Publications
199 results found
Xu Y, Wan W, Dunne F, 2021, Microstructural fracture mechanics: stored energy density at fatigue cracks, Journal of the Mechanics and Physics of Solids, Vol: 146, ISSN: 0022-5096
This paper addresses the mechanistic drivers of short fatigue crack growth using theory, computational crystal plasticity and experimental test and characterisation. The asymptotic theory shows that the crack tip stored energy density is non-singular and finite and can be related to stress intensity, but unlike the latter, it depends on the crystal Burgers vector and intrinsic slip strength. The computational methods allow the stored energy to be calculated accurately at crack tips and show that good agreement is obtained for static cracks with the theory. The experiments allow the crack tip stored energy to be measured, demonstrating intimate microstructural sensitivity, direct correlation with experimental crack growth variations and good quantitative agreement with both asymptotic theory and computational modelling. Hence a new microstructurally-sensitive fracture mechanics has been presented in the context of short cracks within crystalline materials.
Liu Y, Dunne FPE, 2021, The mechanistic link between macrozones and dwell fatigue in titanium alloys, International Journal of Fatigue, Vol: 142, ISSN: 0142-1123
This paper addresses the role of macrozone crystallography and morphology in dwell fatigue in titanium alloy Ti-6Al-4V. Until now, the relationship between macrozones and dwell fatigue damage has remained mechanistically uncertain, but this paper establishes a mechanistic link between macrozones and dwell fatigue damage, and explains the preference for dwell facets to be sub-surface. It also outlines the criteria which are important in a potential definition of a macrozone (or microtextured region). High aspect ratio (>~4) macrozones are particularly damaging when their long-axes are orientated near-normal to the principal loading direction, such that their basal planes are oriented to within ~15° to the principal stress direction. These criteria may be useful in guiding the development of a diagnostic experimental measurement tool (based on EBSD or ultrasonics for example) for macrozone detection in components.
Xu Y, Joseph S, Karamched P, et al., 2020, Predicting dwell fatigue life in titanium alloys using modelling and experiment, Nature Communications, Vol: 11, ISSN: 2041-1723
Fatigue is a difficult multi-scale modelling problem nucleating from localised plasticity at the scale of dislocations and microstructure with significant engineering safety implications. Cold dwell fatigue is a phenomenon in titanium where stress holds at moderate temperatures lead to substantial reductions in cyclic life, and has been implicated in service failures. Using discrete dislocation plasticity modelling complemented by transmission electron microscopy, we successfully predict lifetimes for ‘worst case’ microstructures representative of jet engine spin tests. Fatigue loading above a threshold stress is found to produce slip in soft grains, leading to strong dislocation pile-ups at boundaries with hard grains. Pile-up stresses generated are high enough to nucleate hard grain basal dislocations, as observed experimentally. Reduction of applied cyclic load alongside a temperature excursion during the cycle lead to much lower densities of prism dislocations in soft grains and, sometimes, the elimination of basal dislocations in hard grains altogether.
Carpinteri A, Dunne FPE, Fatemi A, et al., 2020, Special Issue on 'Multiaxial Fatigue 2019': Selected papers from the 12th International Conference on Multiaxial Fatigue and Fracture (ICMFF12), held in Bordeaux, France, on 24-26 June 2019, INTERNATIONAL JOURNAL OF FATIGUE, Vol: 140, ISSN: 0142-1123
Xu Y, Fox K, Rugg D, et al., 2020, Cyclic plasticity and thermomechanical alleviation in titanium alloys, International Journal of Plasticity, Vol: 134, Pages: 1-19, ISSN: 0749-6419
Cyclic behaviour of titanium alloy IMI834 has been investigated with a new thermo-mechanically coupled discrete dislocation plasticity formulation, integrated with experimental observation. The mechanistic basis of the cyclic loading dependence is shown to be the establishment of dislocation pileups and back stress development such that on partial unloading, reversed strain occurs by thermally-activated dislocation escape and reverse glide during a secondary stress hold. Anomalous cyclic strain accumulation in both isothermal and anisothermal stress-loaded alloy IMI834 is explained by the reversed straining and dislocation structure re-arrangement mechanisms. The mechanisms also provide the underpinning explanation for the beneficial effect of elevated temperature excursions in diminishing cyclic creep accumulation and hence reducing dwell fatigue sensitivity in titanium alloys by thermal alleviation.
