Cutting science in biology and engineering

Abstract

On 26–27 October 2015, the Theo Murphy international scientific meeting on ‘Cutting science in biology and engineering’ was held at the Kavli Royal Society Centre, Chicheley Hall, Buckinghamshire, UK. The meeting was organized by Professor Gordon Williams FREng FRS, Professor Tony Atkins FREng, Professor Peter Lucas and Dr Maria Charalambides and it was enabled through the Royal Society scientific programme. It connected scientists from diverse backgrounds and disciplines including Biology and Mechanical Engineering from around the world.

The parting of solids by cutting is a vitally important process for many aspects of science, engineering and industry, as will be outlined below. The aim of the meeting was to bring together research teams investigating cutting from disparate fields in order to advance theoretical and practical knowledge in both academic and commercial settings.

Cutting is a process in which the tip of a tool is placed against a solid object so as to create a fracture that, controlled by the tool, separates new surfaces in a directed manner. Most often, the tool tip is sharp and its edge extended in one direction so as to form a blade. So defined, hardly any human undertaking escapes the employment of a cutting procedure at some point in its course. Moreover, cutting is also a feature of the designs of many parts of animals and plants, particularly those intended for foraging, feeding or protection. Cutting is thus a ubiquitous aspect of life on earth.

To begin with the oldest cutting devices, the variety of cutting forms that have evolved in biological organisms is quite staggering. Beaks, teeth, claws and pincers are familiar examples of cutting devices that animals use variously to forage, feed or defend themselves. Additionally, animals may also use such devices to burrow (cut) into a substrate for protection. Plants often opt to defend themselves from such animal attack with thorns and spines. The biologist studies the structure of all these organisms by dissection with tools such as scalpels and scissors, or by sectioning them finely with a microtome. Cutting is equally a fundamental feature of the health sciences. Humans cut their food up before (with utensils) and during ingestion (with the incisor teeth), and also through mastication, in order to reduce the food intake into smaller pieces suitable for swallowing and digestion. The way that the food responds to these cutting forces will largely affect the product's textural perception. Not just in food preparation at home, but more so that in an industrial setting, involves cutting stages that are crucial to food construction: this involves aspects of processing such as blade wear, end product geometry and waste control.

Surgical tools, both medical and dental, are often cutting tools and biopsies and postmortems also depend on similar designs. In archaeology, the earliest stone tools produced by humans are clearly cutting tools. Interpretations of their use by archaeologists and palaeontologists in terms of manufacturing techniques, such as flint knapping, and understanding their use via the interpretation of marks on bones requires considerable intuitive knowledge of the cutting process. In addition, the separation of material by cutting is commonplace in manufacturing engineering (metal cutting, semiconductor dicing, fabrication of large diffraction gratings), as also in agriculture (harvesting crops and ploughing) and civil engineering (tunnelling and piling). Cutting also features strongly in the timber industry, because tree felling, sawing, veneer production, general carpentry and the preparation of wood surfaces are all types of a cutting process. Less obvious to the lay public, but vital to questions of defence and security are investigations into weapons that can penetrate armour and how armour resists this. Study of the resistance to cutting of mediaeval and ancient metallic and non-metallic armour informs the design of modern body protection.

Owing to its commercial importance, the machining of metals, timber and polymers has received most attention in the research community. Traditional models for cutting such ‘ductile’ materials are couched in terms of plastic flow and friction. However, recent work shows that cutting is actually a branch of elastoplastic fracture mechanics. Inclusion of toughness, as a specific measure of ductility in analyses of force and deformation, resolves quantitatively many otherwise inexplicable results. Cutting in many other application areas has not really been concerned with mechanics and microstructure, other than with an awareness of sharpness. However, a unified theory now exists for the prediction of cutting forces, deformation, surface finish, etc., in materials with properties ranging across the spectrum from ‘hard, stiff and strong’ to ‘soft, compliant and weak’. This theoretical model is beginning to be applied to fields as diverse as the life and health sciences, biology and its related disciplines, arms and armour, abrasive wear, polishing, sculpting and engraving, and to structural integrity analyses involving accidental and unintentional cutting, such as ships hitting rocks.

In the above-mentioned cutting models, concepts from mechanics of materials are employed such as stress and strain, brittleness and ductility, and toughness. These quantities have formal definitions and there are established methods of testing to accurately quantify these. The methods are well known to engineers and physical scientists who use them to create and calibrate analytical models of cutting processes to provide predictions for the cutting forces, energy required and offcut geometry.

Owing to the way in which their subjects are taught and researched, biological scientists, food scientists and medical researchers are usually not familiar with the mechanics of materials science theories and are not required to manipulate mathematics in the way engineers and physical scientists are. Vice versa, the latter group would also benefit from greater knowledge of biological material behaviour and the importance of puncturing and cutting processes in evolution. Mathematical modelling as well as collaboration between engineers and biologists is therefore paramount for accurate interpretation of biological data. In the limited number of cases in archaeology, botany, zoology and palaeontology where models have been employed to aid observations, valuable quantitative data have been generated that explain where qualitative interpretation fails, and may sometimes even refute an earlier qualitative explanation. We believe that not using mathematical modelling has become a barrier to enhanced understanding of the role of cutting processes in a wide range of fields of study, both where cutting is required and where it is not. While, at present, the level of taxonomic sampling in some areas is not yet wide enough to determine the role played by cutting in biological design and function, we certainly believe that many exciting discoveries are yet to be made.

For such progress, the terminology of fracture should be rigorously re-examined. In engineering, there is much confusion and misunderstanding about ‘toughness’ defined in terms of the work required to create a unit area of new separated crack area, and toughness defined in terms of the so-called critical stress intensity factor. In biology and food science, there is a danger that the word ‘toughness’, used to reflect perceptions of texture and ‘mouth feel’, could be confused with its analytical usage. Another topic that requires clarification is the meaning of sharpness and how to interpret data obtained with blunt tools. This remark applies just as strongly to the engineering side as to the biosciences side.

To employ models of cutting in the biosciences, additional equipment will be required, in particular dynamometers to measure cutting forces, but these are readily available. Indeed the process of preparing samples can, in itself, provide mechanical property data. For example, when using an instrumented microtome, the force data can be used to estimate fracture toughness of the material being sliced. Furthermore, in contrast to engineering structural materials, many biological materials have a complex microstructure. Also containing amounts of fluid, particularly when very soft, they present real challenges in obtaining valid data both in experimental and modelling studies. Consequently, it will not always be a process merely of ‘following what the engineers do’. Progress will come from collaboration between researchers having a variety of skills and interests. It follows that it is important to spread this analytical knowledge and this is the aim of this meeting.