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A cracking approach to inventing tough new materials: fracture stranger than friction

Discussion meeting

Starts:

September
202021

09:00

Ends:

September
212021

17:00

Location

Zoom webinar

Overview

Scientific discussion meeting organised by Dr Kevin Kendall FRS, Professor Anthony Kinloch FREng FRS, Professor Neil Alford MBE FREng and Dr Siva Bohm.

Interfering fringes for contacting rubber spheres. Credit: TARRC/MRB

In the 100 years since Griffith published his ground-breaking paper on the energy criterion for cracking, there have been great advances in the understanding of cracks. This meeting reflected on recent observations in the field plus interesting new theories of cracking, as well as discussing how to invent tough new materials, while removing problems in Fracture Mechanics.

The schedule of talks and speaker biographies and abstracts are available below. An accompanying journal issue has been published in Philosophical Transactions of the Royal Society A.

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Schedule of talks

20 September

Session 1 08:45-11:00

Reversible equilibrium fracture and JKR

7 talks Show detail Hide detail

Chairs

Professor John Williams FREng, University of Cambridge, UK

08:45-09:00 Introduction by the Royal Society and Dr Kevin Kendall FRS

09:00-09:10 JKR half a century back

Professor John Williams FREng, University of Cambridge, UK

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09:10-09:25 Advances of the JKR theory of adhesive contact and probing of prestressed cell membranes

Professor Feodor Borodich, Cardiff University, UK

Abstract

Contact probing is the preferable method for studying mechanical properties of very small volumes of materials. Because adhesion caused by van der Waals forces is a universal molecular phenomenon, the nanoindentation problems should normally take into account adhesive interactions. The JKR (Johnson-Kendall-Roberts) theory of adhesive contact is an elegant mathematical theory that includes a combination of the Hertz contact theory, Derjaguin approximation and Griffith criterion. Originally it was introduced for frictionless contact between a rigid spherical indenter and an isotropic half-space. It is shown that the JKR theory may be extended for arbitrary convex, blunt axisymmetric indenters, in particular to indenters whose shapes may be described by power-law profiles of an arbitrary degree. The results are also extended for samples whose materials can be described by linear or linearized models having rotational isotropy of its mechanical properties like transversely isotropic or homogeneously prestressed materials. These extensions allow the researchers to apply the JKR formalism to the biological cell membranes that are prestressed. For the first time, effects caused by molecular adhesion for living cells are analytically studied taking into account the mechanical properties of cell membranes whose stiffness depends on the level of the tensile prestress.

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09:25-09:30 Questions

09:30-09:45 Dynamics of attachment and adhesion: measuring biofouling by AFM

Professor Kathryn Whitehead, Manchester Metropolitan University, UK

Abstract

The Atomic Force Microscope (AFM) is now being widely used to determine the properties and interactions of cell and microbial surfaces and their attachment to substrata. Perpendicular force measurements take place between the AFM tip and surface, and tips may be modified in a number of ways to determine the force of interaction. Perpendicular measurements may also be used on a molecular scale to determine the different regions of chemistry on a cell/microbial surface. ‘Lateral’ force measurements are carried out by depositing microorganisms onto a surface and passing the cantilever tip across them in a raster fashion. By increasing the load on the cantilever tip following each scan, the increased force taken to remove the microbial cells can be mathematically calculated to determine the strength of microbial attachment to the surface. These methods can enhance the results obtained using traditional laboratory methods, and lead to a greater understanding of cell interactions. This work presents an overview of the different uses of AFM to understand the properties of cell and microbial surfaces and their attachment to substrata.

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09:45-09:50 Questions

09:50-09:55 Discussion contribution – Force-based agent-based models of tumour development

Dr Cicely Macnamara, University of Glasgow, UK

Abstract

Once cancer is initiated, with normal cells mutated into malignant ones, a solid tumour grows, develops and spreads within its microenvironment invading the local tissue; the disease progresses and the cancer cells migrate around the body leading to metastasis, the formation of distant secondary tumours. Interactions between the tumour and its microenvironment drive this cascade of events which have devastating, if not fatal, consequences for the human host/patient. Among these interactions, biomechanical interactions are a vital component. In this talk, key biomechanical relationships are discussed through a presentation of modelling efforts by the mathematical and computational oncology community. The main focus is directed, naturally, towards lattice-free agent-based, force-based models of solid tumour growth and development. In such models, interactions between pairs of cancer cells (as well as between cells and other structures of the tumour microenvironment) are governed by forces. These forces are ones of repulsion and adhesion, and are typically modelled via either an extended Hertz model of contact mechanics or using Johnson–Kendall–Roberts theory. This talk will introduce both approaches and should hope to spark lovely discussion. 

