Professor Paul Smith
Academic and research departmentsFaculty of Engineering and Physical Sciences.
Paul is an engineer by background, holding a first Degree in Engineering from the University of Cambridge, where he subsequently carried out doctoral research on the mechanical fastening of carbon fibre reinforced composites.
Joining the University of Surrey in 1986, Paul started as a lecturer in the Department of Materials Science and Engineering. He has held a wide range of posts including that of Head of School of Engineering and as Associate Dean (International). His current role is the Pro-Vice-Chancellor, Executive Dean of the Faculty of Engineering and Physical Sciences.
In research his early interests on mechanical behaviour of composite materials have broadened to encompass a wider range of materials and in particular cast iron assets in the water industry. He is an author on more than 250 journal and conference papers and has presented at numerous international meetings.
University roles and responsibilities
- Pro-Vice-Chancellor, Executive Dean of the Faculty of Engineering and Physical Sciences
Affiliations and memberships
Paul's early research career focused very much on the mechanical behaviour of composite materials, with polymer and ceramic matrices, a field in which he remains active.
He has a particular interest in understanding damage and fracture behaviour and linking this to engineering design methods; he was one of the organisers of WWFE-III, a major evaluation of the current state-of-the-art with regard to predictive models for damage and fracture of composite laminates.
Since the late 1990s, Paul`s research interests have broadened to encompass aspects of the mechanical behaviour of a wider range of materials and in particular cast iron assets in the water industry.
Current research programmes
- Damage accumulation and modelling in composite materials and structures
- Non-destructive evaluation of composite structures
- Impact of composite structures
- Deterioration of buried water distribution pipes
With colleagues at the University of Surrey (in particular Drs Mike Mulheron and David Jesson) Paul has been working with Thames Water for more than 15 years on a range of programmes related to cast iron infrastructure and its management.
This has led to a number of publications relating to the fundamental materials science and engineering of cast iron in service and models which inform the asset management process.
In addition to Thames Water, he is involved currently in projects with a number of companies including BAE Systems, Defence Science and Technology Laboratory, McLaren, NPL and QinetiQ.
Indicators of esteem
European Editor and Editor-in-Chief for Composites part A: applied science and manufacturing
Member of the EPSRC Materials College
Member of the Composites Division Board of the Institute of Materials
Fellow of the Institute of Materials, Minerals and Mining
Chartered Engineer of the Institute of Materials, Minerals and Mining
Former President of ICCM (International Committee for Composite Materials)
World Fellow of ICCM (International Committee for Composite Materials)
A framework for process-related resin selection and optimisation is proposed in the context of research and development for industrial applications of high-pressure resin transfer moulding (HP-RTM). The first stage involves the validation of the reaction kinetics model by differential scanning calorimetry (DSC) and determination of the reaction constants, and the characterisation of viscosity, storage- and viscous-shear moduli by dynamic mechanical analysis (DMA) in a rheometer as a function of time. It also includes capillary pressure measurements for a curing resin impregnating a vertical fibre yarn. Process-related resin selection criteria are based on the optimisation of cycle time, including filling time against gel time, micro-infiltration time and demould time. The proposed framework and the associated test and analysis methodologies have been applied to three epoxy resin systems in connection with carbon fibre reinforcement.
© QinetiQ Ltd 2014.In a manner reminiscent to establishing the 'periodic table', researchers and keen material scientists/engineers have been engaged in intensive activities trying to identify their own characteristic model or discover a unique aspect/failure mode in a composites material. No tangible progress could have possibly been achieved without the relentless efforts made by some 40 dedicated developers of advanced methods for failure criteria for composites. They have been at the core of an international initiative, referred to as the World-Wide Failure Exercise (WWFE). It is aimed at establishing the maturity of existing method and the remaining challenges of building the best method to accurately predict the strength of composites materials. The paper deals generally with the three exercise (WWFE, WWFE-II and WFE-III) which have been conducted over the last 20 years. The focus is on some of the lessons emanating from the latest exercise (WWFE-III).
