Comparing full-field data from structural components with complicated geometries.

A new decomposition algorithm based on QR factorization is introduced for processing and comparing irregularly shaped stress and deformation datasets found in structural analysis. The algorithm improves the comparison of two-dimensional data fields from the surface of components where data is missing from the field of view due to obstructed measurement systems or component geometry that results in areas where no data is present. The technique enables the comparison of these irregularly shaped datasets without the need for interpolation or warping of the data necessary in some other decomposition techniques, for example, Chebyshev or Zernike decomposition. This ensures comparisons are only made between the available data in each dataset and thus similarity metrics are not biased by missing data. The decomposition and comparison technique has been applied during an impact experiment, a modal analysis, and a fatigue study, with the stress and displacement data obtained from finite-element analysis, digital image correlation and thermoelastic stress analysis. The results demonstrate that the technique can be used to process data from a range of sources and suggests the technique has the potential for use in a wide variety of applications.

Transformation of measurement uncertainties into low-dimensional feature vector space.

Advances in technology allow the acquisition of data with high spatial and temporal resolution. These datasets are usually accompanied by estimates of the measurement uncertainty, which may be spatially or temporally varying and should be taken into consideration when making decisions based on the data. At the same time, various transformations are commonly implemented to reduce the dimensionality of the datasets for postprocessing or to extract significant features. However, the corresponding uncertainty is not usually represented in the low-dimensional or feature vector space. A method is proposed that maps the measurement uncertainty into the equivalent low-dimensional space with the aid of approximate Bayesian computation, resulting in a distribution that can be used to make statistical inferences. The method involves no assumptions about the probability distribution of the measurement error and is independent of the feature extraction process as demonstrated in three examples. In the first two examples, Chebyshev polynomials were used to analyse structural displacements and soil moisture measurements; while in the third, principal component analysis was used to decompose the global ocean temperature data. The uses of the method range from supporting decision-making in model validation or confirmation, model updating or calibration and tracking changes in condition, such as the characterization of the El Niño Southern Oscillation.

Real-time quantification of damage in structural materials during mechanical testing.

A novel methodology is introduced for quantifying the severity of damage created during testing in composite components. The method uses digital image correlation combined with image processing techniques to monitor the rate at which the strain field changes during mechanical tests. The methodology is demonstrated using two distinct experimental datasets, a ceramic matrix composite specimen loaded in tension at high temperature and nine polymer matrix composite specimens containing fibre-waviness defects loaded in bending. The changes in the strain field owing to damage creation are shown to be a more effective indicator that the specimen has reached its proportional limit than using load-extension diagrams. The technique also introduces a new approach to using experimental data for creating maps indicating the spatio-temporal distribution of damage in a component. These maps indicate where damage occurs in a component, and provide information about its morphology and its time of occurrence. This presentation format is both easier and faster to interpret than the raw data which, for some tests, can consist of tens of thousands of images. This methodology has the potential to reduce the time taken to interpret large material test datasets while increasing the amount of knowledge that can be extracted from each test.

Detection and tracking of cracks based on thermoelastic stress analysis.

Thermoelastic stress analysis using arrays of small, low-cost detectors has the potential to be used in structural health monitoring. However, evaluation of the collected data is challenging using traditional methods, due to the lower resolution of these sensors, and the complex loading conditions experienced. An alternative method has been developed, using image decomposition to generate feature vectors which characterize the uncalibrated map of the magnitude of the thermoelastic effect. Thermal data have been collected using a state-of-the-art photovoltaic effect detector and lower cost, lower thermal resolution microbolometer detectors, during crack propagation induced by both constant amplitude and frequency loading, and by idealized flight cycles. The Euclidean distance calculated between the feature vectors of the initial and current state can be used to indicate the presence of damage. Cracks of the order of 1 mm in length can be detected and tracked, with an increase in the rate of change of the Euclidean distance indicating the onset of critical crack propagation. The differential feature vector method therefore represents a substantial advance in technology for monitoring the initiation and propagation of cracks in structures, both in structural testing and in-service using low-cost sensors.

Next generation physiologically based kinetic (NG-PBK) models in support of regulatory decision making.

