Papers by Keyword: Shearography

Abstract: Shearography is a whole-field optical technique used to measure the surface displacement derivatives. The digital shearing speckle interferometer (DSSI) allows the acquisition and observation of fringes in real time without having to undergo the photographic wet-process and the filtering procedure. In this paper, the principles of DSSI are introduced. In this method, the undeformed speckle image is firstly digitized and stored into the computer through a CCD camera. The subsequent frames of the deformed object are then subtracted continuously from the stored reference frame, thus a fringe pattern is formed. By counting the fringe order, the displacement derivatives at any point on the surface of the object can be determined. A case study using DSSI method to measure the derivatives of surface displacements is presented.

Abstract: This paper reviews shearography and its applications for testing of aircraft composite structures and honeycomb-based specimen. Shearography is a laser-based interferometry in conjunction with the digital imaging processing technique for full-field measurement of surface deformation. It reveals defects in an object by looking for defect-induced deformation anomalies. It does not require special vibration isolation, and with the development of a small and mobile measuring device (portable inspection system), it can be employed easily in field/factory environments.

Abstract: Defect size of wall thinned pipe is measured by using Speckle Shearing Interferometry (SSI) and Digital Image Correlation (DIC) techniques. A wall thinned defect of a carbon steel pipe was typically caused by flow accelerated corrosion (FAC). As wall thinned pipe can cause a huge accident at the nuclear power plant (NPP), a wall thinned defect should be detected for structure safety. SSI is one of the optical nondestructive techniques and can provide to inspect in real-time and to measure on the whole visible area at a time. DIC is a kind of the visual testing method. This method which uses a stereo vision system can measure the deformation or strain/stress of a structure in 3D. In this paper, ASTM A106 Gr.B carbon steel pipe is used as specimen. When the pressure load is provided by the pressure pump, the out-of-plane deformation along the longitudinal direction of a pipe can be detected quantitatively. Both results of SSI and DIC experiments are compared.

Abstract: There has been a growing interest in the use of composites especially in structural application ranging from aerospace to automotive and marine sectors. However, their performances under impact loading represent one of the major concerns as impacts may occur during manufacture, normal operations and maintenance. This paper presents two novel NDT techniques, thermosonics and digital shearography (DISH) to detect and assess barely visible impact damage (BVID) produced on a stiffened composite wing panel by unknown low energy impacts. Thermosonics is based on synchronized infrared imaging and ultrasonic excitation. Despite the apparent simplicity of the experimental setup, thermosonics involves a number of factors, e.g. acoustic horn location, horn crack proximity, horn-sample coupling etc., that significantly tend to influence both the degree and the period of the excitation. Then, a numerical-experimental procedure for the assessment of the size and depth of delamination by digital shearography (DISH) is proposed. The flaw detection capabilities of DISH have been evaluated by measuring the dynamic response of the delaminated area to applied stresses. The shearographic methodology is based on the recognition of the (0 1) resonance mode per defect. A simplified model of thin circular plate, idealized above each impacted area, is used to calculate the natural frequency of vibrating delamination. The numerical difference between experimental resonance frequencies and those computationally obtained is minimized using an unconstrained optimization algorithm in order to calculate the delamination depth. The results showed that thermosonics is a quick and effective method to detect and localize BVID damage while the combined shearography and optimization methodology was able to size and localize delamination due to low velocity impacts.

Abstract: To reduce weight and improve strength in the aerospace industry, composite structure connected by glue has gained popularity as a replacement for conventional materials and structures. However, the adhesive bonding process is more susceptible to quality variations during manufacturing than traditional joining methods. The integrality, strength and rigidity of product would be broken by disbonding. Investigate the sensitivity and accuracy of the testing method was more impotent to ensure the product quality and testing efficiency. This paper presents a study of detecting sensitivity of sheargoraphy methods when it is used for inspecting disbondinging on honeycomb structures. The potential of shearography method is also outlined.

