Digital Speckle Shearing Interferometry Research Articles

Analysis of Plane Displacement of Pressure Vessels with Defects

Aiming at the problem that the internal defects of the pressure vessel are not easy to detect, the out-of-plane displacement method is used to study it. The out-of-plane displacement field of circular flat plate heads with defects is obtained by the method of elastic theory analysis and compared with the finite element method to verify the accuracy of the finite element method. Based on the finite element method, the effects of vessel material, pressure load, defect shape, and defect size on the first-order out-of-plane displacement derivative are studied. The results show that the material of the container has little effect on the out-of-plane displacement. The first derivative of out-of-plane displacement increases linearly with the increase of pressure, and this relationship will not change with the increase of pressure. With the increase of defect aspect ratio, the maximum value of the first derivative of out-of-plane displacement increases gradually. When δ ˃ 9, the defect is detectable. The research results lay a theoretical foundation for the internal defect detection of pressure vessels based on digital speckle shearing interferometry.

Determination of slope, curvature, and twist from a single shearography fringe pattern using derivative-based regularized phase tracker

A fringe analysis algorithm for determination of slope, curvature, and twist from a single fringe pattern in digital speckle-shearing interferometry is proposed. A method for estimation of biased curvature and twist maps from fringe orientation and fringe density maps is employed. The curvature and twist maps obtained are further processed by B-spline interpolation to achieve high quality curvature and twist maps. A derivative-based regularized phase tracker (RPT) utilizes these predetermined curvature and twist maps for determination of a slope map from a single shearography fringe pattern. The proposed model requires less computational time and it overcomes the limitations of the RPT model. The method is validated with an experimental fringe pattern. The results show that this method is robust against speckle noise and it is able to retrieve accurate slope, curvature, and twist maps from a single shearography fringe pattern.

Phase retrieval in two-wavelength DSSI using a combined filtering method

Phase retrieval in two-wavelength DSSI (digital speckle shearing interferometry) using a combined filtering method is proposed for small deformation derivative measurement. A simultaneous two-wavelength illumination and 3-CCD camera are employed in the experimental setup. The proposed method, which uses a two-wavelength technique does not require the conventional spatial phase unwrapping and has the advantage of good noise suppression for phase retrieval. Experimental results obtained show that using the proposed combined filtering method sensitivity similar to the single wavelength technique can be achieved.

Phase retrieval in DSSI for relatively large deformation

Methods for phase retrieval in digital speckle shearing interferometry (DSSI) with relatively large deformation are proposed using a single wavelength illumination with windowed Fourier filtering (WFF) and two-wavelength simultaneous illumination with iterative sine–cosine average filtering (ISCAF). For relatively large deformation measurement, WFF technique has a higher noise suppression capability than ISCAF technique but requires careful selection of the processing parameters, while ISCAF is a simple filtering with less parameter setting. Both methods for phase retrieval are discussed. Experimental results show that both methods are capable of noise suppression in the phase retrieval for relatively large deformation.

Phase retrieval with a three-frame phase-shifting algorithm with an unknown phase shift

A three-frame phase-shifting algorithm with a constant but unknown phase shift is proposed. The algorithm is based on background-intensity removal prior to phase retrieval to eliminate an undetermined factor in a fringe pattern. The proposed method is validated on three-dimensional profilometry by fringe projection and on deformation measurement by means of digital speckle shearing interferometry. For a fringe pattern with slow-varying background intensity, the background removal is achieved in the frequency domain. For a speckle pattern, a background removal technique is integrated with the three-frame algorithm. In this process, manual intervention is minimal, and high computational speed is achieved. In addition, high-frequency phase signals would not be removed in the noise-reduction process as is the case in the bandpass-filtering technique. Accuracy of the method is discussed.

Using Shearography to Find the Flaws

This article highlights warning signs that a composite panel may be delaminating or thinning, but engineers have a powerful nondestructive testing tool in shearography. This is a laser interferometric method developed originally for full-field observation of surface strains of components. Flaws usually induce strain concentrations around them, and shearography can be employed to detect those flaws. A relatively new variation, digital speckle-shearing interferometry (DSSI), uses a charged-coupled device (CCD) camera and computer image processing to capture and process the interferometric fringe patterns. The digital version is faster than conventional shearography, and does not require any film or Fourier filtering. The main advantages of digital speckle shearing interferometer are the ease and speed with which fringe patterns can be obtained.

Identification of antinodes and zero-surface-strain contours of flexural vibration with time-averaged speckle pattern shearing interferometry

A new time-averaged frame subtraction technique is introduced for vibration analysis by digital speckle shearing interferometry. The technique permits the enhancement of fringes by subtracting two Bessel fringe patterns at different forcing levels. Compared with the phase-shift method, this method is more efficient and easier to implement for qualitative vibration measurement, providing a means for fast inspection of plate vibration behavior. It is also capable of tracing contours of zero strain and locating antinodes on vibrating plates.

Optical distance sensing using digital speckle shearing interferometry

A novel distance sensing technique using digital speckle shearing interferometry is outlined in this paper. The primary incentive for the development of this technique is of one that can be: (i) incorporated into any TV imaging system; (ii) used as a focusing mechanism; (iii) suited to measure from diffuse targets; and (iv) relatively insensitive towards the environment. Results obtained reveal the feasibility of further research and development of this technique for practical applications.

Carrier-modulated Object Step-loading Method of Automated Analysis in Digital Speckle Shearing Interferometry

An object step-loading method has been recently reported to overcome the problems of phase unwrapping and decorrelation in digital speckle shearing interferometry. In this paper, a carrier modulation procedure is incorporated into the object step-loading method in order to improve the phase derivation accuracy. An added advantage with this approach is that the direction of deformation is also revealed.

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.

Digital speckle pattern interferometer for combined measurements of out-of-plane displacement and slope

Fringe patterns depicting the out-of-plane displacement and slope of deformed objects are obtained using digital speckle pattern and digital speckle shearing interferometry respectively. In the study of objects subjected to complex loads, it is often necessary to procure both sets of data in order to facilitate a detailed analysis. In this paper, it is demonstrated that the original design for digital speckle shearing interferometry is able to be employed for this purpose with minimal modifications implemented. It also allows comparative out-of-plane displacement measurements to be made.

Shear measurement in digital speckle shearing interferometry using digital correlation

Accurate computations of surface slope deformation based on digital speckle shearing interferometry fringe patterns require a precise measure of the amount of shear introduced. A previous method of employing image processing, while providing accurate and fast measurements, is limited by the need to place a tape target on the object surface. In this paper, a novel method employing digital correlation is demonstrated to obviate this requirement while maintaining accurate mensurations of shear. It also allows automated measurement procedures to be readily incorporated.

Improved visual detection of flaws by using temporal phase modulation in digital speckle shearing interferometry

The visual detection of flaws by digital speckle shearing interferometry is hampered by a spatial texture form cue deteriorated by speckles. The incorporation of time-varying phase modulations, proposed in this letter, adds a temporal component which helps to improve the differentiation of the flaw-developed fringe pattern from its background.

Automated analysis in digital speckle shearing interferometry using an object step-loading method

Automated analysis in digital speckle shearing interferometry using the conventional two-frame differencing method is hindered by phase unwrapping errors and unresolved fringe patterns due to excessive object load. An object step-loading method with the ability to surmount these limitations is proposed and demonstrated. An added advantage of the proposed method is its ability to yield the entire slope versus load history within one loading cycle.