S.W. Boyd

Integrity of hybrid steel-to-composite joints for marine application

There are many instances where the use of weight-saving polymer composite material for an entire structure is either too complex, too expensive or unfeasible. In these circumstances the use of a hybrid structure can incorporate the benefits of traditional (e.g. steel) construction coupled with the advantages of composite materials [e.g. glass-reinforced polymers, glass-reinforced plastic (GRP)] in weight-critical areas. A number of studies have been carried out on the static strength of hybrid steel-to-composite joints. In the present study, an experimental investigation was undertaken into the fatigue life characterization of a hybrid steel-to-GRP joint. It was found that the fatigue data correlated well with the statistical-based Weibull cumulative distribution function. In addition, post fatigue (in-plane and out-of-plane) residual strength tests were undertaken to ascertain the joint structural performance after cyclic loading. Finite-element-based progressive damage analyses incorporating damage initiation and propagation characteristics, showed good correlation with experimental results.

Fabrication, Testing and Analysis of Steel/Composite DLS Adhesive Joints

This paper describes experimental and numerical techniques to study the structural design of double lap shear joints that are based on thick-adherend steel/steel and steel/composite, with epoxy adhesive. A standard practical fabrication method was used to produce specimens of various dimensions and materials. These specimens consist of 10 mm steel inner adherend and various outer adherend materials including composite and steel of various thicknesses and overlaps. The composite is largely based on carbon fibre-reinforced plastic. The specimens were tested under monotonic tensile loading and the results showed that joint strength depends largely on materials combination and overlap length. The testing also included the use of an advanced imaging system to determine failure initiation and propagation. Two-dimensional finite element analysis (FEA) stress models were applied and showed the importance of modelling the composite layers adjacent to the adhesive bondline in order to account for the critical local stresses. The FEA results also showed that overall shear stress distributions can be used to characterise joint failure. The paper presents the experimental and numerical details with key conclusions.

Nature in Engineering for Monitoring the Oceans: Towards a Bio-Inspired Flexible Autonomous Underwater Vehicle Operating in an Unsteady Flow:

It has long been understood that swimming marine animals have evolved capabilities in terms of speed, manoeuvrability, and efficiency which are desirable for underwater vehicles. Despite this, solutions inspired by nature, or bio-inspiration, are very rarely applied to solve engineering challenges. In particular, it is understood that fish have the ability to alter their mode of swimming to interact with naturally produced vortices as a method of conserving energy and in certain instances extracting energy from a flow. This paper considers whether a bio-inspired flexible autonomous underwater vehicle (AUV) could exploit unsteady flow features to reduce its cost of transport. An analytical model is developed which allows an AUV designer to predict which flow frequencies excite the natural vibration modes of a flexible cylinder. It is demonstrated that by placing a flexible cylinder in an unsteady flow, such as downstream of a bluff body, a similar mechanism to that used by fish may be exploited to move the cylinder upstream with no power input except that extracted from the flow.

Speckle Pattern Characterisation for High Resolution Digital Image Correlation

Digital image correlation (DIC) is an optical technique for full field deformation measurement. The spatial resolution and precision of the measurements are limited by the number of pixels within the image. The use of magnifying optics provides greater spatial resolution images, enabling smaller displacements to be observed with greater accuracy. Increasing the magnification of an image significantly changes the appearance of the non-periodic, stochastic speckle pattern which provides the grey scale contrast necessary for the image correlation method. In the paper a methodology is developed to evaluate the properties of different speckle pattern types under a range of resolutions up to 705 pixel / mm. Numerical deformation of the patterns is also undertaken to evaluate how the changes in the pattern properties affect the accuracy of the DIC measurements.

Composite materials for marine applications – key challenges for the future

This chapter presents the key challenges for the future use of composite materials for marine applications. Five technical challenges have been identified: load transfer mechanisms, safety, life cycle assessment, concurrent engineering and structural health monitoring. These are discussed in the following sections of the chapter. The mechanical behaviour of layered orthotropic structures is considered for both adhesively bonded and hybrid joints, and the strength, fatigue, failure prediction, ageing and optimisation of the connections are described. The safety section discusses the challenge of managing variability and uncertainty when constructing bespoke marine craft, where extensive testing and prototyping are not possible, considering fabrication, strength and through-life behaviour. In the life cycle assessment section, environmental impact is considered using an embodied energy approach. Concurrent engineering approaches that incorporate both the design and production functions in a non-sequential manner are particularly important in the marine industry. A future requirement in this field will be the ability to incorporate design history in optimised design solutions. The structural health monitoring section focuses on state-of-the-art inspection techniques such as vibration-based damage identification approaches and other data-rich experimental mechanics techniques for use in repair intervention strategies. The chapter concludes with comments on the future use of polymeric composite materials for structural marine applications.

