Key Engineering Materials Vols. 293-294

Abstract: Wavelet transform is a powerful technique well suited to non-stationary signal processing. The properties of wavelet are determined by its basis function. In the fields of modal analysis, mechanical condition monitoring and fault diagnosis, impulse responses or transient responses are very common signals to be analyzed. The Laplace wavelet is a single-sided damped exponential wavelet and is a desirable wavelet basis to analyze signals of impulse response. A correlation filtering approach is introduced using the Laplace wavelet to identify the impulse response from vibration signals. Successful results are obtained in identifying the natural frequency of a hydro-generator shaft, and diagnosing the wear fault of intake valve of an internal combustion engine.

Abstract: This investigation considers the identification of fatigue crack growth in aerospace grade steel using Acoustic Emission (AE) techniques. Four specimens with small radius crack initiation sites were investigated under four-point bending (SENB4). A linear array of 4 resonant AE sensors was utilised to locate the crack growth. Detection and location of fatigue crack growth in aerospace grade steel SENB4 specimens using appropriate AE techniques was successful. Time of arrival (TOA) linear location has provided an invaluable tool for separating AE signals arising from fatigue crack growth from the notch from those emanating from the loading points. Analysis of the AE feature data from the notch has allowed initial categorisation of AE from fatigue crack growth. AE data from this research was found to correspond with that reported in previous studies [1] examining Compact Tension (CT) specimens.

Abstract: The capability of piezoelectric wafer active sensors to identifying cracks which are common to metallic structural elements subjected to fatigue loading were explored. A number of laboratory tests were performed to investigate Lamb wave transmission and reception characteristics as well as the sensitivity of the transmitted and reflected signals to the presence of a through-the-thickness crack of various length and orientation in a thin plate. Based on the laboratory tests an optimum strategy for crack detection with the pulse-echo method was investigated. The method can be used for large areas scanning with a small amount of sensors and is favoured for embedded and leave-in-place sensor applications.

Abstract: When evaluating concrete strength, common opinion is that adequate precisions can be achieved only by a particular or even total destruction. However, such methods are not always applied, besides they are very laborious. The NDE methods have a number of merits, when compared with destructive ones: a possibility to find cracks and hidden flaws in concrete; besides, they show good results in testing materials of other types, such as metals and composites. At the same time, application of NDE methods to concretes is difficult because of their complex internal structure. No existing theory can predict these properties of the transmitted wave. Therefore, the main goal of the present work is to propose a theoretical model enabling the wave penetration of ultrasonic wave through a medium with multiple internal obstacles to be described adequately. Practical applications of this ultrasonic method is toward the evaluation of mechanical properties of concrete, where the influence of internal dislocations, such as pores and cracks, is of significant importance.

Abstract: Current analysis of Acoustic Emission (AE) signals is dominated by waveform parameter assessment, such as amplitude, rise-time, absolute energy, initiation frequency etc. whilst the actual waveform collected is often ignored. This paper explores the dispersive behaviour of Lamb Wave propagation as a means of estimating source to sensor distance and source orientation from collected waveforms.

Abstract: A number of structures fail due to the fatigue damage resulting from cyclic mechanical stress. However, as far as coatings are concerned this degradation factor remains underestimated. In the paper the authors compare an impact of repetitive mechanical stress on as-received epoxy coating and on the UV and elevated temperature pre-exposed epoxy film.

Abstract: In this paper we introduce nanoscale description of fracture in a single crystal in order to obtain foundations for modelling damage at more averaged scale. Nanoscale model of fracture is discussed together with nanoscale model of slip plasticity. Fracture systems and slip systems are introduced consistently by means of free energy. They enable us to express critical conditions for initiation of both crack and slip. Evolution equations which describe evo- lution of crack, slip and boundary of slip surface are introduced. Furthermore, we also discuss crack initiation and propagation on active slip surface. With the aid of variables characterizing damage at nanoscale level we discuss the form of damage tensor for more averaged model. One discusses larger context of nanoscale modelling in which the nanoscale models are considered as a segment of multiscale description of behaviour of materials.

Abstract: The ultimate success or failure of a restored tooth is dependent on clinical management such as material choice, cavity design, bonding techniques and others. The current paper adopts a fundamental result in the linear theory of elasticity on the singular stress distribution in bi-materials wedge to analyze the choice of different materials used for the restoration of a tooth. Based on the value of the strongest singularity, different restorative materials are evaluated in terms of the susceptibility to the tooth fracture. Comparable results are reported for amalgam, gold alloys, and ceramic materials. Due to a wide variety of mechanical properties the application of resin composites could lead to better or worse fracture resistance of the restored tooth. The theoretical findings are supported by current clinical reports on longevity of restored teeth.

Abstract: This paper presents new analytical results on the crack tip opening displacement (CTOD) for a through-the-thickness crack in an infinite plate of arbitrary thickness. These results are based on a new fundamental solution for an edge dislocation obtained earlier and published elsewhere. The analytical predictions of CTOD for various ratios of the crack length to the plate thickness are compared with an independent three-dimensional elasto-plastic finite element (FE) study. It is shown that both analytical and numerical results are in a good agreement when the numerical calculations are not affected by the size of the FE mesh and finite boundaries of the FE model.

Abstract: This paper reviews a range of simplified and approximate techniques developed and implemented by the authors in the context of elastic-plastic, elastic-creep and elastic-plastic-creep, monotonic loading of both simple components and complex structures. The primary motivation for the work is to provide more rapid methods for estimating failure variables for damage assessment of structures. The context of the work has included both aerospace structures and tubular frameworks, such as offshore jacket-type structures, both of which belong to a class of structures, which contain repeated units where localised plastic damage may be concentrated.