Thermoreflectance-based approach for surface temperature measurements of thin-film gold sensors

A novel thermoreflectance-based diagnostic tool capable of visualizing spatial and temporal changes in surface temperature is presented. The method uses narrow spectral emission bands of blue [λ = 405 nm with 10 nm full-width-at-half-maximum (FWHM)] and green (λ = 532 nm with 10 nm FWHM) light to monitor the optical properties of gold and thin-film gold sensors, relating changes in reflectivity to temperature through a known calibration coefficient. The system is made robust to tilt and surface roughness variations through the simultaneous measurement of both probing channels with a single camera. Experimental validation is performed on two forms of gold materials heated from room temperature to 200 °C at a rate of ∼100 °C/min. Subsequent image analysis shows perceptible changes in reflectivity in the narrow band of green light, while the blue light remains temperature-insensitive. The reflectivity measurements are used to calibrate a predictive model with temperature-dependent parameters. The physical interpretation of the modeling results is given, and the strengths and limitations of the presented approach are discussed

Effect of texture on elastic precursor decay in magnesium alloy AZ31B

The role of texture in the dynamic yielding behaviour of magnesium alloy AZ31B has been studied in a series of instrumented plate-impact experiments. Specimens with thicknesses between 0.45 and 2 mm were cut parallel and perpendicular to the material extrusion direction and shock loaded to impact stresses around 3 GPa. Frequency-shifted photon doppler velocimetry was performed to capture breakout of the weak shockwave at the rear free-surface. Significant decay of the elastic precursor wave is observed over the examined thickness range. It is shown that shock compression along the extrusion direction (longitudinal loading) exhibits a markedly higher Hugoniot elastic limit (HEL) when compared to the perpendicular direction (transverse loading). Microstructural analysis via Electron Backscatter Diffraction (EBSD) demonstrates a strong basal texture in the material, with the c-axes of the HCP lattice aligned perpendicular to the extrusion direction. It is indicated that the peak elastic stress can be linked to the distribution of angles between the loading direction and the c- and a-axes of the grains, and by extension to the Schmid factors in the three primary slip systems. The present results demonstrate the importance of texture in the time-dependent inelastic deformation of common alloys with highly anisotropic crystal structures such as Mg AZ31B

In-situ visualisation of dynamic fracture and fragmentation of an L-type ordinary chondrite by combined synchrotron X-ray radiography and microtomography

The relationship between the dynamic mechanical properties of stony meteorites and their microstructures was investigated in-situ for an L-type ordinary chondrite using a split-Hopkinson pressure bar apparatus and ultra-high speed phase-contrast X-ray radiography at the European Synchrotron Radiation Facility (ESRF). Synchrotron X-ray microtomography (CT) was performed both prior to and immediately following dynamic compression to correlate key structural features between the initial microstructure and recovered fragments as well as to identify the leading mechanisms for fracture and fragmentation. Real-time visualisation of damage evolution in the specimens revealed the very first cracks to be initiated at the sites of FeNi-metal nodules. These cracks propagated rapidly through the largest group of chondrules (the porphyritic olivine type chondrules) along the loading direction, which led to the formation of column-like fragments. CT analysis of the collected fragments confirmed the dominant mode of fracture to be transgranular with a clear link between FeNi-metal nodule statistics and the size distribution of fragments, emphasising their role in mechanical failure and fragmentation process. The resulting fragmentation was used to validate the predictions of brittle fragmentation models, and found to be in good agreement with the laboratory-scale impacts. In turn, these models can help unravel the consequences of impact-induced fragmentation processes that have helped shape the solar system

Powder metallurgy processing and deformation characteristics of bulk multimodal nickel

cited By 7International audienceSpark plasma sintering was used to process bulk nickel samples from a blend of three powder types. The resulting multimodal microstructure was made of coarse (average size ∼ 135 μm) spherical microcrystalline entities (the core) surrounded by a fine-grained matrix (average grain size ∼ 1.5 μm) or a thick rim (the shell) distinguishable from the matrix. Tensile tests revealed yield strength of ∼ 470 MPa that was accompanied by limited ductility (∼ 2.8% plastic strain). Microstructure observation after testing showed debonding at interfaces between the matrix and the coarse entities, but in many instances, shallow dimples within the rim were observed indicating local ductile events in the shell. Dislocation emission and annihilation at grain boundaries and twinning at crack tip were the main deformation mechanisms taking place within the fine-grained matrix as revealed by in-situ transmission electron microscopy. Estimation of the stress from loop's curvature and dislocation pile-up indicates that dislocation emission from grain boundaries and grain boundary overcoming largely contributes to the flow stress

Investigating shock processes in bimodal powder compaction through modelling and experiment at the mesoscale

Impact-driven compaction is a proposed mechanism for the lithification of primordial bimodal granular mixtures from which many meteorites derive. We present a numerical-experimental mesoscale study that investigates the fundamental processes in shock compaction of this heterogeneous matter, using analog materials. Experiments were performed at the European Synchrotron Radiation Facility generating real-time, in-situ, X-ray radiographs of the shock's passage in representative granular systems. Mesoscale simulations were performed using a shock physics code and set-ups that were geometrically identical to the experiments. We considered two scenarios: pure matrix, and matrix with a single chondrule. Good agreement was found between experiments and models in terms of shock position and post-shock compaction in the pure powder setup. When considering a single grain embedded in matrix we observed a spatial porosity anisotropy in its vicinity; the compaction was greater in the region immediately shockward of the grain, and less in its lee. We introduced the porosity vector, C, which points in the direction of lowest compaction across a chondrule. This direction-dependent observation may present a new way to decode the magnitude, and direction, of a single shock wave experienced by a meteorite in the pas

