Quantitative shearography: error reduction by using more than three measurement channels

Shearography is a noncontact optical technique used to measure surface displacement derivatives. Full surface strain characterization can be achieved using shearography configurations employing at least three measurement channels. Each measurement channel is sensitive to a single displacement gradient component defined by its sensitivity vector. A matrix transformation is then required to convert the measured components to the orthogonal displacement gradients required for quantitative strain measurement. This transformation, conventionally performed using three measurement channels, amplifies any errors present in the measurement. This paper investigates the use of additional measurement channels using the results of a computer model and an experimental shearography system. Results are presented showing that the addition of a fourth channel can reduce the errors in the computed orthogonal components by up to 33% and that, by using 10 channels, reductions of around 45% should be possible.     

Shearography: A novel and practical approach for nondestructive inspection

This paper presents an optical technique referred to as shearography for nondestructive inspection. Shearography is an interferometric method which allows full-field observation of surface displacement derivatives. It reveals flaws in materials by looking for flaw-induced strain anomalies. Both surface and interior flaws can be detected. Because of its numerous advantages, shearography is more practical than holography. The simplicity and the alleviation of vibration isolation requirement make shearography a practical approach for nondestructive testing in field/plant environments.     

Some examples of nondestructive flaw detection by shearography

This paper presents some examples of nondestructive flaw detection using an optical method based on speckle shearing interferometry called shearography. In the method, a structure under study is illuminated by laser and imaged by a special image-shearing camera. After suitable processing, a fringe pattern which represents loci of surface displacement derivatives, i.e., strains, is observed in the image. Since defects in structures usually induce strain concentrations around them and since strain concentrations usually cause perturbations on the surface of structures, shearography reveals defects from anomalies in the recorded fringe pattern. In this work, the technique has been applied to the nondestructive detection of various flaws in plain and welded pipes, composite plates, and other engineering components. A simple model was also developed for the estimation of the depth of disbonds in glassfiber reinforced plastic sheets. Results obtained are good and demonstrate the usefulness of the method as a complement to other conventional NDT techniques.     

Out-of-Plane Strain Measurement in Sandwich Plates with Single Fully Potted Insert by using Digital Shearography

Abstract:  The construction and operation of digital shearography (DS) applied to sandwich plates with a single fully potted insert have been presented. The proposed DS has the advantage of full-field and non-destructive testing, that can measure tiny out-of-plane strain in the elastic region without wasting specimen. For validation purposes, analytical solution analysis (ASA) stemming from our previous study [S. J. Huang, L. W. Chiu, M. T. Kao (2004) Structural Analysis of Sandwich Plates with Inserts – Five Layers Theory, Proc. 2004 AASRC/CCAS Joint Conf .] was conducted. By comparing the results of DS and ASA strain fields throughout the sandwich surface a convincing agreement is revealed, thus successfully showing the full-field strain measurement of the single fully inserted sandwich plates using DS.     

Multiple-image shearography: a direct method to determine curvatures

We present a modified method of shearography, known herein as multiple-image shearography, whereby the curvatures of an object can be measured directly from the resulting fringes. It employs an image-shearing camera that produces three sheared images simultaneously to interfere with each other in the image plane. When film is doubly exposed before and after an object is deformed, three sets of fringes are observed of which one set would depict the second-order derivatives of surface displacement.The theory of the multiple-image shearography technique and its application to curvature measurements in plate bending are presented.     

Evaluating the soundness of bonding using shearography

Shearography is an optical measurement technique invented to overcome several limitations of holography. One distinct advantage is that it alleviates the stringent environmental stability demanded by holography, rendering the technique practical for industrial applications. There are two modes of shearography, one for measuring surface displacement and the other for measuring surface displacement derivatives. This paper discusses the underlying principle of shearography and its application in nondestructive testing, in particular, of laminated composite structures. In flaw characterization, a thin-plate model is used for back-calculating the shape, size and location of debonds. Shearographic nondestructive testing relies on measuring the response of a defect to stresses. Two practical types of stressing for revelation of debonds are described – static loading using vacuum stressing, and dynamic stressing by means of vibrational excitation. In vibrational stressing, both single frequency excitation and broadband excitation can be used. While vacuum stressing is limited to detecting debonds with closed boundaries, the vibrational excitation can be used for detecting debonds with closed as well as opened boundaries. A major drawback associated with the use of single excitation frequency is the need for vibrating the test object within an appropriate frequency range, as otherwise the test data obtained will not readily reveal the debonds. This paper also describes a new method that uses multiple frequency sweep (equivalent to broadband excitation) which fosters rapid detection and unambiguous assessment of the soundness of adhesive bonding.     

