Papers by Author: M.R. Wisnom

Abstract: This paper studies the effect of delaminations on strain maps for a simple cantilever beam. The aim is to build an experimental set-up which allows detecting very slight modifications in the strain maps. The case studied is a single delamination on the mid-plane. The measurement method is the deflectometry technique which enables direct slope measurements on a reflective specimen. The comparison with finite element models clearly indicated that the surface strains bear the information of the extent of the delamination. The second step is to use these surface strains to identify a stiffness reduction map for real impact damages.

Abstract: Acoustic emission (AE) is an attractive technique for the structural health monitoring (SHM) of aerospace systems. To reach its full potential in this role a quantitative approach must be adopted to study damage mechanisms in composite materials. In this paper, some of the practical issues regarding acoustic emission testing in composites are addressed. A model describing Lamb wave propagation through plates is described and used to make phase velocity and attenuation measurements in both aluminium and carbon fibre reinforced plastic plates. Results are then implemented in the frequency domain to conduct an experimental study of normal incidence Lamb wave reflections. Comparisons are made with finite element analysis (FEA) models with good results.

Abstract: Acoustic emission (AE) techniques have obvious attractions for structural health monitoring (SHM) due to their extreme sensitivity and low sensor density requirement. A factor preventing the adoption of AE monitoring techniques in certain industrial sectors is the lack of a quantitative deterministic model of the AE process. In this paper, the development of a modular AE model is described that can be used to predict the received time-domain waveform at a sensor as a result of an AE event elsewhere in the structure. The model is based around guided waves since this is how AE signals propagate in many structures of interest. Separate modules within the model describe (a) the radiation pattern of guided wave modes at the source, (b) the propagation and attenuation of guided waves through the structure, (c) the interaction of guided waves with structural features and (d) the detection of guided waves with a transducer of finite spatial aperture and frequency response. The model is implemented in the frequency domain with each element formulated as a transfer function. Analytic solutions are used where possible; however, by virtue of its modular architecture it is straightforward to include numerical data obtained either experimentally or through finite element analysis (FEA) at any stage in the model. The paper will also show how the model can used, for example, to produce probability of detection (POD) data for an AE testing configuration.

Abstract: Acoustic emission (AE) testing is an increasingly popular technique used for nondestructive evaluation (NDE). It has been used to detect and locate defects such as fatigue cracks in real structures. The monitoring of fatigue cracks in plate-like structures is critical for aerospace industries. Much research has been conducted to characterize and provide quantitative understanding of the source of emission on small specimens. It is difficult to extend these results to real structures as most of the experiments are restricted by the geometric effects from the specimens. The aim of this work is to provide a characterization of elastic waves emanating from fatigue cracks in plate-like structures. Fatigue crack growth is initiated in large 6082 T6 aluminium alloy plate specimens subjected to fatigue loading in the laboratory. A large specimen is utilized to eliminate multiple reflections from edges. The signals were recorded using both resonant and nonresonant transducers attached to the surface of the alloy specimens. The distances between the damage feature and sensors are located far enough apart in order to obtain good separation of guided-wave modes. Large numbers of AE signals are detected with active fatigue crack propagation during the experiment. Analysis of experimental results from multiple crack growth events are used to characterize the elastic waves. Experimental results are compared with finite element predictions to examine the mechanism of AE generation at the crack tip.