M.N. Charalambides

On the analysis of mixed-mode failure

The present paper is concerned with the problem of mixed-mode fracture, especially where such failure occurs along a plane of weakness at a bimaterial interface or by interlaminar failure in polymer fibre-composite materials. Firstly, two different schemes for analysing such mixed-mode failures are discussed. These schemes are: (i) a method based upon a consideration of the local singular field ahead of a crack tip, and (ii) a global method based upon a consideration of the applied energy release rates. Secondly, experimental data taken from the literature are reviewed and it is concluded that the latter scheme results in a more consistent interpretation of the data. Indeed, in several cases the global method clearly gives far better agreement between the theoretical predictions and the observed results. The reasons for this are suggested to be the very localised nature of the singular-dominated region ahead of a crack tip in many test specimens and the relatively large damage zone in the materials studied. Finally, the present work has led to the proposal of a general criterion for fracture under mixed-mode loading.

The analysis of the frictional effect on stress - strain data from uniaxial compression of cheese

Uniaxial compression tests were performed on Gruyere and Mozzarella cheeses. It was observed that shorter samples appeared stiffer when no lubrication was used. This dependence on sample height was eliminated when a synthetic grease lubricant with polytetrafluorethylene (PTFE) was used. Therefore, the true stress-strain curves, i.e. free of frictional effects, were determined. Methods for reproducing these curves using data from unlubricated tests were then sought. It was shown that the true stress-strain curves can be determined by testing samples of increasing heights until the difference between consecutive curves is negligible. The curve corresponding to the tallest sample can then be taken to represent the true stress-strain curve. If size or shape limitations do not allow testing of sufficiently tall samples, quadratic extrapolation of the results may be performed. Alternatively, an iterative finite element analysis could be used. The latter is a more accurate but more time consuming method than the extrapolation procedure. In addition it requires that the coefficient of friction, μ, is known. It was shown that the latter can be derived from an analytical scheme. These values of μ were approximately 0.1 for Gruyere and 0.3 for Mozzarella and they were in close agreement with numerical predictions.

A study of the influence of ageing on the mechanical properties of Cheddar cheese

The aim of this study was to characterize Cheddar cheese in terms of mechanical properties and to relate these with maturing time. Three different types of cheese were studied: sharp Cheddar, mild Cheddar and Monterey Jack. The mechanical behaviour was described in terms of (a) stress-strain curves as obtained from uniaxial compression tests and (b) the fracture toughness values obtained from bending tests on single-edge notched specimens. For each test, the moisture, fat and protein content of the samples were also measured. In order to study the effect of friction between the sample and the loading plates on the measured stress-strain curve, samples of various heights were tested. However, the different sample height led to different applied strain rates. The data from the fracture tests were analysed using the European Structural Integrity Society testing protocol for plastics. A discussion is given on the effect of ageing on the mechanical properties.

Mechanical characterization and micromechanical modeling of bread dough

The mechanical behavior of dough, gluten, and starch was studied in an effort to investigate whether bread dough can be treated as a two phase (starch and gluten) composite material. Mechanical loading tests revealed rate-dependent behavior for both the starch and the gluten constituents of dough. There is evidence from cryo-scanning electron microscopy that damage in the form of debonding between starch and gluten occurs when the sample is stretched. In addition, the Lodge material model was found to deviate from the tension and shear stress-strain test data by a considerably larger amount than from the compression test data. This could indicate that “damage” is dominant along the gluten-starch interface, causing debonding; the latter occurs less under compression loading, but is more prevalent in tension and shear loading. A single-particle finite element model was developed using starch as a filler contained in a gluten matrix. The interface between starch and gluten was modeled using cohesive zone ele...

Large strain time dependent behavior of cheese

This study is aimed at characterizing the nonlinear viscoelastic behavior of hard cheeses using mild cheddar and gruyere as examples. The constitutive model consists of the Van der Waals hyperelastic function and the Prony series. The material constants were calibrated using data from monotonic compression and stress relaxation tests. Based on these constants, finite element simulations of three point bend and wire cutting tests were performed. The finite element predictions were compared with experimental data and good agreement was observed. In addition, successful predictions of the steady-state cutting force in the wire cutting tests have been made through the use of a critical strain failure criterion.