Identification of Specimen Position and Orientation Using Standard Deviation of Intensity in Phase‐Shifting Digital Holography1

Abstract: Phase‐shifting digital holography is a useful method to measure the displacement distribution and the strain distribution of an object surface. The complex amplitude distribution of an object surface is obtained as the complex amplitude distribution at a reconstruction distance. It is, however, difficult to measure the reconstruction distance by actual measurement. We discovered that the standard deviation of the intensity on the reconstructed image becomes the maximum value when the reconstruction distance is the same as the actual optical path length. The displacement distributions are obtained for the x‐, y‐ and z‐directions. When the normal direction of an object surface inclines from the z‐direction, the displacements defined on the xyz‐coordinate system should be transformed into the object coordinate system. It is, therefore, required to develop a measurement method of the orientation of the object to obtain the parameters for transforming from the xyz‐coordinate system into the object coordinate system. In this paper, the method to identify the position and the orientation of a specimen using the standard deviation of the intensity distribution is proposed.     

TV-shearography for measuring 3D-strains

The shearographic interferometry is employed as a nondestructive full field, optical testing and measuring method without contact. Fringes of constant strain (so called isotase, tasis (Greek)=strain) can he observed in real time on the surface of the investigated machine parts and structures of any material and are represented by the shearogram. Using shearography two states of "deformation are recorded by doubly exposing a Holotesl film in an ordinary camera or stored by an electronic image processing system. In the lens of the camera a shearing element inside or outside the focus is integrated or the lateral Michelson shearing interferometer is used. Rigid body motions of the object are not recorded. Local deformation irregularities caused by a defect under or on the surface of the specimen create, strain concentrations; the homogeneous surrounding is poorly superimposed by an interference pattern. The shearogram shows dark and bright fringes which are the functions of the displacement derivative. The holographic interferometry measures the out of plane deformations directly. Terms of the out of plane strain can be determined by the shearographic method as well as the in plane strain fringes which are described in the following.     

Detection of surface strain by three-dimensional digital holography

Three-dimensional digital holography with three object-illuminating beams has been successfully used for the detection of surface strain in metallic objects. The optical setup that uses illuminating beams to irradiate the object from three directions means that all three object surface displacement components, x, y, and z, can be independently calculated and used to find the strain gradients on the surface. The results show the conversion of the complete surface displacement field into a surface strain field. The method is capable of measuring microstrains for out-of-plane surface displacements of less than 10 microm.     

Deformation Analysis of Notched Rubber Specimens

Abstract:  The influence of the initial crack tip radius on the strain distribution in the vicinity of the blunted crack tip was determined experimentally by an optical full-field strain analysis method. These experimental results were compared with the calculations of the commercial finite element code ABAQUS 6.4-1. For the simulation a suitable hyperelastic material law was chosen and fitted to experimental data of three different tests (uniaxial tensile, planar tensile, biaxial tensile). Two different elastomer grades (SBR and EPDM) were selected for the experimental work. The utilized optical full-field strain analysis method based on the image correlation technique was found to be an effective tool to determine strains, strain distributions and gradients near to the crack tip for elastomeric materials. Different material behaviour was observed for the two rubber types investigated. While the crack tip was regularly blunted (half circle shape) for EPDM and the strain gradient was low (less steep), the crack tip was sharp (less blunted) with a higher strain gradient for SBR.     

