Papers by Author: Anna Maria Paradowska

Abstract: Damage accumulation due to fatigue significantly reduces the safety of railway vehicles. Shattered wheel rim failures are the result of large fatigue cracks that propagate roughly parallel to the wheel tread surface. The large stress, most likely due to wheel/rail impact or material discontinuity, is responsible for the initiation of shattered rims. The voids and inclusions of sufficient size in a stress field will also lead to failure of wheels. Significant improvements have been made in recent years to prevent the shattered rim failure. The ‘new’ wheels have a better resistance to the shattered rim failure, due to the fact that the circumferential residual stress on tread of a new wheel must be compressive to comply with requirements of international standard EN 13262. However, this may not necessarily apply for millions of ‘old’ wheels that are still currently in use. At the moment the residual stress measurements are carried out using destructive methods (such as slitting or hole drilling), or using quantitatively ultrasound method obtaining the average stress across the whole section. The main objective of this research was to apply non-destructive neutron diffraction method to quantitatively measure residual stress distribution of the wheel rim in as manufactured condition.

Abstract: Time of flight neutron diffraction method was applied to measure elastic lattice strains in austenitic steel during "in situ" tensile test. Comparing experimental data with self-consistent model, the critical resolved shear stress and hardening parameters were determined for polycrystalline grains. The result allowed us to determine the main component of the stress localization tensor, relating the rate of grain stress with the applied macrostress rate. The evolution of concentration tensor in function of the applied macrostress was analyzed. Finally, the load transfer between grains during yielding of the sample was studied.

Abstract: Neutron diffractometer Engin-X at ISIS was used use in this study to investigate the residual stresses in a section of a multi-pass girth welded thick pipe, of nominal thickness 62 mm, which was made of a ferritic-martensitic steel denoted type P92. Measurements in such large component sections are rare, and have driven the neutron diffraction method to the edge of its capabilities. Significant stresses of over 150 MPa have been found in this pipe section, though post weld heat treatment has been performed. The influences of these welding residual stresses in components at operating temperatures are discussed in terms of their relaxation and high temperature fracture behaviour.

Abstract: Residual strains in plasma sprayed and heat-treated hydroxyapatite (HA) coatings deposited on a titanium alloy (Ti-6Al-4V) substrate were investigated by means of neutron diffraction. Strain measurements were performed in vertical scan (“z-scanning”) mode to provide a through thickness strain profile in the coating and substrate materials. Results are discussed in terms of the influence of heat-treatment on the residual strain profile of these biomedical coatings. This investigation concluded that the heat-treatment had a significant effect on the residual strain profile in HA coatings.

Abstract: Two Japanese long swords (katanas) belonging to the Koto Age (X-XVI century A.D.) were measured through time of flight neutron diffraction to analyze the phases, and the stress and strain distribution, in selected parts of the blades. The swords are representative of two different forging schools (Aoe and Kanesada) and one of the main aims of the measurements was to evidence possible similarities and differences. Two independent experiments were carried out at the ISIS pulsed neutron source using the INES and ENGIN-X diffractometers. The former was employed to map the average phase distribution on two selected cross sections, of each blade, distinguishing among the ridge, the core, and the edge of the blades. In this way, we were able to quantify the coarse distribution of the carbon content and, moreover, we could evidence the presence of martensite. These data were then complemented measuring detailed stress and strain distribution maps on ENGIN-X. As far as the ridge and the core are concerned, the tang data were taken as a reference. These measurements significantly improve the knowledge and understanding of the technology used to produce Japanese swords belonging to the Koto Age.

Abstract: The lattice strains in large tensile deformations, up to the fracture of the sample were measured using neutron TOF method. For the first time, the range of large deformation was studied measuring lattice strain in the deformation neck and using special correction for macrostress value. It was found that during large plastic deformation the lattice stresses arise almost linearly with the macrostress value. The relaxation of elastic strains in some groups of ferritic grains (corresponding to reflections 211 and 200) can be connected with initiation of damage process in the ferritic phase.

Abstract: Forming of metal plates with a high-power laser beam is a flexible materials forming technique. Bending results from the establishment of a steep temperature gradient through the material thickness which leads to non-uniform thermal expansion/contraction and subsequently residual stresses. It is important to characterize these residual stresses as a function of process parameters such as line energy (LE) to optimize treatment conditions and to gain an insight into the mechanism of the formation of the final geometries. Non-destructive neutron diffraction measurements were carried out on ENGIN-X at ISIS and KOWARI at ANSTO to map the residual stress distribution around the heat affected zone (HAZ) of laser deformed mild steel plates for single and multiple passes as a function of line energy (LE), the primary laser forming process parameter. It was found that in the centre of the HAZ, longitudinal residual stresses are tensile and dominant, transverse stresses are predominantly tensile and normal stresses are compressive and close to zero. The residual strain in longitudinal direction increased with LE and number of passes until yielding. Even higher heat input decreased the magnitude of the cusp, but not its total height. The comparison of a stress-free reference sample, measured at both facilities, showed a small discrepancy in the lattice spacing corresponding to ~ 85 μstrain which is insignificant with respect to the experimental values measured.

Abstract: The ternary ceramic Ti3AlC2 has an interesting combination of electrical, thermal and mechanical properties. Single crystal elastic constants under the Reuss approximation for the micromechanical state were obtained by analysing the shifts of neutron diffraction peaks while a polycrystalline sample was subjected to a compressive load varying from 5 to 300 MPa. The values of Young’s modulus and Poisson’s ratio computed from the single crystal compliances are in good agreement with those obtained directly from strain gauges and from the average changes in the a and c unit cell parameters.