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09:55-10:00 Discussion contribution – Nano-cracking in high Ni cathodes – a leading source of degradation in Li-ion batteries

Dr Mel Loveridge, WMG, University of Warwick, UK

Abstract

High nickel cathode materials such as NMC 811 are currently receiving much attention for increasing the energy density in Li-ion batteries. The majority of these materials are based on complex, polycrystalline aggregate-type particles. As the battery ages, the secondary structure starts to develop micro-cracks,
which will begin to affect the performance. Transition metal dissolution (Ni, Co or Mn) often occurs, which can then migrate to the anode and redeposit as a metal on the surface. Such deposits can act as a potential nucleation site for Li to deposit from the electrolyte and create dendritic growths, which can compromise safety and accelerate capacity fade. These findings show the emergence of micro-cracks and how they affect battery performance, but also how the presence of ambient moisture exacerbates the situation.

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10:00-10:05 Discussion contribution – Particle friction and adhesion in process engineering

Professor Aibing Yu, Monash University, Australia

Abstract

This talk will discuss the importance of friction and adhesion between particles in governing the dynamic behaviour of particle systems. It covers the following topics: (1) origins of rolling friction and adhesion; (2) derivation of equations to determine the resulting forces; and (3) a few examples showing their effects on the dynamics of particle systems.    

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10:05-10:45 Discussion

10:45-11:00 Break

Session 2 11:00-13:30

Explaining crack equilibrium

5 talks Show detail Hide detail

Chairs

Professor Feodor Borodich, Cardiff University, UK

11:00-11:10 Cracking theories from Griffith to JKR and beyond

Professor Feodor Borodich, Cardiff University, UK

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11:10-11:25 Computational versus experimental cracking results

Professor Chris Pearce, University of Glasgow, UK

Abstract

A mathematical formulation and numerical modelling framework for brittle crack propagation in heterogeneous materials is presented. The formulation is developed in the framework of configurational mechanics and solved numerically using the finite element method. The local form of the first law of thermodynamics provides an equilibrium condition for the crack front, expressed in terms of the configurational forces. Applying the principle of maximal energy dissipation, it is shown that the direction of the crack propagation is given by the direction of the configurational forces. In combination with the Griffith fracture criterion, these are utilised to determine the position of the continuously evolving crack front. The methodology is extended for heterogeneous materials (spatially varying Young’s modulus and fracture energy) without the need for any further assumptions. Further, Professor Pearce considers problems involving the strong interplay between contact forces and crack evolution, where the crack front is propagating across contact surfaces. The resulting fully implicit computational formulation is able to model objectively complex unstable, non-linear, quasi-static crack propagation.

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11:25-11:30 Questions

11:30-11:45 Phase field fracture methods

Dr Emilio Martinez-Paneda, Imperial College London, UK

Abstract

Phase field fracture methods have gained remarkable popularity in recent years. The reasons are arguably twofold. Firstly, phase field modelling has provided a suitable platform for the simple yet rigorous fracture thermodynamics principles first presented by Griffith. Crack propagation is predicted based on a global energy minimization, without the need for ad hoc criteria. Secondly, phase field formulations have proven to be computationally compelling; advanced fracture features such as complex crack trajectories, crack branching, nucleation, and merging can be captured in arbitrary geometries and dimensions, on the original finite element mesh and without convergence issues.

In this talk, Dr Martinez-Paneda will review the fundamentals of phase field fracture methods and examine their capabilities in delivering predictions in agreement with the classical fracture mechanics theory pioneered by Griffith. New results dealing with the initiation of crack growth and stable crack propagation will be presented, showing how phase field fracture methods satisfactorily approximate classical fracture mechanics predictions and can also reconcile stress and toughness criteria for fracture. Finally, emphasis will be placed on recent extensions of phase field fracture formulations to tackle coupled physical problems of significant technological relevance, such as hydrogen embrittlement or battery degradation.

 

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11:45-11:50 Questions

11:50-11:55 Discussion contribution – speaker tbc

11:55-12:00 Discussion contribution – Cracks in soft solids

Dr Etienne Barthel, ESPCI / PSL / CNRS / Sorbonne Université, France

Abstract

The Griffith crack model is built upon linear elasticity. For soft solids, the standard generalization is the linear viscoelastic model, which has enjoyed numerous but quite equivalent declensions. Although the predictions of these models reflect some features of cracks in soft solids, they are actually unable to account for many observations because all sorts of non-linear phenomena (large strain response, instabilities) step in and play a more and more dominant role as the elastic modulus decreases. Dr Barthel will provide a few illustrations of the shortcomings of the standard model and some ideas of what is presently being done to overcome them.