The fields of toxicology and chemical risk assessment seek to reduce, and eventually replace, the use of animals for the prediction of toxicity in humans. In this context, physiologically based kinetic (PBK) modelling based on in vitro and in silico kinetic data has the potential to a play significant role in reducing animal testing, by providing a methodology capable of incorporating in vitro human data to facilitate the development of in vitro to in vivo extrapolation of hazard information. In the present article, we discuss the challenges in: 1) applying PBK modelling to support regulatory decision making under the toxicology and risk-assessment paradigm shift towards animal replacement; 2) constructing PBK models without in vivo animal kinetic data, while relying solely on in vitro or in silico methods for model parameterization; and 3) assessing the validity and credibility of PBK models built largely using non-animal data. The strengths, uncertainties, and limitations of PBK models developed using in vitro or in silico data are discussed in an effort to establish a higher degree of confidence in the application of such models in a regulatory context. The article summarises the outcome of an expert workshop hosted by the European Commission Joint Research Centre (EC-JRC) - European Union Reference Laboratory for Alternatives to Animal Testing (EURL ECVAM), on "Physiologically-Based Kinetic modelling in risk assessment - reaching a whole new level in regulatory decision-making" held in Ispra, Italy, in November 2016, along with results from an international survey conducted in 2017 and recently reported activities occurring within the PBK modelling field. The discussions presented herein highlight the potential applications of next generation (NG)-PBK modelling, based on new data streams.

An experimental study on the manufacture and characterization of in-plane fibre-waviness defects in composites.

A new method has been developed for creating localized in-plane fibre waviness in composite coupons and used to create a large batch of specimens. This method could be used by manufacturers to experimentally explore the effect of fibre waviness on composite structures both directly and indirectly to develop and validate computational models. The specimens were assessed using ultrasound, digital image correlation and a novel inspection technique capable of measuring residual strain fields. To explore how the defect affects the performance of composite structures, the specimens were then loaded to failure. Predictions of remnant strength were made using a simple ultrasound damage metric and a new residual strain-based damage metric. The predictions made using residual strain measurements were found to be substantially more effective at characterizing ultimate strength than ultrasound measurements. This suggests that residual strains have a significant effect on the failure of laminates containing fibre waviness and that these strains could be incorporated into computational models to improve their ability to simulate the defect.

The stretchability of Mozzarella cheese evaluated by a temperature-controlled 3-prong hook test.

Stretchability is one of the most important functional properties of Mozzarella cheese, but no objective and widely accepted technique exists for evaluation; most of the cheese stretchability tests are influenced by the ambient conditions. This paper demonstrates a technique, which is novel and relatively simple, to evaluate the stretchability of Mozzarella cheese objectively. In an oil bath, melted cheese is stretched by a hook probe, controlled by an Instron tensile tester (Instron Corp., Norwood, MA); cheese strands are lifted from the melted cheese reservoir to a stretch length of 300 mm, when the load and extension data are recorded by the Instron tensile tester. This test overcomes other tests' drawbacks, such as subjective stretching speeds and variations in ambient temperature and humidity. Through the test comparison on standard Mozzarella, the modified stretching test in an oil bath has greater repeatability than the original test without oil bath. From the load and extension curve, the yield load is measured to evaluate the stretchability. In the meantime, the inversion point extension of necking is also measured based on polymer study, producing more repeatable results than the yield load. The modified 3-prong hook test was applied to Mozzarella cheese samples with different processing conditions, including the screw conditions (screw time, speed, and temperature), draining pH, calcium content, and fat content, and significant differences were found between these samples and the control sample.

Three-dimensional automated nanoparticle tracking using Mie scattering in an optical microscope.

The forward scattering of light in a conventional inverted optical microscope by nanoparticles ranging in diameter from 10 to 50nm has been used to automatically and quantitatively identify and track their location in three-dimensions with a temporal resolution of 200ms. The standard deviation of the location of nominally stationary 50-nm-diameter nanoparticles was found to be about 50nm along the light path and about 5nm in the plane perpendicular to the light path. The method is based on oscillating the microscope objective along the light path using a piezo actuator and acquiring images with the condenser aperture closed to a minimum to enhance the effects of diffraction. Data processing in the time and spatial domains allowed the location of particles to be obtained automatically so that the technique has potential applications both in the processing of nanoparticles and in their use in a variety of fields including nanobiotechnology, pharmaceuticals and food processing where a simple optical microscope maybe preferred for a variety of reasons.