5083铝合金搅拌摩擦焊接头拉伸行为研究

目的 研究5083铝合金搅拌摩擦焊接（FSW）的组织、力学性能和拉伸应变,分析接头的拉伸行为。方法 采用数码相机、光学显微镜、电子扫描显微镜等表征分析方法,对焊缝的表面宏观成形、微观组织、断口形貌进行分析;利用拉伸机、三维数字动态散斑应变测量分析系统和显微维氏硬度计对接头的力学性能和拉伸应变进行测试。结果 不同焊接工艺参数下FSW接头的最低抗拉强度为305 MPa,断后延伸率达到了14%以上;焊核区拉伸应变沿板厚方向呈现上高下低和上宽下窄的不均匀梯度分布,发生了较大程度的变形强化,直到拉伸应力达到抗拉强度。断裂失效前300/120接头的最大拉伸应变在晶粒粗大的母材区,500/120和500/200接头的最大拉伸应变则位于晶粒尺寸差异较大的后退侧焊核区与热力影响区交界处。接头拉伸断口宏观上均为45°剪切韧性断裂,微观上均以韧窝韧性断裂为主,而高热输入500/120接头出现脆性断裂特征,其延伸率明显降低。结论 高热力耦合输入使铝合金FSW接头薄弱区发生转变,强韧性降低。     

Comparison of Shearography to Scanning Laser Vibrometry as Methods for Local Stiffness Identification of Beams

Local stiffness of Euler–Bernoulli beams can be identified by dividing the bending moment of a deformed beam by the local curvature. Curvature and moment distributions can be derived from the modal shape of a beam vibrating at resonance. In this article, the modal shape of test beams is measured by both scanning laser vibrometry (SLV) and shearography. Shearography is an interferometric optical method that produces full‐field displacement gradients of the inspected surface. Curvature can be obtained by two steps of derivation of the modal amplitude (in the case of SLV) or one step of derivation of the modal shape slope (in the case of shearography). Three specially prepared aluminium beams with a known stiffness distribution are used for the validation of both techniques. The uncertainty of the identified stiffness distributions with both techniques is compared and related to their signal‐to‐noise ratios. A strength and weakness overview at the end of the article reveals that the shearography is the technique that shows the most advantages.     

Beam-splitting cube for fringe-projection, holographic, and shearographic interferometry

We describe a simple method in which the techniques of fringe-projection, holographic, and shearographic interferometry may be readily realized with a commercial beam-splitting cube, a laser source, and a beam expander. With the use of a computer algorithm, moiré fringes may be derived digitally from the fringes that are projected onto a reference surface and onto an object surface. Successful use of the beam-splitting cube for these optical methods is attributed to the refraction of the two split beams that exit from the two adjacent faces of the cube. By careful orientation of the cube, the two refracted light beams will converge and interfere, resulting in the formation of Young's interference fringes for the fringe-projection method. When the hypotenuse of the beam-splitting cube is placed nearly normal to an illuminated object, both the image and the mirror image of the object will be formed behind the cube where a camera is placed. This optical setup thus forms the basis for double-exposure holography when these two images are fully overlapped and for shearography when the images are slightly laterally displaced.     

Digital recording and reconstruction of holograms in hologram interferometry and shearography

The fundamentals of digital recording and mathematical reconstruction of Fresnel holograms are described. The object is recorded in two different states, and the holograms are stored electronically with a charge-coupled-device detector. In the process of reconstruction the digitally sampled holograms are applied to the different coherent optical methods as hologram interferometry and shearography. If the holograms are superimposed and reconstructed jointly, a holographic interferogram results. If a shearing is introduced in the reconstruction process, a shearogram results. This means that the evaluation technique, e.g., hologram interferometry or shearography, can be influenced by numerical methods.     

Strains measured on plane and curved surfaces by means of the shearographic method - Part 1

The shearographic interferometry is employed as a nondestructive full field, optical testing method without contact. Fringes of constant strain (so called isotase, tasis (Greek)=strain) can be observed in real time on the surface of the investigated machine parts and are represented by the shearograms. Using shearography two states of deformation are recorded by doubly exposing a Holotest film in an ordinary camera or stored by an image processing system. In the telelens of the camera a shearing element inside or outside the focus is integrated. Rigid body motions of the specimen are not recorded. Local deformation irregularities caused by a defect under or on the surface of the specimen create strain concentrations; the homogeneous surrounding is poorly superimposed by an interference pattern, e.g., in the case of the internal pressure differential for testing tubes¹. The shearogram shows dark and bright fringes which are the functions of the displacement derivative. The holographic interferometry measures the out of plane deformations directly. A term of the out of plane strain can be determined by the shearographic method² as well as the in plane strain fringes under certain conditions which are described in the following. For this purpose the fringe patterns generated by the basic types of loading are presented and compared with the photoelastic patterns showing obvious analogies. The magnitude of the shearographic strain components is nearly the same when measuring by means of strain gauges or determining theoretically as is shown for one case.     