In order to assess the remaining life of cast iron assets in the water sector, an understanding of their fracture and fatigue characteristics is necessary. The present work is concerned with the toughness and Paris crack growth behaviour of cast iron materials with a range of micro-structures, taken from trunk mains currently in service. When considered with other data from the literature, the results from the present study enable the range of fatigue crack growth behaviour likely to be seen in service to be quantified. The role of microstructure in fracture and fatigue behaviour is discussed. Calculations of fatigue life based on integration of the Paris law are then carried out and compared with previously published data for samples from cast iron distribution mains. The results from these investigations support the development of asset management tools for use in the water industry. (C) 2010 Elsevier B.V. All rights reserved.
Renovation techniques such as use of liners to extend the lifetime of pipe networks are well established in the water industry. As well as spanning gaps and bridging holes, interactive liners are capable of surviving circumferential (or ring) failure, as may be experienced by distribution mains under flexural loading; movement of the host main following failure leads to tensile and shear displacements of the liner near the fracture. Experiments were carried out on cast iron host pipes containing a simulated transverse fracture, with thin-walled (similar to 3 mm) medium density polyethylene liners having a variety of longitudinal gaps. The fracture plane was subjected to shear displacements up to similar to 50 mm and longitudinal separations up to similar to 20 mm. Pipes were held at a pressure of 10 bar for about 12 days to allow creep deformation to take place, while the ability of the liner to withstand the applied displacement and separation was determined. It was demonstrated that the liner material investigated could withstand lateral deflections of the order of 50 mm for considerable periods of time. The implications of this finding on current design guidelines need further consideration. However, if the outcomes are favourable, this could lead to increased use of interactive liners, with potential benefits to both the water industry and the customer.
Bio-derived fibres and resins are of increasing interest as alternatives to petrochemicals in the production of so-called environmentally friendly composite materials. However, whilst the majority of systems consider complete replacement, another route is to look at the constituents that are required to give certain properties, including the content of diluents; a third is to identify ‘hot spots’ in manufacturing. This paper considers these three possibilities in the context of the production of a resin system, and presents results from a life cycle assessment. The aim of this study was to make qualitative assertions based on quantitative estimates. The current work provides a practical assessment of the contribution of the manufacturing process of a multi-part resin formulation to a range of environmental impacts. As a part of this, a multi-stage methodology, the first of its kind, which is more relevant for the batch processes used to manufacture many structural thermosetting polymer systems, was developed. This was applied to a range of resins, some of which include bio-mass derived precursors. For the boundary conditions used, the indications are that the impacts due to taking the constituents and processing them to produce the resin system are insignificant compared with those due to producing the feedstocks in the first place. Surprisingly, whether the feedstocks were from fossil resources or were bioderived was of little significance. As a consequence of the analysis, it has been demonstrated that whilst a manufacturer can make significant savings through careful management of plant and the supporting energy mix, significant improvements to the environmental impacts of resin systems can be made through the choice of particular monomers.
Engineered Cement Composites (ECC) materials have the potential to be used in civil engineering applications where a level of ductility is required to avoid brittle failures. However uncertainties remain regarding mechanical performance, physical properties, shrinkage and durability. In the present work, specimens containing cement powder and admixtures have been manufactured following two different processes and tested mechanically. Multiple matrix cracking has been observed in both tensile and flexural tests and this leads to “strainhardening” behaviour. The results have been correlated with sample density and porosity and it is suggested that higher levels of porosity do not necessarily lead to a loss of the strain hardening capacity. Shrinkage has been investigated and it is shown, consistent with the literature, that shrinkage can be reduced both by controlling the initial environment to which the material is exposed and by the use of additives. Durability was assessed by flexure testing of beams specimens aged for different times. Initial testing (up to one year) indicates that the specimen retain ductility, although the initial cracking threshold increases with time – which may have implications for longer aging times.
The laser treatment of ceramics can lead to increased concentrations of hydroxyl ions on the surface, resulting in improved adhesive bond strength in quasi-static tests. Whether the improvement can be translated to armor applications is investigated here. The ballistic testing of composite-backed, surface treated and non-treated ‘control’ alumina and silicon carbide panels was undertaken. The failure locus of the ceramic to adhesive/composite joint and the qualitative degree of damage were assessed. Laser surface treated samples performed better than control samples, with silicon carbide moving from single shot to multi-shot capability, thus giving significant advantages for the deployment of these materials.
