Real-Time Radiographic Investigation of Weld Pool Depression

Real-time radiography has been utilized by us previously as a vision system in remote arc welding process monitoring [1]. Direct information on weld penetration extracted from real-time radiographic images of the solidified weld was used in feedback to adjust the welding conditions to maintain weld quality [2]. The disadvantage of this method is the time (and space) delay in extracting information on weld penetration which leads to the appearance of small weld areas with lack of penetration. In subsequent work weld penetration was controlled by the use of radiographic information on weld pool depression to eliminate the time delay thus providing feedback before weld solidification [3]. In this implementation the method was applied to control bead-on-plate weld penetration only with no weld joint involved. A weld made on a joint which must be filled by liquid metal during welding is more complicated than a bead-on-plate weld

Ultrasonic Evaluation of Interfacial Properties in Adhesive Joints: Evaluation of Environmental Degradation

It is known that failure of adhesive joints can occur either in the bulk region of an adhesive layer (cohesive mode) or along an interface between the adhesive and an adherent (interfacial or adhesion mode). Properly manufactured adhesive joints fail in the cohesive mode, while after environmental degradation the failure mode becomes predominantly interfacial. Therefore we conclude that humid environments affect mostly the interfacial region of a joint. This indicates the importance of non-destructive assessment of interface properties. While methods of cohesive property evaluation have progressed significantly, much more effort is required to develop interface evaluation methods. A development of this type is undertaken in this work

Second Order Asymptotic Boundary Conditions for Modeling of Imperfect Interface

The interface between two solids has properties differing from those of the bulk media. The actual structure of such an interface depends on the particular type of solid contact: contact formed during solidification, metallurgical solid bond, dry mechanical or lubricated contact, et cetera. The classical boundary conditions which are satisfied for an infinitely thin perfect bond are not adequate to describe wave interaction with such an imperfect interface.</p

Efficient Reconstruction of Elastic Stiffnesses in Isotropic and Anisotropic Plates

Because composite materials are often used in safety critical applications, such as in aerospace, it is desirable to have a quick and reliable confirmation of their material properties. The aim of this paper is to introduce the idea of inferring elastic and/or viscoelastic material properties from plate wave measurements with a minimum amount of data. Material characterization of anisotropic materials has been a focus point of research for many years. A good review of this literature can be found in a recent review article [1]. Rokhlin and collaborators [2–4] have also studied this problem extensively, using propagation characteristics and the transmission coefficient to reconstruct elastic properties. The advantage of reconstructing the stiffnesses from the zeros of the transmission coefficient lies the the fact that in contrast to the zeros of the reflection coefficient the transmission coefficient zeros are independent of the fluid properties. A drawback of this method is that the plate must be accessible from both sides.</p

Measurements of Elastic Constants of Thin Al2O3 and SiC/Al Composite using Coupled Ultrasonic Plate Modes

An ultrasonic technique utilizing coupled ultrasonic plate modes for the measurement of elastic constants has been suggested in our previous studies [1–3]. The technique is based on measurements of obliquely incident ultrasonic beam zero-transmission angles and reconstruction from these angles of the composite elastic constants. Such a technique is particularly useful for measuring elastic constants of anisotropic plates and it has a unique capacity to measure in-plane elastic constants of thin anisotropic plates

Ultrasonic Guided Waves for Lap Splice Joint Inspection in Aging Aircraft

The integrity and reliability of adhesive joints in structural applications has long been a critical issue. Failure of these bonds during service could be catastrophic. In particular, the aviation industry due to its progressively aging fleet calls for a reliable non-destructive testing technique to characterize the bond quality. Several NDE techniques were developed each with its own advantages and disadvantages. [1–2] Utilization of ultrasonic guided waves offer a reliable and time efficient method. [3] Following several laboratory studies this technique was tested on a service damaged aircraft at Sandia National Laboratories in Albuquerque, NM. A Double Spring Hopping Probe (DSHP) was developed and tested on this test bed. A protocol for the inspection of lap splices and tear strap was established. The concepts of utilization of wave structure in mode selection, angle and frequency tuning for optimal use of the Lamb type guided waves are addressed. Highlights of our recent work on aging aircraft inspection are presented in this paper. The work expands on our earlier presentations. [4

Finite-element modelling of elastic wave propagation and scattering within heterogeneous media

The scattering treated here arises when elastic waves propagate within a heterogeneous medium defined by random spatial fluctuation of its elastic properties. Whereas classical analytical studies are based on lower-order scattering assumptions, numerical methods conversely present no such limitations by inherently incorporating multiple scattering. Until now, studies have typically been limited to two or one dimension, however, owing to computational constraints. This article seizes recent advances to realize a finite-element formulation that solves the three-dimensional elastodynamic scattering problem. The developed methodology enables the fundamental behaviour of scattering in terms of attenuation and dispersion to be studied. In particular, the example of elastic waves propagating within polycrystalline materials is adopted, using Voronoi tessellations to randomly generate representative models. The numerically observed scattering is compared against entirely independent but well-established analytical scattering theory. The quantitative agreement is found to be excellent across previously unvisited scattering regimes; it is believed that this is the first quantitative validation of its kind which provides significant support towards the existence of the transitional scattering regime and facilitates future deployment of numerical methods for these problems

Theoretical Study of High Frequency Ultrasonic Wave Attenuation in Polycrystalline Materials

Three different regimes for scattering of ultrasonic waves in poly-crystalline materials exist, depending on the ratio of the mean grain size to the wavelength: (i) the low frequency (Rayleigh) region with scattering-induced attenuation proportional to the fourth power of the frequency and to the cube of the mean grain diameter, (ii) the medium frequency (stochastic) region with scattering proportional to the square of the frequency and to the mean grain diameter, and (iii) the high-frequency (geometric) region with scattering independent of frequency

Lamb Wave Scattering from Rivets

For structures with large surface areas, a full integrity evaluation can be a time-consuming operation. Lamb wave techniques allow this evaluation to be performed with waves propagating along one dimension of the inspection area while the probing transducers are moved in the perpendicular dimension, giving information about the presence of flaws within the entire scanned area. For riveted structures the scattering of the Lamb waves from the rivets is often the dominant feature in the measured response, masking the more subtle effects of Lamb wave interactions with the flaws of interest [1]. In this paper we consider the scattering of lowest mode symmetric and antisymmetric Lamb waves from model rivets, and derive analytic expressions for the scattered fields. With solutions of this type the disruptive effects of the rivets can be “processed out” of measured data in order to expose the signals which are due to the flaws in the structure