Papers by Author: W. George Ferguson

Abstract: Fatigue cracking reported in a lighting pole on an elevated bridge structure near Wellington raised the question of how to better design for and predict the fatigue life of lighting poles subject to wind induced fatigue. There have been concerns as to the reliability and currency of the methods commonly used in New Zealand. The paper therefore reviews current international design methods and describes the development of an improved fatigue design method for lighting poles in New Zealand. The new method uses fracture mechanics based crack growth formulations in conjunction with a modified J.D. Holmes Method for wind response analysis of the pole to varying wind speeds. Cumulative crack growth is calculated iteratively rather than using an S-N curve based Palmegran-Miner summation. Wind spectra used in the method are developed from long term meteorological records at representative locations. Software has been prepared to enable quick assessment of the expected fatigue life of lighting poles, and associated gear openings and holding down bolts The method and software has been calibrated with reference to full scale laboratory fatigue proof testing of representative base stubs and natural wind response testing of a 12 m high lighting pole.

Abstract: During the mid 1990s earthquakes in Northridge, California, and Kobe, Japan, illustrated a lack of understanding of the behaviour of structural steels exposed to seismic loads. Under this type of load regime, structural steel members are subjected to fully plastic load cycles and unexpectedly brittle failures resulted. This paper presents a simple, yet powerful, method for predicting the accumulation of damage in a steel element, based on its toughness. In addition the damage parameter chosen provides an accurate prediction of when failure of the element can be expected to occur. The damage accumulation model developed allows for the deconvolution of complex load histories, such as could be expected to occur during a seismic event, in a systematic, stepwise manner. This approach is ideally suited to automation and could readily be implemented into a finite element model.

Abstract: Precipitation hardening, or aging hardening, is one of the most widely adopted techniques for strengthening of aluminium alloys. During the precipitation process, three major mechanisms are involved: i.e. nucleation, growth and coarsening. Kampmann and Wagner have developed a powerful and flexible numerical approach (KWN model) for dealing with concomitant nucleation, growth and coarsening and thus capable of predicting the full evolution of the particle size distribution. KWN model has been successfully applied to a number of aluminium alloy systems, such as 2xxx, 6xxx and 7xxx. However, most of these modelling works were focused on the wrought aluminium alloys, few had applied to the casting aluminium alloys. In the present modelling work, the microstructure evolution is modeled based on the KWN model and then a strength model based on the well established dislocation theory is used to evaluate the resulting change in hardness or yield strength at room temperature. Then the modelling is applied to casting aluminium alloys A356 and A357. And the modelling results are validated by comparing with own experimental results and the results obtained from the open literature.

Abstract: Nanoindentation technique has been widely used for measuring mechanical properties from a very small volume of material. The hardness measured using the depth sensing nanoindentation technique often decreases with increasing indentation size, the so called indentation size effect (ISE)[1, 2]. It has been generally acknowledged that the ISE in crystalline materials originates from the density change of geometrically necessary dislocations (GND) needed to accommodate a permanent indentation imprint. Conventionally, to characterize an ISE often requires a series measurement of hardness values at different indentation size. Based on the celebrated Oliver-Pharr scheme[3]. We propose a method to derive the ISE from the loading curve of one single indentation test. The application and limitation of the proposed method will be discussed.

Abstract: A method for assessing likelihood of brittle fracture in cyclically loaded steel assemblies subjected to inelastic strains is proposed. The method proposed is based upon relationships between monotonic and cyclic endurance of steel specimens proposed by Kuwamura and Takagi, and analysis of crack tip opening displacement (CTOD), Charpy V-Notch (CVN) and tensile results of pre-strained, fatigue pre-cracked and side-grooved specimens of constructional steel. The proposed method allows the influence of displacement ductility classification (as used in seismic design of structures), notch geometry, and cyclic strain amplitude history on crack initiation to be incorporated into a single design analysis approach. Small scale CTOD testing of steel materials with various levels of pre-strain may be used to identify stress intensity and crack tip displacement at crack initiation for use in the analysis. The integration of a fracture mechanics based approach to analysing stress intensity in conjunction with assembly plastic deformation characteristics derived from finite element modeling offers the promise of an improved approach to steel assembly design for cyclic plastic endurance and should result in more reliable structures and reduced need for large scale testing. This has particular relevance to the structural design of seismic resisting steelwork assemblies which are expected to develop dependable ductile behaviour under high strain variable amplitude cyclic actions.