Three measurement techniques used to measure the glass transition temperature (Tg) have been subjected to a critical comparison; dynamic mechanical analysis (DMA), thermomechanical analysis (TMA) and differential scanning calorimetry (DSC). A new procedure, whereby different specimens are tested over a range of heating rates, has been used in order to eliminate the effects of thermal lag and determine a Tg independent of heating rate (Tg(0)). It has been shown that for measurements of Tg(0) for composites, the DMA thermal lag ‘corrected’ method gave the most reliable data. The work has provided additional guidance on these techniques that could usefully be incorporated in future standards, to improve precision, comparisons and consistency of Tg measurement.
The purpose of the study is to accelerate the development of ceramic materials for armour applications, by substantially increasing the information obtained from a high-energy projectile impact event. This has been achieved by modifying an existing test configuration to incorporate a block of ballistic gel, attached to the strike face of a ceramic armour system, to capture fragments generated during the ballistic event such that their final positions are maintained. Three different materials, representative of the major classes of ceramics for armour applications, alumina, silicon carbide and boron carbide, have been tested using this system. Ring-on-ring biaxial disc testing has also been carried out on the same materials. Qualitative analysis of the fracture surfaces using scanning electron microscopy and surface roughness quantification, via stereoimaging, has shown that the fracture surfaces of biaxial fragments and ballistic fragments recovered from the edges of the tile are indistinguishable. Although the alumina and boron carbide fragments generated from areas closer to the point of impact were also similar, the silicon carbide fragments showed an increase in porosity with respect to the fragments from further away and from biaxial testing. This porosity was found to result from the loss of a boron-rich second phase, which was widespread elsewhere in the material, although the relevance of this to ballistic performance needs further investigation. The technique developed in this work will help facilitate such studies.
Graphene nano platelets cross-linked with elemental sulphur have been used as supercapacitor electrode material to provide successful energy storage in a structural device. Chemical crosslinking of the composite produces a mechanically stable material, with both high conductivity and surface area. Characterisation was conducted using scanning electron microscopy and energy dispersive X-ray spectroscopy. Different concentrations of graphene-sulphur are investigated, along with addition of conductive carbon black and multiwall carbon nanotubes. The effects of these variables on the performance of the sulphur cross-linked graphene as a supercapacitor electrode are presented through impedance spectrometry, cyclic voltammetry and galvanostatic charge-discharge. Analysis of the structural performance of the material is conducted by flexural three-point-bend testing.
Late-stage fatigue damage of an E-glass/epoxy 3D orthogonal non-crimp textile composite loaded in the warp direction has been investigated using a combination of mechanical testing, X-ray micro computed tomography (μCT), optical microscopy and finite element modelling. Stiffness reduction and energy dissipated per cycle were found to be complementary measurements of damage accumulation, occurring in three stages: a first stage characterised by rapid changes, a more quiescent second stage, followed by a third stage where the (decreasing) stiffness and (increasing) energy dissipation change irregularly and then rapidly, to failure. Microscopy of specimens cycled into the transition between the second and third stages showed macroscopic accumulations of fibre fractures in sections of warp tows which lying adjacent to the surface weft tows which are crowned-over by the Z-tows. At these locations, the warp tow fibres are subjected to stress concentrations both from transverse weft tow matrix cracks and resin pocket cracks.