Investigating shock processes in bimodal powder compaction through modelling and experiment at the mesoscale

Impact-driven compaction is a proposed mechanism for the lithification of primordial bimodal granular mixtures from which many meteorites derive. We present a numerical-experimental mesoscale study that investigates the fundamental processes in shock compaction of this heterogeneous matter, using analog materials. Experiments were performed at the European Synchrotron Radiation Facility generating real-time, in-situ, X-ray radiographs of the shock’s passage in representative granular systems. Mesoscale simulations were performed using a shock physics code and set-ups that were geometrically identical to the experiments. We considered two scenarios: pure matrix, and matrix with a single chondrule. Good agreement was found between experiments and models in terms of shock position and post-shock compaction in the pure powder setup. When considering a single grain embedded in matrix we observed a spatial porosity anisotropy in its vicinity; the compaction was greater in the region immediately shockward of the grain, and less in its lee. We introduced the porosity vector, C, which points in the direction of lowest compaction across a chondrule. This direction-dependent observation may present a new way to decode the magnitude, and direction, of a single shock wave experienced by a meteorite in the past

Ultra-high-speed X-ray imaging of shock-induced cavity collapse in a solid medium

The phenomenon of cavity collapse has long been of interest because of the dramatic and highly localised increases in pressure and temperature that can occur during the collapse process. Due to the constraints imposed by optical imaging systems, existing experimental work has largely been limited to cylindrical cavities in transparent liquid and gel media. We present an ultra-high-speed synchrotron X-ray imaging study of the shock-induced collapse of spherical cavities in polymethyl methacrylate (PMMA), performed at the European Synchrotron Radiation Facility (ESRF). A multi-camera imaging system allowed multiple radiographs to be captured per event, revealing the time evolution of sub-surface structures during collapse, such as jet, toroid and crack formation. Shock states were achieved through plate impact experiments, using both a single-stage and two-stage gas gun, generating a wide range of shock pressures between 0.49 and 16.60 GPa. Data extracted from the radiographs suggest a transition from strength-dominated to hydrodynamic collapse, which is complete at approximately 4.80 GPa

The mechanical response of commercially pure copper under multiaxial loading at low and high strain rates

In this paper, we present the dynamic response of commercially pure copper subjected to combined tension-torsion loads representative of real case impact scenarios. Experiments were conducted both quasi statically, at a strain rate equal to 10−3 s−1, and dynamically at strain rates in the region between 500 s−1 and 1000 s−1. All high rate experiments were conducted using a novel Split Hopkinson Tension-Torsion Bar instrumented with high-speed photographic equipment. The dynamic combined loading experiments demonstrate the capability of the apparatus to generate longitudinal and torsional stress waves which are synchronised upon loading of the specimen. The presented data show that dynamic equilibrium conditions and nearly steady strain rates were achieved during the experiments. Additionally, the analyses of the loading paths show that nearly proportional strain loading was attained during testing. The measured experimental results illustrate, for the first time, the failure stress locus of the material over a wide range of stress states including pure torsion, shear-dominated combined tension-shear, tension-dominated combined tension-shear and plain tension. The quasi-static and dynamic failure envelopes are herein presented in the normal stress vs shear stress space to motivate the development of accurate and effective constitutive models. To conclude, the Drucker-Prager criterion was employed to approximate the failure loci and to assess the rate sensitivity of the material. A moderate asymmetry of the uniaxial ultimate stresses in tension and compression is predicted both at quasi-static and dynamic strain rates

Experimental analysis of the multiaxial failure stress locus of commercially pure titanium at low and high rates of strain

The mechanical response and failure mechanism of commercially pure titanium subjected to combined tension-torsion loading are studied experimentally at strain rates ranging from 10−3 s−1 to 103 s−1. A novel tension-torsion split Hopkinson bar (TTHB) equipped with a high speed camera was utilised during high-rate experiments, while quasi-static tests were conducted using a universal screw-driven machine. The multiaxial dynamic experiments demonstrate the ability of the developed TTHB apparatus to achieve synchronisation of longitudinal and torsional waves upon loading the specimen, to satisfy the dynamic equilibrium of the specimen and to attain constant strain rate loading. The failure envelope of commercially pure titanium was analysed over a wide range of stress states including pure torsion, shear-dominated combined tension-shear, tension-dominated combined tension-shear, and plain tension. The analyses of the loading paths show that these were nearly proportional in terms of strain. The multiaxial failure stress locus was constructed in the normal versus shear stress space from experiments conducted at low and high rates of strain. The Drucker-Prager criterion was employed to approximate the failure envelope and to assess its rate sensitivity. The failure stress locus of commercially pure titanium and its rate dependence are reported for the first time. The TTHB apparatus developed allows the definition of the failure stress locus of aerospace materials directly from experiments and, therefore, the evaluation of the existing failure/yielding criteria