Lu X, Dunne FPE, Xu Y, 2020, A crystal plasticity investigation of slip system interaction, GND density and stored energy in non-proportional fatigue in Nickel-based superalloy, International Journal of Fatigue, Vol: 139, ISSN: 0142-1123
A dislocation and gradient-based crystal plasticity finite element study of fatigue has been carried out for nickel-based superalloy RR1000 in order to investigate detrimental non-proportional effects on fatigue life. Six differing multiaxial loading cycles including both proportional and non-proportional paths have been addressed and a critical stored energy density criterion employed for fatigue life. Non-proportional paths are shown to lead to higher numbers of intragranular slip system activations, reflecting experimental observations. These give higher geometrically necessary dislocation (GND) densities resulting from slip system interaction occurring through latent hardening effects in the model. The higher GND densities in turn drive up local stress and stored energy densities, thereby leading to lower predicted fatigue lives, in keeping with non-proportional fatigue experiments in the alloy considered. Intragranular slip system interaction may be the mechanistic explanation for non-proportional effects in fatigue of engineering alloys.
Wan W, Dunne FPE, 2020, Microstructure-interacting short crack growth in blocky alpha Zircaloy-4, International Journal of Plasticity, Vol: 130, Pages: 1-15, ISSN: 0749-6419
Microstructurally short fatigue crack growth in blocky alpha Zircaloy-4 is experimentally investigated in cyclic three-point bend testing. The short crack propagation is sensitive to the local microstructure with respect to grain crystallographic orientation and grain boundaries. Polycrystals with predominant c-axis texture aligned out-of-plane and normal to loading give alternating crack paths along prismatic planes. Samples with c-axis texture aligned in plane and normal to loading typically show straight paths along prismatic planes, sometimes tortuous paths, but always crystallographic. Prismatic <a>-direction crack growth rate is low compared to that for prismatic <c>-direction growth for given loading. Hence the crystallographic plane within which cracks grow is important for determining overall growth rate. For tortuous cracks, with the predominant c-axis texture in plane and normal to loading, crack growth occurs along basal, prismatic and pyramidal planes, deflecting from one slip plane to another during transgranular propagation.
Chen B, Janssens KGF, Dunne FPE, 2020, Role of geometrically necessary dislocation density in multiaxial and non-proportional fatigue crack nucleation, International Journal of Fatigue, Vol: 135, ISSN: 0142-1123
Experimental and crystal plasticity modelling studies have been carried out to investigate nonproportionality and stress state effects in fatigue in a 316 stainless steel and nickel-basedsuperalloy RR1000 which have substantial effects on fatigue life. Stored energy density hasprovided a reasonably consistent and unifying explanation for the experimental observationsof fatigue life in axial, torsional, in-phase proportional tension and torsion, and nonproportional loading regimes. A single fatigue quantity (the critical stored energy density,equating to new surface energy) has been shown to provide good qualitative and reasonablequantitative prediction of the experimental observations of the complex loading, providing amechanistic explanation for the fatigue behaviour. For the case where significant densities ofGNDs develop (for the fine-grained nickel), the latter is found to differentiate the proportionaland non-proportional fatigue lives and its contribution to the local stored energy is crucial forcapturing the correct fatigue lives under the differing loadings.
