COMPOSITE AND SMART STRUCTURES GROUP (CSSG)

Department of Aeronautical Engineering, Building J07 University of Sydney,
Sydney, NSW, Australia, 2006.
Tel: 61-2-9351 2338 Fax: 61-2-9351 4841

ABSTRACTS


William Higgs - M.E.(Res)
An experimental orthotic knee joint has been designed for use as the articulating member of a knee-ankle-foot orthosis (KAFO). The novelty of this orthotic joint, called the Airstrut, is attributed to its inflatable structure, which may permit knee-swing during the swing phase of gait and stability during the stance phase of gait. The aim of this investigation is to investigate the effects of finite inflation on the subsequent buckling response of the Airstrut during simulated knee flexion bending loads. This paper develops a finite element solution for three incremental Airstrut configurations. The first lateral buckling mode and its corresponding bending moment is used to determine the structural limit of the inflated Airstrut. A non-linear material and geometrically non-linear finite element solution is obtained for the inflation load. A Rik's method solution was then obtained to examine the postbuckling response of the Airstrut. In addition, an eigenvalue extraction was performed to study the intrinsic influence of air pressure on the structural stability of Airstrut. Necessarily, a two-dimensional finite element solution was obtained to determine geometric sizing of the KAFO side struts- for compatibility with the Airstrut knee joints. Finally, experimental data was obtained to verify the theoretical results. It is found that a simple eigenvalue extraction provides a conservative estimate of the critical buckling moment for preliminary design purposes. Furthermore, there is good correlation between theory and experiment and exists noticeable interaction between the Airstrut and pressurized air prior to final buckling for this type of orthotic knee joint.

Adrian Rispler - PhD

The goals of this dissertation were to investigate methods of increasing the efficiency of highly loaded aircraft type joints. The thesis can be divided into five main sections. The first section provides some background to the type of joints that can be found on an aircraft. A brief literature review of optimisation methods is also presented in this section. The section is completed with some theoretical background to be employed in subsequent Chapters.

The second section deals with the structural optimisation of bonded joints. Following a brief review of previous research conducted in this field, different optimisation methods are presented and employed in the shape optimisation of the adherends of bonded joints. A heuristic method known as ESO (Evolutionary Structural Optimisation Method) is described. In summary, the method is based on finite element analysis and consists in discarding the unstressed material of a loaded structure in an incremental stepwise fashion. Applications of this method to achieve minimisation of maximum peak stresses in the adhesive follow. In one case, ESO was applied to the shaping of adherends and on the other case to the shaping of an adhesive fillet. In the shape optimisation of adherends, only the portion of adherend within the overlap region is allowed to evolve. Large reductions of peak stresses at the tip of the adhesive were achieved for both single and double lap joints. This is followed by the shape optimisation of adhesive fillets to minimise the stress concentration at the tip of the adhesive. Thus the adherend shape remains unchanged throughout the optimisation process. This method was also successful in achieving significant reductions on peak stresses within the adhesive fillet.

Also in the second section, a modification of the ESO method is introduced for the optimisation of composite pin-loaded joints. This is preceded by presenting the different failure modes commonly found in composite materials and a survey of different methodologies which are currently available to the designer for the reduction of peak stresses on pin-loaded joints. The modified approach proposed, consists of changing the finite element's property of highly stressed elements around a loaded hole in a stepwise fashion, allowing the formation of an insert. This insert provides a localised plastic zone, which reduces the stress concentration on the composite material providing stress relief. The application of this new method to the optimisation of pin-loaded joints resulted in a large reduction in stress concentration around loaded holes. The prediction of stress concentration reductions around loaded holes was correlated by mechanical testing, employing photoelastic analysis to determine full strain fields in the vicinity of the loaded hole. The correlation was very successful both in terms of magnitude and patterns of full strain field, proving the potential of this new method. The method was then applied to multiple load cases and on two holes in a series or in a parallel configuration.

The third section of the thesis deals with the analysis of T-joints used also in marine structures. Experimental studies were conducted employing different filler types at the resin rich area normally found in web/flange intersections. Also, the flange and web thicknesses of T-joints were varied to assess the influence of geometrical parameters on pull-out strength. Finite element analysis was conducted to correlate the no-insert configuration with the thinnest web. Different types of analysis were employed to study this situation. The most accurate failure prediction was achieved with a two-dimensional orthotropic plane strain analysis employing the actual geometry of the plies (as measured by an electronic microscope). Stiffness and failure initiation zone were also accurately predicted using this analysis.

Section four deals with the analysis, design, manufacturing and testing of an aircraft composite hinge. Composite hinges were manufactured of thermoplastic and thermoset materials and tested to failure. Thermoplastic hinges achieved a larger failure load when compared to thermoset hinges. The failure mode was, in all instances, by delamination.

Finally, the fifth section presents conclusions from the different investigations performed on bonded joints, pin-loaded joints, T-joints and the design and testing of a composite hinge. This is followed by recommendations for future work.

Clinton Chee - PhD

The application of static shape control was investigated in this thesis particularly for a composite plate configuration using piezoelectric actuators. A new electro-mechanically coupled mathematical model was developed for the analysis and is based on a third order displacement field coupled with a layerwise electric potential concept. This formulation, TODL, is then implemented into a finite element program. The mathematical model represents an improvement over existing formulations used to model intelligent structures using piezoelectric materials as actuators and sensors. The reason is TODL does not only account for the electro-mechanical coupling within the adaptive material, it also accounts for the full structural coupling in the entire structure due to the piezoelectric material being attached to the host structure. The other significant improvement of TODL is that it is applicable to structures which are relatively thick whereas existing models are based on th in beam/plate theories. Consequently, transverse shearing effects are automatically accounted for in TODL and unlike first order shear deformation theories, shear correction factors are not required.

The second major section of this thesis uses the TODL formulation in static shape control. Shape control is defined here as the determination of shape control parameters, including actuation voltage and actuator orientation configuration, such that the structure that is activated using these parameters will conform as close as possible to the desired shape. Several shape control strategies and consequently algorithms were developed here. Initial investigations in shape control has revealed many interesting issues which have been used in later investigations to improve shape controllability and also led to the development of improved algorithms. For instance, the use of discrete actuator patches has led to greater shape controllability and the use of slopes and curvatures as additional control criteria have resulted in significant reduction in internal stresses. The significance of optimizing actuator orientation and its relation to piezoelectric anisotropy in improving shape controllability has also been presented. Thus the major facets of shape control has been brought together and the algorithms developed here represent a comprehensive strategy to perform static shape control.


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