Shearography with syncrhonized pressure stressing

Non-destructive evaluation (NDE) techniques of optical video-based speckle interferometry are gaining importance as inspection tools, particularly by the aerospace industry [1,2]. An optical technique such as shearography is attractive to the NDE community largely because of its non-contacting nature, full-field measurement and fast inspection results. However, in order for this optical interferometric method to become widely used as an NDE tool, this technique must be made to be robust enough to operate in noisy environments typically found in industry settings. In this paper, we address these issues for the case of detection of disbonds using shearography in conjunction with synchronized pressure stressing

Crack-tip deformation field measurements using coherent gradient sensing

A real time, full field, lateral shearing interferometry - coherent gradient sensing (CGS) - has recently been developed for investigating fracture in transparent and opaque solids. The resulting interference patterns are related to the mechanical fields by means of a first order diffraction analysis. The method has been successfully applied to quasi-static and dynamic crack tip deformation field mapping in homogeneous and bimaterial fracture specimens

Characterization of the Interface Roughness of Coatings Based on Ultrasonic Reflection Coefficient Amplitude Spectrum

In order to nondestructively characterize the interface roughness of coatings effectively, the ultrasonic reflection coefficient amplitude spectrum (URCAS) involving interface roughness was derived based on the phase screen approximation theory [1]. The interface roughness was determined by a two-parameter inversion combined with a cross-correlation algorithm. For homogeneous coatings, the effects of ultrasonic wavelength λ, beam coverage, and shape variations of the coating on the roughness measurements were analyzed through numerical calculation. A series of simulations indicated that measurement errors were less than 10% when the relationship between interface roughness and wavelength satisfied Rq=1.5%λ~12%λ approximately. For inhomogeneous coatings, the attenuation coefficient shows a non-negligible effect on the URCAS. A new URCAS suited for roughness measurement of inhomogeneous coatings was formulated by considering the relationship of attenuation coefficient α(f) on frequency f, which was determined by simulations. Ultrasonic experiments were carried out on standard roughness specimens and tungsten carbide (WC) coating specimen utilizing delay line transducers. The standard roughness specimens were shown in Fig 1, whose roughness Rq were 8.5μm, 14.2μm, and 28.6μm measured by confocal laser scanning microscope (CLSM), respectively. The WC coating was sprayed on stainless steel by high velocity oxygen fuel (HOVF). Experimental results show that the roughness of standard roughness specimens obtained by the proposed ultrasonic measurement are in good agreement with the LCM observations, and the relative errors are less than 8.5%. For inhomogeneous WC coatings, the absolute error of roughness measurement is less than 2.5μm and the relative error is less than 20% between ultrasonic and metallographic methods

On the Domain of Dominance of the Asymptotic Elastodynamic Crack-Tip Fields

A substantial part of the experimental data in dynamic fracture mechanics has been obtained under the assumption that the two-dimensional asymptotic elastodynamic stress-intensity factor field (the KdI-field) is dominant over at least the region around the crack-tip over which the experimental measurements are made. The validity of this assumption is investigated in this thesis both experimentally and through finite-element simulations of the experiments. The experiments reported in this work were on 4340 steel, three-point bend specimens loaded dynamically using a drop-weight tower. The two cases of dynamically loaded stationary cracks and dynamically propagating cracks were considered. An optical configuration is proposed that leads to a bifocal high-speed camera capable of focusing on two different planes simultaneously. This was used in conjunction with the method of caustics to measure the apparent stress-intensity factor simultaneously from two different regions (initial-curves) around the crack-tip. If the initial-curves lie within the domain of dominance of the asymptotic field, the measured values of the dynamic stress-intensity factor must agree to within experimental error. By suitably adjusting the optical set-up, a range of initial-curves was scanned in an attempt to map the domain of dominance of the KdI-field. The impact hammer and supports of the drop-weight loading device were instrumented in order to monitor the time dependent loads acting on the specimen. These loads were subsequently used as boundary tractions in dynamic two- and three-dimensional finite-element simulations of the experiments. The simulations were carried only up to the point of crack initiation. Comparison of the numerical simulations with the experimental results help in identifying the role of three-dimensionality and transient conditions on the measured stress-intensity factor values. On the basis of both the experimental results as well as the numerical simulations, no sizeable annulus of dominance for the asymptotic elastodynamic field was found for the laboratory situation studied. It appears that the assumption of an underlying KdI-dominant (or two-dimensional) field might not hold to a level of accuracy that would warrant many of the conclusions made in the literature regarding the crack-initiation toughness values as well as the uniqueness of the dynamic fracture toughness - crack velocity relation or its specimen and acceleration dependence.</p

Adaptive Hetorodyne Line-Probe Interferometer for Enhanced Directionally-Sensitive Detection of Ultrasound

