep characteristics of geogrid based on modelling and experimental method

The creep characteristics of geogrid have significant influence on the analysis of retaining wall's mechanical behaviour and deformation characteristics. This paper uses digital speckle pattern interferometer to observe the model test of a retaining wall with embedded geogrid, and analyses the stress and strain distribution of soil behind the retaining wall according to the observation data. The mechanical model of stress distribution of geogrid is established in this paper, and the creep model of geogrid is also established to simulate the time-related behaviour of geogrid. In order to prove the rationality and reliability of the model proposed in this paper, the PFC2D software platform is employed to verify the numerical model of the reinforced retaining wall, and the creep model of geogrid is included into the PFC2D software. The calculation results show that the displacement of the retaining wall can be significantly reduced because of the anchoring effect of the geogrid. The time-dependent displacement of the retaining wall can approximately meet the logarithmic relationship. Therefore, the creep model of geogrid proposed in this paper is effective and reliable, and the method proposed in this paper can provide valuable reference information for similar engineering practice.

Experimental Evaluation of Residual Stresses in Pipes Manufactured by UOE and ERW Processes

Pipes are basic elements used in the construction of pipelines for the long-distance transportation of oil and gas and their derivatives. They can be manufactured by cold forming processes such as UOE and ERW, both widely used in the oil and gas industry. These processes produce high levels of non-uniform plastic deformation, which introduce a new state of residual stress into the material. In some cases, these stresses combine with mechanical stresses generated by external loads leading to service failures, interrupting the transmission line and increasing the risk of accidents. Therefore, determining in advance the residual stress distribution in pipes is an important task which involves the evaluation of the structural integrity. Six pipe samples obtained by the UOE and ERW processes were measured and evaluated using a portable optical device that combines radial in-plane digital speckle pattern interferometer (DSPI) with the incremental hole-drilling technique to measure residual stresses. The experimental results indicate a distinct residual stress distribution for each manufacturing process, while the measured residual stress distributions in the longitudinal and circumferential directions were similar at all measurement locations along an individual pipe.

Automatic Frames Extraction and Visualization From Noisy Fringe Sequences for Data Recovering in a Portable Digital Speckle Pattern Interferometer for NDI

We report on a method for recovering data from a simple portable Digital Speckle Pattern Interferometer, we used for monitoring structural behavior of a painting on wood, hanging on a wall, outside of laboratory conditions, without anti-vibration devices. In such a situation, fringes produced by the object displacements were affected by unpredictable distortions caused by environment vibrations. However, an sufficient number of suitable, i.e., undistorted or barely distorted, fringe patterns usable for processing was found. We performed multiple acquisitions at a frame rate as high as possible. The main task was picking out usable interferograms from large amount of frames. We developed a PC-based method based on jointly analysis of spectral content and fringe image sharpness as selection rules. The selected frames were utilized for off-line processing by using an approach based on Hilbert Transform and Phase Unwrapping via MAx-flow (PUMA) algorithm. We obtained a collection of displacement-maps, that allowed for evaluating the whole structure deformations, caused by environmental thermo-hygrometric fluctuations.

Radial in-plane digital speckle pattern interferometer combined with instrumented indentation

This paper presents a modular device based on digital speckle pattern interferometry (DSPI) which is combined with instrumented indentation. The interferometric module uses a diffractive optical element that confers radial in-plane sensitivity enabling the measurement of whole displacement field generated by the shallow indentation print on the surface of the material under testing. The indentation module uses a piezoelectric loading cell and an inductive transducer to simultaneously measure the loading applied on the ball indenter tip as well as its penetration on the material under testing. A mechanical/hydraulic scheme was developed to achieve a high loading capability with a compact indentation module, suitably sized with the interferometric module. A finite element simulation was carried out for a generic low carbon steel material without residual stresses and under a tensile external loading of 25%, 50% and 75% of its yielding stress. In the same way, a steel bar was experimentally indented by using the compact indenter module and the radial in-plane displacements around the indentation were measured with the measurement module. Good agreement was found between the simulated and measured displacement fields. In addition, the influence of the tensile load on the measured displacement fields was clearly observed by the measurement module.

Full-field optical deformation measurement in biomechanics: Digital speckle pattern interferometry and 3D digital image correlation applied to bird beaks

In this paper two easy-to-use optical setups for the validation of biomechanical finite element (FE) models are presented. First, we show an easy-to-build Michelson digital speckle pattern interferometer (DSPI) setup, yielding the out-of-plane displacement. We also introduce three-dimensional digital image correlation (3D-DIC), a stereo photogrammetric technique. Both techniques are non-contact and full field, but they differ in nature and have different magnitudes of sensitivity. In this paper we successfully apply both techniques to validate a multi-layered FE model of a small bird beak, a strong but very light biological composite. DSPI can measure very small deformations, with potentially high signal-to-noise ratios. Its high sensitivity, however, results in high stability requirements and makes it hard to use it outside an optical laboratory and on living samples. In addition, large loads have to be divided into small incremental load steps to avoid phase unwrapping errors and speckle de-correlation. 3D-DIC needs much larger displacements, but automatically yields the strains. It is more flexible, does not have stability requirements, and can easily be used as an optical strain gage.

