Stuart James Wildy

A new passive defect detection technique based on the principle of strain compatibility

This paper discusses the development of a new passive technique of on-line damage detection based on the most fundamental concept in continuum mechanics: strain compatibility. The main feature of this technique is its invariance to changing operating conditions. The technique is quite general and can be applied to structures made of isotropic or anisotropic materials and structural components experiencing elastic or plastic deformations. A few practical situations, involving the development of crack damage in plate structures, are considered to demonstrate the feasibility of this crack detection technique, as well as its potential for many practical applications. Future work will focus on the experimental validation of the technique.

Reconstruction of baseline time-trace under changing environmental and operational conditions

Compensation of changing environmental and operational conditions (EOC) is often necessary when using guided-wave based techniques for structural health monitoring in real-world applications. Many studies have demonstrated that the effect of changing EOC can mask damage to a degree that a critical defect might not be detected. Several effective strategies, specifically for compensating the temperature variations, have been developed in recent years. However, many other factors, such as changing humidity and boundary conditions or degradation of material properties, have not received much attention. This paper describes a practical method for reconstruction of the baseline time-trace corresponding to the current EOC. Thus, there is no need for differentiation or compensation procedures when using this method for damage diagnosis. It is based on 3D surface measurements of the velocity field near the actuator using laser vibrometry, in conjunction with high-fidelity finite element simulations of guided wave propagation in free from defects structure. To demonstrate the feasibility and efficiency of the proposed method we provide several examples of the reconstruction and damage detection.

Low-cost, open-source, collapsible, air-transportable, field-manufacturable telecommunications tower

Temporary telecommunications systems are often deployed in humanitarian situations. Such systems typically require or operate more effectively with the antennae elevated above surrounding structures, maximizing line-of-sight coverage. Achieving this elevation for all deployments is problematic for several reasons. First, chartered or scheduled commercials airlines continue to play a prominent role in delivering relief personnel and equipment during the acute phase of many disasters and related events. This places limits on the size and weight of equipment that can be delivered during this critical period. Second, any repairs or modification to equipment must rely on limited local resources, and thus it is desirable for equipment to be constructed of common building materials. Third, weather conditions are often poor, requiring structures to be sufficiently strong. This stands in tension with the desire that the equipment be as low-cost as possible to maximize the number of units that can be deployed. In this paper we describe a low-cost, portable, airline luggage compliant, collapsible telecommunications tower designed as a student project that meets various Australian standards, can be easily erected in seven minutes and costs less than US

Detection of delamination damage in a composite laminate beam utilising the principle of strain compatibility

600 in small quantities, and for which all custom parts can be 3D printed in the field. A prototype of this tower weighs just 19kg, and survived a 30-day deployment, including winds up to 22m/s (80km/hour), and is rated to survive much stronger winds. The designs for this tower have been open-sourced for replication and use by any party.

Investigation of thermo-mechanical properties of slurry based thermal barrier coatings under repeated thermal shock

This paper presents an experimental investigation of a new method for damage detection based on the most fundamental concept in continuum mechanics: strain compatibility. Compliance with this principle implies a deformed material is free from discontinuities, which are indicative of many types of structural damage. Therefore the principle of strain compatibility, in its ability to identify discontinuities, is very promising as a new foundation for future research into non-destructive evaluation and structural health monitoring technologies. The proposed method has many advantages compared to existing damage detection techniques, such as its invariance to material properties, type and intensity of loading, and the geometry of the structure. In this paper, a proposed formulation of the strain compatibility equation for beam structures, which is invariant to loading intensity, is presented. An experimental investigation of the proposed algorithm was conducted on a delaminated cantilever beam, utilising a PSV-3D scanning laser vibrometer. The experiment demonstrated that the strain compatibility technique can accurately locate delamination damage in composite beam structures.

Effect of plate thickness and load history on fatigue crack growth

Thermal Barrier Coatings have existed for over 40 years, and with in the last 15 years their use in industrial applications has dramatically increased. Thermal Barrier Coatings (TBCs) are currently used in gas turbines, diesel engines, throughout aerospace and nuclear power industries. The purpose of TBC is to reduce temperature and thermal stresses, and, as a result, increase the reliability and life of load-bearing components subjected to high temperature or temperature flux. However, TBCs often fail under thermal cyclic loading with reliability still being the major issue impeding their wide-spread applications. The focus of this work is on experimental investigations of zirconia/nickel graded TBC system, subject to thermal shock loading. The graded TBC systems were fabricated utilising a recently developed slurry spray manufacturing technique. This is a robust technique, and is able to cover large and curved surfaces at low cost, and provides many advantages in comparison with its alternatives. This paper describes the developed technique and presents selected results of thermo-mechanical and fracture testing of the TBCs including graded coatings fabricated using this new technique.

Accuracy of Quasi-Static Bending Strain Measurement using Scanning Laser Doppler Vibrometry (SLDV)

A new theoretical approach is presented for investigating fatigue crack growth in plates of finite thickness. The developed approach utilizes a modified strip-yield analysis and the concept of plasticity-induced crack closure. A number of typical fatigue crack growth phenomena are investigated including the thickness effect on constant amplitude fatigue crack growth, retardation due to a tensile overload cycle, and short crack growth from sharp notches. Theoretical predictions are compared with experimental data and are found to be in very good correlation.

Scanning laser vibrometer for non-contact three-dimensional displacement and strain measurements

This paper presents an investigation into the use of 1D scanning laser Doppler vibrometry for the measurement of quasi-static bending strain, on a beam, from measured out-of-plane deflections. To calculate the bending strain from measured deflections, a Savitzky-Golay differentiation filter is used. An experimental investigation was conducted to determine how the strain estimate would be affect by the spatial interval between measurement points, amplitude of loading, and the numerical parameters used within the Savitzky-Golay differentiation filter. A number of findings, from the investigations undertaken are presented. The bending strain measurement technique is best approximated by using a third-order polynomial in the Savitzky-Golay differentiator filter. An accurate bending strain measurement can be produced at any spatial interval, as long as the filter width spans a fixed distance. In addition, changes in applied load have no effect on the accuracy of the technique. This technique has significant application for the measurement of localised bending strain on the surface of structures or materials that are cyclically loaded and subjected to harsh environmental conditions, such as elevated temperatures.