Papers by Selim Barhli
Scripta Materialia, 2017
Pre-cracked samples of unpoled, polycrystalline, soft ferroelectric lead zirconate titanate were ... more Pre-cracked samples of unpoled, polycrystalline, soft ferroelectric lead zirconate titanate were subjected to electrical and mechanical loading whilst using synchrotron X-ray diffraction to map strain-fields. Clear evidence is found of switching around the crack tip and development of a crack-wake. A J-integral estimate based on strain-field data provided a sensitive measure of strain changes during loading. Electrical loading did not cause crack advance or crack-tip switching; this provides evidence for electrically permeable crack models. There was also enhancement of electrically-driven switching in the crack-wake region. The results provide improved understanding and a resource for testing fracture models in ferroelectrics.
Carbon, 2017
The strain field of a crack in polygranular isotropic nuclear graphite, a quasi-brittle material,... more The strain field of a crack in polygranular isotropic nuclear graphite, a quasi-brittle material, has been studied during stable fracture propagation. Synchrotron X-ray computed tomography and strain mapping by diffraction were combined with digital volume correlation and phase congruency image analysis to extract the full field displacements and elastic crystal strains. The measured displacement fields have been analysed using a Finite Element method to extract the elastic strain energy release rate as a J-integral. Non-linear properties described the effect of microcracking on the elastic modulus in the fracture process zone. The analysis was verified by the good agreement of the predicted and measured elastic strain fields when using the non-linear model. The intrinsic critical elastic strain energy release rate for mode I crack propagation is approximately 200 J m-2 .
Digital image correlation has been routinely used to measure full-field displacements in many are... more Digital image correlation has been routinely used to measure full-field displacements in many areas of solid mechanics , including fracture mechanics. Accurate segmentation of the crack path is needed to study its interaction with the microstructure and stress fields, and studies of crack behaviour, such as the effect of closure or residual stress in fatigue, require data on its opening displacement. Such information can be obtained from any digital image correlation analysis of cracked components, but it collection by manual methods is quite onerous, particularly for massive amounts of data. We introduce the novel application of Phase Congruency to detect and quantify cracks and their opening. Unlike other crack detection techniques, Phase Congruency does not rely on adjustable threshold values that require user interaction, and so allows large datasets to be treated autonomously. The accuracy of the Phase Congruency based algorithm in detecting cracks is evaluated and compared with conventional methods such as Heaviside function fitting. As Phase Congruency is a displacement-based method, it does not suffer from the noise intensification to which gradient-based methods (e.g. strain thresholding) are susceptible. Its application is demonstrated to experimental data for cracks in quasi-brittle (Granitic rock) and ductile (Aluminium alloy) materials.
A novel method has been developed based on the conjoint use of digital image correlation to measu... more A novel method has been developed based on the conjoint use of digital image correlation to measure full field displacements and finite element simulations to extract the strain energy release rate of surface cracks. In this approach, a finite element model with imported full-field displacements measured by DIC is solved and the J-integral is calculated, without knowledge of the specimen geometry and applied
loads. This can be done even in a specimen that develops crack tip plasticity, if the elastic and yield behaviour of the material are known. The application of the method is demonstrated in an analysis of a fatigue crack, introduced to an aluminium alloy compact tension specimen (Al 2024, T351 heat condition).
Recent developments of synchrotron X-ray sources and dedicated high-energy beamlines are now enab... more Recent developments of synchrotron X-ray sources and dedicated high-energy beamlines are now enabling strain measurements from large volumes of industrially relevant metallic materials. Such capability is allowing the validation of novel and alternative non-destructive experimental methods of strain measurement or computational models of complex deformation processes. This study describes the first dynamic contact strain measurement of a ball bearing using stroboscopic Energy Dispersive X-ray Diffraction (EDXD). The experiment probed the dynamic contact strain in the outer raceway of a test bearing. The inner raceway of the bearing was attached to a shaft rotating at 150 revolutions per minute and the outer raceway, where the measurements were made, was fixed in a stationary bearing housing. A triggering system was used to synchronise the data acquisition of the EDXD detector with the bearing rotation. Specifically, diffraction data was acquired, stroboscopically, from the material volume within the raceway, in a known location, when the ball was positioned directly below it. A total of 20 seconds of accumulated diffraction signal was recorded, acquiring 2 milliseconds of data per revolution, providing diffraction patterns of sufficient quality for the dynamic contact strain to be measured. Macromechanical stress field was calculated from the micromechanical strains measured from five lattice planes. This allowed a comparison of the experimentally measured stress field and that of finite element (FE) simulations. Good agreement was observed between the FE results and experimental measurements indicating the applicability of this novel dynamic strain measurement technique for tribological systems.