Abstract: Optical Full Field Techniques (OFFT) are more and more utilized by mechanical laboratories. Among these methods, interferometry techniques (mainly composed of Speckle/Grating Interferometry or Speckle/Grating Shearography) are more difficult to use in a mechanical lab context, because of their sensitivity to external vibrations (except shearography), and because of the global lack of optical culture of mechanical engineers. Speckle-based methods are of great practical interest for the users, but their signal to noise ratio (SNR) is affected by the rigid body motion of the specimen. Here, the speckle decorrelation is minimized at local scale directly using the SNR. First, a shearography experiment is modeled to characterize the recorrelation procedure for a rigid body motion, a constant strain map and finally a high degree of localization. The mean noise level is found to be 6 times higher than a fully-correlated phase map for a 1 pixel speckle size. Last, a first application to a single-ply fabric composite lamina is shown. Resulting strain maps are of high quality with a very low spatial resolution (4 pixels). The local bending / global tension coupling effect is clearly put in evidence.

Abstract: In the present work, thermal expansion coefficients of a number of ceramic coatings were studied by a non-destructive technique (NDT) known as shearography. Ceramic coatings, i.e., a white enamel and a yellow Acrylic Lacquer on a metallic alloy, i.e., carbon steels, were investigated at a temperature range simulating the severe weather temperatures in Kuwait, especially between the daylight and the night time temperatures, 20-60 0C. The investigation was focused on determining the in-plane displacement of the coating, which corresponds to the thermal deformation (strain), with respect to the applied temperature range. A mathematical relationship was derived along with the experimental data. The mathematical relationship described the thermal deformation of a coated film as a function of temperature. Furthermore, results of shearography indicate that the technique is found very useful NDT method not only for determining the thermal expansion coefficients of different coatings, but also the technique can be used as a 2D- microscope for monitoring the deformation of various coatings in at a submicroscopic scale.

Abstract: Shearography, a type of speckle interferometry, is a powerful tool for nondestructive testing (NDT) because it is insensitive to environmental vibration disturbance and easy to detect strain concentration in an internal defect with noncontact and whole field. The shearing amount in Shearography has influence on determining defect size and source location. Thus the shearing amount must be optimally adjusted to get accurate evaluation. This paper investigates the defect size and source location according to the change of shearing amount in artificial slender defects along longitudinal direction of pipeline. Based on the investigation, a new technique is proposed for the quantitative determination of defect size and source location.

Abstract: In the present work, thermal expansion coefficients of a number of organic coatings were studied by a non-destructive technique (NDT) known as shearography. An organic coating, i.e., epoxy, on a metallic alloy, i.e., carbon steels, was investigated at a temperature range simulating the severe weather temperatures in Kuwait especially between the daylight and the night time temperatures, 20-60 0C. The investigation focused on determining the in-plane displacement of the coating, which amounts to the thermal deformation (strain ) with respect to the applied temperature range. Along with the experimental data, a mathematical relationship was derived describing the thermal deformation of a coated film as a function of temperature. Furthermore, results of shearography indicate that the technique is very useful NDT method not only for determining the thermal expansion coefficients of different coatings, but also the technique can be used as a 2Dmicroscope for monitoring the deformation of the coatings in real-time at a submicroscopic scale.

Abstract: When a sandwich structure is loaded in uniaxial in-plane compression the skin over the debonded region may buckle allowing further propagation of the debond, leading to structural collapse. Hence how to detect the shape and size of debonded defect, and to trace propagation trend of the defect are key factor in the process of damage evaluation of subsurface defect. In the paper, a nondestructive testing system is developed based on liquid crystal cell and Wollaston shearing prism. Nematic liquid crystal has the property of modulating the intensity and phase of light, and the intensity will not change during the phase modulating. According to this property, the phase-shifting of digital shearography are performed successfully. The experimental results prove that we can not only perform detection of the defect size and shape in subsurface of sandwich structure, but also trace the further propagation of debonded defect because of local buckle. This indicates that the method of digital shearogrephy will offer an effective measurement tool to research the buckle behavior of debond defect in subsurface of sandwich under uniaxial in-plane compression.