Bergsmo A, Dunne FPE, 2020, Competing mechanisms of particle fracture, decohesion and slip-driven fatigue crack nucleation in a PM nickel superalloy, International Journal of Fatigue, Vol: 135, Pages: 105573-105573, ISSN: 0142-1123
Fatigue cracks may initiate around non-metallic inclusions via particle fracture, particle decohesion and slip-driven nucleation. Cohesive zone techniques within microstructurally faithful crystal plasticity modelling validated by micromechanical experiments (HR-DIC and HR-EBSD) are employed to investigate these nucleation phenomena. Particle fracture and decohesion lead to stress redistribution which influences subsequent energy storage driving slip-driven fatigue crack nucleation. Particle fracture and decohesion strengths were determined and using a stored energy criterion, the number of cycles to initiation of the fatigue microcrack was predicted. A threshold applied stress below which decohesion and fracture do not occur was obtained, thus modestly increasing fatigue life.
Poole B, Barzdajn B, Dini D, et al., 2020, The roles of adhesion, internal heat generation and elevated temperatures in normally loaded, sliding rough surfaces, International Journal of Solids and Structures, Vol: 185-186, Pages: 14-28, ISSN: 0020-7683
The thermal effects of plastic and frictional heat generation and elevated temperature were examined along with the role of adhesion in the context of galling wear, using a representative crystal plasticity, normally loaded, sliding surface model. Galling frequency behaviour was predicted for 316L steel. Deformation of the surfaces was dominated by the surface geometry, with no significant effect due to variations in frictional models. Plastic and frictional heating were found to have a minimal effect on the deformation of the surface, with the rapid conduction of heat preventing any highly localised heating. There was no corresponding effect on the predicted galling frequency response.Isothermal, elevated temperature conditions caused a decrease in galling resistance, driven by the temperature sensitivity of the critical resolved shear stress. The extent of deformation, as quantified by the area of plastically deformed material and plastic reach, increased with temperature. Comparisons were made with literature results for several surface amplitude and wavelength conditions. Model results compared favourably with those in the literature. However, the reduction in predicted galling resistance with elevated temperature for a fixed surface was not as severe as observations in the literature, suggesting other mechanisms (e.g. phase transformations, surface coatings and oxides) are likely important.
Prastiti NG, Xu Y, Balint DS, et al., 2020, Discrete dislocation, crystal plasticity and experimental studies of fatigue crack nucleation in single-crystal nickel, International Journal of Plasticity, Vol: 126, Pages: 1-14, ISSN: 0749-6419
Dislocation configurational energy and stored energy densities are determined in discrete dislocation and crystal plasticity modelling respectively and assessed with respect to experiments on single crystal nickel fatigue crack nucleation. Direct comparisons between the three techniques are provided for two crystal orientation fatigue tests. These provide confirmation that both quantities correctly identify the sites of fatigue crack nucleation and that stored energy density is a reasonable approximation to the more rigorous dislocation configurational energy. GND density is shown to be important in locating crack nucleation sites because of its role in the local configurational energy density.
Piglione A, Yu J, Zhao J, et al., 2020, Micro-mechanisms of Cyclic Plasticity at Stress Concentrations in a Ni-Based Single-Crystal Superalloy, Pages: 333-340, ISSN: 2367-1181
Ni-based single-crystal superalloys are high-temperature materials used for turbine blades in jet engines. Fatigue damage can pose a major threat to the integrity of such components in operation. Traditionally, TEM-based studies on the fatigue behaviour of superalloys has been studied by investigating cyclic plasticity in the bulk of the material. When the cyclic loads are nominally elastic, however, such investigation may not contribute to the understanding of the alloy’s fatigue behaviour, since plastic micro-strains are confined to regions near stress raisers such as microstructural defects and are therefore randomly distributed. In turn, the plastic micro-strains near the ‘critical’ stress raiser, i.e. the one that acts as nucleation site for the dominant crack, govern fatigue life by inducing early crack initiation, and are therefore the key to capture the material’s fatigue behaviour. Hence, this work is concerned with the experimental characterisation of cyclic plasticity at the initiation site in a Ni-based single-crystal superalloy at 800 °C tested with nominally elastic cyclic loads. Such investigation was carried out by focused ion beam (FIB) lift-outs and subsequent transmission electron microscopy (TEM) studies. It is shown that deformation is significantly more pronounced near the ‘critical’ stress concentration; in addition, deformation is rather homogeneous across large regions surrounding the stress raiser, with remarkably different deformation modes compared to those observed in the bulk of the specimens and from those expected in superalloys tested in similar conditions. The investigation of local cyclic plasticity at stress concentrations promises therefore to provide new insight into fatigue crack initiation in Ni-based superalloys.