A Robust Achromatic DSPI Interferometer for Measurement in Polar Coordinates

Abstract This paper presents a new concept of a digital speckle pattern interferometer with radial in-plane sensitivity. The interferometer uses a special diffractive optical element and a particular design that makes it non sensitive to laser wavelength. A compact design makes it robust enough to be used for in-field measurement. Applications of this interferometer to build robust devices for 2D displacement, mechanical stresses and residual stresses measurement are presented in this paper.

Mechanical stress measurement by an achromatic optical digital speckle pattern interferometry strain sensor with radial in-plane sensitivity: experimental comparison with electrical strain gauges

This paper shows the optical setup of a radial in-plane digital speckle pattern interferometer which uses an axis-symmetrical diffractive optical element (DOE) to obtain double illumination. The application of the DOE gives in-plane sensitivity which only depends on the grating period of the DOE instead of the wavelength of the laser used as illumination source. A compact optical layout was built in order to have a portable optical strain sensor with a circular measurement area of about 5 mm in diameter. In order to compare its performance with electrical strain sensors (strain gauges), mechanical loading was generated by a four-point bending device and simultaneously monitored by the optical strain sensor and by two-element strain gauge rosettes. Several mechanical stress levels were measured showing a good agreement between both sensors. Results showed that the optical sensor could measure applied mechanical strains with a mean uncertainty of about 5% and 4% for the maximum and minimum principal strains, respectively.

Evaluation of the piezoelectric behaviour produced by a thick-film transducer using digital speckle pattern interferometry

This paper presents an interferometric measurement of the out-of-plane deflections produced by a piezoelectric transducer, manufactured by thick-film deposition of a ceramic paste over an alumina substrate, when is subjected to a DC electric voltage. It is shown that a digital speckle pattern interferometer with an incorporated phase-shifting facility allows the measurement of nanometer displacements generated by the piezoelectric device. These measurements are used to evaluate the effective piezoelectric charge constant along the polarization direction ( d 33) eff that characterizes the thick-film transducer.

Achromatic digital speckle pattern interferometer with constant radial in-plane sensitivity by using a diffractive optical element

We report on a digital speckle pattern interferometer that applies a binary diffractive optical element (DOE) to generate double illumination and radial in-plane sensitivity. The application of the DOE ensures independence on the wavelength of the laser used as an illumination source. Furthermore, in-plane sensitivity only depends on the grating period of the DOE. An experimental setup was built allowing the measurement of a set of radial in-plane displacement fields either using a red laser as a light source or a green one. When displacement fields computed from the measured optical phase maps obtained with a red or a green laser were compared, two main results were observed: (a) deviations between mean values ranged only up to 7 nm and (b) phase maps presented the same amount of fringes. In addition, phase maps measured with the red laser were processed as they were obtained with green light. For this case, deviations have ranged only up to 0.5 nm. On the other hand, a set of measurements performed changing the DOE by a conical mirror showed clearly that radial in-plane sensitivity increased when the red laser was changed by the green one.

Two Dimensions Residual Stresses Analysis Through Incremental Groove Machining Combined with Electronic Speckle Pattern Interferometry

In this study a new residual stress determination method in two directions simultaneously is presented. This method is based on stresses relaxation in a groove that is machined incrementally. The residual stresses relaxation occurs simultaneously from both the depth and the length of the groove. Thus, measuring the surface strain field generated by the relaxation enables to determine the stress gradient both along the depth and the length of the groove. To measure the surface strain in a direction perpendicular to the groove, a digital speckle pattern interferometer is used. This method is suitable when the residual stress field in the structure varies in the depth as well as along the surface of the part, like for example in a welded structure. The method is tested here on an aluminium plate in which a central band has been shot peened.

Measurement of large cryogenic structures using a spatially phase-shifted digital speckle pattern interferometer

The James Webb Space Telescope (JWST) Backplane Stability Test Article (BSTA) was developed to demonstrate large precision cryogenic structures' technology readiness for use in the JWST. The thermal stability of the BSTA was measured at cryogenic temperatures at the Marshall Space Flight Center (MSFC) X-Ray Calibration Facility (XRCF) and included nearly continuous measurements over a six-week period in the summer of 2006 covering the temperature range from ambient down to 30 Kusing a spatially phase-shifted digital speckle pattern interferometer (SPS-DSPI). The BSTA is a full size, one-sixth section of the JWST primary mirror backplane assembly (PMBA). The BSTA, measuring almost 3 m across, contains most of the prominent structural elements of the backplane and is to our knowledge the largest structure ever measured with SPS-DSPI at cryogenic conditions. The SPS-DSPI measured rigid body motion and deformations of BSTA to nanometer-level accuracy. The SPS-DSPI was developed specifically for the purposes of this test and other tests of large cryogenic structures for JWST.