Distribution networks are critical in providing continuous potable water supplies to households and businesses. Trunk mains are the major arteries of the distribution network and convey large volumes of water over long distances. Worldwide, much of this infrastructure is made of ageing cast iron and is deteriorating at different rates. Many of these mains are beginning to approach the end of their service lives (with some already exceeding their design life) and consequently out of large populations of pipes, some are failing, although some still have considerable residual life. Trunk main failures can have significant social, health and safety, environmental and economic impacts. It is therefore imperative to prevent the wide-scale failure of trunk mains through the implementation of proactive asset management strategies. Such approaches require accurate condition assessment data across the network in conjunction with deterioration modelling to predict how the assets' condition and performance changes over time. This work, being part of a wider collaborative project, has outlined a deterioration modelling framework on the basis of existing physical probabilistic failure models and research focussing on residual mechanical properties, corrosion and the NDT detection of flaws. The developed deterioration model can be used to characterise individual pipes (deterministic approach), as well as the cohort/network modelling of pipes (probabilistic approach). Deterioration is assumed to be predominantly based on corrosion. Previously this has been dealt with in a rather simplistic manner. The broader work has, on the one hand,shown that corrosion mechanisms are rather different than previously thought and, on the other, that their effect on a given pipe can be variable. A corrosion model capable of simulating the distribution of corrosion properties of the primary defects is to be incorporated within the proposed modelling framework and the development of important aspects of this model are discussed here. © 2014 WIT Press.
This paper presents investigations to create a structural supercapacitor with activated carbon fabric electrodes and a solid composite electrolyte, consisting of organic liquid electrolyte 1 M TEABF4 in propylene carbonate and an epoxy matrix where different compositions were considered of 1:2, 1:1 and 2:1 w/w epoxy: liquid electrolyte. Vacuum-assisted resin transfer moulding was used for the impregnation of the electrolyte mixture into the electrochemical double layer capacitor (EDLC) assembly. The best electrochemical performance was exhibited by the 1:2 w/w epoxy: liquid electrolyte ratio, with a cell equivalent-in-series resistance of 160 cm2 and a maximum electrode specific capacitance of 101.6 mF g-1 while the flexural modulus and strength were 0.3 GPa and 29.1 MPa, respectively, indicating a solid EDLC device.
As part of an on going programme to characterise the residual properties and understand the failure mechanisms of in-service grey cast iron water pipes, the fatigue crack propagation behaviour of grey cast iron samples has been studied. Specimens were sourced from three ex-service pipes. For each pipe the microstructure and composition were characterised and the fracture toughness was determined. The fatigue behaviour was investigated in terms of the crack growth rate (da/dN) as a function of the applied stress intensity factor range. Clear differences in the fatigue behaviour of the samples from different pipes were observed. The result from these investigations, which indicate that microstructural differences play a role in mechanical behaviour, will support the development of asset management tools for use in the water industry.
Considering the low specific capacitance of structural solid supercapacitors, which is due to the low ion diffusivity in solid electrolytes and the small specific surface area of some structural electrodes such as carbon fiber fabrics, novel structural supercapacitor designs are proposed and evaluated in this study based on supercapacitor-functional sandwich composite materials. Typical electrochemical double layer capacitors (EDLCs) are proposed with liquid organic electrolyte 1 M TEABF4 in PC (propylene carbonate). In the innovative sandwich structured composites, supercapacitors are embedded in the skins and integrated in the honeycomb core where the aluminium faces of the core constitute the current collectors of the supercapacitor-functional core. The sandwich composite material exhibited a flexural modulus of 5.07 GPa and a flexural strength of 413.9 MPa. The EDLCs embedded in the skins increased the skin flexural modulus and strength by 47% and 56%, respectively, for embedded lateral EDLCs, and by 91% and 106%, respectively, for embedded lateral and longitudinal EDLCs. Compared to typical EDLCs with the same electrolyte, the structural supercapacitors in this study demonstrated superior specific electrode capacitance, Csp,el = 153 F g-1 for the honeycomb supercapacitor and Csp,el = 95.7 F g-1 for the skin supercapacitor, translating to overall structural composite material performance of 0.68 Wh/m2honeycomb and 30.5 W/m2honeycomb for the supercapacitor-functional honeycomb, and 0.02 Wh/m2skin and 5.4 W/m2skin for the supercapacitor-functional skin.
A 2-D finite element model has been developed to simulate crack growth (net-tension and shear-out failures) in composite bolted joints. Results from the model have been compared with a similar approach from the literature and experimental data for a woven fabric system. Agreement is reasonable in each case.