Zheng Z, Eisenlohr P, Bieler TR, et al., 2020, Heterogeneous Internal Strain Evolution in Commercial Purity Titanium Due to Anisotropic Coefficients of Thermal Expansion, JOM, Vol: 72, Pages: 39-47, ISSN: 1047-4838
Chen L, James Edwards TE, Di Gioacchino F, et al., 2019, Crystal plasticity analysis of deformation anisotropy of lamellar TiAl alloy: 3D microstructure-based modelling and in-situ micro-compression, International Journal of Plasticity, Vol: 119, Pages: 344-360, ISSN: 0749-6419
Detailed microstructure characterisation and in-situ micropillar compression were coupled with crystal plasticity-based finite element modelling (CP-FEM) to study the micro-mechanisms of plastic anisotropy in lamellar TiAl alloys. The consideration of microstructure in both simulation and in-situ experiments enables in-depth understanding of micro-mechanisms responsible for the highly anisotropic deformation response of TiAl on the intra-lamella and inter-lamella scales. This study focuses on two specific configurations of lamellar microstructure with the interfaces being aligned and to the loading direction. Microstructure-based CP-FEM shows that longituginal slip of super and ordinary dislocations are most responsible for the plastic anisotropy in the micropillar while the anisotropy of the micropillar is due to longitudinal superdislocations and longitudinal twins. In addition, transversal superdislocations were more active, making the deformation in the micropillar less localised than that in the micropillar. Moreover, the CP-FEM model successfully predicted substantial build-up of internal stresses at interfaces, which is believed to be detrimental to the ductility in TiAl. However, as evidenced by the model, the detrimental internal stresses can be significantly relieved by the activation of transverse deformation twinning, suggesting that the ductility of TiAl can be improved by promoting transverse twins.
Zheng Z, Dunne FPE, 2019, Effects of grain size, orientation, and source density on dislocation configurational energy density, JOM, Vol: 71, Pages: 2576-2585, ISSN: 1047-4838
The effects of grain size, source density, and misorientations on the dislocation configurational energy area density are investigated using two-dimensional discrete dislocation plasticity. Grain boundaries are modeled as impenetrable to dislocations. The considered grain size ranges from 0.4μm2 to 8.0μm2 . The configurational energy area density displays a strong size dependence, similar to the stress response. Two sets of materials are considered, with low and high source/obstacle density. The high-source-density specimens exhibit negative configurational energy, implying that the dislocation structure is more stable than for isolated dislocations . The contribution of misorientation to the configurational energy density is analyzed using specimens with a single orientation or a checkerboard arrangement. The configurational energy density is found not only to depend on the dislocation spacing but also to be related to the local stress states. Low source densities lead to higher (positive) configurational energy densities.
Zheng Z, Prastiti NG, Balint DS, et al., 2019, The dislocation configurational energy density in discrete dislocation plasticity, Journal of the Mechanics and Physics of Solids, Vol: 129, Pages: 39-60, ISSN: 0022-5096
Dislocation configurational energy is the term assigned to describe the elastically-stored energy associated with the interaction of dislocations and their structures. It is the energy which is over and above that from the summation of the dislocation line energies when considered isolated and non-interacting. It is therefore different to the free energy and the stored energy. This paper presents a formulation for its determination utilising discrete dislocation plasticity. The total geometrically necessary (GND) and statistically stored dislocation density mean free distance allows the configurational energy density to be determined, thus providing a length scale over which the configurational energy is stored. This quantity is assessed in polycrystals undergoing fatigue loading showing that clear microstructural locations, often associated with high GND density, become established at which the progressive, cyclic, increasing configurational energy occurs. A higher length scale crystal plasticity stored energy density has recently been introduced which attempts to capture local dislocation configurational energy density as an indicator of fatigue crack nucleation and growth. The former is compared and assessed against the dislocation configurational energy density in this paper.