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12:00-12:30 Discussion

12:30-13:30 Break

Session 3 13:30-15:30

Inventing new materials: concrete cracking

6 talks Show detail Hide detail

Chairs

Professor Neil Alford MBE FREng, Imperial College London, UK

13:30-13:40 New cement materials by toughening

Professor Neil Alford MBE FREng, Imperial College London, UK

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13:40-13:55 Fracture toughness of 1D and 2D-nanoreinforced cement via scratch testing

Professor Ange-Therese Akono, Northwestern University, USA

Abstract

Cement is the most widely consumed material globally, with the cement industry accounting for 8% of human-caused greenhouse gas emissions. Aiming for cement composites with a reduced carbon footprint, this study investigates the potential of nanomaterials to improve mechanical characteristics. Specifically, this study investigates the fracture response of Portland cement reinforced with 1D and 2D carbon-based nanomaterials, such as carbon nanofibres, multiwalled carbon nanotubes, helical carbon nanotubes, and graphene oxide nanoplatelets. Novel processing routes are shown to incorporate 0.1–0.5 wt% of nanomaterials into cement using a quadratic distribution of the ultrasonic energy. Scratch testing is used to probe the fracture response by pushing a sphero-conical probe against the surface of the material under a linearly increasing vertical force. The fracture toughness is then computed using a nonlinear Fracture Mechanics model. Nanomaterials are shown to bridge nanoscale air voids leading to pore refinement for graphene-reinforced cement. An improvement in fracture toughness is observed in cement nanocomposites, with a positive correlation between the fracture toughness and the mass fraction of nanofiller for graphene-reinforced cement. An increase in the water penetration resistance is also observed. Last, but not least, this study will investigate the influence of nanomaterials of the rate-dependence of the fracture behaviour and on the fracture response following exposure to elevated temperatures. In brief, this study illustrates the potential of nanomaterials to toughen cement and improve its performance.

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13:55-14:00 Questions

14:00-14:15 Corrosion-induced cracks and failure in reinforced concrete

Dr Mohammad Mehdi Kashani, University of Southampton, UK

Abstract

A significant number of older major infrastructure artefacts, located in an aggressive environment, suffer from material ageing and deterioration. Among different types of deterioration, the corrosion of reinforcing bars in reinforced concrete (RC) structures and bridges is the most common type of deterioration mechanism. In the current practice of Bridge Management Systems (BMS), however, the determination of the condition states (CSs) of deteriorated bridges is highly dependent on the opinion of experienced inspectors. Taking such complexity into account, this presentation presents a new stochastic predictive methodology using a nonhomogeneous Markov process, which directly relates the visual inspection data (corrosion rate and crack widths) to the structural vulnerability of deteriorated concrete bridges. This methodology predicts the future condition of corrosion-induced damage (concrete cracking) by linking Structural Vulnerability Analysis (SVA) and a discrete-time Markov chain model.

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14:15-14:20 Questions

14:20-14:25 Discussion contribution – Fatigue cracking of viscoelastic bitumen for asphalt roads

Dr Yuqing Zhang, Aston University, UK

Abstract

Fatigue cracking is one of the main distresses in asphalt roads. Bitumen, a binder in asphalt mixtures that pave the road surface layers, fundamentally determines the resistance to the fatigue cracking. This study presents mechanical models for quantifying the initiation and propagation of the fatigue cracking in bitumen. A dynamic shear rotational cracking (DSR-C) model was derived based on viscoelastic damage mechanics to predict the cracking length in bitumen when subjected to a rotational shear fatigue load. The DSR-C model was proved, by comparing with time sweep fatigue test results, being capable of accurately predicting the cracking length at different temperatures, frequencies, loading and aging conditions. A viscoelastic Griffith model was derived as a crack initiation criterion, which was validated by surface energy measurements based on contact angle tests. A pseudo J-integral based Paris’ Law was employed to obtain the crack propagation speed for the fatigue crack growth. The model coefficients were determined from the steady stage of crack growth curve and proved to be material properties that are temperature dependent but independent of loading amplitudes or frequencies. Future study will be focused on defining fatigue failure to determine fatigue life of the bitumen.

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14:25-14:30 Discussion contribution – Toughening epoxy with oxide nanoparticles

Associate Professor Manjeet Singh Goyat, University of Petroleum & Energy Studies, India

Abstract

Over the past three decades, toughening of epoxy polymers by oxide nanoparticles gained significant attention. The brittle nature of epoxy polymers limits their use in advanced structural applications where safety is a major concern. Rigid spherical oxide nanoparticles offered several advantages over other nanostructures for reinforcing epoxy polymers such as low toxicity, non-conductivity, availability, economical, and low aspect ratio (∼1) making them easy to disperse in viscus or semi-viscous polymers. Ultrasonic dual mixing (UDM) offered novel prospects to improve the mechanical properties of nanoparticles-reinforced epoxy composites by organized dispersion of oxide nanoparticles. The tensile fracture displayed a strong epoxy-nanoparticle interface that led to the enhanced mechanical properties due to leading toughening mechanisms such as crack deflection, plastic deformation, and particle pullout. However, single-edge-notch 3-point-bending (SENB) fracture revealed significant enhancement in the fracture toughness and fracture energy due to crack deflection, plastic deformation, crack front bowing and crack pinning toughening mechanisms. The oxide nanoparticles have an inordinate competence to toughen the epoxy polymers.

