Cutting science in biology and engineering

Cutting is a process in which a tool is used to create a new surface in a solid. The creation of new surfaces is a fracture process so that cutting may be regarded as fracturing. However, fracture is usually driven by forces remote from the failure point while cutting involves forces being applied locally via the tool. The crucial factor in both processes is the energy per unit area required to create the new surfaces which is the notion introduced by A.A. Griffith. An understanding of the processes involved requires an energy balance to be drawn up to include the fracture energy, Gc, and other forms of energy dissipation. These include plastic deformation and friction and the latter is particularly important in cutting when the tool, or blade, rubs on the material being cut.

The analysis will be illustrated by considering the removal of a surface layer by a sharp wedge. If all the deformation is elastic, and hence there is no permanent deformation, and there is no friction then Gc is given simply by the cutting force divided by the cut width, This is similar to the splitting of wood though friction usually occurs and must be included. If the removed layer deforms plastically chip curling and shearing occurs and there is additional dissipation. For this rather simple system there are a series of transitions from elastic bending of the chip to firstly plastic bending and then to plastic shearing as the wedge angle of the tool increases.

Professor Gordon WIlliams FREng FRS, Imperial College, UK

Professor Gordon WIlliams FREng FRS, Imperial College, UK

Professor Williams has been in the Mechanical Engineering Department at Imperial College London since 1960 after first being an apprentice at the RAE Farnborough.  He did his PhD with Hugh Ford on the then, new topic of mechanical properties of polymers.  This proved to be an inspired choice since polymer science and engineering grew very rapidly with major companies developing new materials.  His interest included all mechanical properties but over time the emphasis moved more to fracture and particularly Fracture Mechanics which was a rapidly developing field.  He published a book “Fracture Mechanics of Polymers” in 1982.  His interest broadened to include the fracture of composites and adhesives and providing standard test methods.

He became involved in cutting theory some 15 years ago as a method for measuring the toughness of soft materials including polymers such as polyethylene but also to foods such as cheese.  Biological materials such as gels have been a natural extension of this activity and are a considerable challenge.

The fracture toughness of soft solids such as foods is often needed for correlations to important parameters such as sensory attributes or release rates of contained substances (e.g. salt) and as input in model simulations of industrial food processes or mastication. Soft solids are more difficult to test and analyse due to their high compliance and strong non-linearity as well as their strong dependence on loading rate. This behaviour makes conventional fracture tests used for structural materials unsuitable for soft solids. Cutting tests in the form of wire or blade cutting offer an attractive alternative for such materials.

Cutting tests in the form of wire and blade cutting are presented for a range of food materials including cheese, gels and pet foods. The fracture toughness calculated from the cutting test data are compared to independent fracture tests for validation purposes. Methods enabling modelling of fracture initiation and propagation in such soft solids will be presented. Possible failure criteria based on critical maximum strain and cohesive zone models are employed and their accuracy is judged by comparing with experimental data. The studies enable the effect of strain rate on fracture toughness of this class of materials to be determined.

Dr Maria Charalambides, Imperial College London, UK

Dr Maria Charalambides, Imperial College London, UK

Dr Charalambides, CEng, MIMechE, is a Reader in Mechanics of Materials in the Department of Mechanical Engineering at Imperial College London. Prior to her appointment at Imperial as a Lecturer in 1997, she was a Research Associate in the same Department and a Senior Scientist at the National Physical Laboratory in Teddington, Middlesex.

Her published research includes material modelling and mechanical characterisation of soft polymeric solids, micromechanics models of particulate filled polymeric composites, experimental and numerical modelling of industrial food processes such as rolling, extrusion and cutting, development of inverse indentation methods material characterisation of polymers , and fracture and deformation in paint and adhesive coatings.

She has published approximately thirty peer reviewed journal publications and thirty conference papers in these research areas. Maria has a First Class B.Eng (Hons) in Mechanical Engineering from Imperial College London. She also holds a PhD in Mechanical Engineering, from Imperial College London, on the Delamination of fibre reinforced composites.