Engineered Cement Composite (ECC) materials have the potential to be used in applications where a level of pseudo-ductility under tensile stress is required. Most previous work has focussed on comparatively thin specimens. For future civil engineering applications, however, it is imperative that the behaviour of thicker specimens is understood. In the present work, specimens containing cement powder, water, polymeric fibres and admixtures were manufactured following two different processes and tested in tension. Multiple matrix cracking was observed during tensile testing, leading to a pseudo-ductile behaviour. Complementary measurements of sample density and porosity suggest that a high porosity could be linked with an enhanced tensile strain-to-failure whereas high density is associated with a high maximum stress. The fibre dispersion, assessed by scanning electron microscopy, indicated that mechanical performance was enhanced with increasing proportion of fibres aligned along the tensile test axis, and this orientation can be linked to the manufacturing process.
This study presents novel investigations of sulphur-graphitic nanoplatelet (S-GNP) and sulphur-microwave expanded graphene oxide (S-MWGO) composite electrodes for structural electrochemical double layer capacitors (EDLCs) with liquid organic electrolyte 1 M TEABF4 (tetraethylammonium tetrafluoroborate) in propylene carbonate (PC). Elucidating the chemical structure of these electrodes, XPS (X-ray photoelectron spectroscopy) and Raman spectroscopy indicated the presence of CSSC links while mixed EDX (energy dispersive X-ray spectroscopy) elemental maps displayed elemental S outlining the edges of nanoplatelets, concluding the presence of S-links between nanoplatelets. While S-linking improved the mechanical properties and ensured structural integrity of the produced monoliths without the need of any binder, it also decreased the specific surface area of the resulting materials. Furthermore, additional sulphur might have been trapped in other forms, amounting to up to 26 wt% sulphur in the composite graphitic and graphene oxide-based electrodes. Three-point bend testing yielded that an S-GNP-MWCNT monolith with 20 wt% S and 0.24 wt% MWCNT exhibited similar mechanical properties to those of a rigid polyurethane foam. The same S-GNP-MWCNT monolith exhibited an average electrode capacitance of 12.2 F g−1 during discharge at 2.2 mA/cm2. An S-MWGO-MWCNT monolith electrode with 9.6 wt% S, 16.4 wt% carbon black and 0.24 wt% MWCNT exhibited an average electrode capacitance of 64.9 F g−1 during discharge at 2.2 mA/cm2 but higher resistance than the S-GNP electrodes.
An advanced method for joining fibre reinforced polymers to metallic substrates has been investigated. The solution was shown to offer improvements in strength, toughness (as indicated by the area under the load-displacement curve) and damage tolerance (residual strength after impact) under a range of test conditions.
Grey cast iron water pipe networks have been installed around the world, often 100–180 years ago. Cohorts (which can be defined by age, size, casting technology and geographical location, to specify but a few groups) degrade at different rates due to environmental and in-service issues, which can lead to a significant loss in mechanical performance. Hence, the management of these assets can be extremely problematic in terms of identifying priorities. The current paper considers the causes of such degradation, the consequences for defining accurate and up-to-date condition assessment protocols and hence the type and urgency of rehabilitation strategies. It follows that understanding the integrity/life expectancy of water networks requires non-destructive evaluation (NDE) of large-diameter cast iron trunk mains, with particular reference to the kinds of defects that are likely to be present and the issues that make assessment difficult. From this, recommendations are outlined for asset managers required to specify NDE protocols, based on an understanding of the nature of the material and conditions in the field.
An experimental and theoretical study of the effects of 90° ply cracking on the thermal expansion coefficients of crossply laminates has been carried out. It has been found experimentally that reductions in the coefficient of thermal expansion of up to 50% are caused by 90° ply cracks induced mechanically, although considerable care is needed in the experimentation. This behavior was modeled using a simple shear-lag analysis, and the resulting analytical expressions are compared with other approaches available in the literature. The growth of matrix cracks in a model GFRP system under severe thermal cycling (77 to 373 K) is investigated. The changes in expansion coefficient are affected by the growth of 0° ply cracks in addition to the 90° ply cracks. The crack growth rate/cyclic strain energy release rate range data are compared with those reported previously for mechanical fatigue cycling of similar material. The two data sets are consistent if plotted in terms of a fracture mechanics parameter which aims to account for the temperature dependence of material properties.