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14:30-14:35 New tough ceramics via graphene based reinforcements

Dr Cristina Ramírez, Institute of Ceramics and Glass, ICV-CSIC, Spain

Abstract

Over the past decade, a new family of ceramic matrix composites has been developed from the incorporation of homogeneously dispersed graphene-based fillers (graphene nanoplateles, graphene oxide sheets, or graphene nanoribbons) into the ceramic matrix. These composites have shown a significant increment of their fracture toughness accompanied by other electrical and thermal functionalities, which make them potentially attractive for a wide range of applications. Here, the principal observations on the reinforcing mechanisms responsible for this improvement are briefly reviewed, discussing the relation with graphene platelets orientation, morphology, and functionalization.

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14:35-15:15 Discussion

15:15-15:30 Break

Session 4 15:30-17:00

Cracks and defects

5 talks Show detail Hide detail

Chairs

Professor Rob Ritchie FREng ForMemRS, University of California, Berkeley, USA

15:30-15:40 Review of defects in the Griffith theory

Professor Rob Ritchie FREng ForMemRS, University of California, Berkeley, USA

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15:40-15:55 Making and breaking diamond laminates

Professor Stefan Rosiwal, Friedrich-Alexander Universität, Germany

Abstract

Diamond foils are exceptionally strong yet brittle. One approach to make ceramic foils less susceptible to brittle fracture is to introduce interfaces into the material that provide pathways for crack deflection. The group tries to produce strong yet tough diamond/metal laminates (DMLs) from freestanding diamond foils using a brazing process or by alternating coating processes CVD (diamond) and PVD (metal). The mechanical behaviour was characterized via three-point bending (3PB) where the brazed laminates exhibit step-like fracture. Crack deflection at interfaces induces toughening within the laminates. At approx. 3.0 MJ/m3 diamond/metal laminates exhibit more than twice the fracture energy of monolithic diamond foils while maintaining 90% stiffness and about 70% nominal strength. The group finds that the diamond-to-metal interface plays a critical role: it must be strong enough to enable the transfer of shear stress, while being weak enough to deflect a crack.

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15:55-16:00 Questions

16:00-16:15 Trains and boats and planes: living with cracks in transport

Professor Roderick Smith FREng, Imperial College London, UK

Abstract

Our increased understanding of the micro-scale mechanisms involved in crack extension has been gained under extremely well-defined laboratory conditions. Applying this fund of knowledge to real problems is by no means trivial. This presentation will illustrate some of the difficulties by using examples from various transport applications. 

The greatest benefit has been the use by the aircraft industry which has learned to live with cracks whist at the same time increasing performance. Shipping, on which world trade depends, has only recently adopted modern analysis of cracks in design and life evaluation, but big issues remain in an industry where cracks can be measured in metres rather than microns. Railways are perhaps the most conservative of industries yet were responsible for the identification and early attempts to analyse fatigue at their inception around 1840. The particular conditions of the wheel rail interface, influence damage in rails, wheels and axles.

The key lies in defining material properties which mirror those which reflect service conditions: this similitude is difficult to achieve if the global let alone local, loading conditions are unknown. It is the author’s experience that this knowledge is ill defined in many applications. 

Failures are often associated with competing mechanisms, fatigue, corrosion, wear, creep etc. The measurement of the dimensions of cracks in real components still remains challenging. The need to reduce weight, save fuel and reduce emissions has led to increasing pressures to design lighter structures by using lighter materials, which themselves present new challenges in property definition, fabrication and joining.

 

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16:15-16:20 Questions

16:20-16:25 Discussion contribution – Paradox: defects can toughen adhesives

Professor Anthony Kinloch FREng FRS, Imperial College London, UK

Abstract

Highly-crosslinked epoxy polymers are widely used as structural adhesives in very demanding engineering applications. Such thermosetting polymers have many excellent properties such as high stiffness, good creep resistance and good thermal properties. However, their crosslinked microstructure tends to make them very brittle materials. Nevertheless, the controlled formation of well-dispersed defects, such as micro-voids, in these adhesives may lead to a significant increase in their toughness. The main toughening mechanisms that are initiated via the formation of such micro-voids are identified. It is then shown how these observations have led to a wide range of epoxy adhesives which are significantly toughened via the addition of rubbery, nano-silica and nano-carbon, etc. particles.

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16:25-16:30 Discussion contribution – Toughness of wood and wood composites

Professor Lars Berglund, KTH Royal Institute of Technology, Sweden

Abstract

Wood is essentially a porous 'all-polymer' nanocomposite of high degree of anisotropy. We experience wood as a very tough material, and the high toughness for crack growth perpendicular to the orientation of fibres is well established. Recently, transparent wood biocomposites have been fabricated intended for building applications, where the pore space of wood is filled with a polymer of suitable refractive index. For large structures, the toughness in the weakest crack growth direction is critical and results from such a study are presented.