This talk will discuss and define sharpness from a practical and scientific point of view, across all aspects of the use of sharp edge cutting technology from micro surgical cutting, to shaving, to food processing, to industrial processes. Methods of sharpness measurement, both human subjective assessment to scientific measurement and quantitative measurements will be described, including their drawbacks and implications.

The talk will explain in detail the physical and material requirements of a sharp edge, the implications on cutting performance in terms of energy consumption and quality of the resultant cut and condition of the cut item. Sharpening techniques will be described, illustrating how incorrect sharpening can lead to significantly inferior performance both in terms of how sharp a blade is, but also how long a cutting edge will last.

An overview of the instrumentation used to quantify sharpness, edge durability and cutting edge geometries will be given and how they can be used both within blade / knife manufacture and importantly in re-sharpening. The talk will describe a number of real life stories, indicating significant financial losses, some incurred by World class companies, through ignorance of the simple but essential application of sharpness technology.

Mr Roger Hamby, CATRA, UK

Mr Roger Hamby, CATRA, UK

Roger Hamby read production engineering at Trent University  and has practiced it for the whole of the last 45 years.
In the last 37 years he has worked for the Cutlery and Allied Trades Research Association ( CATRA) in Sheffield  as Senior Engineer and Director of Research. In this post Roger has been instrumental in leading the research and development of  knife/ blade sharpening systems, sharpness/ edge durability testing technology / instrumentation and a greater understanding of the technology and practical problems of maximising  cutting  performance and efficiency in relation to sharp edge cutting used within the food, plastics, medical  and household sectors. Today CATRA is regarded as one of the World’s foremost authorities on  cutting technology  and suppliers of a unique range of test equipment and testing services.

Why it is easier to cut when there is motion along, as well as perpendicular to, a cutting blade is explained and the analysis is applied to optimisation of blade geometries to produce minimum damage in cut surfaces.

The formation of grooves by scratching is an example of cutting with more than one edge. The mechanics are investigated and applied to the topic of abrasive wear. It is shown that not only hardness and contact load is important, but also the fracture toughness of the surface material.

Cutting the same material at different depths of cut results in a spectrum of different forms of offcut, from ductile continuous ribbons at small depths to brittle ‘knocking lumps out’ at large. The associated forces vary from steady, through oscillating, to intermittent. Which behaviour occurs depends on the depth of cut relative to a material length parameter given by [ER/k2] = [KC/k]2 where E is Young’s modulus, R is fracture toughness, k is the shear yield strength and KC is the critical stress intensity factor. Glass has low KC/k and is normally brittle, but it can be machined with continuous chips at micrometre depths of cut; steel has high KC/k and normally machines in a ductile manner, but at metre depths of cut becomes brittle. This behaviour represents a ‘cube-square’ scale effect and supports the contention that analysis of cutting is a branch of elastoplastic fracture mechanics. Its connexion with allometry (scaling in biology) is discussed.

Professor Tony Atkins FREng, University of Reading, UK

Professor Tony Atkins FREng, University of Reading, UK

Professor Tony Atkins is emeritus professor of mechanical engineering at the University of Reading and a visiting professor at Imperial College London. He was elected the the Royal Society of Engineering in 2002. 

Most dentists pay little heed to what is actually happening at the end of their turbine but as they proceed to prepare a cavity within a tooth for restoration. However, there are many surface preparation factors that will influence how well our adhesive and non-adhesive materials will work. Many of these will relate to the methods used to prepare the cavity in the first place such as diamond, tungsten carbide and steel burs.  Cutting of the root dentine is also needed in preparation prior to endodontic treatment, with significant commercial investment for the development of flexible cutting instruments based around nickel titanium alloys.

