J-integral analysis: An EDXD and DIC comparative study for a fatigue crack

Synchrotron Energy Dispersive X-ray Diffraction (EDXD) and Digital Image Correlation (DIC) have been applied to map simultaneously the 2D elastic strain and displacement fields of a propagating fatigue crack in the HAZ of a welded Cr2Ni4MoV bainitic steel. The position of the crack tip was tracked via a phase congruency analysis of the displacement field, and also by detection of its cyclic plastic zone. Both types of full field data provided independent inputs to finite element/J-integral analyses that directly quantified the elastic cyclic stress intensity factor range applied to the crack. No knowledge was required of the specimen geometry, crack length or applied loads. The agreement between the two analyses in this controlled study shows that strain mapping by synchrotron EDXD can provide a reliable method to study the crack fields in more complex problems, such as interactions between crack closure, residual stresses and applied loading

A 3D full-field study of cracks in a nuclear graphite under mode I and mode II cyclic dwell loading conditions

Three‐dimensional (3D) full‐field deformation around crack tips in a nuclear graphite has been studied under mode I and mode II cyclic dwell loading conditions using digital volume correlation (DVC) and integrated finite element (FE) analysis. A cracked Brazilian disk specimen of Gilsocarbon graphite was tested at selected loading angles to achieve mode I and mode II cyclic dwell loading conditions. Integrated FE analysis was carried out with the 3D displacement fields measured by DVC injected into the FE model, from which the crack driving force J‐integral was obtained using a damaged plasticity material model. The evolution of near‐tip strains and the J‐integral during the cyclic dwell loading was examined. Under cyclic dwell, residual strain accumulation was observed for the first time. The results shed some light on the effect of dwell time on the 3D crack deformation and crack driving force in Gilsocarbon under cyclic mode I and II loading conditions

In situ observation of compressive deformation of an interconnected network of zinc oxide tetrapods

Zinc oxide tetrapods have remarkable functional and mechanical properties with potential applications in different fields including nanoelectronic and optoelectronic sensing, functional composites and coatings, as well as energy harvesting and storage. Based on the 3D shape of these microparticles, they can be assembled into highly porous (up to 98%) macroscopic ceramic framework structures that can be utilized as a versatile template for the fabrication of other multi-scaled foam-like materials. Here we investigated the three-dimensional structure of low density interconnected zinc oxide tetrapod networks by high resolution X-ray computed tomography. In situ observations during mechanical loading show inhomogeneous development of anelastic strain (damage) during compression, and homogeneous elastic recovery on unloading. Individual tetrapods are observed to deform by arm rotation to accommodate strain

Exploring factors affecting undergraduate medical students’ study strategies in the clinical years: a qualitative study

The aim of this study is to explore the effects of clinical supervision, and assessment characteristics on the study strategies used by undergraduate medical students during their clinical rotations. We conducted a qualitative phenomenological study at King Saud Bin Abdulaziz University for Health Sciences, College of Medicine, Riyadh, Saudi Arabia during the period from November 2007 to December 2008. We conducted semi-structured focus groups interviews with students and conducted individual interviews with teachers and students to explore students’ and clinical teachers’ perceptions and interpretations of factors influencing students’ study strategies. Data collection was continued until saturation was reached. We used Atlas-ti Computer Software (Version 5.2) to analyse the data, apply the obtained themes to the whole dataset and rearrange the data according to the themes and sub-themes. Analysis of data from interviews with twenty-eight students and thirteen clinical supervisors yielded three major themes relating to factors affecting students’ study strategies: “clinical supervisors and supervision”, “stress and anxiety” and “assessment”. The three themes we identified played a role in students’ adoption of different study strategies in the “community of clinical practice”. It appeared that teachers played a key role, particularly as assessors, clinical supervisors and as a source of stress to students

New techniques for in-situ observations of crack growth behaviour

Experimental observations are essential for the validation for models for crack growth behaviour, particularly in the short crack regime. This paper reports the use of several new methods applied to in-situ short crack observation, including 3D observations of microstructure and cracking by synchrotron tomography techniques, and 2D optical digital image correlation. The examples presented include stress corrosion of stainless steels in aqueous environments, and brittle fracture of of nuclear graphite, where these new methods enable the visualisation of cracks in circumstances in which they would generally be regarded as practically non-observable

