[Show abstract][Hide abstract] ABSTRACT: The combination of fringe projection (FP) and two-dimensional digital
image correlation (2D-DIC) has been proposed in previous work 
as an alternative method to obtain displacement maps in the three
spatial directions. However, if a telecentric lens is not employed in
the experimental setup, the in-plane displacements obtained with
2D-DIC are influenced by the out-of-plane displacements occurring
during deformation. Nevertheless, this error can be corrected if the
out-of-plane displacements are known, for instance from measurements
using the FP technique. In this paper a novel methodology based on the
combination of FP and 2D-DIC is employed to perform the correction
of the in-plane displacements, and is applied to several experimental
examples. Results are compared and validated with those obtained using a
commercial 3D-DIC system showing an average displacement error of
4% for X-displacements and 6.5% for Y-displacements.
Optics and Lasers in Engineering 01/2014; · 1.92 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The need to provide strong evidence of the validity of predictions from computational solid mechanics models used in engineering design decisions is discussed. A new procedure is proposed, based on image decomposition, for reducing the dimensionality of strain field data from models and experiments and then comparing the resultant feature vectors via a simple linear correlation in which validation is deemed to be achieved when the coordinate pairs from the two feature vectors lie within a scatter band defined by the minimum measurement uncertainty. The procedure is illustrated by some simple examples that allow the advantages and drawbacks of the approach to be highlighted. It is anticipated that the procedure could become part of a corporate plan or regulatory process for verification and validation of computational solid mechanics models.
The Journal of Strain Analysis for Engineering Design 01/2013; 48(1):36-47. · 0.88 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Experimental strain analysis, structural health monitoring and non-destructive testing and evaluation are regarded as separate disciplines that, in general, are deployed independently at different phases in the life cycle of an engineering component, i.e. in the design process, in service and after an event or service period, respectively. It is proposed that the integrated use of these three disciplines is advantageous and beneficial in terms of reduced capital and operational costs for critical and safety-relevant components, as well as, in validating simulations, in both quantifying and reducing risk of unexpected failure, and in estimating remanent life. We propose the foundation of this integration to be data-rich strain fields measured and compared quantitatively, with each other and with data from simulations, at temporal intervals during the life of a component.
The Journal of Strain Analysis for Engineering Design 01/2013; 48(1):48-58. · 0.88 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The use and results of the procedure published by Standardisation Project for Optical Techniques of Strain measurement (SPOTS) for a successful calibration of a digital image correlation (DIC) system are described. The details of the calibration specimen used are discussed together with procedure and criteria that must be met to achieve an acceptable calibration. The DIC system was evaluated over a strain range of 289 to 2110 µstrain, with a resulting calibration uncertainty ranging from 14 to 28.7 µstrain. The optical strain measurements were obtained from images taken directly from the bare metal surface, which had been prepared with grit paper, as opposed to generating a speckle pattern by painting the surface.
[Show abstract][Hide abstract] ABSTRACT: A novel approach is introduced to map the mesoscale plastic strain distribution resulting from heterogeneous plastic deformatio in complex loading and component geometries, by applying the discrete Fourier transform (DFT) to backscattered electron (BSE images of polycrystalline patches. These DFTs are then calibrated against the full width at half the maximum of the centra peak of the DFTs collected from the same material tested under in situ scanning electron microscopy uniaxial tensile conditions, which indicates a close relationship with the global tensile strain.
In this work, the technique is demonstrated by measuring the residual strain distribution and plastic zone size around a fatigu crack tip in a commercially pure titanium compact tension specimen, by collecting BSE images in a 15×15 array of 115 μm squar images around the fatigue crack tip. The measurement results show good agreement with the plastic zone size and shape measure using thermoelastic stress analysis.