An approach to the simulation of fluid-structure interaction in the aortic valve.

A pair of finite element models has been employed to study the interaction of blood flow with the operation of the aortic valve. A three-dimensional model of the left ventricle with applied wall displacements has been used to generate data for the spatially and time-varying blood velocity profile across the aortic aperture. These data have been used as the inlet loading conditions in a three-dimensional model of the aortic valve and its surrounding structures. Both models involve fluid-structure interaction and simulate the cardiac cycle as a dynamic event. Confidence in the models was obtained by comparison with data obtained in a pulse duplicator. The results show a circulatory flow being generated in the ventricle which produces a substantially axial flow through the aortic aperture. The aortic valve behaves in an essentially symmetric way under the action of this flow, so that the pressure difference across the leaflets is approximately uniform. This work supports the use of spatially uniform but temporally variable pressure distributions across the leaflets in dry or structural models of aortic valves. The study is a major advance through its use of truly three-dimensional geometry, spatially non-uniform loading conditions for the valve leaflets and the successful modelling of progressive contact of the leaflets in a fluid environment.

On the opening mechanism of the aortic valve: some observations from simulations.

Viewed from the standpoint of mechanical engineering design, the aortic valve is impressive. However, our understanding of its mechanics is limited by our inability to study its in vivo function closely and in detail. Computer simulation methods offer an alternative approach and a first step towards the construction of a more complete cardiac model is described. The model includes the aortic valve, its leaflets and their supporting root, and the sinuses modelled as nonlinear materials. An explicit finite element code has been used to examine the time-varying displacements of the structure that was subject to pressure distributions, which included left ventricular, aortic and thoracic pressures. It was shown that the leaflets of the valve open by a combination of root expansion in a radial direction and leaflet movement in the direction of blood flow. This was compared to a model in which the aortic root was stiffened significantly, and it was found that this modified valve opened by leaflet folding to give a much smaller orifice. These findings, concerning the importance of root expansion, are in agreement with earlier experimental observations.

Simulation of damage in a porcine prosthetic heart valve.

A model of a bioprosthetic porcine valve has been produced which simulates the non-linear elastic behaviour of the fixed tissue and the re-inforcement of the leaflets by collagen fibre bundles. The loading on the model is a spatially uniform but temporally varying distribution of pressure. Bending is allowed in the tissue of the leaflets and is shown to be significant in determining the behaviour and failure modes of the leaflets. The simulation of the undamaged valve is validated against in vitro pulse duplicator studies and a simple fluid--solid interaction simulation. Progressive damage is introduced into three models of the valve at a location where tears have been commonly found in vivo. It was found that during the opening and closing of the valve the tip of the tears were subject to mode III or tearing displacements, but that in the fully closed or diastolic state the tear tip was subject to mode I or opening displacements such that it would be expected to propagate parallel to the line of attachment to the stent. The tear tip stresses increased with the length of the tear so that the rate of tearing would be expected to increase with length.

Influence of anisotropy on the mechanical behaviour of bioprosthetic heart valves.

Chemically modified pericardium is commonly used in the fabrication of bioprosthetic heart valves. This material exhibits non-linear elastic behaviour and, as for most other biological soft tissues, it is orthotropic in its extensibility. The influence of the natural orthotropy of pericardium on the mechanical behaviour of pericardial heart valves during the whole cardiac cycle has been studied, using the finite element method. A model of the leaflet of a bicuspid valve has been created, defining the material of the tissue as orthotropic non-linear elastic. Two preferential orthogonal orientations of the tissue have been analysed (axial and circumferential). The results show that even a small amount of orthotropy (an orthotropy index of 1.5 has been used) can significantly affect the mechanical behaviour of the valve, and that an appropriate orientation of the fibres can contribute to optimizing the stress distribution in the leaflets.

Three-dimensional stress analysis of polypropylene leaflets for prosthetic heart valves.

The effect of changing the modulus and thickness of the material in the leaflets of an artificial heart valve has been investigated. This has been achieved with a finite element model of the valve having approximately 2300 thin shell elements. The valve motion and the resulting stresses are modelled dynamically during closure, and subsequent pressurisation. The stresses decrease as the leaflets are made thicker and the modulus is increased. Local and global thickening has been investigated. The highest stresses appear at the tops of the stent posts in the regions of the commissures. When the modulus is too low, or the leaflets are too thin, the valve prolapsed.