A cure modelling study of an unsaturated polyester resin system for the simulation of curing of fibre-reinforced composites during the vacuum infusion process:

This study presents the cure kinetics and cure modelling of an ambient curing unsaturated polyester (UP) resin system for its cure simulation in the vacuum infusion (VI) process. The curing of the UP resin system was investigated using differential scanning calorimetry (DSC). The dynamic DSC test measurements were conducted to find out the ultimate heat of reaction and enable experimental conversion determination for the isothermal curing. The empirical autocatalytic cure kinetics model incorporating the Arrhenius law represented the cure behaviour. The results of the cure kinetics study, the cure model, the material properties and the boundary conditions were the inputs in PAM-RTM software for the simulation of the degree of cure and the exothermic temperature during the infusion and the room temperature curing stages. The simulation results were compared with experimentally measured data. A vacuum infusion (VI) experiment involving a non-crimp glass fibre preform was performed in order to monitor the curing using thermocouples and validate the temperature simulation result. It was shown that the degree of cure and the exothermic temperature of a room temperature curing thermoset resin system during the VI process could be predicted through the steps of this study.

Dynamic analysis of composite marine structures using full-field measurement

Composite materials are increasingly used in structural applications within the marine industry. Due to the geometric complexity of marine structures, there is a practical requirement that they are assembled by joining smaller component pieces using either mechanical fasteners or adhesive bonding. In this paper Digital Image Correlation (DIC) is used to provide full-field analysis of the complex strain fields generated within an adhesively bonded composite single lap joint. Tests are undertaken quasi-statically and at high rate, demonstrating a significant change in the assembly response between laboratory testing conditions and dynamic loading events typical of the marine environment. The work demonstrates the potential of applying full-field experimental technique to provide detailed analysis of complex structural problems, typical of large marine structures.

Experimental Study of Dynamic Behaviour of Aluminum/Aluminum and Composite/Composite Double Lap Joints

This paper presents an experimental investigation on the behaviour of structural adhesive bonding under quasi-static and moderately high loading rates. It addresses the effects of the loading rate on the strength of the adhesively bonded joints under dynamic tensile. A comparison has been achieved between the strength and the damage of specimens’ made of aluminium and lamina substrates. High rate tests showed ringing in the force/displacement curves.

Fatigue life and residual strength analysis of steel-composite joints

There are many instances where the use of weight saving composite materials for an entire structure is either; too complex, too expensive or unfeasible. In these circumstances the use of a hybrid structure can incorporate the benefits of traditional construction materials, for example steel, coupled with the advantages of composite materials in weight critical areas. In the present study, an investigation was undertaken into the fatigue life characterisation of a hybrid joint for marine application. In addition the residual strength of the joint, after a fixed number of fatigue cycles, was assessed under axial compression and bending loads. A progressive damage model was developed to predict the location of major stress concentrations, the path of damage and subsequent loss in stiffness of the joint under axial compression.

Tidal turbine blade selection for optimal performance in an array

In order to maximize tidal energy capture from a specific site free stream devices are situated in arrays. In an array the downstream evolution of the wake generated by a rotating tidal energy conversion device influences the performance of the device itself, the bypass flow to either side as well as the performance of any downstream device. As such it is important to design a turbine that can perform efficiently and effectively in these circumstances. Use of passively adaptive composite blades for horizontal axis tidal turbines has been shown to improve performance in fluctuating inflows. Active adaptation and/or bi-directional hydrofoil sections could be implemented in order to optimize performance throughout the tidal cycle. This paper considers the performance in an array of four free stream turbines implementing standard rigid blades, wholly bi-directional blades, passively adaptive blades and actively adaptive blades. The method used to evaluate the performance of tidal current turbines in arrays couples an inner domain solution of the blade element momentum theory with an outer domain solution of the Reynolds averaged Navier Stokes equations. The annual energy capture of four devices with each blade type in a staggered array is then calculated for a single tidal cycle and compared.