IN-PLANE INSENSITIVE DOUBLE-APERTURE DIGITAL SHEAROGRAPHY FOR SLOPE MEASUREMENT

An improved in-plane insensitive double-aperture digital speckle shearing interferometric technique is proposed to measure the first derivative of out-of-plane displacement （slope）. The temporal phase-shifting method is used for the quantitative analysis of fringes. The designed system employs a double-aperture arrange- ment placed in front of the imaging lens. A glass wedge covers one of the two apertures to introduce a laterally shear. The experimental specimen is a circular aluminum plate, clamped along its edge and subjected to both out- of-plane deflection and in-plane rotation. Experimental results show that the fringes obtained from the proposed optical configuration represent pure slope contour distributions, and that the contributions from the in-plane dis- placement components are completely eliminated. Theoretical and experimental results are in good agreement.

A Compact Laser Shearography System for On-Site Robotic Inspection of Wind Turbine Blades

Shearography is an optical technique in the field of nondestructive evaluation(NDE)of various materials.Its main advantages are that it is noncontact type and can cover a large area in a single inspection.As a result,although it has been widely acknowledged as an effective technique particularly for NDE of composite materials to detect subsurface defects such as delamination,disbond,cracks,and impact damages,the use of shearography for on-site inspection of wind turbine blades(WTBs)has not been reported.This is due to wind causing structural vibration in the WTB.The solution in this paper is to make the shearography sit on the WTB during inspection when the WTB is parked,so that the relative motion between the shearography and the WTB is minimized within the tolerance of the shearography system.The ultimate goal of the solution is to enable a robot-assisted shearography system to inspect the WTB on-site.This paper presents the research work on a new shearography design for integration with a robotic climber for on-site WTB inspection.The approach is tested and evaluated in experimental settings,and a comparative assessment of the approach with other robotic NDE techniques is carried out.The results demonstrate the potential benefits and suitability of the approach for on-site robotic inspection of WTBs.

Measurement of absolute three-dimensional displacement gradients using tri-color quad-beam digital shearography

A tri-color quad-beam digital shearography is proposed to achieve the measurement of absolute three-dimensional(3D) displacement gradients.Four laser beams with three different center wavelengths are symmetrically irradiated to the object surface from the upper and lower left and right directions.Four phase maps are then extracted from the two interferograms obtained from two shots.Based on these four phase maps,the absolute 3D displacement gradients are determined.This means of absolute 3D displacement gradient measurement effectively improves the measurement capability of digital shearography and expands its application range.

A robust phase extraction method for overcoming spectrum overlapping in shearography

The advantage of spatial phase-shifting shearography is its ability to extract the phase from a single speckle pattern;however,it often faces spectrum overlapping,which seriously affects phase quality.In this paper,we propose a shearography phase-extraction method based on windowed Fourier ridges,which can effectively extract phase information even in the presence of severe spectrum overlapping.A simple and efficient method was applied to determine the parameters of the windowed Fourier ridges,and a linear variation window was used to match the phase-extraction requirements for different frequency coordinates.A numerical simulation was quantitatively conducted to compare the phase-extraction results of the proposed method with those of the conventional method for various cases,and a shearography system was built with two types of objects to demonstrate the feasibility of the proposed method.

Quality evaluation of rubber-to-metal bonded structures based on shearography

The quality of bonding is often a concern in rubber-to-metal bonded parts with regard to the integrity and stability of the structure. In this study, shearography has been used to detect the out-of-plane deformation of the defects caused by thermal and vacuum stresses. A sample of the vulcanized styrene-butadiene rubber bonded to steel with known artificial voids was prepared and during bonding process a region was glued with thick adhesives. The rubber surface of the sample was inspected in thermal radiation and vacuum modes respectively with a self-designed shearography device. Meanwhile, a numerical simulation was conducted to predict the out-of-plane deformation of the rubber surface in these two stress modes. Results from the numerical simulation and the experiments indicated that the debonding defects could be inspected in both loading modes. In the thermal radiation mode the region with thick adhesives could be identified successfully. This study provides a guideline for quality control of rubber-to-metal structures using an optical method.

Shearography and its applications-a chronological review

This paper presents the activities in the field of shearography in chronological order and highlights the great potential of this holographic measurement technology.After a brief introduction,the basic theory of shearography is presented.Shear devices,phase-shift arrangements,and multiplexed shearography systems are described.Finally,the application areas where shearography has been accepted and successfully used as a tool are presented.

Influence of welding parameters on fracture toughness and fatigue crack growth rate in friction stir welded nugget of 2024-T351 aluminum alloy joints

Friction stir welding （FSW） was performed on 2024-T351 aluminum alloy plates. Metallographic analysis, Vickers microhardness and XRD tests were conducted to determine the properties of the welded zone. FE simulation of the FSW process was implemented for the different welding conditions to extract the residual stress and stress intensity factor （SIF）. Fracture and fatigue behaviors of the welds which have the initial crack in the nugget zone and the crack orientation along the welding direction, were studied based on standard test methods. Fracture behavior of the welds was also evaluated by shearography method. The results showed that the tool rotational and traverse speeds affect the fracture toughness and fatigue crack growth rate. FSW provides 18%-49% reductions in maximum fracture load and fracture toughness. A slight diminution in fracture toughness of the joints was observed for lower traverse speed of the tool, and at higher traverse or rotational speeds, increasing the probability of defects may contribute to low fracture toughness. Fatigue crack propagation rate of all welds was slower than that of the base metal for low values of stress intensity factor range ΔK （ΔK〈13 MPa·m^1/2）, but is much faster for high values of ΔK.

New multiplexed system for synchronous measurement of out-of-plane deformation and two orthogonal slopes

We propose a novel system for synchronous measurement of out-of-plane deformation and two orthogonal slopes using a single camera. The linearly polarized reference beam introduced by an optical fiber interferes with the unpolarized object beam to measure the out-of-plane deformation. A modified Mach–Zehnder interferometer is used to measure the two orthogonal slopes of the out-of-plane deformation. One of the object beams of the Mach–Zehnder interferometer is an unpolarized beam, and the other object beam is split into two orthogonal linearly polarized object beams by a polarizing prism. The two beams are orthogonally polarized. Hence, they will not interfere with each other. The two polarized beams respectively interfere with the unpolarized beam to simultaneously measure the two orthogonal slopes of the out-of-plane deformation. In addition, the imaging lens and apertures are respectively placed in three optical paths to independently control the carrier frequencies and shearing amounts. The effectiveness of this method can be proved by measuring two pressure-loaded circular plates.

Successes and challenges in non-destructive testing of aircraft composite structures

Composite materials are increasingly used in the aerospace industry.To fully realise the weight saving potential along with superior mechanical properties that composites offer in safety critical applications,reliable Non-Destructive Testing(NDT)methods are required to prevent catastrophic failures.This paper will review the state of the art in the field and point to highlight the success and challenges that different NDT methods are faced to evaluate the integrity of critical aerospace composites.The focus will be on advanced certificated NDT methods for damage detection and characterization in composite laminates for use in the aircraft primary and secondary structures.