The growth of transverse ply cracks in composite laminates has been investigated both theoretically and experimentally. Some of the closed-form strain energy release rate based analyses of this problem in the literature have been compared and extensions to these approaches are presented. These models have been shown to be consistent with an alternative approach based on an approximate expression for the stress intensity factor at the tip of a growing transverse ply crack. An experimental study of transverse ply crack growth has been carried out using a simple model array of transverse ply cracks in a glass/epoxy laminate. By making the transverse ply sufficiently thick, the specimen compliance was found to change measurably as individual cracks grow. Hence, the strain energy release rate could be determined experimentally (via the compliance relationship) and compared with analytical predictions. Agreement was found to be satisfactory.
This paper presents an investigation into the behaviour of a number of pipe liner materials, with the specific aim of determining their ability to remain intact during failure of the cast iron host main. Three different liner systems have been evaluated along with an unlined control. One of the liners (epoxy resin lining) is a non-structural technique, whereas the other two (Subcoil and a new development) are semi-structural (or interactive) liners. Metallographic analysis and tensile tests were carried out on small samples cut from the cast iron host pipe in order to characterise the basic properties of the cast iron. Metallography revealed a microstructure typical of a grey cast iron, consisting of acicular graphite flakes with some rosettes; etching with 2% Nital acid etch revealed the presence of pearlite. Tensile tests on small samples cut from the pipes indicated significant non-linearity in the stress-strain response. Lined cast iron pipes were tested to failure in four-point bending. A circumferential notch was machined into the wall of some of the pipes, in order to simulate the reduction in host-pipe strength due to corrosion. In addition some tests were carried out under applied internal water pressure, to determine if this had any effect on liner behaviour. Both interactive liners survived host-pipe failure, whilst, as expected, the non-structural liner did not. A simple method was developed which enabled the bending moment curvature relationship in a bend test to be modelled from tensile data; this model gave satisfactory agreement with the experimental data.
The in-service strength degradation, as a result of corrosion, of cast iron water distribution pipes has been investigated. The strengths of 1 m lengths of pipe extracted from the ground have been measured in either 3- or 4-point bending and the size of the controlling defect has been estimated by visual examination of the fracture surface. The application of Weibull statistics to the bend test data demonstrates that there is bimodal behaviour which suggests that there are two populations of flaws present. It is postulated that the larger flaw size population is associated with corrosion pits that form during the process of graphitisation, while the smaller flaw size population is associated with the inherent flaws within the (brittle) cast iron pipe material. A critical pit depth is identified at the transition between the two competing flaw populations, where there is a change in slope on the Weibull plot. It is shown also that the residual strength/pit depth data are described equally well by either of the two conventional analyses, i.e. loss of section and fracture mechanics. © 2002 Thames Water Utilities Ltd. Published by Elsevier Science Ltd. All rights reserved.
A high profile international activity is currently underway to assess the maturity of well established methodologies for the prediction of damage (matrix cracking and delamination) and ultimate failure in composite laminates. The activity is known as the 3 World-Wide Failure Exercise (WWFE-III). The predictions are made 'blindly' by the originators of those well established methodologies, who accepted an invitation to take part in the exercise. The organisers of the WWFE-III have provided the participating groups (originators) with comprehensive material property data and a full description of 13 challenging Test problems to be solved and used in their analysis. In this paper, an up-date is given regarding the progress made by the participants for applying their models to solve the specified Test Cases. A wide variety of approaches have been implemented and some of the results are described briefly. Copyright ©?QinetiQ Ltd 2011.