A four-point bending test allowing stable crack growth in transparent wood is carried out, and the analysis is based on crack bridging laws. Digital image correlation measurements of the strain field are combined with finite element modeling to estimate elastic constants of the orthotropic wood composite during the fracture test. Crack bridging laws are estimated for three different materials. Somewhat unexpectedly, addition of a polymer matrix is not improving toughness of transparent wood, and micromechanisms of crack growth are discussed in order to explain this. The complexity of wood and wood composites makes engineering design more difficult, although it also creates opportunities for tailored combinations of strength and toughness in biocomposites which of great interest for sustainable development in the building sector.

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16:30-17:00 Discussion

21 September

Session 5 09:00-11:15

Composite fracture

6 talks Show detail Hide detail

Chairs

Professor Anthony Kinloch FREng FRS, Imperial College London, UK

09:00-09:15 Advances in composites cracking principles

Professor Anthony Kinloch FREng FRS, Imperial College London, UK

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09:15-09:20 Questions

09:20-09:35 Impact of CFRP composites

Professor John Dear, Imperial College London, UK

Abstract

Experimental and theoretical modelling studies on the behaviour of continuous carbon-fibre/polymer matrix composites subjected to a relatively low velocity or high-velocity impact, using a rigid, metallic impactor. Drop-weight and gas-gun tests are employed to undertake the low-velocity and high-velocity impact experiments, respectively. The carbon fibre composites are based upon a thermoplastic poly (ether-ether ketone) matrix (termed CF/PEEK) or a thermoset toughened-epoxy matrix (termed CF/Epoxy). The results clearly reveal that the CF/PEEK composites exhibit the better impact performance. A three-dimensional (3-D) Finite Element Analysis (FEA) model incorporating an elastic-plastic (EP) damage model is developed to simulate the impact response of carbon-fibre reinforced-plastic (CFRP) composites. To validate the predictive capability of the model, experimental results are obtained from relatively low-velocity impact tests on CFRP plates employing either a matrix of a thermoplastic polymer, ie poly(ether-ether ketone), or a thermosetting epoxy polymer. The developed 3-D EP model is shown to model successfully the experimentally-measured impact behaviour of the CFRP composites.

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09:35-09:40 Questions

09:40-09:55 Computational modelling of CFRP composites for crashworthiness assessment

Professor Brian Falzon, Queen’s University Belfast, UK

Abstract

The airframes of the latest generation of wide-body passenger aircraft are primarily made from CFRP composites and estimated to be around 20% lighter than their aluminium counterpart, delivering a commensurate reduction in fuel consumption. As the industry transitions towards zero-emissions flight, further emphasis is being placed on reducing structural weight. A similar challenge faces the automotive industry where the comparatively heavier electric powertrain, compared to an internal combustion engine powertrain, is driving major lightweighting research and development programmes. 

The aerospace and automotive industries have been at the forefront of incorporating computational modelling to reduce the cost of product development and certification. The use of composites has brought with it a new set of challenges in developing reliable and robust simulation tools. Modelling the energy absorbing capacity of composite structures undergoing crushing, is particularly challenging. 

This paper provides an overview of recent developments in the computational modelling of composite crushing. Numerical strategies to enhance accuracy and robustness are presented. The fidelity of the numerical solution is highly dependent on the reliability of the material data generated from appropriate material characterisation tests. Not all such tests conform to an agreed standard and the testing at different strain rates presents additional complexity. Approaches for scaling up the computational models are also discussed. 

 

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09:55-10:00 Questions

10:00-10:15 Reinforcing polymers with low dimensional nanocarbons

Professor Ian Kinloch, University of Manchester, UK

Abstract

Graphene, given its excellent intrinsic properties, should be an ideal reinforcement for stiffening, strengthening and toughening polymeric matrices. However, the reported performance of graphene reinforced elastomers, thermoplastics and thermosets is found to vary significantly in the literature. This talk will explore the challenge in realising the promise of graphene, and hence by extension other nanomaterials, through the characterisation and testing of model polymer composite systems, measuring interfacial micromechanics using Raman spectroscopy and analytical models. In particular, the roles of graphene morphology and dispersion and the nature of the graphene-polymer interface will be discussed.

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10:15-10:20 Questions

10:20-10:25 Discussion contribution – Fracture and fatigue delamination propagation in DCB specimens: comparison of two material systems

Professor Leslie Banks-Sills, Tel Aviv University, Israel

Abstract

Double cantilever beam (DCB) specimens composed of carbon fibre reinforced polymer laminate composites were tested. Two material systems were investigated. In the first, the specimens were fabricated from 15 plies of a plain woven prepreg (G0814/913) arranged in a multi-directional (MD) layup. The plies alternated with yarn in the 0°/90°- directions and +45°/-45°- directions with the delamination between these two ply types. For the second material type, the specimens were fabricated from 19 plies with the delamination between a unidirectional fabric and a woven ply with yarn in the +45°/-45°- directions. The remainder of the plies were woven, with yarn alternating between the 0°/90° and +45°/-45°- directions. This laminate was produced by means of a wet-layup.

