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.
Ceramic armour must offer protection against armour piercing threats at low weight and affordable cost. As a possible means of improving armour, a range of SiC-B4C composites have been produced and characterised. The degree of contact between the two phases has been quantified and shown to have a strong effect on the densification and microstructure in these materials. This understanding has enabled independent variation of microstructural parameters which are normally interrelated. These were; porosity, SiC:B4C mass ratio, B4C distribution in a SiC matrix and SiC grain size distribution. To assess effects of each of these parameters on ballistic performance V50 testing was carried out, using 7.62 mm armour piercing rounds. The amount of porosity is shown to have a slight effect on V50 and a marked effect on scatter in V50. The pore size distribution is also shown to be important; across a range of pairs of materials with similar total pore volumes but differing pore size distributions, larger pores consistently give lower V50. SiC:B4C mass ratio does not seem to greatly affect V50, potentially allowing application specific cost/weight balances at constant protection level. B4C distribution has a strong effect. In general, for B4C features with diameters ranging from 1 m to 100 m, the coarser features performed better. Using coarse B4C particles in a SiC matrix, a V50 of approximately 980 ± 20 m s-1 at a density of 3.00 g cm-3 was achieved reproducibly. This material is preferred due to a combination of relatively lower cost, reduced density and repeatability. Knoop indentation has been used to derive possible merit indices which could potentially be used to rank ballistic materials. These includes analysis of failure probability of indents and the indentation size effect. A preliminary study indicates ballistic impacts may affect SiC polytype composition.
Throughout use, military equipment is subjected to everyday wear and tear. Some may be significant in its own right, some will interact with other damage, and some will fatigue with use. Under these circumstances, how will equipment that has been in the field deal with an actual ballistic event, or other primary duty issue? Current assessment methodologies ensure safety standards are met, but detailed evaluation of components requires transportation from site. Minimising transport of equipment would reduce costs and fuel usage, and also save lives. The current work considers the use of digital image correlation (DIC) for non-destructive evaluation (NDE), with a particular focus on the assessment of combat helmets. To optimize component loading, the use of pressure differentials and single-point mechanical loading were trialed. Finite element analysis (FEA) suggests pressure differentials produce a greater likelihood for the detection of component damage via surface strain discontinuities. By contrast, single point loading produces highly concentrated strain in the region of contact, whilst minimal strains result for the rest of the component. The optimization of component speckling has also been considered, leading to the development of a novel approach using customisable transfer paper, which can be printed with a pattern specific to a given test geometry. This allows greater standardization, faster application, and increased accuracy, compared with traditional approaches, such as spray painting a speckle pattern. Following these experiments, NDE of an entire military helmet was investigated using a portable test rig. With the method for helmet loading in concept stage, proof that the technique can detect damage is presented via five case studies. The variety of materials and testing processes show the novel approach for component speckling has direct use for the completion of, and external to, the primary goal of the project: “to develop a DIC technique with the potential for portable damage detection of helmets”.
Within this project, investigations have been made into the materials and processes involved in resin transfer moulding (RTM), the process used to produce McLaren Automotive’s mono-cell, used as the monocoque in its various models. Following an assessment of the literature surrounding RTM and an analysis of the state-of-the-art technology on the market, fundamental material property data was obtained for multiple components used in the production of the mono-cell. These materials include matrix resin, carbon fibre, preform binder, assembly adhesive, structural foams, primer and aluminium. A framework for process-related resin selection and optimisation was produced to increase prospective matrix system assessment and reduce the cost of doing so through an efficient regime of investigations into cycle time, including filling time against gel time, micro-infiltration time and demould time. Reaction kinetics modelling using differential scanning calorimetry (DSC) and characterisation of viscosity, storage- and viscous-shear moduli by dynamic mechanical analysis (DMA) in a rheometer were used in the initial down selection while further investigations were performed using capillary pressure measurements of curing resin impregnating a fibre yarn. Investigations were carried out into the optimised usage of binder and adhesive additives in conjunction with fibre reinforcements in the RTM process. Their effect on the processability and final product quality was assessed through the use of mechanical testing and microscopy. Structural foams and metallic inserts were investigated as comoulded assemblies for their processability and in-service performance through the use of mechanical and contact angle testing, microscopy and DSC measurements. The data obtained from the investigations into each material and their interactions with each other and the high-pressure RTM (HP-RTM) process as well as the methodologies of investigation have been adopted to optimise McLaren Automotive’s mono-cell production process through the timely and cost-effective production of data transferrable to large scale composite component manufacture.
