Validity and sensitivity of a human cranial finite element model: Implications for comparative studies of biting performance

Finite element analysis (FEA) is a modelling technique increasingly used in anatomical studies investigating skeletal form and function. In the case of the cranium this approach has been applied to both living and fossil taxa to (for example) investigate how form relates to function or infer diet or behaviour. However, FE models of complex musculoskeletal structures always rely on simplified representations because it is impossible completely to image and represent every detail of skeletal morphology, variations in material properties and the complexities of loading at all spatial and temporal scales. The effects of necessary simplifications merit investigation. To this end, this study focuses on one aspect, model geometry, which is particularly pertinent to fossil material where taphonomic processes often destroy the finer details of anatomy or in models built from clinical CTs where the resolution is limited and anatomical details are lost. We manipulated the details of a finite element (FE) model of an adult human male cranium and examined the impact on model performance. First, using digital speckle interferometry, we directly measured strains from the infraorbital region and frontal process of the maxilla of the physical cranium under simplified loading conditions, simulating incisor biting. These measured strains were then compared with predicted values from FE models with simplified geometries that included modifications to model resolution, and how cancellous bone and the thin bones of the circum-nasal and maxillary regions were represented. Distributions of regions of relatively high and low principal strains and principal strain vector magnitudes and directions, predicted by the most detailed FE model, are generally similar to those achieved in vitro. Representing cancellous bone as solid cortical bone lowers strain magnitudes substantially but the mode of deformation of the FE model is relatively constant. In contrast, omitting thin plates of bone in the circum-nasal region affects both mode and magnitude of deformation. Our findings provide a useful frame of reference with regard to the effects of simplifications on the performance of FE models of the cranium and call for caution in the interpretation and comparison of FEA results

Fringe Pattern Analysis in Wavelet Domain

We present a full-field technique for single fringe pattern analysis based on wavelet transform. Wavelets technique is a powerful method that quantifies at different scales how spatial energy is distributed. In the wavelets domain, fringe pattern analysis requires spatial modulation by a high-frequency carrier. We realize the modulation process numerically by combining the fringe pattern and its quadrature generated analytically by spiral phase transform. The first application concerns the speckle denoising by thresholding the two-dimensional stationary wavelet transform (2D-swt) coefficients of the detail sub-bands. In the second application, the phase derivatives are estimated from the 1D-continuous wavelet transform (1D-cwt) and 2D-cwt analysis of the modulated fringe pattern by extracting the extremum scales from the localized spatial frequencies. In the third application, the phase derivatives distribution is evaluated from the modulated fringe pattern by the maximum ridge of the 2D-cwt coefficients. The final application concerns the evaluation of the optical phase map using two-dimensional discrete wavelet transform (2D-dwt) decomposition of the modulated fringe pattern. The optical phase is computed as the arctangent function of the ratio between the detail components (high-frequency sub-bands) and the approximation components (low-frequency sub-bands). The performance of these methods is tested on numerical simulations and experimental fringes

Application of digital speckle interferometry to visualize surface changes in metallic samples immersed in Cu(NO3)2 solutions

Digital speckle pattern interferometry (DSPI) has been applied to analyze surface corrosion processes in a metallic sample immersed in a 0.1 M Cu(NO3)2 solution. The corrosion process induces changes in the surface and in the solution refractive index. A detailed analysis of the DSPI measurements has been performed to obtain a two-dimensional visualization of the surface changes and an evaluation of the refractive index changes of the solution. The possibilities of DSPI for measuring surface changes in these conditions have been analyzed.Funding of this research by Spanish MINECO and the European FEDER Program (Project MAT2011-22719) and by Gobierno de Aragón (Research groups T12 and T76) is gratefully acknowledged. Authors would also like to acknowledge the use of Servicio General de Apoyo a la Investigación-SAI, Universidad de Zaragoza.Peer Reviewe

DEVELOPMENT OF PHASE RETRIEVAL TECHNIQUES IN DISPLACEMENT AND DISPLACEMENT DERIVATIVES MEASUREMENT

Ph.DDOCTOR OF PHILOSOPH

Biomechanical Evaluation of Decellularized and Crosslinked Corneal Implants Manufactured From Porcine Corneas as a Treatment Option for Advanced Keratoconus