3D cellular automata fracture model for porous graphite microstructures

Nuclear graphite has a complex porous microstructure, which depends on raw materials and manufacturing process; porosity can change with radiolytic oxidation and also in the absence of oxidation with very high neutron fluences. Porosity directly affects the fracture process and the graphite tensile strength. To understand the effects of porosity on component strength and its relation to small specimen data, microstructure sensitive models are needed that can simulate the statistics of strength of porous microstructures, also addressing size and strain gradients effects such as notches. This requires multi-scale models that capture the key microstructural features with sufficient fidelity, and also with sufficient computational economy to simulate component behaviour. To achieve this, an innovative technique to calculate the elastic stress distribution in a 3D porous solid under uniaxial or biaxial tension has been developed that uses cellular automata. Synthetic microstructures with arbitrary distributions of pore sizes and shapes are created that simulate realistic microstructures; a fracture algorithm simulates failure initiation and crack growth. The model calculates the tensile strength of a microstructure volume for any arbitrary failure criteria; the critical strain energy release rate is used as an example to demonstrate how porosity affects the fracture process. The presented Cellular Automata (CA) model is at least an order of magnitude more efficient than finite element methods of equivalent discretisation; CA are also scale independent and well suited for parallel computing. This would allow large volumes of representative microstructures to be simulated, with a Monte-Carlo based approach to investigate strength variability

Intergranular crack nucleation in polycrystalline alumina

Crack nuclei in pure and Cr-doped aluminas with average grain size between 1.5 μm and 3.6 μm have been studied using digital image correlation of optical images, to observe the interaction between microstructure, residual stress and damage development. Individual intergranular crack nuclei within areas comprising tens of thousands of grains were studied to measure crack surface lengths and crack opening displacements as a function of load, prior to unstable fracture. Grain orientations in the vicinity of intergranular crack nuclei, and the grain boundary planes, has been characterised by trace analysis and electron backscatter diffraction. This allows estimation of the thermal stresses that were sufficient to crack the grain boundaries

Influence of microstructure and stress on short intergranular stress corrosion crack growth in austenitic stainless steel type 304

Intergranular stress corrosion cracking (IGSCC) causes failures in austenitic stainless steels when the appropriate electrochemical, metallurgical and mechanical conditions exist. In this study, the effects of time, applied stress, residual stress and microstructure on population of short crack nuclei has been investigated in sensitised type 304 austenitic stainless steel, tested under static load in an acidified potassium tetrathionate (K2S4O6) environment. Statistical analysis, using the Gumbel distribution method, enables analysis of the growth rate of short crack nuclei. This methodology is being developed, in order to quantitatively evaluate the influence of grain boundary engineering and surface finishing on crack nucleation

Short fatigue cracks in austempered ductile cast iron (ADI)

The growth of short fatigue cracks was investigated in an austempered ductile cast iron (wt% 3.6C, 2.5Si, 0.6Mn, 0.15Mo, 0.3Cu), austenitized at 870 °C and then austempered at 375 °C for 2 h. At stress amplitudes close to the fatigue limit endurance limit of 107 cycles, subcritical crack nuclei initiated at graphite nodules. The crack nucleus decelerated and arrested after propagating a short distance. The position of an arrested crack tip was characterized using an electron backscatter diffraction technique, demonstrating that short fatigue cracks in austempered ductile cast iron (ADI) can be arrested by boundaries such as those between ausferrite sheaves or packets and prior austenite grains. Refinement of the prior austenite grain size decreased the size of subcritical crack nuclei. It is proposed that the arrest and retardation of short crack nuclei are controlled by the austenite grain size and graphite nodule size. This determines the fatigue endurance limit

3D cellular automata fracture model for porous graphite microstructures

Nuclear graphite has a complex porous microstructure, which depends on raw materials and manufacturing process; porosity can change with radiolytic oxidation and also in the absence of oxidation with very high neutron fluences. Porosity directly affects the fracture process and the graphite tensile strength. To understand the effects of porosity on component strength and its relation to small specimen data, microstructure sensitive models are needed that can simulate the statistics of strength of porous microstructures, also addressing size and strain gradients effects such as notches. This requires multi-scale models that capture the key microstructural features with sufficient fidelity, and also with sufficient computational economy to simulate component behaviour. To achieve this, an innovative technique to calculate the elastic stress distribution in a 3D porous solid under uniaxial or biaxial tension has been developed that uses cellular automata. Synthetic microstructures with arbitrary distributions of pore sizes and shapes are created that simulate realistic microstructures; a fracture algorithm simulates failure initiation and crack growth. The model calculates the tensile strength of a microstructure volume for any arbitrary failure criteria; the critical strain energy release rate is used as an example to demonstrate how porosity affects the fracture process. The presented Cellular Automata (CA) model is at least an order of magnitude more efficient than finite element methods of equivalent discretisation; CA are also scale independent and well suited for parallel computing. This would allow large volumes of representative microstructures to be simulated, with a Monte-Carlo based approach to investigate strength variability