There has been a growing interest in the last decade for the use of composite materials in the automotive industry, due to their low weight and high energy absorption capabilities. Predicting the response under impact conditions of these lightweight materials is crucial for both design and reduction of manufacturing and testing during the development of new vehicles. However, the complexity of composites makes it difficult to predict their behaviour using Finite Element Analysis (FEA). Hence, this research looks into the development of a reliable Finite Element methodology to simulate lightweight automotive crash structures, combining composite materials with polymeric foams in a sandwich panel. To achieve that goal, firstly a calibration process of a constitutive model for crushable foams was developed, including the proposed experimental tests to characterise the polymer. The approach was then validated by correlating a mixed-mode indentation tests with its corresponding Finite Element model and results from Digital Image Correlation (DIC). Secondly, different configurations of the crash structure were tested under mixed-mode loading conditions at quasi-static rates, based on the side-pole impact test. These experiments provided a better understanding with regard to the performance of each configuration. Furthermore, they were used to develop a robust FE methodology to accurately predict their behaviour, including good correlations in terms of load-displacement and failure mechanisms. Finally, the methodology was employed to design an impact test on a similar crash structure. The tests were conducted, and the outcomes were used to improve the Finite Element models, showing good agreement in terms of loads and energy absorption capability. It was found that the structures that were tested did not experience composite crushing, highlighting the need of further optimisation of the sandwich structure. Therefore, having validated the numerical methodology, FEA can now be used to assess the influence of different design approaches to improve the performance of this type of crash structures, prior to the manufacturing and testing of new full-size components.
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.
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.
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 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.
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.
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.
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).
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.
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.
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.
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.
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.
SL Ogin, S Topal, L Baiocchi, AD Crocombe, P Potluri, PJ Withers, M Quaresimin, PA Smith, MC Poole, AE Bogdanovich (2015)Late-stage fatigue damage in a 3D orthogonal non-crimp woven composite: an experimental and numerical study, In: Composites Part A: Applied Science and Manufacturing79pp. 155-163
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
© 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).
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.
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.
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.
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.
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.
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
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.
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.
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.
RA Shenoi, SSJ Moy, LC Hollaway, PA Smith (2004)Untitled, In: PROCEEDINGS OF THE INSTITUTION OF CIVIL ENGINEERS-STRUCTURES AND BUILDINGS157(1)pp. 1-2
THOMAS TELFORD SERVICES LTD
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.
