A study of residual stresses and their effect on thermo mechanical fatigue in complex geometries

It is known that residual stresses within engine components, such as turbine housings, can combine with service generated stresses and cause unexpected failures during operation, therefore it is important that all the stresses (residual and induced, compressive and tensile) are fully characterised and understood. The use of neutrons as a tool to measure the strains within a material is well established; however, applying this technique to complex engineering components can prove challenging. This research investigates the measurement of residual stresses in complex geometries found within the turbine housing component of a turbocharger using neutron diffraction. The effect of various production methods on residual stress distributions is also explored. Successful strain measurements were taken using the Engin-X instrument at the ISIS spallation source from three turbine housings selected from various stages in the manufacturing process, allowing a study of the effect of heat treatment and machining on stress magnitudes and direction. The turbine housing consists of various sections greater than the maximum 60mm path length of the neutrons, therefore path lengths must be carefully chosen to achieve acceptable neutron count rates. Engin-X benefits from an automated experimental setup to make the selection of this limited path length easier on complex shapes. The turbine housings were mounted on to a positioning table allowing translation in X, Y, Z directions and also rotation in , . Each of the housings were scanned using laser scanners and this in conjunction with the virtual path length measurement software SScanSS allowed automated measurements of acceptable path lengths to be made. On this occasion measurements in one principal direction were measured and the correct measurement methodology established. Continuation of this work was then carried out on SALSA at the ILL reactor source. Measurements were made on turbine housings, one as cast and one heat treated. The internal divider wall of each turbine housing was examined as this is an area where crack initiation can occur. The results showed that, heat treatment can reduce compressive residual stresses. However, compressive stresses are thought to slow the onset of crack initiation and could be beneficial in the material, this will be investigated further in future work. It is hoped that this information will be used to improve production methods and result in improved simulation methodologies to allow accurate predictions of thermal fatigue and fracture locations to be established. The authors wish to thank Dr Jon James (Open University) for his help in setting up the experiment and for the use of the SScanSS software

Three Dimensional Mapping of Texture in Dental Enamel

We have used synchrotron x-ray diffraction to study the crystal orientation in human dental enamel as a function of position within intact tooth sections. Keeping tooth sections intact has allowed us to construct 2D and 3D spatial distribution maps of the magnitude and orientation of texture in dental enamel. We have found that the enamel crystallites are most highly aligned at the expected occlusal points for a maxillary first premolar, and that the texture direction varies spatially in a three dimensional curling arrangement. Our results provide a model for texture in enamel which can aid researchers in developing dental composite materials for fillings and crowns with optimal characteristics for longevity, and will guide clinicians to the best method for drilling into enamel, in order to minimize weakening of remaining tooth structure, during dental restoration procedure

Neutron diffraction studies of magnetostrictive Fe–Ga alloy ribbons

Melt-spun Fe–Ga ribbons were prepared and some ribbons were annealed at 1000 °C for 1 h then slowly cooled to room temperature. X-ray diffraction patterns revealed no evidence of texture and only bcc phase in the as-quenched ribbons. However, high-resolution neutron diffraction patterns gave more information on the structure of these ribbons. Only diffractions from the disordered bcc A2 phase were found in as-quenched ribbons with 15, 17.5, and 19.5 at. % Ga content, without any trace of satellite peaks or splitting peaks from the proposed Ga–Ga pairing superlattice structure. The broadening of the base of the �110� peaks for all samples except the as-quenched 15 at. % Ga ribbon might indicate the existence of some kind of short range ordering. Ribbons developed L12 phase after annealing especially in the Fe 19.5 at. % Ga ribbon where the formation of L12 phase reduced the Ga content in the remaining A2 phase and decreased its lattice parameter dramatically. D03 phase formed in the as-quenched 22.5 at. % Ga ribbon and the following annealing treatment transformed more A2 phase into D03 phase

Crystallographic and magnetic identification of secondary phase in orientated Bi5Fe0.5Co0.5Ti3O15 ceramics

