Darren J. Hughes

Residual Strain Measurement by Synchrotron Diffraction

Third generation synchrotron X-ray sources such as the European Synchrotron Radiation Facility and the Advanced Photon Source (USA) have made very intense beams of very high energy X-rays available for the first time. At energies in excess of 60 keV penetration lengths of the order of centimetres are possible in most engineering materials. The associated low scattering angles limit the strain measurement directions available at depth. Gauge dimensions as small as microns and sub-second measurement times give the technique unique characteristics, making 2 and 3 dimensional strain mapping economically feasible. The current state of the art is reviewed and the potential assessed, primarily using illustrative case studies made at the ESRF. These include the measurement of near surface strains caused by peening, TIG welding stresses for the development of finite element models, the mapping of crack bridging during fatigue crack growth in Ti/SiC fibre composites and crack field mapping in 3D.

The Use of Combs for Evaluation of Strain-free References for Residual Strain Measurements by Neutron and Synchrotron X-ray Diffraction

The measurement of residual strain using diffraction techniques relies on the determination of a change in lattice parameter relative to a reference or “strain-free” lattice parameter. Therefore, o...

Connecting the macro and microstrain responses in technical porous ceramics. Part II: microcracking

Following previous study on non-microcracked porous ceramics (SiC and alumina), we studied the micro and macrostrain response of honeycomb porous microcracked ceramics under applied uniaxial compressive stress. Cordierites of different porosities were compared. Both macroscopic and microscopic strains were measured, by extensometry and neutron diffraction, respectively. Lattice strains were determined using a single diffraction peak (steady-state neutron source) in both the axial and the transverse sample directions. Complementarily, we measured the macroscopic Young’s modulus of these materials as a function of temperature, at zero load, using high-temperature laser ultrasound spectroscopy. This allowed having a non-microcracked reference state for all the materials investigated. Confirming our previous study, we observed that macrostrain relaxation occurs at constant load, which is not observed in non-microcracked compounds, such as SiC. This relaxation effect increases as a function of porosity. Moreover, we generally observed a linear dependence of the diffraction modulus on porosity. However, for low and very high applied stress, the lattice strain behavior versus stress seems to be influenced by microcracking and shows considerable strain release, as already observed in other porous microcracked ceramics. We extended to microcracked porous ceramics (cordierite) the macro to microstrain and stress relations previously developed for non-microcracked ceramics, making use of the integrity factor (IF) model. Using the whole set of data available, the IF could also be calculated as a function of applied stress. It was confirmed that highly porous microcracked materials have great potential to become stiffer and more connected.

Thermal and Mechanical Response of Industrial Porous Ceramics

In this study, the mechanical behavior of porous thermally microcracked ceramics has been compared with that of solely porous materials, under compressive applied stress. The different aspects of the micro and macroscopic stress-strain curves have been inserted into a coherent analytical model and compared with finite element modeling calculations. The agreement between experiments and models is very good. It is shown that mechanical microcracking, as opposed to thermal, introduces an irreversible aspect in the deformation mechanisms of porous ceramics. In this concern, mechanical loads differentiate themselves from thermal cycling. This leads for instance to a change of the Young’s modulus as a function of applied load, which qualifies those materials as visco-elastic.

High hydrostatic pressure equipment for neutron scattering studies of samples in solutions

The design of new high pressure equipment for structural and dynamical studies on samples in solution is described. We present two sample cells for applying pressures up to 150 and 700 MPa (i.e. 1.5 and 7 kbar), respectively. These cells are mounted on special sticks and inserted into the calorimeter of a cryostat to regulate the temperature. Different parts of the equipment – the pressure controller, the sticks and the cells – are described. In addition, radiography tests which were performed with neutrons in situ at the Institut Laue Langevin to verify the tightness of the cells and the hydrostatic transmission of the pressure to the sample are presented. First results on model lipids are in agreement with former results by R. Winter et al. [Towards an understanding of the temperature/pressure configurational and free-energy landscape of biomolecules, J. Non-Equilib. Thermodyn. 32 (2007), pp. 41–97].

