ABSTRACT
The appraisal is strongly focussed on challenges associated with the nuclear sector, however these are representative of what is generally encountered by a range of engineering applications. Ensuring structural integrity of key nuclear plant components is essential for both safe and economic operation. Structural integrity assessments require knowledge of the mechanical and physical properties of materials, together with an understanding of mechanisms that can limit the overall operating life. With improved mechanistic understanding comes the ability to develop predictive models of the service life of components. Such models often require parameters which can be provided only by characterisation of processes occurring in situ over a range of scales, with the sub-micrometre-scale being particularly important, but also challenging. This appraisal reviews the techniques currently available to characterise microstructural features at the nanometre to micrometre length-scale that can be used to elucidate mechanisms that lead to the early stages of environmentally-assisted crack formation and subsequent growth. Following an appraisal of the techniques and their application, there is a short discussion and consideration for future opportunities.
ABSTRACT
In this work, porosity-property relationships of quasi-brittle materials are explored through a combined experimental and numerical approach. In the experimental part, hemihyrate gypsum plaster powder ( CaSO 4 · 1 / 2 H 2 O ) and expanded spherical polystyrene beads (1.5-2.0 mm dia.) have been mixed to form a model material with controlled additions of porosity. The expanded polystyrene beads represent pores within the bulk due to their light weight and low strength compared with plaster. Varying the addition of infill allows the production of a material with different percentages of porosity: 0, 10, 20, 30 and 31 vol%. The size and location of these pores have been characterised by 3D X-ray computed tomography. Beams of the size of 20 × 20 × 150 mm were cast and loaded under four-point bending to obtain the mechanical characteristics of each porosity level. The elastic modulus and flexural strength are found to decrease with increased porosity. Fractography studies have been undertaken to identify the role of the pores on the fracture path. Based on the known porosity, a 3D model of each microstructure has been built and the deformation and fracture was computed using a lattice-based multi-scale finite element model. This model predicted similar trends as the experimental results and was able to quantify the fractured sites. The results from this model material experimental data and the lattice model predictions are discussed with respect to the role of porosity on the deformation and fracture of quasi-brittle materials.
ABSTRACT
Thermal barrier coatings (TBC) are used widely on a range of components that operate at high temperatures. We report measurement of the factor that is required to convert the Raman shift to stress for air plasma sprayed yttria (7 wt %) stabilized tetragonal zirconia (ZrO(2)) (YSZ) thermal barrier coatings. The factor is evaluated for the as-coated condition and also following a heat treatment at 1000 °C for 1050 h. Two Raman bands at 608 cm(-1) and 640 cm(-1) have been investigated in a diamond anvil cell under hydrostatic pressure up to ~24 GPa. In the range of zero to ~1.6 GPa, a linear behavior was observed in terms of the shifts of these two Raman bands with a gradient similar to dense bulk tetragonal ZrO(2). From these measurements the factors to convert wavenumber shift to stress have been derived. The application of these conversion factors to stress measurement in TBC coated test specimens and components is discussed.
