Anand Asundi

Two-dimensional digital image correlation for in-plane displacement and strain measurement: a review

As a practical and effective tool for quantitative in-plane deformation measurement of a planar object surface, two-dimensional digital image correlation (2D DIC) is now widely accepted and commonly used in the field of experimental mechanics. It directly provides full-field displacements to sub-pixel accuracy and full-field strains by comparing the digital images of a test object surface acquired before and after deformation. In this review, methodologies of the 2D DIC technique for displacement field measurement and strain field estimation are systematically reviewed and discussed. Detailed analyses of the measurement accuracy considering the influences of both experimental conditions and algorithm details are provided. Measures for achieving high accuracy deformation measurement using the 2D DIC technique are also recommended. Since microscale and nanoscale deformation measurement can easily be realized by combining the 2D DIC technique with high-spatial-resolution microscopes, the 2D DIC technique should find more applications in broad areas.

Phase error analysis and compensation for nonsinusoidal waveforms in phase-shifting digital fringe projection profilometry

The nonlinear intensity response of a digital fringe projection profilometry (FPP) system causes the captured fringe patterns to be nonsinusoidal waveforms and leads to an additional phase measurement error for commonly used three- and four-step phase-shifting algorithms. We perform theoretical analysis of the phase error owing to the nonsinusoidal waveforms. Based on the derived theoretical model, a novel and simple iterative phase compensation algorithm is proposed to compensate the nonsinusoidal phase error. Experiments show that the proposed algorithm can be used for effective phase error compensation in practical phase-shifting FPP.

Fiber metal laminates: An advanced material for future aircraft

Abstract Fiber Metal Laminates (FML) consist of thin, high strength aluminium alloy sheets alternately bonded to plies of fiber-reinforced epoxy adhesive. They provide an ideal combination of metals and composites that results in a material, which combines the best features of organic matrix composites and metals, without sharing their individual disadvantages. FML offer substantial weight savings relative to current metallic structures. Further, the number of parts required to build a component may be dramatically less than the number of parts needed to construct the same component of metal alloy. This can lead to labour savings, sometimes offsetting the higher price of the present materials. These features, together with superior fatigue behaviour, damage tolerant properties, inherent resistance to corrosion, good fire resistance for safety improvement, make FML very attractive candidate materials for future aircraft structures [1–3]. Later a new concept apply on this hybrid material: Fiber-Metal Laminates with Splice or Spliced Laminates. The development of spliced laminates has been a logical step after the identification of the favorable behavior of FML. Spliced laminates may provide a good solution obtaining substantially increased dimensions of spliced products. The splicing concept offers the same benefit (20 – 50% weight savings) as for a regular FML panel, but for much wider panels (>4 meters). This increased width capability can result in a significant reduction in manufacturing cost. These attributes make spliced laminates promising candidates for fuselage and lower wing materials for the next generation of Very Large Civil Transport (VLCT) aircraft and the Ultra High Capacity Aircraft (UHCA) for 600 to 800 passengers [4].

Studies of digital microscopic holography with applications to microstructure testing

We propose an in-line digital microscopic holography system for testing of microstructures. With the incorporation of a long-distance microscope with digital holography, the system is capable of imaging test microstructures with high resolution at sufficient working distances to permit good illumination of samples. The system, which was developed in an in-line configuration, achieves high imaging capacity and exhibits properties that are favorable for micromeasurement. We demonstrate the performance of the system with experiments to determine the displacement of a silicon microcantilever and with investigations of the microscopic resolution capability.