Crystallization and morphology development in polyethylene–octakis(n-octadecyldimethylsiloxy)octasilsesquioxane nanocomposite blends

The dispersion, morphology and crystallization kinetics of low density polyethylene (LDPE)–octakis(n-octadecyldimethylsiloxy)octasilsesquioxane (POSS) nanocomposite blends was investigated. Novel octakis(dimethylsiloxy)octasilsesquioxane (Q8M8H) molecules were octafunctionalised with octadecyl alkyl-chains (Q8C18) and blended with 0.25–10 wt% loadings into a commercial LDPE. Time-resolved Small- and Wide-Angle X-ray Scattering (SAXS/WAXS), thermal and microscopy techniques were used to elucidate the POSS dispersal, crystalline morphology and crystallization kinetics of the host polymer. POSS particles dispersed well in the host polymer up to 5% wt loading and acted as nucleating agents without disrupting the crystal lattice of the PE. Above 5% wt loading the POSS aggregated, reduced the bulk crystallinity and hindered the crystallization process. The aggregation of POSS is attributed to increased POSS–POSS interactions whereby the POSS molecules self-assemble in an interdigitated manner. The results were compared with an analogous LDPE–T8C18 POSS cage blend at 10% wt loading. In complete contrast, the T8 POSS particles disperse well in the host polymer being effective nucleating agents and increased the bulk crystallinity. This may have important implications in the processing of polyolefins where the T8 system acts to accelerate crystallization whereas the Q8 system retards it.

Synchrotron X-Ray Measurements of Residual Stress in a Worn Railway Rail

In recent years significant research activity has been focused tqwards understanding the processes of railway rail fatigue. A number of serious incidents have been attributed to rail fractures due to rolling contact fatigue cracking. The mechanisms of rail failure are not fully understood but some are related to the relationship between defects and the residual stress field in the rail. Synchrotron X-ray diffraction has been employed to measure residual strain in a worn US rail section. Residual stresses are derived from the measured strains. It is shown that the use of high energy X-radiation can reveal the detailed stress distribution in a steel rail head.

Determination of the fracture behaviour of axial splitting tubes and the numerical prediction of their energy absorption capabilities

Axial splitting of tubes has been used as an energy absorption mechanism since the 1980s. The simulation of the fracture behaviour of splitting tubes, key to accurately predict the splitting mechanism behaviour, has not been studied. This paper explores the fracture strain and the strain state local to the tip of the crack of the splitting tubes, and examines the accuracy of a numerical crush model using the acquired strain fracture data. The measurement of the fracture strain and the strain state local to the tip of the crack on the E355 steel tube surface was performed with a digital image correlation method. The tubes were fabricated to simulate the axial splitting mechanism used in railway energy-absorbing applications. Four- and six-notch initiators were cut in the specimens. The fracture strain data were included in a LS-DYNA numerical model. The friction coefficient was inferred by correlation of the simulation to the quasi-static compression test results. It was found that the digital image correlation method determined the fracture strain as ϵf = 0.60 for both types of specimens, with four and six notches. The strain ratio measured by the digital image correlation software was α = −0.03 and −0.11 for specimens with four and six notches, respectively. The inferred friction coefficient value was μ = 0.07 for both specimens with four and six notches. This value agrees with the value quoted in the literature for a similar energy-absorbing mechanism.

Stress distributions in multilayer laser deposited Waspaloy parts measured using neutron diffraction

During layered manufacturing of functional metallic parts by laser direct metal deposition (LDMD) residual stresses are generated by the transient thermal fields. Various methods to reduce or control the development of these stresses have been previously attempted. This paper reports a novel method using the pulse parameters of a diode laser deposition system as a tool for residual stress control. Stresses in LDMD manufactured Waspaloy blocks are calculated from strain measurements in three directions carried out at the neutron diffraction beam line SALSA at the European ILL facility, Grenoble, France. The results show tensile stresses close to the top of the structures of similar magnitude for all parameters and large compressive stresses normal to the deposition surface close to the base.

Precise Determination Of Residual Stresses In Large Specimens By Neutron Diffraction

Experimental errors in neutron residual strain imaging are often underestimated, mis-understood or simply ignored. The choice of beam defining optics can play a large role in the magnitude of these errors. In this paper we show that the use of a traditional slit beam defining system with large specimens can lead to several sources of errors. Large specimens usually require necessarily large distances between the slits and the instrument reference point (gauge volume). At large distances, the slit system can lead to significant underestimation of the residual strain magnitude as well as poor definition of the gauge volume. We show that for large specimens, the use of radial focussing collimators reduces these effects, leading to significant improvement in measurement reliability. Specifically, we show application of radial collimators at the SALSA instrument of the Institut Laue Langevin neutron source in Grenoble, France.