The UK advanced gas-cooled reactor (AGR) nuclear power stations are graphite moderated. Throughou... more The UK advanced gas-cooled reactor (AGR) nuclear power stations are graphite moderated. Throughout reactor life, fast neutron irradiation and radiolytic-oxidation increase the risk of cracking in graphite components from the stress-concentrating keyways. Forward predictions of reactor integrity require knowledge of the strengths of in-service components, which are estimated from data that includes the flexural strength of reactor-extracted specimens and notched feature tests of non-irradiated graphite. The notch sensitivity of irradiated graphite is not monitored at present; knowledge of this parameter would increase confidence in the conservatism of the current assessments. This work is part of a project that aims to quantify the effects of loading mode and geometry on notch sensitivity, with the objective of developing a measurement methodology for small test specimens. The deformation fields ahead of wedge-loaded keyhole-shaped notches specimens of non-irradiated Gilsocarbon have been quantified by in situ synchrotron X-ray computed tomography (XCT), analysed by digital volume correlation (DVC), and compared with finite element (FE) simulations with experimentally accurate boundary conditions. Deviations from linear-elasticity were mapped to investigate non-linear damage development ahead of blunt and sharp features, with the aim of quantifying the damage-law for non-irradiated graphite. Heterogeneous behaviour was observed between the microstructural phases, but no significant concerted non-linear deformation at the notch tip prior to failure. The developed techniques will be used to study damage development in notched four-point bend testing of small Gilsocarbon samples.
Neutron diffraction and synchrotron X-Ray diffraction and imaging have been applied to study, in ... more Neutron diffraction and synchrotron X-Ray diffraction and imaging have been applied to study, in situ, the mechanical response to tensile and bending loading of polygranular Gilsocarbon nuclear grade near-isotropic graphite (grade IM1-24). Digital image correlation of X-ray radiographs and digital volume correlation of tomographs allow measurement of bulk elastic moduli and examination of the heterogeneity of deformation in the microstructure. Both the neutron and X-ray studies show the application of tensile strain reduces the bulk elastic modulus. A permanent set is observed to develop with applied tensile strain. The elastic strains within the graphite crystals were measured by diffraction; a cross-correlation analysis method has been applied for greater speed, robustness and improved precision in the measurement of the change in basal plane separation distance. In compression, a linear relation is observed between the elastic strains in the graphite crystals and the applied strain. In tension, this relationship is non-linear. The results are discussed with respect to the distribution of elastic and inelastic strain within the graphite microstructure. It is deduced that the significant residual elastic strains in the as- manufactured graphite are relaxed by microcracking as tensile strain is applied.
Conference Papers by Selim Barhli
The analysis by diffraction of polycrystalline materials can determine the full tensor of the ela... more The analysis by diffraction of polycrystalline materials can determine the full tensor of the elastic strains within them. Point-by-point maps of elastic strain can thus be obtained in fine-grained engineering alloys, typically using synchrotron X-rays or neutrons. In this paper, a novel approach is presented to calculate the elastic strain energy release rate of a loaded crack from two-dimensional strain maps that are obtained by diffraction. The method is based on a Finite Element approach, which uses diffraction data to obtain the parameters required to calculate the J-integral via the contour integral method. The J integral is robust to uncertainties in the crack tip position and to poor definition of the field in the crack vicinity, and does not rely on theoretical assumptions of the field shape. A validation of the technique is presented using a synthetic dataset from a finite element model. Its experimental application is demonstrated in an analysis of a synchrotron X-ray diffraction strain map for a loaded fatigue crack in a bainitic steel.
Full field mapping of displacements between successive images by digital image correlation is a p... more Full field mapping of displacements between successive images by digital image correlation is a powerful and well-established technique, used in fields as diverse as geo-tectonics, engineering mechanics and materials science. Analysis of three-dimensional images, such as computed X-ray tomographs, is also becoming routine. These techniques provide new ways to study and quantify deformation and failure processes: recently they have been applied to detect and study cracks and defects in engineering materials, for instance by coupling the displacement analysis with finite element codes to readily extract the crack propagation strain energy release rate (J Integral). Such analyses increase the richness of the data obtained, for example providing information on the mode of loading, and are suitable for the analysis of engineering components under complex states of stress.
This work has highlighted areas where the development of image correlation methods that are optimised for analysis of discontinuities would be beneficial, for better detection of small cracks and the early development of damage against the background displacement field; improved precision in crack displacement field measurement by intelligent “masking’ or analysis algorithms and better integration with finite element software packages to make use of advanced tools for 2D and 3D deformation analysis.