Object Grating Method Application in Strain Determination on CTOD Tests

Abstract:  Standard fracture toughness tests require standard specimens with the presumption that mechanical properties are uniform in the crack growth direction. Standards for crack tip opening displacement (CTOD) fracture tests prescribe remote crack mouth opening displacement, which can lead to inadequate results in the case of heterogeneous materials properties. This paper describes the application of an object grating method (OGM) on the fracture behaviour of a heterogeneous specimen. Fracture behaviour is described by measuring deformation on the surface of a specimen, in terms of CTOD and, consequently, by strain determination. An OGM is advantageously used when measuring modified CTOD tests on two specimens with an initial crack in a macroscopic heterogeneous welded joint. Results significantly show that fracture behaviour depends on the material in the vicinity of the crack tip concerning the direction of crack propagation.     

Real-time extended dynamic range imaging in shearography

Extended dynamic range (EDR) imaging is a postprocessing technique commonly associated with photography. Multiple images of a scene are recorded by the camera using different shutter settings and are merged into a single higher dynamic range image. Speckle interferometry and holography techniques require a well-modulated intensity signal to extract the phase information, and of these techniques shearography is most sensitive to different object surface reflectivities as it uses self-referencing from a sheared image. In this paper the authors demonstrate real-time EDR imaging in shearography and present experimental results from a difficult surface reflectivity sample: a wooden panel painting containing gold and dark earth color paint.     

Full-field assessment of the damage process of laminated composite open-hole tensile specimens. Part I: Methodology

The present paper is the first part of a study dedicated to the assessment of the damage taking place in composite open-hole tensile specimens using full-field strain measurements. It concentrates on the thorough validation of the methodology used to process the full-field data. First, the grid method used to provide the two components of the in-plane displacement field at the surface of the test specimens is described. Then, the data processing procedure to obtain the displacement maps during the test is discussed. The main part of the paper is dedicated to the derivation of strains from displacements, investigating two procedures: local differentiation and polynomial fitting. The effect of the grid is also investigated. Finally, successful experimental results are presented.     

Full Field Tensile Strain Shearography of Welded Specimens

Digital shearography is a coherent optical method used in conjunction with digital image processing. It allows the shearogram, which depicts directly displacement derivatives, in real time to be observed and to be evaluated numerically. Strains are functions of the displacement derivatives. Thus, the shearogram contains the strain information, but usually it includes both the in‐plane strain, e.g. ?u/?x, and the out‐of‐plane component, e.g. ?w/?x. In order to get the pure in‐plane strain as well as the pure out‐of‐plane component, two linearly independent directions of illumination (usually the same but mutual and sequential illuminations) are introduced in the measuring device. The shearograms for each illuminating direction are evaluated by applying the phase shifting technique. The result by subtracting the phase maps of the two shearograms yields a new fringe pattern depicting the pure in‐plane strain component and the result by adding the phase maps corresponds to the pure out‐of‐plane component. The theory and an application to non‐destructive tensile testing are demonstrated in this paper. Its applications for determining the strain distribution of the homogeneous and inhomogeneous material and an error analysis are presented.     

Laser Welding of Zr41Ti14Cu12Ni10Be23 Bulk Metallic Glass: Experiment and Temperature Field Simulation

We investigate laser bonding of Zr₄₁Ti₁₄Cu₁₂Ni₁₀Be₂₃ bulk metallic glass (BMG) using experiments and finite element simulation. The specimen plates with 5 × 30 × 1.3 mm³ are welded together successfully and the welded joint maintains fully amorphous under the welding parameters of 1.3 kW and 7 m min^?1. To examine in detail the thermal stability of the welded joint, we employ the SYSWELD software to simulate the distribution of temperature field and thermal cycle curves. The simulation results are consistent with the laser welding experiments, and there is no intersection point between the thermal cycle curves and time–temperature‐transformation diagram, indicating that the joint retains amorphous structure.