The high failure rate of the Yttria Partially Stabilized Zirconia (YPSZ)-porcelain interface in dental prostheses is influenced by the micro-scale mechanical property variation in this region. To improve the understanding of this behavior, micro-scale fracture toughness profiling by nanoindentation micropillar splitting is reported for the first time. Sixty 5 μm diameter micropillars were machined within the first 100 μm of the interface. Berkovich nanoindentation provided estimates of the bulk fracture toughness of YPSZ and porcelain that matched the literature values closely. However, the large included tip angle prevented precise alignment of indenter with the pillar center. Cube corner indentation was performed on the remainder of the pillars and calibration between nanoindentation using different tip shapes was used to determine the associated conversion factors. YPSZ micropillars failed by gradual crack propagation and bulk values persisted to within 15 μm from the interface, beyond which scatter increased and a 10% increase in fracture toughness was observed that may be associated with grain size variation at this location. Micropillars straddling the interface displayed preferential fracture within porcelain parallel to the interface at a location where nano-voiding has previously been observed and reported. Pure porcelain micropillars exhibited highly brittle failure and a large reduction of fracture toughness (by up to ~90%) within the first 50 μm of the interface. These new insights constitute a major advance in understanding the structure-property relationship of this important bi-material interface at the micro-scale, and will improve micromechanical modelling needed to optimize current manufacturing routes and reduce failure.
Imaging of domains is a key step in understanding the microstructure and hence the properties of ferroelectric single
crystals. This understanding is essential for exploiting engineered domain configurations to achieve enhanced
performance. In this paper, single crystals of Barium Titanate are observed by reflection topography using unfocussed
monochromatic synchrotron X-ray light. A 10 x 10 mm polished surface of an unpoled crystal was mapped to form a
composite image, indicating a fine structure of a- and c-domains. By making use of the angular separation of the
diffracted reflections and specimen rocking, the relative tilts between adjacent domains about two orthogonal axes were
found. Angular resolution better than 0.1mrad in tilt measurements allowed the local elastic curvature of lattice planes to
be observed. The resulting composite images show well defined boundaries between regions of distinct microstructure,
and give an indication of the proportion of the domain types present. Over large regions of the crystal the domain
structure was finer than the X-ray camera resolution of 6.5μm; AFM and SEM imaging of domains was then used to
confirm the typical domain spacing. The results are interpreted in the context of models of compatible microstructure in
tetragonal crystals using microscopy of etched crystals to assist the interpretation. The technique shows promise for
mapping fine microstructure in single crystals, through the use of high resolution X-ray cameras, and is successful in
revealing lattice orientation information that is not normally available in optical or AFM measurements.
We introduce a new technique for the determination of the material property characterizing the resistance to ductile fracture from a single tensile test on an unnotched specimen. The property known as the essential work of fracture (EWF) is usually associated with the specific energy, per unit cross sectional area, consumed during ductile fracture in a double edge notched tensile (DENT) specimen. This energy is referred to as 'essential' in order to distinguish it from the non-essential energy consumed on distributed plastic deformation accompanying fracture, but not required for material separation. In the present study we consider tensile tests carried out on unnotched dog-bone (DB) tensile specimens carrying large numbers of markers and incorporating continuous measurement of elongation between any two markers using a laser scanning extensometer. In a single test it is therefore possible to obtain multiple load-elongation curves for a large number of tensile specimens. This data is analyzed by separating contributions to specimen elongation made by distributed (pre-softening) and localized (post-softening) plastic deformation. We demonstrate on a series of tests the evaluation of essential and non-essential work of necking and tearing for an aluminum alloy subjected to different heat treatments, and compare the results with those obtained from conventional DENT tests.
We present the formulation for finding the distribution of eigenstrains, i.e. the sources of residual stress, from a set of measurements of residual elastic strain (e.g. by diffraction), or residual stress, or stress redistribution, or distortion. The variational formulation employed seeks to achieve the best agreement between the model prediction and some measured parameters in the sense of a minimum of a functional given by a sum over the entire set of measurements. The advantage of this approach lies in its flexibility: different sets of measurements and information about different components of the stress-strain state can be incorporated. We demonstrate the power of the technique by analysing experimental data for welds in thin sheet of a nickel superalloy aerospace material. Very good agreement can be achieved between the prediction and the measurement results without the necessity of using iterative solution. In practice complete characterisation of residual stress states is often very difficult, due to limitations of facility access, measurement time or specimen dimensions. Implications of the new technique for experimental analysis are all the more significant, since it allows the reconstruction of the entire stress state from incomplete sets of data.
Conference Committee Involvement (2)
SPIE Micro+Nano Materials, Devices, and Systems
7 December 2015 | Sydney, New South Wales, Australia