Currently corneal transplantation is the main treatment for late-stage keratoconus; however, transplantation procedures are accompanied by significant risk of post-surgical complications; this in addition to supply limitations imposed by a worldwide shortage of human donor corneas, has driven the development of alternative therapies. One such therapy is the use of corneal implants derived from porcine corneas (Xenia®, Gebauer Medizintechnik GmbH, Neuhausen, DE). In contrast to human donor tissue, these implants can be produced on demand and due to the processes used pose no risks for host-immune rejection. Their use has already been demonstrated clinically in patients for preventing the progression of topographic changes in keratoconus whilst improving visual acuity. The implants are derived from natural tissue and not standardised synthetic material, whilst this likely reduces the risk of issues with bio-incompatibility, there is inevitably variability in their intrinsic mechanical properties which requires investigation. Here, speckle interferometry is employed to examine the biomechanical properties, in response to physiologically representative forces, of native porcine corneal tissue prior to processing and after a proprietary 4-stage process involving decellularization, washing, compression and crosslinking. The control lenticules had an average Young’s modulus (E) of 11.11 MPa (range 8.39–13.41 MPa), following processing average E of the lenticules increased by 127% over that of the unprocessed tissue to 25.23 MPa (range 18.32–32.9 MPa). The variability in E of the lenticules increased significantly after processing suggesting variability in the propensity of the native tissue to processing. In summary, it is possible to produce thin (<90 µm) lenticules from porcine corneas with enhanced stiffness that are effective for treating late-stage keratoconus. Due to the observed variability in the responses of lenticules to processing, interferometry could be a useful technique for ensuring quality control in commercial production via biomechanical screening

An examination of the response of East Asian lacquer films to changes in environmental conditions