Optical methods provide a non-contact method of detecting ultrasound at the surface of a test object. Unlike conventional piezoelectric transducers, which require a couplant, optical detection provides an absolute calibration of the ultrasonic displacement amplitude. In addition, they can have a broader bandwidth and a higher spatial resolution of detection than conventional piezoelectric transducers. All these advantages however typically come at the expense of sensitivity. The best extant optical detectors still suffer from a two order of magnitude sensitivity gap with respect to conventional piezoelectric transducers. In this paper, we describe an adaptive heterodyne interferometer receiver using wave mixing in photorefractive bismuth silicate (BSO) crystals which is configured as a line receiver that is directionally most sensitive to ultrasound impinging normal to the line, and is significantly less sensitive to ultrasound impinging in other directions. Such a system is attractive in situations where the ultrasonic scatter from a specific direction is to be selectively pulled out in the presence of scatter from other “noise” sources. The line probe system also provides a way to bridge the sensitivity gap that optical detection thus far has suffered vis-à-vis piezoelectric detection. Results of applications to nondestructive testing of metal surfaces are presented.</p

Nondestructive Evaluation Using Shearing Interferometry

Coherent shearing interferometry involves the interference of a coherent optical wavefront with a spatially shifted version of itself. The resulting interference pattern carries information which for small shears (spatial shifts) can be related to the gradients of the phase of the wavefront. The primary advantage of this optical technique is that it is relatively insensitive to rigid body motion. A coherent wavefront that is transmitted through a body or is reflected from the surface of a body will carry information about the resulting stress state or deformation of the body. This information can be used for nondestructive evaluation applications using optical shearing methods in order to identify defects such as cracks and disbonds. In this paper, we will first give a brief review of various shearing methods, and then describe in detail the use of Coherent Gradient Sensing, a diffraction grating shearing technique that was developed by Tippur, Krishnaswamy and Rosakis [1,2,3], for the optical detection of cracks in bodies.</p

Noise reduction techniques for electronic speckle interferometry

Video-based speckle interferometric methods such as electronic speckle pattern interferometry (ESPI) allow us to measure full-field surface deformation of a diffuse object. In this paper we show, in a first step, that the susceptibility of ESPI to noise can be substantially reduced [1] by synchronizing the optical interferometer and the object stressing system with the CCD image acquisition and processing system, and by performing what amounts to a repetitive sequence of rapid ESPI tests. In this manner, a stable fringe pattern can be obtained as long as the ambient noise is of sufficiently lower frequency than the video acquisition rate (typically 30 Hz). This scheme will be referred in this paper as reference-updating subtractive correlated ESPI to distinguish it from classical ESPI. We then take this issue further with a hybrid additive-subtractive decollated ESPI technique that will freeze out unwanted environmental noise of frequencies much higher than video rates [2], The basic principle of this proposed technique can be summed up as follows: (i) acquire speckle images containing information about the same two deformed object states in every frame of the image acquisition sequence, (ii) decorrelate the speckles between every frame, and (iii) compare every consecutive pair of speckle images to extract visible fringes corresponding to the two deformed states of the test object.</p

EMAT Generation and Laser Detection of Single Lamb Wave Modes

Non-contact and couplant-free nondestructive testing methods are desired in certain situations. There are several designs of non-contact transducers that have been developed. Electromagnetic acoustic transducers (EMATs) and laser-based ultrasonic sensors are well known non-contact transducers. EMATs are typically of limited bandwidth. EMATs and LBU both have lower sensitivity than conventional piezoelectric transducers, and are therefore typically useful in situations where there is sufficient signal strength, but where non-contact is necessary (EMATs and LBU), or where high spatial resolution (LBU) is needed. Thus, ultrasonic NDE schemes with various configurations have been utilized; these configurations include using EMATs as both generator and receiver [1–2], using lasers as both generator and receiver[3–4], and using a laser as source and EMATs as receiver [5].</p

A Fiber Optic Ultrasound Sensor for Monitoring the Cure of Epoxy

Epoxy is a common matrix material in fibrous composites and is frequently used with: kevlar, glass, carbon, and boron fibers. Composite materials with an epoxy matrix are processed in an autoclave which applies temperature and pressure to the part during cure. Temperature and pressure facilitate the crosslinking of epoxide groups which react with amine groups present in the epoxy resin. Current methods of hardening fiber/epoxy composites utilize predetermined values of temperature and pressure in the cure cycle. These values are specified by the manufacturer and do not account for batch to batch variations in the epoxy resin which affect the cure. Possible variations in the epoxy resin include: chemical composition, water content, resin fiber content, temperature history, and humidity [1].</p

Crack detection in fuselage panels by a narrow-band laser-based ultrasonic system

Surface acoustic waves can be used for the characterization of mechanical properties of materials, as well as to investigate the near-surface region of a solid for cracks and other flaws by probing for the presence of scattering sources. In the non-destructive characterization of solids, laser generation of ultrasound as well as interferometric detection of the surface waves are particularly attractive in view of the non-contacting nature of such systems. In recent studies, accurate detection of surface wave speed and attenuation have been shown to be possible by the use of dual-probe laser interferometers[1,2]. A number of authors have also shown the feasibility of generating ultrasound through the use of high-power lasers that generate acoustic waves either by thermoelastically heating the solid, or by ablating the material. For nondestructive applications, however, it is undesirable to ablate the material, and hence one must confine laser generation to the thermoelastic regime. However, in view of the relatively low sensitivity of typical optical detection systems, the generated surface waves have to be sufficiently strong and can be obtained only in the ablation regime if a single laser source is used.</p