A comparative study of linear and nonlinear simulations of the leaflets in a bioprosthetic heart valve during the cardiac cycle.

Two geometrically identical models of the leaflets of a bicuspid bioprosthetic heart valve have been constructed using finite elements. The boundary conditions applied to the models were also identical but a linear material model has been used in one and a nonlinear elastic model in the other. The models were fullscale and contained 2600 Belytschko-Lin-Tsai shell elements which allowed the variation of stress through the thickness of the leaflet to be modelled. A time-varying, spatially-uniform pressure differential was applied across the leaflets to model their behaviour during a complete cardiac cycle. The simulation was performed using a dynamic, explicit, time-stepping, finite element code. A comparison of the two models showed that the nonlinear model was more responsive to the time-varying pressure wave, and deformed into more complex shapes during the opening and closing phases which induced lower compressive but higher tensile stresses in the leaflets.

Distribution of load in an oral prosthesis system: an in vitro study.

Although the long-term success of osseointegrated endosseous implants for the support of fixed dental prostheses has been reported, the increasingly routine and widespread use of implant-supported prostheses has led to problems associated with their structural integrity. To assess the service life of the components of the prosthetic system, a knowledge of the loads transmitted through the system is needed. This paper reports in vitro findings for forces and bending moments transmitted through the abutments from a fixed dental prosthesis to its supporting implants. The results have been compared to previous theoretical analyses that generally overestimate the maximum tensile forces. The effect of the length of distal cantilevers and of missing gold screws as the result of loosening or fracture is also considered. It is proposed that bending moments may play a larger role in failures than previously believed.

Tightening characteristics for screwed joints in osseointegrated dental implants.

The significance of tightening abutment screws and gold cylinders to osseointegrated fixtures with the correct torque is demonstrated, and a simple relationship between applied torque and screw preload is derived by use of mechanical engineering principles. The principles of a number of tightening methods are outlined and assessments made of their accuracy. The difference between optimum and design torque is highlighted. The necessity and means of achieving optimum torque to ensure a reliable joint in clinical practice is discussed.

Are hip fractures caused by falling and breaking or breaking and falling? Photoelastic stress analysis.

The distribution of stress in the heads of femurs has been assessed using an engineering technique known as photoelastic stress analysis. The aim of the investigation was to locate and evaluate the probable sites of structure failure in femurs with various axes of loading. The analysis shows that the maximum stress in the upper femur occurs in the cervical region during vertical loading. Lateral loading does not produce significant stress patterns at the sites of extracapsular hip fracture. It is concluded that cervical hip fractures in individuals suffering from osteoporosis result from a combination of fatigue damage and axial muscular compression, rather than forces generated by a fall. This has significance for the medico-legal interpretation of the cause of death following femoral neck fracture.

Breast cancer recurrence after lumpectomy and irradiation: role of mammography in detection.

Between 1977 and 1989, 102 of 1,145 patients treated with lumpectomy and radiation as an alternative to mastectomy required subsequent biopsy for suspected ipsilateral recurrence. The authors reviewed the mammograms of 58 of those patients for whom at least two sets of mammograms were available, including one set obtained within 3 months of the subsequent repeated biopsy. Of 38 biopsy-proved recurrences, 13 (34%) were detected with mammography alone, 17 (45%) with palpation alone, and eight (21%) with both mammography and palpation. While standard mammographic criteria for biopsy were followed, distortions and calcifications inherent to posttreatment appearances necessitated judicious modifications. As anticipated, sensitivities and positive predictive values showed more impairment within the lumpectomy quadrant. In addition, all cases of purely in situ cancer were detected solely with mammography. Mammography plays an important complementary role to physical examination in posttreatment follow-up. On the basis of these findings, a follow-up algorithm for the ongoing evaluation of these patients was developed.

Theoretical analysis of the fatigue life of fixture screws in osseointegrated dental implants.

Metal fatigue failure of the gold screw used to retain a fixed prosthesis to Brånemark osseointegrated fixtures/abutments has been analyzed theoretically. Mechanical engineering principles show the importance of appropriate preload being applied through the gold screw to the gold cylinder and abutment. The significance of the screw design and necessity of applying the correct torque to achieve a long fatigue life for the screw are also described. The consequence of misalignment of a gold cylinder to an abutment is discussed.