Fracture toughness resistance curves, as well as fatigue delamination propagation properties were determined. A master curve based on the Hartman-Schijve approach to fatigue was found for each material system. It was seen that fatigue delamination propagation in the wet-layup is faster than that in the prepreg.

 

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10:25-10:30 Discussion contribution – Viscoelastic matrix of natural composites

Professor Stanislav Gorb, Kiel University, Germany

Abstract

Natural fibrous composites including wood, bone, skin, insect cuticle, etc are supplemented by the matrix composed (depending on particular biological material) of proteins, lipids, lignin. The matrix of biological composites is often hydrated and has tight bond to the fibres (and that is why it has strong adhesive properties) and viscoelastic behaviour. In this presentation, a short overview about matrices of different biological composites will be provided and more detailed information on propolis, which is a sticky substance used by bees to seal their hive and protect the colony against pathogens will be discussed. 

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10:30-11:00 Discussion

11:00-11:15 Break

Session 6 11:15-13:15

Learning from nature

5 talks Show detail Hide detail

Chairs

Dr Sivasambu Bohm, Imperial College London, UK

11:15-11:30 Applicability of AFM to cancer-related cell changes

Professor Małgorzata Lekka, Institute of Nuclear Physics, Polish Academy of Sciences, Poland

Abstract

Biomechanics-related changes have been recognised to be crucial in various pathologies. It ranges from diseases where the mechanical change is evident due to genetic modifications of cytoskeleton-membrane linkage like in muscular dystrophies to those where the alterations are only experimentally observed, eg in cancer. The determination of mechanical properties of living cells as an indicator of cancer progression has become possible with the development of such local measurement techniques as atomic force microscopy (AFM). Its most important advantage is a nanoscopic character implying that very local alterations can be quantified. In this research, AFM-derived mechanical properties of single cells and tissues have been studied in the context of cancer development. The results gathered from AFM measurements of various cancers show that, for most cancers, individual cells are characterised by the lower apparent Young’s modulus, denoting higher cell deformability. Its value depended on various factors, like the properties of substrates used for cell growth, force loading rate, or indentation depth. Despite this, the results proved the AFM capability to recognise mechanically altered cells. This can significantly impact the development of methodological approaches toward the precise identification of pathological cells. They would allow for more effective detection of cancer-related changes. 

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11:30-11:35 Questions

11:35-11:50 Bone fracture

Professor Rob Ritchie FREng ForMemRS, University of California, Berkeley, USA

Abstract

The structure of human cortical bone evolves over multiple length-scales from its nanoscale collagen and hydroxyapatite constituents to osteonal structures at near-millimetre dimensions. To resist fracture, bone’s toughness derives intrinsically through plasticity (fibrillar sliding) at structural-scales below a micron and extrinsically through mechanisms (crack deflection/bridging) at larger structural-scales. Biological factors, eg, ageing and bone diseases such as vitamin-D deficiency and osteogenesis imperfecta, can diminish fracture resistance over multiple length-scales by degrading both bone’s intrinsic and extrinsic toughness. The issue of bisphosphonates for treating osteoporosis will also be addressed and how these drugs can lead to atypical femoral fractures (AFFs) in a small percentage of patients who are susceptible to this drug. Bisphosphonates are excellent for treating the issue of bone quantity, but with respect to bone quality, the toughening mechanisms in bone biopsies taken from people with AFFs are clearly affected; specifically, AFFs are found to be associated intrinsically with diminished fibrillar sliding due to excessive cross-linking, and extrinsically with diminished crack deflection associated with increased, more homogenized, mineralization.

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11:50-11:55 Questions

11:55-12:00 Discussion contribution – Drying of blood droplets

Professor Alex Routh, University of Cambridge, UK

Abstract

When a drop of blood is placed on a solid substrate and dried a range of morphologies are observed, with many similarities to regular dried colloidal dispersions. 

As with regular colloidal dispersions a front of consolidated material appears at the edge and propagates horizontally across the droplet, towards the centre. For blood, this consolidation front appears to halt a set distance from the edge and this is ascribes to gelation of the bulk fluid.

For the fully dried droplet, an accumulation of material at the edge is often observed, called a coffee ring. This edge ridge appearance is dependent on the original volume fraction of solid material as well as the drop-substrate contact angle. Theoretical predictions have been made for regular dispersions with decent agreement between theory and experiment. For blood there is considerable discrepancy in the morphological prediction which is ascribed the protein induced Marangoni flow.

Cracks are observed within the central region of the droplet, which is assumed to have gelled. The density of cracks seems to vary between patients according to their physiology.