Proceedings of The Royal Society A Mathematical Physical and Engineering Sciences 08/2012; 468(2144):2399-2415. · 2.38 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: This paper expounds a revised characterisation of the elastic stresses ahead of a crack tip in polycarbonate which takes account of the elastic–plastic boundary stresses induced by the presence of the crazed region that surrounds a crack. The advanced experimental techniques used in this work have provided insights into fractography, identification of the crazed region and location of the crack tip position (using confocal laser scanning microscopy and scanning electron microscopy). In addition, the four-parameter model of crack tip stresses has led to modified definitions for crack tip stress intensity factors which explicitly account for craze-induced shielding effects on the fatigue crack growth rate in polycarbonate. The model is generic and offers the potential for increased understanding of fatigue crack growth in polycarbonate.
[Show abstract][Hide abstract] ABSTRACT: For holography and speckle interferometry the calibration of the
sensitivity is a must, because illumination and observation directions
vary across the field of view. A numerical estimate or a static
calibration using rigid body motions is standard, and reference
materials exist for static strain calibration. Recently, reference
materials for the dynamic calibration of optical instruments of
displacement and strain measurement were designed and prototypes were
manufactured in the European FP7 project ADVISE. We review the
properties of the reference material and the concept of traceability for
the field of displacement values by using a calibrated single point
transducer. The mode shape is assessed using out-of-plane DSPI, Finite
Element Analysis as well as analytic solutions of the plate vibration.
We present measurements using stroboscopic DSPI on the reference
material under acoustic excitation and compare the measured mode shapes
to the ones predicted by FE analysis. We apply different comparison
methodologies based on point-by-point deviations and on decomposition of
the mode shapes into a set of orthogonal basis functions. The latter
method is well suited to assess stability and reproducibility of a mode
shape. Finally, the deviations are used to estimate the reference
material uncertainty which is an essential parameter for determining the
calibration uncertainty. Uncertainty contributions of the DSPI set-up
are taken into account. To conclude, the application area and
limitations of the reference material are discussed.
[Show abstract][Hide abstract] ABSTRACT: Finite element model updating is an inverse problem based on measured structural outputs, typically natural frequencies. Full-field responses such as static stress/strain patterns and vibration mode shapes contain valuable information for model updating but within large volumes of highly-redundant data. Pattern recognition and image processing provide feasible techniques to extract effective and efficient information, often known as shape features, from this data. For instance, the Zernike polynomials having the properties of orthogonality and rotational invariance are powerful decomposition kernels for a shape defined within a unit circle. In this paper, full field strain patterns for a specimen, in the form of a square plate with a circular hole, under a tensile load are considered. Effective shape features can be constructed by a set of modified Zernike polynomials. The modification includes the application of a weighting function to the Zernike polynomials so that high strain magnitudes around the hole are well represented. The Gram-Schmidt process is then used to ensure orthogonality for the obtained decomposition kernels over the domain of the specimen. The difference between full-field strain patterns measured by digital image correlation (DIC) and reconstructed using 15 shape features (Zernike moment descriptors, ZMDs) at different steps in the elasto-plastic deformation of the specimen is found to be very small. It is significant that only a very small number of shape features are necessary and sufficient to represent the full-field data. Model updating of nonlinear elasto-plastic material properties is carried out by adjusting the parameters of a FE model until the FE strain pattern converges upon the measured strains as determined using ZMDs.
Journal of Physics Conference Series 07/2011; 305(1):012011.
[Show abstract][Hide abstract] ABSTRACT: The self-belief, motivation, tendency to procrastinate and learning styles of engineering students are discussed. It is proposed that engineering has developed an idiom and a learning approach that favours the dominant client, i.e. men, while simultaneously undermining the self-efficacy and motivation of women. Thematic coherence and teaching within a context that is familiar to students have been shown previously to be effective approaches for engaging students and are extended here to utilise the common experiences of all students to initiate the learning cycle. These approaches are combined with a template for teaching that uses the 5Es (Engage, Explore, Explain, Elaborate and Evaluate) in order to render the fundamentals of engineering more accessible to all students. This methodology can be introduced by individual instructors, who will be rewarded by students who are more engaged, more motivated and more likely to give a higher rating to the instructor and the course.