As a precursor to a study of erosive near behaviour of ceramic matrix composites fracture by indentation and single particle impact has been studied in two glass-ceramic/silicon carbide fibre composite systems. The damage has been characterised and quantified using a combination of confocal scanning laser microscopy and scanning electron microscopy. Lateral cracks which form approximately parallel to the surface, have been found to be the predominant damage event. In the calcium alumino-silicate (CAS)/Nicalon system, lateral cracks tend, to form in regions of the matrix which have a high local fibre volume fraction, whilst in the barium magnesium alumino silicate (BMAS)/Tyranno system they tend to avoid fibre-rich regions. These results are consistent with an analysis of residual thermal stresses in the two systems. In CAS/Nicalon the coefficient of thermal expansion of the matrix is greater than that of the fibre. This puts the matrix into axial tension at room temperature with the stress increasing with local fibre volume fraction. In BMAS/Tyranno the reverse in the case. Thus in both systems, the observed damage is a consequence of the residual stress as well as the stress due to the contact event.
Cements, which are intrinsically brittle materials, can exhibit a degree of pseudo-ductility when reinforced with a sufficient volume fraction of a fibrous phase. This class of materials, called Engineered Cement Composites (ECC) has the potential to be used in future tunneling applications where a level of pseudo-ductility is required to avoid brittle failures. However uncertainties remain regarding mechanical performance. Previous work has focused on comparatively thin specimens; however for future civil engineering applications, it is imperative that the behavior in tension of thicker specimens is understood. In the present work, specimens containing cement powder and admixtures have been manufactured following two different processes and tested in tension. Multiple matrix cracking has been observed during tensile testing, leading to a “strain-hardening” behavior, confirming the possible suitability of ECC material when used as thick sections (greater than 50 mm) in tunneling applications.
In previous work we have used a two-dimensional finite element model to predict the strength of GFRP woven fabric double-lap joint bolted joints that fail in the net-tension mode. The failure criterion was based on a fracture mechanics approach, incorporated within a XFEM framework, developed and validated previously for open-hole failure. Results were compared with experimental data obtained from clamped joints. While agreement between model and experiment showed promise, there are features of the problem, in particular the effect of bolt clamp-up and the associated load transfer as a result of friction, which cannot necessarily be captured with the limits of a two-dimensional model. The present work has therefore developed a three-dimensional model and applied it to the same data set. The effect of clamp-up torque is incorporated by modelling the bolt and washers and introducing a bolt tension, which enables the influence on frictional load transfer and the in-plane stress distributions to be incorporated within the model. The predictions for joint strength were in good agreement with experimental data up to the values of w/d for which the failure mechanism was observed experimentally to change to the bearing failure mode. Copyright © (2012) Asian-Australasian Association for Composite Materials (AACM).
The applicability of Weibull statistics to the condition assessment of cast iron water distribution pipes has been considered. The effect of Weibull modulus, characteristic strength, sample size and mode of loading (tension or flexure) on the strength of cast iron water distribution pipes is investigated. The strength distribution of cast iron samples cut from sections of five different water distribution pipes recovered from the ground have been characterized. Strengths have been measured in flexure, at two different temperatures (ambient and 0 degrees C), and in tension at ambient temperature using two different sample sizes. It is shown that characteristic strength values in flexure decrease with increasing size of graphite flake and that there is no significant difference between the results at the two temperatures investigated. For samples of the same volume tested in tension and flexure, the reduced strength measured in tension is consistent with Weibull predictions. However, the strength of large samples tested in tension was not significantly different from the small samples, perhaps because the samples were of the same thickness and conventional Weibull scaling is not applicable. Finally, using a method which treats a large pipe as an assembly of small samples, the strength distributions from the small samples tested in tension are used to make a prediction of the strengths of 1 m span sections of pipe loaded in three-point bending, which were reported in previous work. The predicted pipe strengths are close to the lower end of the measured pipe strength distribution. Overall, this work suggests that Weibull analysis is a useful tool to examine the strength distribution of removed from cast iron water pipes and so has the potential to contribute in the assessment of asset condition.
Polymeric foams are used extensively as the core of sandwich structures in automotive and aerospace industries. Normally, several experiments are necessary to obtain the required properties to model the response of crushable foams using finite element analysis (FEA). Hence, this research aims to develop a simple and reliable calibration process for extracting the physical parameters which are required by the material model available in the commercial FE package Abaqus. To do this, a set of experimental tests, including uniaxial compression, uniaxial tension and shear punch tests, is proposed. All the experimental tests were also simulated, and generally, good correlations between experiments and numerical models were obtained. The validity of the overall approach was finally demonstrated using an indentation test in which the foam was subjected to a more complex mixed mode loading. During these indentation tests, digital image correlation was used to observe full-field strain distribution in the foam under the indenter. Good agreement between the experimental results and the numerical predictions was found for load–displacement response, failure mode and strain distribution.