Tooth-cutting interactions have been examined microscopically for over twenty five years using a variety of microscopic techniques; in particular, video-rate confocal microscopy. This has given a unique insight into how many of the procedures that we take for granted are achieved in clinical practice, by showing microscopic video images of the cutting as it occurs within the tooth. Cutting techniques with high and low torque handpieces, dry and wet airbrasive applications and laser cutting have all been imaged microscopically. The influence of these preparation modalities on both sound and diseased (carious) enamel and dentine is profound: especially when the integrity of the remaining tooth surface has such a strong influence on success or failure for bonding adhesive materials. 

Professor Timothy Watson, King's College London, London

Professor Timothy Watson, King's College London, London

Tim Watson is Professor of Biomaterials and Restorative Dentistry and Honorary Consultant in Restorative Dentistry at King’s College London Dental Institute, Guy’s Hospital and established the Biomaterials, Biomimetics & Biophotonics Research Group. Much of his research is based on the microscopic imaging of new operative techniques and adhesive restorative materials, with extensive publications in the fields of microscopy, dental materials and operative dentistry.  He was one of the founder inventors of the application of bioactive glasses in operative dentistry.  He is a past president of the Dental Materials Group of the International Association for Dental Research and current IADR board member for the European Region.  He is past president for the British society for Oral & dental Research. Twenty PhD students have graduated under his supervision over the last 20 years.  He has world-wide lecturing commitments and his research group has attracted over £6M in research grants from Government, Charity and Industrial sources.

Surgical robotics is gaining in popularity due to the ever-increasing need for more complex, accurate, but less invasive surgery, which conventional instruments are holding back. Amongst the many embodiments at different stages of research and development, needle steering holds promise, but has yet to demonstrate real clinical impact. This minimally invasive procedure involves the insertion of a thin, flexible needle through the skin and into tissue, for application to a variety of clinical needs, including diagnostics, therapy and therapy monitoring. The ability to steer the needle enables the surgeon to avoid obstacles and counteract targeting inaccuracies due to tissue deformation (due to tool-tissue interactions) and organ motion (due to e.g. breathing and pulsatile forces), but these first need to be appropriately modelled and tracked, which remains an open challenge.

In this presentation, the complexity of tool-tissue interaction modelling will be reviewed in the context of exciting new research on a steerable needle, codenamed STING, which is inspired by the ovispotior (or egg laying channel) of parasitic wasps. The biomimetic foundations of this unique design will be discussed, followed by an overview of several unique approaches to the characterisation and modelling of the needle behaviour. These have been developed specifically for STING, with the aim of understanding and optimising its performance in surgery, but many of the results have broader implications. The talk will end with a summary of the current state of the field and an outlook of what is to come.

Dr Ferdinando Rodriguez y Baena, Imperial College London, UK

Dr Ferdinando Rodriguez y Baena, Imperial College London, UK

Dr Ferdinando Rodriguez y Baena is a Reader in Medical Robotics in the Department of Mechanical Engineering at Imperial College, where he leads the Mechatronics in Medicine Laboratory. His expertise and track record lie in the development of bespoke mechatronics systems in medicine. He has developed commercial systems for orthopaedic surgery (the Acrobot orthopaedic robot and the Navigator computer assisted surgery system, now of Stryker Inc.), as well as several surgical and pre-surgical prototypes, including an advanced needle steering system (funded by the European Research Council), a unique cooperative controller for robot assisted soft tissue surgery (Dynamic Active Constraints), and novel implant designs/surgical instruments for improved performance in joint replacement. Dr Rodriguez y baena is the outgoing Chair of the Robotics and Mechatronics Technical Professional Network of the IET, an Associate Editor for the IEEE Robotics and Automation Magazine, the Chair of the Programme Committee of the International Society for Computer Assisted Orthopaedic Surgery, a Leverhulme Prize winner (engineering), and a current ERC grant holder. He has published over 150 papers and has secured in excess of £4M in research funding to date.

Insects and mammals, in general, consume each other or terrestrial plants; in other words they eat organisms with structures that resist gravity. Insects have a tough exoskeleton while mammals have an endoskeleton. The leaves and stems of vascular plants, in contrast, have tough structures around each cell. The scale, heterogeneity and temporal variability of food affect the adaptations for cutting and processing food. The requirements for processing insects as food therefore differ from those for processing relatively large animals. Herbivores face the added complication that the physical properties of plant food can vary with environmental factors.

