Nonlinear mixing of non-collinear guided waves at a contact interface.

Wave mixing offers several practical benefits relative to harmonic generation for detecting both distributed and localised damage. An analytical model is proposed for predicting the frequency, mode and direction of propagation of the mixed modes arising from the nonlinear mixing of two incident guided-wave modes at an interface exhibiting contact acoustic nonlinearity. These predictions are validated by a finite-element (FE) analysis involving a unilateral contact law that models both clapping and frictional sliding at the interface. This analysis also provides quantitative insights regarding the optimal interaction angle between the incident waves in the case of shear-horizontal SH0 modes, and the dependence of the mixed-wave amplitude on the load ratio (i.e. ratio of contact stress to normal stress across the interface due to both incident waves). The non-collinear mixing of guided waves is also investigated experimentally for various values of contact stress and incident stress amplitude, demonstrating the existence of mixed waves, as well as showing that interface mixing leads to a higher amplitude mixed wave than that due to the background material nonlinearity. This higher amplitude combined with great flexibility in the choice of incident wave parameters (frequency, mode and direction) makes wave mixing an attractive practical approach for detecting contact acoustic nonlinearity at crack-like defects and interfaces. Experimentally, the maximum value of mixed-wave amplitude is obtained at the same value of contact stress for the various incident stress amplitudes, whereas the FE model shows a maximum at a unique value of the load ratio. This difference may be a consequence of surface roughness, which is not included in the FE model.

Statistical data for the tensile properties of natural fibre composites.

This article features a large statistical database on the tensile properties of natural fibre reinforced composite laminates. The data presented here corresponds to a comprehensive experimental testing program of several composite systems including: different material constituents (epoxy and vinyl ester resins; flax, jute and carbon fibres), different fibre configurations (short-fibre mats, unidirectional, and plain, twill and satin woven fabrics) and different fibre orientations (0°, 90°, and [0,90] angle plies). For each material, ~50 specimens were tested under uniaxial tensile loading. Here, we provide the complete set of stress-strain curves together with the statistical distributions of their calculated elastic modulus, strength and failure strain. The data is also provided as support material for the research article: "The mechanical properties of natural fibre composite laminates: A statistical study" [1].

A computer simulation study of imaging flexural inhomogeneities using plate-wave diffraction tomography.

This paper investigates the feasibility of plate-wave diffraction tomography for the reconstruction of flexural inhomogeneities in plates using the results of computer simulation studies. The numerical implementation of the fundamental reconstruction algorithm, which has recently been developed by Wang and Rose [C.H. Wang, L.R.F. Rose, Plate-wave diffraction tomography for structural health monitoring, Rev. Quant. Nondestr. Eval. 22 (2003) 1615-1622] is investigated addressing the essential effects of applying the discrete form of the Fourier diffraction theorem for solving the inverse problem as discussed by Kak and Slaney [A.C. Kak, M. Slaney, Principles of Computerized Tomographic Imaging, IEEE Press, New York, 1988] for the acoustic case, viz. diffraction limited sensitivity, influence of weak scatterer assumption, damage location and scatter field data processing in time and Fourier space as well as experimental limitations such as finite receiver length and limited views. The feasibility of the imaging technique is investigated for cylindrical inhomogeneities of various severities and relative position within the interrogation space and a normal incident interrogation configuration. The results show that plate-wave diffraction tomography enables the quantitative reconstruction of location, size and severity of plate damage with excellent sensitivity and offers the potential for detecting corrosion thinning, disbonds and delamination damage in structural integrity management applications.

An experimental and finite element poroelastic creep response analysis of an intervertebral hydrogel disc model in axial compression.

A hydrogel intervertebral disc (IVD) model consisting of an inner nucleus core and an outer anulus ring was manufactured from 30 and 35% by weight Poly(vinyl alcohol) hydrogel (PVA-H) concentrations and subjected to axial compression in between saturated porous endplates at 200 N for 11 h, 30 min. Repeat experiments (n=4) on different samples (N=2) show good reproducibility of fluid loss and axial deformation. An axisymmetric nonlinear poroelastic finite element model with variable permeability was developed using commercial finite element software to compare axial deformation and predicted fluid loss with experimental data. The FE predictions indicate differential fluid loss similar to that of biological IVDs, with the nucleus losing more water than the anulus, and there is overall good agreement between experimental and finite element predicted fluid loss. The stress distribution pattern indicates important similarities with the biological IVD that includes stress transference from the nucleus to the anulus upon sustained loading and renders it suitable as a model that can be used in future studies to better understand the role of fluid and stress in biological IVDs.

Determination of the regression rate of a fast moving solid/liquid interface using ultrasonics.

This paper reports on an ultrasonic measurement system and its application for in situ real-time measurement of very fast regression rates (>200 mm/s) of the melting interface (RRMI) produced when burning particular metals such as aluminium at high pressures. The RRMI is referred to as the rate at which a solid/liquid interface moves along a metallic rod while burning in an oxygen-enriched atmosphere. The ultrasonic transducer and associated equipment used to drive and record the transducer's output signal and conversion of this output into a regression rate is described. Aluminium rods were burned in pure gaseous oxygen at pressures up to 69 MPa (10,000 psia) where the RRMI was calculated at 204+/-2 mm/s. Other tests with a variety of sample materials, geometric shapes and test conditions were also conducted. The resulting RRMI's calculated with the ultrasonic measurement system compare excellently with rates obtained using a visual review of the same tests and with published results (where available).

The influence of backward wave transmission on quantitative ultrasonic evaluation using Lamb wave propagation

In view of the various novel quantitative ultrasonic evaluation techniques developed using Lamb wave propagation, the influence of an important related phenomenon, backward transmission, is investigated in this paper. Using the discrete layer theory and a multiple integral transform method, the surface displacement and velocity responses of isotropic plates and cross-ply laminated composite plates due to the Lamb waves excited by parabolic- and piston-type transmitting transducers are evaluated. Analytical expressions for the surface displacement and velocity frequency response functions are developed. Based on this a large volume of calculations is carried out. Through examining the characteristics of the surface displacement and velocity frequency response functions and, especially, the different propagation modes' contributions to them, the influence of the backward wave transmission related to quantitative ultrasonic nondestructive evaluation applications is discussed and some important conclusions are drawn.