Wilson D, Wan W, Dunne FPE, 2019, Microstructurally-sensitive fatigue crack growth in HCP, BCC and FCC polycrystals, Journal of the Mechanics and Physics of Solids, Vol: 126, Pages: 204-225, ISSN: 0022-5096
Microstructurally sensitive fatigue crack growth in four material systems with BCC, FCC and HCP crystallography was investigated through integrated crystal plasticity eXtended Finite Element (XFEM) modelling and experiment. The mechanistic drivers for crack path tortuosity and propagation rate have been investigated and crack propagation found to be controlled by crack tip stored energy and the crack direction by anisotropic crystallographic slip at the crack tip. Experimentally observed microstructurally-sensitive fatigue crack path tortuosities and growth rates in titanium alloy (Ti-6Al-4V), ferritic steel, nickel superalloy and zirconium alloy (zircaloy 4) have been shown to be captured, supporting the underpinning mechanistic arguments. Very short crack growth is dominated by local slip, but with increasing length, crack tip stresses begins to predominate, increasing the availability of slip systems and giving smaller amplitude oscillations between slip systems. This leads to overall crack paths which are in fact crystallographic but which appear not to be. Key features of crack retardation at grain boundaries, changes in rate resulting from crystallography, and intragranular crack path deflections have been experimentally observed and captured.
Wilson D, Dunne FPE, 2019, A mechanistic modelling methodology for microstructure-sensitive fatigue crack growth, Journal of the Mechanics and Physics of Solids, Vol: 124, Pages: 827-848, ISSN: 0022-5096
A mechanistic methodology for simulating microstructurally-sensitive (tortuosity and propagation rate) fatigue crack growth in ductile metals is introduced which utilises the recently introduced dislocation configurational stored energy as the measure of the driving force. The model implements crystal plasticity finite element simulations using the eXtended Finite Element Method (XFEM) to represent the crack. Two methods of predicting the direction of growth (based on the crystallographic slip or the maximum principal stress) are compared. The crystallographic slip based direction model is shown to predict microstructurally-sensitive fatigue crack growth in single crystals which displays many features of path tortuosity that have been observed experimentally. By introducing a grain boundary, the crystallographic model is shown to capture behaviour similar to that observed experimentally including crack deflection and retardation at the grain boundaries. Finally, two experimental examples of fatigue cracks growing across three grains are analysed, and the model is shown to capture the correct crystallographic growth paths in both cases.
Waheed S, Zheng Z, Balint D, et al., 2019, Microstructural effects on strain rate and dwell sensitivity in dual-phase titanium alloys, Acta Materialia, Vol: 162, Pages: 136-148, ISSN: 1359-6454
In this study, stress relaxation tests are performed to determine and compare the strain rate sensitivity of different titanium alloy microstructures using discrete dislocation plasticity (DDP) and crystal plasticity finite element (CPFE) simulations. The anisotropic α and β phase properties of alloy Ti-6242 are explicitly included in both the thermally-activated DDP and CPFE models together with direct dislocation penetration across material-interfaces in the DDP model. Equiaxed pure α, colony, Widmanstatten and basketweave microstructures are simulated together with an analysis of the effect of α grain size and dislocation penetration on rate sensitivity. It is demonstrated that alloy morphology and texture significantly influence microstructural material rate sensitivity in agreement with experimental evidence in the literature, whereas dislocation penetration is found not to be as significant as previously considered for small deformations. The mechanistic cause of these effects is argued to be changes in dislocation mean free path and the total propensity for plastic slip in the specimen. Comparing DDP results with corresponding CPFE simulations, it is shown that discrete aspects of slip and hardening mechanisms have to be accounted for to capture experimentally observed rate sensitivity. Finally, the dwell sensitivity in a polycrystalline dual-phase titanium alloy specimen is shown to be strongly dependent on its microstructure.