The single fibre pull-out (SFPO) test has been used to investigate the interfacial interaction between a glass fibre and a polyester matrix system. However, mechanical data alone cannot explain fully the mechanisms of failure, and time-of-flight secondary-ion mass spectrometry (ToF-SIMS) has been utilised to gain insight into the interfacial chemistry of adhesion. The present work employs ToF-SIMS for the forensic examination of fibre surfaces following a SFPO test. Regions of interest have been selected for retrospective spectral analysis. Results are presented which lead to the description of a failure model based upon these complementary analytical techniques. ToF-SIMS has revealed a difference in the surface chemistry at the fibre tip compared to the bulk of the pulled out region, which correlates with stress transfer models in the literature showing higher stress states existing at the embedded fibre tip region. The application of the methodology to nano-modified polyester matrix composites is discussed.
This thesis focuses on the subject of damage in composite materials and structures, in particular delaminations arising from an impact event and subsequent Mode I and Mode II loading and fatigue delamination growth. Interlaminar fracture toughness values have been calculated from an experimental study for DCB and ENF specimens. Specimens with artificial inserts at two different interfaces were used along with specimens with delaminations introduced from an impact event. The standard analysis method for both Mode I and Mode II has been adapted to account for the delamination away from the mid plane. For Mode I loading, the load to initiate delamination growth from experimental results is in good agreement with the predicted results from the adapted Mode I equation. For Mode II loading, crack migration did not appear obvious from the experimental study, and an adapted equation accounting for delaminations away from the mid plane has been successfully used. A fatigue study on a structural element loaded both in-plane and out of plane has highlighted the complex nature of damage growth in composite structures. The study has highlighted the issues of delamination investigation using the ultrasonic NDT technique, whereby non-critical delamination growth is sometimes masked by the more dominant delamination and as such the complex growth of delaminations within a structure is difficult to quantify using this technique.
Surface treatments of silicon carbide have been investigated with the aim of improving the strength of the bond between the ceramic and an epoxy adhesive. Three surface conditions have been characterised; as-fired, air re-fired and KrF laser processed. A number of characterisation techniques have been used to determine the morphological and chemical changes that have occurred to the surface. Scanning electron microscopy of the re-fired and laser processed samples showed surfaces that appeared glassy, with the laser processed surface showing a different morphology. X-ray photoelectron spectroscopy indicated both treatments had oxidised the surface and the laser processed surface also had a greater concentration of hydroxyl groups. The wettability of both surfaces had improved and the laser processed surface was found to be highly hydrophilic. Mechanical testing of joints prepared with this technique showed them to have the highest strength in tension, with the locus of failure being cohesive. © 2013 The Authors.
This paper details a quality control test for polymeric composite interfaces independent of reinforcement type and geometry. Experimentation has shown the capability of AFM indentation in characterising interfacial mechanical property variation with focus on measurement quantification to produce elastic modulus maps at the micro- and nano-scale
The SFPO test and the SFFT have been utilised to determine the interfacial properties of a glass fibre and two resin materials; i) polyester resin (PR) ii) ormosil nanomodified polyester resin (NR). Both methods led to similar trends in the test data. Failure mechanisms were studied using a range of techniques.
Matrix ply cracking is the most common damage to form when a laminate is loaded, and is of considerable significance for the integrity of a composite structure. The overall aim of the present work is to provide validated constitutive relations for crack accumulation in off-axis plies under mixed mode loading. The results presented in this paper include experimental investigations to describe the development of the cracking and the development of finite element-based models of cracked laminates. The effect of matrix cracking on the residual stiffness of various laminates is determined both experimentally and using finite element simulation. The ratio of modes in different angle ply laminates and the associated criteria for matrix crack initiation are explored.