This work addresses the material properties and behaviour of Japanese lacquers (urushi) coatings similar to those found on the Mazarin Chest, an important Japanese lacquerware artefact currently displayed in the Toshiba Gallery of Japanese Art at the Victoria and Albert Museum (V&A) in London. For almost four centuries, the Mazarin Chest has been displayed in uncontrolled environmental conditions and has been exposed to a range of lighting conditions. As a result, the Mazarin Chest has deteriorated and started to suffer from different kinds of damage. However, the optimal conservation approach to repairing this damage is not known and as a consequence, research on urushi and related materials is of great interest. For the first time, the effect of changing relative humidity (RH) on the response of urushi is investigated by examining the deflection of a glass substrate coated with a thin film of urushi. Phase shifting interferometry was employed to measure this deflection, from which the in-plane stress developed in the system due to the expansion mismatch in the bilayer was calculated. This was performed for aged (exposed to ultra-violet radiation) and non- aged urushi films. The film stress was observed over 66 hrs under 30%, 36% and 42% RH, while the stress response was observed over 7 hrs during exposure to 60%, 54% and 48% RH. During exposure to 30%, 36% and 42% RH and for both non-aged and aged urushi films, tensile in-plane stress was observed. It was seen to reach a peak value then relax over a longer time scale. The stresses develop in the non-aged urushi films were found to be higher than the stresses developed in the aged urushi films. The peak stress values for non-aged and aged urushi films were found to increase when increasing the difference between the storage RH (75%) and the target RH. When the non-aged and aged urushi films were subjected to a step change from low RH (30%, 36% and 42%) to high RH (60%, 54% and 48% RH), a compression inplane stress was observed and in this case, the aged urushi films exhibited higher stresses than those developed in non-aged urushi films. II To predict the behaviour of the urushi films, a simple 1D model of stress has been proposed. It shows reasonable agreement against the experimental stress measurements when the RH changes from 75% to 30%, 36% and 42% respectively, for both nonaged and aged urushi films. Furthermore, the model is extended to predict the stress response as a function of depth for non-aged urushi layers above an aged layer. For both the upper urushi layer (non-aged) and the lower urushi layer (aged), the model suggests that decreasing the layer depth results in an increase in the stress levels. In addition to the stress response measurements and in order to fully characterise the mechanics of urushi, mechanical properties were determined for non-aged and aged urushi films under different RH levels. These including the moisture diffusion coefficients, the elastic modulus, the tensile strength at break, the elongation at break, and the viscoelastic properties. The moisture diffusion coefficients were determined from the sorption and desorption curves when urushi films, non-aged and aged, subjected to step changes in RH. The results showed that the moisture diffusion coefficients were independent of the moisture content for non-aged and aged urushi. A strong dependence of the elastic modulus and the elongation at failure on the RH has been observed for non-aged and aged urushi films. At all strain rates used and as the RH increases, a significant reduction in tensile elastic modulus and an increase in the elongation at failure were observed. As a result of the UV ageing, no plastic deformation was observed in the stress-strain curves at any RH or tensile speeds. Creep recovery behaviours of non-aged and aged urushi under 30%, 50% and 75% RH and different stress levels were studied. The overall deformation levels at 75% RH for non-aged films were found to be higher than that at 30% RH and the aged films exhibited less stiffness. This trend reflects the strong influence of water, as a plasticizer, on urushi films. The variation of the elastic recovery with the applied stress and the RH for non-aged and aged urushi films were determined from the recovery behaviour. The results showed that the amount of elastic recovery at 30% RH for non-aged urushi was higher III than that for aged urushi films at all stress values. At 50% RH and 75% RH, the aged urushi films recovered to a greater degree than non-aged urushi films. The 4-element Burger‟s model was used for quantitative characterisation of the creep recovery curves to determine the viscoelastic properties for urushi films and their dependence on the RH and UV ageing. It was found that the RH has a strong effect on the viscoelastic properties for both non-aged and aged urushi films. A significant decrease in these parameters has been observed when the RH has been increased suggesting that the films tend to become soft as a result of the high mobility of the molecular chains under elevated RH. We have proposed a methodology to identify the effectiveness of the Japanese traditional consolidation processes that are used to consolidate the formation of microcracks on lacquerware surfaces. A rectangular aluminium substrate, covered with a thin film of aged urushi, was mechanically loaded using three point bending device and the in-plane urushi surface displacement profile was obtained through phase shifting digital speckle pattern interferometry (DSPI). A comparison of the displacement profiles before and after creating a ‟v‟ notch along the film showed an anomalous profile around the notch. While after consolidate the notch, no anomaly in the displacement profiles were observed. The potential for DSPI to play a key role in investigating consolidation mechanics has been demonstrated with the detection of the surface displacement around a notch before and after consolidation

Phase shifting speckle interferometry for dynamic phenomena.

The paper presents an algorithm able to retrieve the phase in speckle interferometry by a single intensity pattern acquired in a deformed state, provided that the integrated speckle field is resolved in the reference condition in terms of mean intensity, modulation amplitude and phase. The proposed approach, called throughout the paper "one-step", can be applied for studying phenomena whose rapid evolution does not allow the application of a standard phase-shifting procedure, which, on the other hand, must be applied at the beginning of the experiment. The approach was proved by an experimental test reported at the end of the paper

Characterising delamination in composite materials : a combined genetic algorithm - finite element approach

A novel delamination identification technique based on a low-population genetic algorithm for the quantitative characterisation of a single delamination in composite laminated panels is developed, and validated experimentally The damage identification method is formulated as an inverse problem through which system parameters are identified. The input of the inverse problem, the central geometric moments (CGM), is calculated from the surface out-of-plane displacements measurements of a delaminated panel obtained from Digital Speckle Pattern Interferometry (DSPI). The output parameters, the planar location, size and depth of the flaw, are the solution to the inverse problem to characterise an idealised elliptical flaw. The inverse problem is then reduced to an optimisation problem where the objective function is defined as the L2 norm of the difference between the CGM obtained from a finite element (FE) model with a trial delamination and the moments computed from the DSPI measurements. The optimum crack parameters are found by minimising the objective function through the use of a low-population real-coded genetic algorithm (LARGA). DSPI measurements of ten delaminated T700/LTM-45EL carbon/epoxy laminate panels with embedded delaminations are used to validate the methodology presented in this thesis.EThOS - Electronic Theses Online ServiceGBUnited Kingdo