This paper reviews some of this recent work on the analysis of 2D and 3D damage in engineering materials, and describes developments in quantitative analysis of defects by image correlation. The examples covered include brittle crack propagation in nuclear graphite, fatigue loading in magnesium alloys and indentation damage in brittle and ductile materials.
Talks by Selim Barhli
Nuclear graphite is treated as a linear elastic material in engineering design; Graphite is, howe... more Nuclear graphite is treated as a linear elastic material in engineering design; Graphite is, however, a heterogeneous quasi-brittle material, with non-linear mechanical behavior and the development of a micro-cracked fracture process zone, which can cause strength to vary with size. Small test specimens from nuclear graphite, which are extracted either from operating reactors or used in material test reactor (MTR) accelerated experiments, provide the data to predict the performance of structural components; it is necessary to have confidence that such small specimen tests are representative and conservative. The objective of this work is to better understand how the microstructure of a coarse grained polygranular graphite accommodates applied strain, and the effect of this applied strain on its mechanical properties. To study this, it is necessary to be able to observe, in situ, the relationship between the applied strains, the total strains in the material’s microstructure and the elastic strains in the crystals.
This presentation summarises progress in work to observe deformation and fracture in nuclear graphite, using synchrotron X-ray tomography and digital volume correlation to measure three-dimensional strain fields. High precision synchrotron diffraction studies on strained samples and the fracture process zone of propagating cracks provide new insights into the inelastic deformation of graphite. Microcracked fracture process zones are common to quasi-brittle materials as diverse as high toughness monolithic ceramics, polymeric and natural biological composites, geological minerals and even volcanic structures. Experimental methods that support the study and modeling of damage development are thus important to a wide range of problems, beyond nuclear graphite.
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Papers by Selim Barhli
loads. This can be done even in a specimen that develops crack tip plasticity, if the elastic and yield behaviour of the material are known. The application of the method is demonstrated in an analysis of a fatigue crack, introduced to an aluminium alloy compact tension specimen (Al 2024, T351 heat condition).
Conference Papers by Selim Barhli
This work has highlighted areas where the development of image correlation methods that are optimised for analysis of discontinuities would be beneficial, for better detection of small cracks and the early development of damage against the background displacement field; improved precision in crack displacement field measurement by intelligent “masking’ or analysis algorithms and better integration with finite element software packages to make use of advanced tools for 2D and 3D deformation analysis.
This paper reviews some of this recent work on the analysis of 2D and 3D damage in engineering materials, and describes developments in quantitative analysis of defects by image correlation. The examples covered include brittle crack propagation in nuclear graphite, fatigue loading in magnesium alloys and indentation damage in brittle and ductile materials.
Talks by Selim Barhli
This presentation summarises progress in work to observe deformation and fracture in nuclear graphite, using synchrotron X-ray tomography and digital volume correlation to measure three-dimensional strain fields. High precision synchrotron diffraction studies on strained samples and the fracture process zone of propagating cracks provide new insights into the inelastic deformation of graphite. Microcracked fracture process zones are common to quasi-brittle materials as diverse as high toughness monolithic ceramics, polymeric and natural biological composites, geological minerals and even volcanic structures. Experimental methods that support the study and modeling of damage development are thus important to a wide range of problems, beyond nuclear graphite.
loads. This can be done even in a specimen that develops crack tip plasticity, if the elastic and yield behaviour of the material are known. The application of the method is demonstrated in an analysis of a fatigue crack, introduced to an aluminium alloy compact tension specimen (Al 2024, T351 heat condition).
This work has highlighted areas where the development of image correlation methods that are optimised for analysis of discontinuities would be beneficial, for better detection of small cracks and the early development of damage against the background displacement field; improved precision in crack displacement field measurement by intelligent “masking’ or analysis algorithms and better integration with finite element software packages to make use of advanced tools for 2D and 3D deformation analysis.
This paper reviews some of this recent work on the analysis of 2D and 3D damage in engineering materials, and describes developments in quantitative analysis of defects by image correlation. The examples covered include brittle crack propagation in nuclear graphite, fatigue loading in magnesium alloys and indentation damage in brittle and ductile materials.
This presentation summarises progress in work to observe deformation and fracture in nuclear graphite, using synchrotron X-ray tomography and digital volume correlation to measure three-dimensional strain fields. High precision synchrotron diffraction studies on strained samples and the fracture process zone of propagating cracks provide new insights into the inelastic deformation of graphite. Microcracked fracture process zones are common to quasi-brittle materials as diverse as high toughness monolithic ceramics, polymeric and natural biological composites, geological minerals and even volcanic structures. Experimental methods that support the study and modeling of damage development are thus important to a wide range of problems, beyond nuclear graphite.