 

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12:00-12:05 Discussion contribution – 3D printing for orthopaedic fracture fixation

Dr Laura Leslie, Aston University, UK

Abstract

Osteoporosis is a condition which causes bones to lose strength and density and can cause fragility fractures. Worldwide, around 1 in 3 women and 1 in 5 men aged over 50 will experience a fragility fracture due to osteoporosis. However, current orthopaedic screws are often poorly suited to osteoporotic bone and the design and test process for new screw designs is inhibited by the highly expensive manufacturing process and long lead time. 

In this study Dr Leslie shows that the use of 3D printing can be a valid method of rapidly and cheaply producing a variety of different screw designs to test efficacy before taking those designs on for further testing.

Six different wood screws were reverse-engineered and 3D printed in polymeric resin on a Stereolithography (SLA) machine. These printed wood screws and their equivalent metal counterparts were inserted into synthetic bone blocks (Sawbones PCF5 and PCF10). Pull-out tests were conducted in accordance with ASTM 543-13. Results showed a correlation between five of the six metal vs 3D printed tests. In addition, new orthopaedic screws were designed and tested which display a greater holding power in osteoporotic bone.

 

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12:05-12:10 Discussion contribution – Observing ice-cracks from space

Dr Oliver Marsh, British Antarctic Survey, UK

Abstract

Ice shelf instability and marine ice-cliff retreat are two mechanisms that lead directly to ice loss to the ocean. The rates of fracture associated with these mechanisms are the source of the largest uncertainties in the sea level projections used to plan coastal adaptation worldwide. Cracks extending tens of kilometres across ice shelves can be mapped from space using high-resolution optical and radar satellites such as Sentinel 1 & 2 (ESA) and TerraSAR-X (DLR), providing a catalogue of observations against which to test fracture models. Here Dr Marsh shows satellite imagery of pure tensile fractures, shear fractures, buckling and branching cracks, developing on ice shelves over timescales from days to decades. They discuss the large iceberg calving events from the Brunt Ice Shelf in February 2021 and the Ronne Ice Shelf in May 2021 and the development of the fractures leading to iceberg release. Spatial heterogeneity in fracture toughness and rheology are not well constrained, but the rapidly increasing availability of satellite data provides a route to better understanding these mechanisms.

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12:10-12:30 Discussion

Session 7 13:15-15:00

Interfacial cracking

4 talks Show detail Hide detail

Chairs

Dr Kevin Kendall FRS, Adelan Ltd, UK

13:15-13:30 Graphene production by cracking and industrial applications

Dr Sivasambu Bohm, Imperial College London, UK

Abstract

Graphene has found its use in numerous industrial applications due to its unique properties. While its impermeable and conductive nature can replace currently used anticorrosive toxic pigments in coating systems, graphene can be an important component as a next-generation additive for many industrial applications. The current bottlenecks in using graphene and graphene oxide are the availability of cost-effective, high-quality materials from vein graphite and their effective incorporation into the product matrices. 

On overcoming these factors, graphene may attract significant demands in terms of volume consumption. Graphene can be produced on industrial scales and cost-effective top-down routes such as chemical, electrochemical, and/or high-pressure mechanical exfoliation. Graphene depending on end applications can be chemically tuned and modified via functionalisation so that easy incorporation into product matrices is possible. This paper discusses different production methods and their impact on the quality of graphene produced in terms of energy input. Graphene with an average thickness below five layers were produced by few methods with varied defects. However, a higher yield of graphene with a lower number of layers was produced by the high-pressure exfoliation route. Graphene additive role in Li-ion batteries will be discussed and commercial development of industrial energy storage applications.

 

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13:30-13:35 Questions

13:35-13:50 The importance of interfacial shear: friction vs fracture

Professor Daniele Dini, Imperial College London, UK

Abstract

Just like cracking and fracture at material interfaces are significantly affected by the application of shear, the behaviour of frictional and adhesive contacts is also strongly affected and often controlled by the inherent resistance of the surfaces to applied tangential stresses. In this respect, there are many analogies between fracture and contact mechanics, with both fields being strongly affected by the mechanisms controlling how systems respond to interfacial shear, which is in turn governed by a plethora of physical, chemical and mechanical phenomena across scales. The talk will start by looking at key aspects of frictional contacts, including surface representations, the breakdown of continuum theories at the nano- and microscales, as well as multiscale and multiphysics aspects for analytical and computational models relevant to applications spanning a variety of sectors. The discussion will then move to the study of the interplay between interfacial shear stress, adhesion and deformations (especially for soft materials) in adhesive contact mechanics, which are rarely captured simultaneously at the local level and lead to diverse, sometimes contradicting, predictions in terms of evolution of contacting interfaces. Finally, methods using fracture mechanics techniques to study contacts will be critically reviewed, providing an overview of the advantages and limitations of cross-pollination between these two classical and yet everlasting fields.