Structural health monitoring of smart composite materials by using EFPI and FBG sensors

Abstract Structural health monitoring (SHM) including the real-time cure monitoring and non-destructive evaluation (NDE) in-service is very important and definitely demanded for safely working of high performance composite structures in situ. It is very difficult to carry out by using conventional methods. A unique opportunity was provided to real-time monitor the health status of composite structures by using embedded fiber optic sensors (FOSs). In this paper, the extrinsic Fabry–Perot interferometer (EFPI) and fiber Bragg grating (FBG) sensors are real-time employed to simultaneously monitoring the cure process of CFRP composite laminates with and without damage. The results show that both embedded EFPI and FBG sensors could be used to monitor the cure progress of composite materials and detect the occurred damage on-line during the fabrication of composite structures. Furthermore, the NDE of smart composite laminates embedded both EFPI and FBG sensors are performed by using the three-point bending test. The experimental results present that the flexural strain of CFRP composite laminates with damage is more than that of CFRP laminates without damage under same load as we expected. Both EFPI and FBG sensors also show the excellent correlation during the cure monitoring and bending test.

FAST PHASE-UNWRAPPING ALGORITHM BASED ON A GRAY-SCALE MASK AND FLOOD FILL

Phase-unwrapping algorithms, an active and interesting subject in recent years, are important in a great number of measurement applications. Active research is being undertaken to develop reliable and high-speed procedures. The current process uses a gray-scale mask and the flood-fill concept from image processing for phase unwrapping. The algorithm unwraps phase from an area with higher reliability to one with lower reliability. In addition to robustness, the speed of the algorithm proposed is much faster than conventional routines. The experimental results of different algorithms are compared by analysis of a tooth plaster and a photoelastic specimen.

Real-time cure monitoring of smart composite materials using extrinsic Fabry-Perot interferometer and fiber Bragg grating sensors

Real-time cure monitoring of composite materials is very important to improve the performance of advanced composite materials. It is very difficult to monitor the cure process online using conventional methods. Fiber optic sensors in smart composite materials provide a unique opportunity to monitor the cure process of composite materials in real time by using embedded sensors. In this paper, extrinsic Fabry-Perot interferometer (EFPI) and fiber Bragg grating (FBG) sensors are embedded in carbon/epoxy composite laminates and used to monitor the cure process simultaneously. Furthermore, measurements of residual strains of composite laminates during the cure have been performed. The results show that both EFPI and FBG sensors can be used to monitor the strain development of composite laminates with and without damage during cure. An excellent correlation between the EFPI and FBG sensors is presented.

Imaging analysis of digital holography

In this study we focus on understanding the system imaging mechanisms given rise to the unique characteristic of discretization in digital holography. Imaging analysis with respect to the system geometries is investigated and the corresponding requirements for reliable holographic imaging are specified. In addition, the imaging capacity of a digital holographic system is analyzed in terms of space-bandwidth product. The impacts due to the discrete features of the CCD sensor that are characterized by the amount of sensitive pixels and the pixel dimension are quantified. The analysis demonstrates the favorable properties of an in-line system arrangement in both the effective field of view and imaging resolution.

Properties of digital holography based on in-line configuration

Digital holography for micromeasurement is an active re- search topic. With respect to the requirement of realizing characteriza- tion of micro-scale structures in microelectromechanical systems with high resolution and accuracy, in-line configuration is studied in this paper as the fundamental structure of a digital holography system. A math- ematical model based on Fourier optics is developed to analyze the digital recording mechanism and properties of the system in comparison with those of the commonly used off-axis arrangement. Theoretical analysis and experimental results demonstrate that in-line configuration is advantageous in enhancing the system performance. Besides the re- laxed requirement of spatial resolution on the CCD sensors and the greater flexibility of the system, higher lateral resolution and lower speckle noise can be achieved.

Least-squares calibration method for fringe projection profilometry considering camera lens distortion

By using the least-squares fitting approach, the calibration procedure for fringe projection profilometry becomes more flexible and easier, since neither the measurement of system geometric parameters nor precise control of plane moving is required. With consideration of camera lens distortion, we propose a modified least-squares calibration method for fringe projection profilometry. In this method, camera lens distortion is involved in the mathematical description of the system for least-squares fitting to reduce its influence. Both simulation and experimental results are shown to verify the validity and ease of use of this modified calibration method.