Oxide materials which exhibit both ferroelectricity and ferromagnetism are of great interest for sensors and memory applications. Layered bismuth titanates with an Aurivillius structure, (BiFeO3)nBi4Ti3O12, can possess ferroelectric and ferromagnetic order parameters simultaneously. It has recently been demonstrated that one such example, Bi5Fe0.5Co0.5Ti3O15,where n = 1 with half the Fe3+ sites substituted by Co3+ ions, exhibits both ferroelectric and ferromagnetic properties at room temperature. Here we report the fabrication of highly-oriented polycrystalline ceramics of this material, prepared via molten salt synthesis and uniaxial pressing of high aspect ratio platelets. Electron backscatter images showed that there is a secondary phase within the ceramic matrix which is rich in cobalt and iron, hence this secondary phase could contribute in the main phase ferromagnetic property. The concentration of the secondary phase obtained from secondary electron microscopy is estimated at less than 2.5 %, below the detection limit of XRD. TEM was used to identify the crystallographic structure of the secondary phase, which was shown to be cobalt ferrite, CoFe2O4. It is inferred from the data that the resultant ferromagnetic response identified using VSM measurements was due to the presence of the minor secondary phase. The Remanent magnetization at room temperature was Mr ≈ 76 memu/g which dropped down to almost zero (Mr ≈ 0.8 memu/g) at 460 oC, far lower than the anticipated for CoFe2O4

Influence of residual stresses on thermo-mechanical fatigue in turbine housings

This poster presents the results of a neutron investigation of the residual stresses generated during the production of the turbine housing component of a turbocharger with the aim of improving simulation methodologies to predict the locations of thermal fatigue and fracture locations and hence improve the performance of the turbochargers. Initial strain measurements were made on ENGIN-X (Rutherford Appleton Laboratory, UK) on three turbine housings selected from various stages in the manufacturing process. Measurements were made on a region of the housing where failures (cracks) have been recorded after accelerated durability tests, although these have never been predicted by FEA. Measurements were also made using SALSA (ILL, France) on two turbine housings, one as cast and one heat treated. In this experiment the internal divider wall was examined in detail as this is an area where crack initiation can occur during operation. Our results show that heat treatment can reduce compressive residual stresses. However, as compressive stresses are thought to slow the onset of crack initiation and could be beneficial in the material, questions about the efficacy of current production methods have now been raised. These results will be compared with the results of FEA simulations, and suggestions for modifying the simulation methodology and/or production route will be discussed. The authors wish to thank Dr Jon James (Open University) for his help in setting up the ENGIN-X experiment and for the use of the SScanSS software

Strain analysis of 15th century guns and projectiles

In this proposal we request 1 day on D2B and 8 days on SALSA to collect compositional and microstructural information on a series of battlefield artefacts using neutron diffraction. Examination of both projectiles and gun barrels will enable us to determine phase fractions and characterize surface features (i.e., scratching and wear) on the projectiles and inside of the barrel formed during firing, in addition to casting flaws (voids) and density gradients in the cast metal during construction. Knowledge of the strains experienced during construction and firing will prove critical information and add to our understanding of gunpowder weapons technology and its development during the 15th and 16th centuries

Temperature dependent crystallization of amorphous Y67Fe33 studied using kinetic small angle neutron scattering

Temperature-resolved small angle neutron scattering (SANS) has been used to study the nucleation, growth kinetics and crystallite morphology in the Y–Fe system. Crystallization from amorphous Y67Fe33 to the YFe2 Laves phase via a novel ‘YFe’ intermediate phase has been followed to completion as a function of temperature from 180 to 500 ◦C. The SANS results agree well with published kinetic neutron diffraction data. Below 390 ◦C, diffraction data suggest that SANS arises solely from the contrast between crystalline Y and the Fe-rich amorphous matrix. Between 390 and 410 ◦C all temperature variables are seen to form a sharp peak. This suggests that critical scattering occurs at Tc ≈ 400 ◦C. This critical scattering implies that full crystallization of Y67Fe33 occurs over a very narrow (∼20 ◦C) temperature range

Measurement of residual stresses in the turbine housings of turbochargers

Emission regulations in Europe are extremely stringent and only turbocharged and charged-air-cooled engines are able to comply with current requirements. To meet these performance, durability and legislative conditions, a turbine housing must be capable of operating in harsh environments where vibration is always present and exhaust temperatures can reach 800°C, however many turbine housings are at the limits of operation with regard to strength and durability under these conditions, and failures are recorded on accelerated durability tests. It is now of great importance to understand the fatigue performance of these components and the conditions that determine this performance. In this experiment residual stress measurements will be made of three turbine housings in areas where cracks are commonly observed. We estimate that 4 days of beam time will be required