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13:50-13:55 Questions

13:55-14:00 Discussion contribution – Controlled interface cracking: 3D printing

Dr Laura Zorzetto, University of Liège, Belgium and Max Planck Institute of Colloids and Interfaces, Germany

Abstract

In polyjet printing photopolymer droplets are deposited on a build tray, levelled off by a roller and cured by UV light. This technique is convenient to fabricate composites combining soft and stiff phases. Considering the layer‑by‑layer nature, interfaces between different photopolymers can be formed either before or after UV curing. The group characterized the properties of interfaces in 3D printed composites at different length-scales. They used nanoindentation to measure the spatial variation in mechanical properties across bimaterial interfaces at the micrometer level. It was found that interfaces formed by deposition after curing were sharp; whereas those formed before curing showed blending of the two materials over a length-scale bigger than individual droplet size. To characterize the impact of finite‑size interfaces, the group fabricated and tested composites having compliant and stiff layers alternating along different directions. Interface differences measured at the microscopic level had a macroscopic influence also on the mechanical behaviour both in bending and in tension. Eventually, the group produced bimaterial composites joined by an interface formed either before or after curing. Crack propagation along such interfaces was greatly influenced by the curing, as interfaces formed before curing were more resistant to crack opening with respect to interfaces formed after curing.

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14:00-14:05 Discussion contribution – Fluid-driven fractures in weak sands; a numerical approach

Dr Paula Gago, Imperial College London, UK

Abstract

Weakly cemented granular materials or soft rocks are of great importance in many industrial and geological processes. They are intermediate between soils and rocks, sharing common characteristics with both. They have low strength and stiffness, poor core integrity and stress dependent porosity and permeability.  

Fluid-driven fractures in these materials are generally shaped as flow channels (narrow paths presenting higher permeability than the matrix) with some degree of branching. 

The group has found that there is an interplay between material strength and its (inhomogeneous) stress distribution underlying the behaviour of fracture development. Here Dr Gago will discuss this interplay. To this end, numerical simulations and high-performance computing were used. 

 

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14:05-14:30 Discussion

14:30-15:00 Break

Session 8 15:00-16:15

Fracture, the future: improvements in fracture mechanics

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Chairs

Professor Rob Ritchie FREng ForMemRS, University of California, Berkeley, USA

15:00-15:15 Overview of innovations in tough materials

Professor Rob Ritchie FREng ForMemRS, University of California, Berkeley, USA

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15:15-15:20 Questions

15:20-15:35 Cracking thermoplastic composite welded joints

Dr Irene Fernandez Villegas, Delft University of Technology, The Netherlands

Abstract

To design and build lightweight and safe thermoplastic composite welded structures we need to understand and be able to predict the strength and crack propagation in the welded joints. To this purpose, using test methods and standards developed for adhesively bonded joints is common practice among researchers invested in the development of welding technologies. However, different welding processes introduce different macro-, meso- and micro-structural features in the welded joints that might have a profound effect on the results on the tests. Therefore, knowledge about the welding process is of great importance in order to prudently interpret the results of mechanical tests in welded joints. To illustrate the importance of such a holistic approach, this presentation compiles a series of examples of how specific features of different types of welded joints affect the results of strength and fracture mechanics tests and might result in erroneous conclusions if a reductionist approach were to be followed.

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15:35-15:40 Questions

15:40-15:50 Brittle strength changes with sample size by the Griffith energy principle

Dr Kevin Kendall FRS, Adelan Ltd, UK

Abstract

The purpose of this paper is to show that brittle test samples give a huge size effect that can take several different forms depending on the sample geometry, crack position and mode of force application. In general, the crack equilibrium force is not proportional to the sample cross-section area. Sometimes crack equilibrium force depends on dimension d or d1/2 and sometimes the force is independent of area, for example in peel or lap joint cracking. With adhering spheres, the crack force at equilibrium depends on sphere diameter, not on contact area, leading to an error of 103 in predicting the adhesion strength of nanoparticles using Galileo’s original stress criterion of failure. This big size effect arises from the potential energy term in the conservation theory, not considered by Griffith but dominating certain cracks. These examples illustrate the fact that strength of a brittle material containing a crack is an unsatisfactory concept because the cracks absorb surface energy driven by volume energy terms or by potential energy terms or a mixture of the two, leading to a disconnection between applied cracking force and sample cross-section area.  

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15:50-15:55 Questions

15:55-16:15 Discussion

Session 9 16:15-17:00

Panel discussion

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Chairs

Dr Kevin Kendall FRS, Adelan Ltd, UK

16:15-17:00 Panel discussion

Dr Irene Fernandez Villegas, Delft University of Technology, The Netherlands
Professor Małgorzata Lekka, Institute of Nuclear Physics, Polish Academy of Sciences, Poland
Professor Kathryn Whitehead, Manchester Metropolitan University, UK
Professor Ian Kinloch, University of Manchester, UK
Professor Rob Ritchie FREng ForMemRS, University of California, Berkeley, USA

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