Chen B, Janssens K, Dunne F, 2019, Multiaxial and non-proportional microstructure-sensitive fatigue crack nucleation, 12th International Conference on Multiaxial Fatigue and Fracture (ICMFF), Publisher: E D P SCIENCES, ISSN: 2261-236X
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Zheng Z, Stapleton A, Fox K, et al., 2018, Understanding thermal alleviation in cold dwell fatigue in titanium alloys, International Journal of Plasticity, Vol: 111, Pages: 234-252, ISSN: 0749-6419
Dwell fatigue facet nucleation has been investigated in isothermal rig disc spin tests and under anisothermal in-service engine conditions in titanium alloy IMI834 using α-HCP homogenised and faithful α-β lamellar microstructure crystal plasticity representations. The empirically observed facet nucleation and disc failure at low stress in the isothermal spin tests has been explained and originates from the material rate sensitivity giving rise to soft grain creep accumulation and hard grain basal stresses which increase with fatigue cycling until facet nucleation. The α-HCP homogenised model is not able to capture this observed behaviour at sensible applied stresses. In contrast to the isothermal spin tests, anisothermal in-service disc loading conditions generate soft grain slip accumulation predominantly in the first loading cycle after which no further load shedding nor soft grain creep accumulation is observed, such that the behaviour is stable, with no further increase in hard grain basal stress so that facet nucleation does not occur, as observed empirically. The thermal alleviation, which derives from in-service loading conditions and gives the insensitivity to dwell fatigue dependent on the temperature excursions, has been explained. A stress-temperature map for IMI834 alloy has been established to demarcate the ranges for which the propensity for dwell fatigue facet nucleation is high, threatening or low.
Wilson D, Zheng Z, Dunne F, 2018, A microstructure-sensitive driving force for crack growth, Journal of the Mechanics and Physics of Solids, Vol: 121, Pages: 147-174, ISSN: 0022-5096
A stored energy based methodology for calculating the driving force for crack growth is introduced which can capture the highly local microstructural sensitivity. This has been implemented in the context of crystal plasticity finite element simulations with explicit representation of the crack with the eXtended Finite Element Method (XFEM), with non-local approaches for both stored energy and J-integral calculation. The model is shown to have good agreement with discrete dislocation plasticity (DDP) models in terms of the crack-tip dislocation configurational energy, and with experimental observations of long and very short (microstructurally-sensitive) cracks for both fracture toughness and crack growth rate data. The method is shown to capture the microstructural sensitivity, in contrast with the widely used J-Integral method. By modelling different crack lengths, the diminution of the microstructural sensitivity with increasing crack length is quantified and a critical length defined above which the microstructural sensitivity is insignificant.
Barzdajn B, Paxton AT, Stewart D, et al., 2018, A crystal plasticity assessment of normally-loaded sliding contact in rough surfaces and galling, Journal of the Mechanics and Physics of Solids, Vol: 121, Pages: 517-542, ISSN: 0022-5096
An investigation of rough metal to metal contacting surfaces under normal load and undergoing sliding has been carried out with explicit representation of measured surface profiles within a crystal plasticity finite element formulation in which grain size, texture and slip properties are incorporated. A new metric called plastic reach has been introduced for contacting surfaces which reflects both the magnitude of the local surface asperity plasticity and its spatial reach. This quantity has been shown to obey a power law relationship with the applied normal load for sliding contact which in turn has been related to a hazard function. In this way, a new methodology to predict the galling frequency that follows a Weibull distribution has been established. Additionally, a quantitative definition of galling for the class of metal on metal contacting surfaces is considered. The predicted galling frequency distribution for a 316 stainless steel has been compared with independently experimentally measured galling frequencies showing qualitative agreement of the distributions. An assessment of confidence limits has also therefore been provided for the modelling methodology.