Every day, water networks across the developed world are relied on by billions of people to provide them with a fresh supply of water. Many of these networks are comprised of pipes made from grey cast iron and may have been in service for up to 150 years. Despite their age, some parts of these networks continue to operate with little degradation, whereas in other areas they degrade rapidly: more recently laid pipes are being outlived by their forerunners. In such networks, it is the trunk mains (pipes between 12-60” [300 mm to 1500 mm] in diameter) that are of great concern, since they pose the greatest risk of failure and are already bursting more frequently. Accurate NDE is required to enable the mains in poor health with the highest risk of failure to be identified and replaced before they burst. A review of the published literature has shown that whilst there are many NDE techniques to choose from, many are not practical for application to the mains. The review process also highlighted the kinds of defects present in grey cast iron and an initial stress analysis using strength models and material data published in the literature has suggested defect sizes approaching 5 mm must be able to be detected to prevent catastrophic pipe failure. Ultrasonic inspection has been investigated and shown to work effectively on uncorroded cast iron. Speed of sound values between 4100 – 4600 m s-1 have been observed across several pipes. A speed of sound of 2950 ± 80 m s-1 has been measured for graphitic corrosion, however, inspection on corroded main has not been possible. A complementary magnetic technique, with the potential to scan pipe rapidly in order to identify mains in need of further investigation, as well as providing supplementary condition data, has been trialled and shown to detect corrosion layers up to 6 mm thick. A methodology using a 3D scanner to accurately determine the “ground truth” pipe condition has been developed. This methodology proved to be successful and provided corrosion measurements that were in-keeping with those obtained through standard pit depth measurements. Further, the data showed that traditional pit depth measurements do not always find the deepest external corrosion pits, particularly where the surrounding geometry is complicated. This methodology was used in a live comparison exercise of two, commercially available techniques. This comparison highlighted problems with the surface preparation required by some techniques, which can be quite damaging, and with some proprietary post-processing algorithms – the raw data can be more useful. From this assessment process, it has been possible to specify very detailed schedule for the testing of new NDE techniques in the future.
Shaving is an everyday act for many people and Gillette is at the forefront of this market. The complex process of designing a razor involves understanding the interaction between the cartridge and the face which are complicated systems in their own right. Wet shaving is a complex tribological process for which the mechanisms and parameters are not well understood. The time and high cost associated with designing razors are a major driving force for developing a technical model of shaving. Friction has been identified as an important parameter influencing consumer relevant attributes such glide and comfort. This thesis focused on breaking the problem down into two key areas, skin friction and hair cutting friction. By combining in-vivo and in-vitro testing capabilities, the key parameters affecting skin friction were determined and quantified. Due to the limited knowledge of the relative contribution of adhesion and deformation friction to total friction in the biotribology field, this thesis has confirmed past results and expanded on previous knowledge regarding the relative proportion of adhesion and deformation in three lubrication cases, namely, dry, water and oil contacts. Empirical models of skin friction for these three cases were developed to estimate the relative proportion of adhesion and deformation friction. The primary parameters affecting relative proportion of adhesion and deformation included the contact lubrication, probe material, sliding speed, and probe geometry. Further, the results indicated for the oil contact case, for high normal loads and sliding speeds, deformation friction contributed as much as 50% of the total friction. Hair cutting friction was also investigated focusing on two parameters, hair density and hair cutting profile. These two parameters significantly affected hair cutting friction, where increasing hair density and the area under the curve (hair cutting profile) increased hair cutting friction significantly. Two case studies were considered that combined data from skin friction and hair cutting to estimate the relative proportion of adhesion, deformation and hair cutting friction to shaving friction. The results showed, for contacts with water as a lubricant, hair cutting and adhesion friction contribute on average the same proportion (40-40%) and depends on the type of hair cutting profile considered. For contacts with oil as a lubricant, relative contribution of hair cutting friction significantly increases and can be as high as 80% of the shaving friction depending on the hair cutting profile considered.