European Journal of Engineering Education 06/2011; 36(3):211-224.
[Show abstract][Hide abstract] ABSTRACT: The goal of the Deconstructing Engineering Education Programmes project is to revise the mechanical engineering undergraduate curriculum to make the discipline more able to attract and retain a diverse community of students. The project seeks to reduce and reorder the prerequisite structure linking courses to offer greater flexibility for students. This paper describes the methods used to study the prerequisites and the resulting proposed curriculum revision. The process involved dissecting each course into topics at roughly the level of a line in a syllabus, editing the list of topics, associating prerequisites and successors to each topic and then using a genetic algorithm to produce clusters of topics. The new curriculum, which consists of 12 clusters, each of which could be a full year course, is quite different from the traditional curriculum.
European Journal of Engineering Education 06/2011; 36(3):269-283.
[Show abstract][Hide abstract] ABSTRACT: A reference material and a series of standardized tests have already been developed for respectively calibrating and evaluating
optical systems employed for measuring in-plane static strain (for draft standard see: www.twa26.org). New work has commenced
on the design of a reference material (RM) for use with instruments or systems capable of measuring three-dimensional displacements
and strains during dynamic events. The rational decision-making process is being utilized and the initial stages have been
completed, i.e. the identification and weighting of attributes for the design, brain-storming candidate designs and evaluation
of candidate designs against the attributes. Twenty-five attributes have been identified and seven selected as being essential
in any successful design, namely: the boundary conditions must be reproducible; a range of in-plane and out-of-plane displacement
values must be present inside the field of view; the RM must be robust and portable; there is a means of verifying the performance
in situ; and for cyclic loading it must be possible to extract data throughout the cycle. More than thirty candidate designs
were generated and have been reduced to nine viable designs for further evaluation. In parallel with this effort to design
a reference material, work is also in progress to optimize methodologies for conducting analyses via both simulations and
experiments. Image decomposition methods are being explored as a means to making quantitative comparisons full-field data
maps from simulations and experiments in order to provide a comprehensive validation procedure.
[Show abstract][Hide abstract] ABSTRACT: Nanoparticles are by definition too small to be visible in an optical microscope and devices such as scanning electron microscopes
must be used to resolve them. However electron beams quickly lead to cell death and so it is difficult to study the interaction
of nanoparticles with living cells in order to establish whether such interactions could be damaging to the cell. A simple
modification to a conventional inverted optical microscope is proposed here which renders the location of nanoparticles readily
apparent and permits tracking of them in threedimensions. Particles in the range 100nm to 500nm have been tracked with a temporal
resolution of 200ms. The technique, although motivated by the desire to study the interaction of nanoparticles with cells,
has a wide range of potential applications in the fields of food processing, pharmaceuticals and nano-biotechnology.
[Show abstract][Hide abstract] ABSTRACT: Most of the existing algorithms used for processing phase-shifted photoelastic data attempt to compute the unambiguous or
demodulated isoclinic map in order to obtain the unambiguous or continuous isochromatic map. However, in some cases experiments
on engineering components yield isoclinic maps that are severely corrupted due to the interaction between isoclinics and isochromatic.
The result is that some of these algorithms fail in the direct demodulation of isoclinic maps from phase-shifted photoelastic
data. An indirect way to obtain the isoclinic map by computing first the unambiguous isochromatic map is presented. The employed
approach is based on a regularisation process that, by minimising a cost function, selects the appropriate value of the relative
retardation angle at each pixel. In this way, an unambiguous map can be straightforwardly unwrapped and calibrated to generate
an isochromatic map. The unambiguous isoclinic angle map is then calculated using the regularized isochromatic map. The process
has been demonstrated to be robust and reasonably quick for crack tip fringe patterns.
[Show abstract][Hide abstract] ABSTRACT: 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.
Journal of Microscopy 03/2011; 243(2):172-8. · 1.63 Impact Factor