Nondestructive Testing for Environmental Degradation of Adhesive Joints

The work described here is an ultrasonics based experimental study which aims to address the lack of a reliable technique for detecting strength loss in adhesive joints after exposure to hot wet environments. This is manifested as a change in the failure mode of an adhesive system from a cohesive failure in the as-made condition, that is failure through the adhesive, to an adhesive failure, failure between the adhesive and adherend, after exposure to a hot, wet environment. This work has been concerned with the bonding of aluminum using two part epoxy adhesive. The reason for the change in failure mode is thought to lie in changes in the oxide layer which is present between the aluminum and the epoxy. The oxide layer generally has a porous structure into which epoxy can penetrate, forming a micro-composite layer, referred to as the interlayer. It is the detection of changes in this interlayer which present the biggest problem to current N.D.T. techniques for adhesive joints [1]. This is largely a problem of size, the interlayer being typically no larger than a few microns thick, sandwiched between several hundred microns of epoxy and several millimetres of aluminum. It is the need to detect changes in such a thin layer through such a thick layer which presents the biggest problem

Measurement of Reflectance Function for Layered Structures Using Focused Acoustic Waves

In the ultrasonic NDE of layered materials and structures, such as bonded joint, coating, and in particular the composite material, the surface or Lamb wave velocity or the reflection and transmission coefficient are measured, to determine for examples, the elastic constants, the anisotropy and the integrity of the materials, etc. A commonly used technique to determine locally the surface or Lamb wave velocity V g is based on the measurement of the reflection minima or the transmission maxima at oblique incidence of the test sample. It is supposed that at the critical incident angle θ c where the reflection coefficient appears the minima, the surface or Lamb waves are favorably generated and V g =V o/sinθ c where V 0 is the wave speed in coupling liquid. So, the determination of the reflection function is essential and important. In general, the acoustic reflection or transmission coefficient of a layered medium depends on the wave incident angle θ, the wave frequency ƒ and the orientation angle φ if the material is anisotropic. To obtain the whole information of this reflectance function R(θ,φ,ƒ), one needs to insonify the structure at varying incident and orientation angles and do the frequency spectroscopy using the wide-band transducer

Wave interaction with defects in pressurised composite structures

There exists a great variety of structural failure modes which must be frequently inspected to ensure continuous structural integrity of composite structures. This work presents a Finite Element (FE) based method for calculating wave interaction with damage within structures of arbitrary layering and geometric complexity. The principal novelty is the investigation of pre-stress effect on wave propagation and scattering in layered structures. A Wave Finite Element (WFE) method, which combines FE analysis with periodic structure theory (PST), is used to predict the wave propagation properties along periodic waveguides of the structural system. This is then coupled to the full FE model of a coupling joint within which structural damage is modelled, in order to quantify wave interaction coeffcients through the joint. Pre-stress impact is quantified by comparison of results under pressurised and non-pressurised scenarios. The results show that including these pressurisation effects in calculations is essential. This is of specific relevance to aircraft structures being intensely pressurised while on air. Numerical case studies are exhibited for different forms of damage type. The exhibited results are validated against available analytical and experimental results

Development of capillary electrochromatography for the determination of aldehydes and ketones in air

The objectives in this study were to determine the through-thickness elastic constant of fiber-epoxy/metal composites with ultrasonic measurements and determine the elastic constants of the constituent layers. The specimens that we studied consist of alternating layers of aluminum (2024 T3) and fiber-epoxy [1], all of which are 200 to 300 μm thick. As Figure 1 indicates the outer two layers of the composite are aluminum, so that there is always one additional layer of aluminum in the composite. The fiber-epoxy layers consist of a unidirectional layup of aramid fibers for the aramid-epoxy/aluminum specimens. For the glass-epoxylaluminum specimen, the fiber-epoxy layers consist of two sublayers of unidirectional fibers where the fibers of one sublayer lie at 90° with respect to the other sublayer. Because the fibers of both sublayers are perpendicular to the transmitting transducer, the ultrasound perceives the two sublayers as a single layer

An Experimental Evaluation of the Resonant Ultrasonic Spectroscopy Method

For several years we have used Resonant Ultrasonic Spectroscopy (RUS) for routine determination of elastic-stiffness coefficients [1]. RUS has also been used elsewhere as reviewed by Maynard [2] and by Migliori [3]. Therefore, it is important experimentally establish the uncertainty of the RUS method. Previously, we have compared RUS with pulse-echo superposition and with other resonance methods [4,5] In this paper, we describe the process we used to determine that the RUS method has a 0.15% uncertainty, which is comparable with our best short-pulse time-of-flight measurement methods