Herbivores range from the smallest insects that can live and feed inside a single leaf to larger insects and the smallest mammals that consume individual leaves but are still small enough to be able to select parts of leaves. Cutting adaptations relate to the fine scale properties of the food. On the other hand the largest mammals process food in bulk and the implications of this are not well understood.

Grazing (grass eating) in large mammals has generally evolved from browsing (eating dicotyledonous plants). Browse is commonly assumed to be relatively easy to physically process, compared with a diet of grasses (a group of monocotyledonous plants) that are considered to be tough and fibrous. These assumptions are reviewed. Browsers are potentially faced with a highly heterogeneous diet, particularly when living in seasonal environments. Cutting adaptations appear to include clearance of fractured components of the diet away from the cutting edges. In grazers, the more complicated cutting patterns of the teeth may relate not so much to relative toughness, or even the pattern of the veins, but to the bulk processing of very long, thin and narrow leaves.

Professor Gordon Sanson, Monash University, Australia

Professor Gordon Sanson, Monash University, Australia

Gordon Sanson is primarily interested in the evolution of mechanical processing of plants by herbivores and seeks to understand tooth function by integrating tooth wear and its causes with diet properties and selection, body size, digestibility, behaviour, and longevity. He has explored how plants defend themselves from herbivory, chemically and mechanically, at both the insect and mammal scale. He has been a Thomas Sidey Visiting Professor to the University of Otago Faculty of Dentistry, President of the Royal Society of Victoria (Australia) and Head of the School of Biological Sciences at Monash University. He was a co-founder of the John Monash Science School for secondary students at Monash as an innovator of science education. In recent years he has increasingly been involved in education. As Director of eEducation he promoted the integration of pedagogy, classroom design and technology and has industry and teaching awards.

Tooth wear in mammals gradually removes features of the working surfaces of teeth vital for breaking down food particles. This loss of tooth tissue is a major factor limiting the lifespans of individuals in the wild. The threat to enamel, the main contact tissue of tooth crowns, is mainly mechanical, stemming from contacts with hard objects including silica(tes) from soils, amorphous plant silica and also perhaps woody plant tissue such as seed coats. Shallow marks have been documented extensively on worn tooth enamel in microwear studies, forming the main indication of what has transpired during wear events. The circumstances of this mechanical wear qualify as a type of cutting process in which the foreign object (siliceous particle or seed shell) can be viewed as the ‘tool’ creating change in the enamel surface. During any one contact, there are two possibilities: either the enamel surface is re-arranged via plastic deformation (thus not losing material within that single event) or a chip or ribbon of enamel is created that results in immediate tissue loss (definitive of wear).  The factors affecting what transpires are well-known to include the relative hardness of the contacting materials, angulation of the contacting surfaces and friction. Less well-known is the effect that enamel toughness plays in protecting against wear, and the critical depth to which enamel is disturbed. In collaboration with a large group of collaborators that include A.G. Atkins, we have attempted displacement-controlled cutting tests on enamel designed to simulate wear conditions accurately and discover the effective value of the toughness of enamel at this scale. From this, we suggest that the standard ‘R-curve’ approach to biological mineralized tissues, which would have toughness values building from low values upwards with increasing scale, has to be revised for enamel. 

Professor Peter Lucas, Kuwait University, Kuwait

Professor Peter Lucas, Kuwait University, Kuwait

Peter Lucas is Professor of Bioclinical Sciences (Faculty of Dentistry), Kuwait University, holding a BSc from University College London and PhD and DSc from the University of London. He worked at the National University of Singapore, University of Hong Kong and George Washington University before joining Kuwait in 2011. He has about 120 full journal papers, plus one book (Dental Functional Morphology – 2004, reviewed in both Nature and Science). His research revolves around feeding processes.