Lan B, Carpenter MA, Gan W, et al., 2018, Rapid measurement of volumetric texture using resonant ultrasound spectroscopy, Scripta Materialia, Vol: 157, ISSN: 1359-6462
This paper presents a non-destructive evaluation method of volumetric texture using resonant ultrasound spectroscopy (RUS). It is based on a general theoretical platform that links the directional wave speeds of a polycrystalline aggregate to its texture through a simple convolution relationship, and RUS is employed to obtain the speeds by measuring the elastic constants, where well-established experimental and post-processing procedures are followed. Important lower-truncation-order textures of representative hexagonal and cubic metal samples with orthorhombic sample symmetries are extracted, and are validated against independent immersion ultrasound and neutron tests. The successful deployment of RUS indicates broader applications of the general methodology.
Zhao C, Stewart D, Jiang J, et al., 2018, A comparative assessment of iron and cobalt-based hard-facing alloy deformation using HR-EBSD and HR-DIC, Acta Materialia, Vol: 159, Pages: 173-186, ISSN: 1359-6454
Three iron-based alloys (Nitronic 60, Tristelle 5183 and RR2450) and a cobalt alloy (Stellite 6) are studied using bend-testing to induce progressive straining and both high resolution DIC and EBSD are utilized to provide quantitative characterization of the deformation mechanisms. The roles of austenite, ferrite and carbide/silicide phases are investigated, together with how each contributes to slip activation and localisation, GND development and hardening through to particle pull-out and fracture. The observed mechanisms are discussed in the context of galling performance.The results suggest that a distribution of fine precipitates, both intra-granular and at grain/phase boundaries, promote more homogeneous and distributed slip, and the development of distributed higher densities of GNDs. The latter promotes hardening which in turn also facilitates homogeneity of deformation and potentially better galling resistance. A uniform size of fine precipitates is also helpful; large silicides lead to particle fracture and pull-out, likely highly damaging under conditions of sliding contact and galling.
Lan B, Britton TB, Jun T-S, et al., 2018, Direct volumetric measurement of crystallographic texture using acoustic waves, Acta Materialia, Vol: 159, Pages: 384-394, ISSN: 1359-6454
Crystallographic texture in polycrystalline materials is often developed as preferred orientation distribution of grains during thermo-mechanical processes. Texture dominates many macroscopic physical properties and reflects the histories of structural evolution, hence its measurement and control are vital for performance optimisation and deformation history interogation in engineering and geological materials. However, exploitations of texture are hampered by state-of-the-art characterisation techniques, none of which can routinely deliver the desirable non-destructive, volumetric measurements, especially at larger lengthscales. Here we report a direct and general methodology retrieving important lower-truncation-order texture and phase information from acoustic (compressional elastic) wave speed measurements in different directions through the material volume (avoiding the need for forward modelling). We demonstrate its deployment with ultrasound in the laboratory, where the results from seven representative samples are successfully validated against measurements performed using neutron diffraction. The acoustic method we have developed includes both fundamental wave propagation and texture inversion theories which are free from diffraction limits, they are arbitrarily scalable in dimension, and can be rapidly deployed to measure the texture of large objects. This opens up volumetric texture characterisation capabilities in the areas of material science and beyond, for both scientific and industrial applications.
Erinosho TO, Collins DM, Todd RI, et al., 2018, Statistical effects in X-ray diffraction lattice strain measurements of ferritic steel using crystal plasticity, Materials and Design, Vol: 153, Pages: 159-165, ISSN: 0264-1275
The influence of statistics on calculated lattice strains has been studied by comparing crystal plasticity finite element (CPFE) calculations with strains measured experimentally. Experimentally, when Bragg's law is obeyed, a plane normal must lie within a narrow orientation range (∼ 0.02° for synchrotron diffraction), or Bragg tolerance. However, CPFE models consider only a small number of grains compared to experiments, necessitating a justification of the statistically representative volume. It also becomes necessary to assess the threshold of Bragg tolerance allowable for the determined statistically representative volume. In this study, an 8 × 8 × 8 model was deemed as statistically representative such that only small benefits are obtained in terms of lattice strain calculations by adopting larger models such as 10 × 10 × 10. Based on the selected model, an allowable Bragg tolerance of approximately 5° was calculated. Also highlighted was the coupling between lattice strain, texture, hardening and applied boundary condition which are discriminators that will affect the choice of model size and Bragg tolerance threshold.