Engineered Cement Composite (ECC) materials have the potential to be used in civil engineering applications where a level of pseudo-ductility is required. Of particular interest is the possibility of eliminating the steel from reinforced cementitious structures ensuring that no long-term corrosion exists, which is especially relevant for hydraulic tunnels. Uncertainties remain, however, with regard to the mechanical performance, physical properties, durability and shrinkage of these materials, especially when they are used in thick sections for large scale engineering structures. The current work has studied ECCs in this light. The physical properties such as density, porosity and fibre dispersion and orientation are also of interest: this forms the classic materials engineering triangle of the links between material composition and manufacturing process, the microstructure and mechanical properties. A cementitious matrix, reinforced with polymeric fibres, has been manufactured using two different processes and fibre types. Specimens have been tested in tension and flexure, and multiple matrix cracking has been observed, which leads to a pseudo-ductile behaviour. Enhanced mechanical performance in tension is in line with a greater fibre alignment and higher levels of porosity do not necessarily lead to a loss of pseudo-ductility. Flexure testing shows a pseudo-ductile behaviour maintained for over three years. The fibre surface coating and the interfacial properties are relatively stable, which is in line with the maintaining of the pseudo-ductility. Theoretical models suggest that the results are in line with the ACK model, particularly for a fibre volume fraction which considers fibre orientation and fibre pull-out is likely to be responsible for the pseudo-ductile behaviour of the material. ECC materials tend to exhibit a high shrinkage on cure; this could result in cracking, which could compromise the longevity of structures. Methods for controlling shrinkage include the controlling of the environment and use of additives such as powder micro-silica.
The only widely-accepted method of gauging the ballistic performance of a material is to carry out ballistic testing; due to the large volume of material required for a statistically robust test, this process is very expensive. Therefore a new test, or suite of tests, that employ widely-available and economically viable characterisation methods to screen candidate armour materials is highly desirable; in order to design such a test, more information on the armour/projectile interaction is required. This work presents the design process and results of using an adapted specimen configuration to increase the amount of information obtained from a ballistic test. By using a block of ballistic gel attached to the ceramic, the fragmentation generated during the ballistic event was captured and analysed. In parallel, quasi-static tests were carried out using ring-on-ring biaxial disc testing to investigate relationships between quasi-static and ballistic fragment fracture surfaces. Three contemporary ceramic armour materials were used to design the test and to act as a baseline; Sintox FA alumina, Hexoloy SA silicon carbide and 3M boron carbide. Attempts to analyse the post-test ballistic sample non-destructively using X-ray computed tomography (XCT) were unsuccessful due to the difference in the density of the materials and the compaction of fragments. However, the results of qualitative and quantitative fracture surface analysis using scanning electron microscopy showed similarities between the fracture surfaces of ballistic fragments at the edges of the tile and biaxial fragments; this suggests a relationship between quasi-static and ballistic fragments created away from the centre of impact, although additional research will be required to determine the reason for this. Ballistic event-induced porosity was observed and quantified on the fracture surfaces of silicon carbide samples, which decreased as distance from centre of impact increased; upon further analysis this porosity was linked to the loss of a boron-rich second phase. Investigating why these inclusions are lost and the extent of the effect of this on ballistic behaviour may have important implications for the use of multi-phase ceramic materials as armour.
Measurement of the degree of cure of composite materials is vital to both research and manufacture of these materials. The glass transition temperature (Tg) is a measurable material property that can be used as an indicator of the degree of cure. The three most common thermal analysis techniques used to measure Tg are DMA, TMA and DSC (i.e. dynamic mechanical analysis, thermomechanical analysis and differential scanning calorimetry). There is a current need to improve the experimental methods and analysis of data when using these techniques, where issues such as thermal lag can negatively impact data precision. In this work, a method using multiple tests at different heating rates has been applied to these three techniques to eliminate the effect of thermal lag as well as assess other variables that can influence test data; specimen moisture condition, specimen thickness and fibre type. It was shown that while thermal lag can be accounted for, there are remaining slight differences between DMA, TMA and DSC Tg data, which can be expected due to the different response modes involved (e.g. mechanical, thermal expansion, calorimetric). For DMA testing, a simple relationship has been proposed, relating heating rate and specimen thickness, which can account for the effect of thermal lag when comparing data obtained for specimens of different thicknesses or for the same thickness at different heating rates; the relationship is supported by relevant experimental evidence. It was shown for materials with different degrees of cure that the relationship between Tg and degree of cure followed the same trend regardless of differences in Tg measured by the three techniques. Preliminary experiments indicated that FTIR showed promise for measurement of the degree of cure of composite materials, in addition to measurements by DSC.