Fracture investigation in starch-based foods.

The study of oral processing and specifically cutting of the food piece during mastication can lead towards optimization of products for humans or animals. Food materials are complex biocomposites with a highly nonlinear constitutive response. Their fracture properties have not been largely investigated, while the need for models capable of predicting food breakdown increases. In this study, the blade cutting and the essential work of fracture (EWF) methodologies assessed the fracture behaviour of starch-based pet food. Tensile tests revealed rate-dependent stiffness and stress softening effects, attributed to viscoplasticity and micro-cracking, respectively. Cutting data were collected for 5, 10 and 30 mm s−1 sample feed rates, whereas the EWF tests were conducted at 1.7, 3.3 and 8.3 mm s−1 crosshead speeds corresponding to average crack speeds of 4, 7 and 15 mm s−1, respectively. A reasonable agreement was achieved between cutting and EWF, reporting 1.26, 1.78, 1.76 kJ m−2 and 1.52, 1.37, 1.45 kJ m−2 values, respectively, for the corresponding crack speeds. These toughness data were used in a novel numerical model simulating the ‘first’ bite mastication process. A viscoplastic material model is adopted for the food piece, combined with a damage law that enabled predicting fracture patterns in the product.

Modelling the deformation of a confectionery wafer as a non-uniform sandwich structure

The aim of this research was to model the mechanical behaviour of wafers found in various confectionery products in order to optimise the manufacturing stage. Compression and bending tests showed that the mechanical behaviour of the wafer was characteristic of a brittle foam. The internal microstructure of the wafer sheet was examined with an optical microscope which showed that the wafer possessed a sandwich structure with a porous core between two denser skins. An analytical model was developed to calculate the individual moduli of the wafer core and skin sections. These modulus values were used in a finite element (FE) model which consisted of a simple repetitive geometry. The FE model simulated the linear deformation of the wafer under compression and bending. The predictions from the analytical and numerical models were compared. They were found to agree in compression but deviated under bending due to the large mismatch of the core and skin moduli.

Cutting science in biology and engineering

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.

On modeling the large strain fracture behaviour of soft viscous foods

Mastication is responsible for food breakdown with the aid of saliva in order to form a cohesive viscous mass, known as the bolus. This influences the rate at which the ingested food nutrients are later absorbed into the body, which needs to be controlled to aid in epidemic health problems such as obesity, diabetes, and dyspepsia. The aim of our work is to understand and improve food oral breakdown efficiency in both human and pet foods through developing multi-scale models of oral and gastric processing. The latter has been a challenging task and the available technology may be still immature, as foods usually exhibit a complex viscous, compliant, and tough mechanical behaviour. These are all addressed here through establishing a novel material model calibrated through experiments on starch-based food. It includes a new criterion for the onset of material stiffness degradation, a law for the evolution of degradation governed by the true material’s fracture toughness, and a constitutive stress-strain resp...

Extrusion of unleavened bread dough: experiments and simulations

An experimental and numerical study on ram extrusion of bread dough was conducted in order to develop predictive models for the pressures involved, as well as the deformation of the extruded dough. Such studies are needed as high pressures can potentially lead to significant degassing, tearing, and shearing of the dough and hence poor bread quality; the latter limits the use of extrusion processes which would otherwise be a cost-effective forming process. A laboratory extrusion rig was designed, with dies of varying angles and exit radii. Rate dependent behavior was observed from tests conducted at different extrusion speeds, and higher extrusion pressure was reported for dies with smaller exit radius or larger die angle. A simulation of extrusion was performed to predict the extrusion pressure as well as the extrudate swell, as a function of die geometry and extrusion rate. A continuum approach was taken in the constitutive model of dough which is a starch filled system in a protein matrix. A nonlinear v...