Zhang Z, Wilkinson A, Preuss M, et al., 2018, Quantitative investigation of micro slip and localization in polycrystalline materials under uniaxial tension, International Journal of Plasticity, Vol: 108, Pages: 88-106, ISSN: 0749-6419
Micro slip activation and localization inTi-6Al-4Vdeformed in tension havebeen examinedquantitatively using high-resolution (HR) digital image correlation (DIC),HR-electron backscatter diffraction(EBSD)and crystal plasticityfinite element modelling. The measured polycrystal slip, strain, lattice rotation and geometrically necessary dislocation (GND) densitydistributionsare generally well capturedby thea prioricrystal plasticity model based on the rate-sensitive properties of α-titanium. An overall slip trace analysis showed over 80% agreement betweenHR-DIC and crystal plasticity modellingofthe primary slip activation. Thetexture beneath the characterised free-surface has been found toaffectthe local slip, stress distribution, lattice curvature and GND density andthreetexturevariationshave been considered.Grain-levelslip trace analysis shows that the crystal plasticity modelling can capture single (straight)slip, multiple slip activation andcomplex wavyslip.The latterhas been found to result from the interactionof independently activated basal andprismatic slip systems with commonslip direction. Initial inter-granular misorientations greater than about 5ohave been shown to influence the subsequent micromechanical grain behaviour including slip, lattice rotation and GND density. This work contributes to the understanding of slip localization and load shedding in dwell fatigue in polycrystalline hexagonal materials.
Zhang Z, FPE D, 2018, Phase morphology, variants and crystallography of alloy microstructures in cold dwell fatigue, International Journal of Fatigue, Vol: 113, Pages: 324-334, ISSN: 0142-1123
This paper examines microscale crystal slip accumulation, cold creep, and stress redistribution (load shedding) related to dwell fatigue in a range of α–β Ti alloy microstructures. The role of basal slip and prism slip is evaluated in load shedding in a rogue grain combination. The results enrich the Stroh dislocation pile up interpretation of dwell by accounting for the anisotropic rate dependence of differing slip systems together with morphology.Microstructural morphology has been found to play an essential role in cold creep and load shedding in dwell fatigue. Basketweave structures with multiple α variants have been shown to give the lowest load shedding for which the mechanistic explanation is that the β lath structures provide multiple, small-scale α variants which inhibit creep and hence stress relaxation, thus producing more uniform, diffuse stress distributions across the microstructure through microscale kinematic confinement, imposed by multi (α)-to-single (β) BOR relations (i.e. multiple α variants sharing the same parent β grain). The critical consequence of this is that alloys typically having multi-variant basketweave structure (e.g. Ti-6246), remain free of dwell fatigue debit whereas those alloys associated with globular colony structures (e.g. Ti-6242) suffer significant dwell debit. This understanding is important in microstructural design of titanium alloys for resisting cold dwell fatigue.
Zheng Z, Waheed S, Balint D, et al., 2018, Slip transfer across phase boundaries in dual phase titanium alloys and the effect on strain rate sensitivity, International Journal of Plasticity, Vol: 104, Pages: 23-38, ISSN: 0749-6419
Dislocation transmission through α/β phase boundaries in titanium alloys is studied using integrated crystal plasticity (CP) and discrete dislocation plasticity (DDP) modelling techniques, combined with experimental micro-pillar compression test results. Direct dislocation transmission together with the nucleation of new dislocations ahead of a pile-up at an α/β interface, termed indirect slip transfer, are both assessed and their role in controlling microstructure-dependent strain rate sensitivity considered. A critical shear stress criterion for direct slip transfer across an α/β interface in Ti-6242 has been established by capturing the local slip penetration through the phase boundary using CP and DDP comparisons with experimental two phase micro-pillar compression. The competition between direct and indirect slip transfer has been investigated using a single Frank-Read source DDP model. Direct slip transfer is found to occur only under specific conditions which have been quantified. The strain rate sensitivity of dual phase titanium alloys is demonstrated to depend on average pile-up size which is significantly influenced by α/β morphology.
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