Calibration of spatially phase-shifted DSPI for measurement of large structures

We present a method for the calibration of a spatially phase-shifted digital speckle pattern interferometer (SPS-DSPI), which was designed and built for the purpose of testing the James Webb space telescope (JWST) optical structures and related technology development structures. The need to measure dynamic deformations of large, diffuse structures to nanometer accuracy at cryogenic temperature is paramount in the characterization of a large diameter space and terrestrial based telescopes. The techniques described herein apply to any situation, in which high accuracy measurement of diffuse structures are required. The calibration of the instrument is done using a single-crystal silicon gauge. The gauge has four islands of different heights that change in a predictable manner as a function of temperature. The SPS-DSPI is used to measure the relative piston between the islands as the temperature of the gauge is changed. The measurement results are then compared with the theoretical changes in the height of the gauge islands. The maximum deviation of the measured rate of change of the relative piston in nm/K from the expected value is 3.3%.

High-speed phase-stepped digital speckle pattern interferometry using a complementary metal-oxide semiconductor camera

A digital speckle pattern interferometer based on a complementary metal-oxide semiconductor (CMOS) camera is described. The temporal evolution of dynamic deformation is measured using interframe phase stepping. The flexibility of the CMOS detector is used to identify regions of interest with full-field time-averaged measurements and then to interrogate those regions with time-resolved measurements sampled at up to 70 kHz. A numerical and analytical investigation shows that the maximum surface velocity that can be reliably measured with interframe phase stepping corresponds to +/-0.3 times the surface velocity at which the interferogram is sampled at the Nyquist limit.

Digital speckle pattern interferometric (DSPI) and thermo-graphic investigations on the thermal responds in human teeth

Human dentine is a composite material made up of constituents of varying thermal properties, which when subjected to changes in temperature experiences complex thermal effects. In this study, the response of functionally adapted dentine to temperature changes in physiological range is analyzed using a digital speckle pattern interferometer (DSPI) and thermography in conjunction with an advanced digital fringe processing technique. The advanced digital fringe processing is conducted using an asymmetric and central peak Gaussian filter, in order to remove noise and, subsequently, enhance the quality of the images. This investigation demonstrates the responds of functionally adapted dentine to temperature changes in its own plane and out of plane. Distinct pattern of out-of-plane and in-plane displacement was observed on the dentine by the DSPI analysis. The thermo-graphic analysis was used to rationalize the above nature of deformation in the dentine. These analyses showed that the inner, cervical dentine exhibited conspicuous deformation at the maximum temperature rise and equilibrated last during temperature drop.

Measurement of complex surface deformation by high-speed dynamic phase-stepped digital speckle pattern interferometry

We describe a high-speed digital speckle pattern interferometer incorporating a line-scan camera and a waveguide phase modulator for the measurement of complex deformation (vibration phase and amplitude) at audio acoustic frequencies. Experimental data show continuous phase-stepped recovery of out-of-plane surface deformation in one dimension, obtained at 100 kHz with 2pi/20-rad (0.02-mum) displacement resolution, for surface velocities of 3.2 mm s>(-1) .

Computer-aided interferometric measurements of drift and phase shifter calibration for digital speckle pattern interferometry

A phase-shifting digital speckle pattern interferometer (DSPI) has been calibrated using a Michelson interferometer. Calibration consists in measuring phase shifts generated by piezoelectric transducers (PZTs) and also to determine drift of the phase with time in the interferometer arms. The calibration fringes are displayed live on the TV monitor and then processed slice by slice after recombination following their recording sequence (PZT voltage or time). Dark fringes are detected to compute the wavelength with good accuracy (λ/170 theoretical) and the optical path difference is stored in a datasheet for every slice. This leads to the measurements of the drift and the calibration of the phase shifters with an accuracy about λ/20. Out-of-plane and in-plane optical fiber DSPIs have been successfully characterized using the same algorithm. The method presented also enables inexpensive data processing. Moreover the real-time measurement of the interferometer drift enables feedback loop active control during DSPI measurements.

Semi-automatic phase shift calibration using digital speckle pattern interferometry

A semi-automatic method for calibrating phase shifting devices using a simple speckle pattern interferometric arrangement has been demonstrated. The method applies a simple two-beam digital speckle pattern interferometer (DSPI) and a personal computer with a frame grabber to achieve this purpose. The principle of the technique, together with the optical arrangement, is described and the system is then used for the calibration of a phase shifting device based on a piezoelectric transducer (PZT). The calibration results are presented, and advantages and limitations of the technique are discussed. Calibrating a PZT of 3.5 μm full expansion in one-direction takes no more than 17 s and the RMS error of the calibration is 0.6% in terms of the derived sensitivity of the phase-shifting device. The error in phase measurement by a four-step phase stepping algorithm using the calibrated phase shifting device is of the order of 0.20 °(RMS) or 1 1800 of a wave.

Improved DSPI configuration for the inspection of components in the production line

In this paper, a digital speckle pattern interferometer configuration for the enhanced nondestructive inspection of components in the production line is presented. The setup used is adapted from the original design employed for digital speckle shearing interferometry. The theoretical principle behind the technique is outlined and its application in the detection of poor bolting in plates is demonstrated.