Fracture behaviour of refractory ceramics after cyclic thermal shock

Two commercially available refractory ceramic materials primary used as substrates for fast firing of porcelain stoneware were investigated. The first one, commercially known as CONC, contains cordierite and mullite in the ratio 50:50. The REFO refractory composite material with coarser microstructure compared to CONC has a cordierite-to-mullite ratio of 50:45 and the balance is filled by quartz. Both materials were exposed to water-quench tests from 1250 degrees C, applying various numbers of thermal cycles (shocks). Subsequently the fracture toughness was evaluated on both as-received and shocked samples using the Chevron notched specimen technique. The results were analysed with respect to the microstructure damage caused by the thermal loading. Scanning electron microscopy was used to analyse both microstructure and fracture surfaces in samples with different thermal loading history

NANOPARTICLE-MODIFICATION OF NICU-BASED ALLOY 400 FOR LASER POWDER BED FUSION

NiCu-based Alloy 400 is a material being frequently used in corrosive environments wherefore it is applied in several industries like the maritime sector or chemical processing [1]. Numerous functional parts made of this alloy, like heat exchangers or liquid-carrying tubes for instance, may withstand harsh environments to a certain extend but still, at high temperatures and especially in carbon-rich atmospheres, component failure occurs due to poor metal dusting and creep resistance [2–5]. Reinforcing the base alloy system with nanoparticles using gas atomization and subsequent laser powder bed fusion (LPBF) can counteract such material failure [6]. Hence, in this work, titanium was added to Alloy 400 and atomized under nitrogen atmosphere in order to cause TiN nanoparticle formation in the microstructure of printed components.Mechanical Engineerin

Graduate Students as Academic Writers: Writing Anxiety, Self-Efficacy, and Emotional Intelligence

Researchers interested in psychological factors affecting writers in higher-education institutions, or academic writers, are concerned with internal variables affecting writing productivity; however few empirical studies explore these factors with samples of students who are in the process of earning master’s or doctoral degrees (i.e., graduate students). In this study, we examined writing anxiety, self-efficacy, and emotional intelligence in a sample of graduate students at a large, research-intensive university in the United States. Using a survey, we collected measures on these variables in addition to demographic information from the participants. We then used the measures to descriptively compare groups of students with similar characteristics and to run three regression models to identify which variables best predicted writing anxiety. Our findings indicate self-efficacy is a statistically significant and large predictor of writing anxiety while emotional intelligence (EI) is not, though descriptive data showed moderate effects between EI and first language (i.e., whether or not a student reported English as a first language). In the presence of self-efficacy, gender remained a significant predictor of writing anxiety, while first language did not. We discuss implications for future research and practice focused on helping graduate student academic writers succeed

Effects of Age and Loading Velocity on the Delamination Strength of the Human Aorta

Delamination strength is the mechanical property which plays a key role in the pathological process referred to as Arterial Dissection. This dissection, known especially for its occurrence in the thoracic aorta, is manifested by a separation of the layers of an artery wall, and may end with total rupture and internal haemorrhaging. Although its incidence is relatively rare, from 3 to 6 cases per 100 000 per year, it is a life-threating disease with a significant lethality [1-3]. The exact conditions under which the dissection is initiated, and as a crack propagates through the arterial wall, remain an open topic in computational as well as experimental mechanics. The aim of our study is to contribute to the deepening of our knowledge of Arterial Dissection, by collecting experimental data which is suitable for the purpose of showing how the delamination strength measured in the peeling experiments depends on age and anatomical location. In addition to the effects of age and location, our study also focuses on the effect of loading rate. The experimental branch of our research is complemented by a computational modelling of the delamination interface, in which we are looking for a numerical characterization of the material parameters describing discontinuity propagation. An XFEM model of the peeling experiment is built in Abaqus, which in our approach plays the role of the regression analysis, incorporating the cohesive zone (CZ) in order to model the delaminating arterial layers. The main objective is to obtain a detailed description of a set of constitutive parameters, which would be age- and location-specific. Our present data suggest that delamination strength strongly depends on age, and furthermore, the anatomical site also seems to be a significant factor. On the other hand, the loading velocity does not cause significant changes in results

Effect of hybrid polymer coating of Bioglass® foams on mechanical response during tensile loading

A simplified two-dimensional finite elements model was created for a polyvinyl alcohol (PVA) coated BioglassH strut undergoing tensile stresses (loading mode I). The strengthening contributions due to the infiltration of coating into surface cracks and coating's stiffness were evaluated in terms of stress intensity factor KI and tensile stresses σyy in the proximity of the crack tip. The infiltration of the coating until the crack tip resulted as themost effective criterion for the struts strengthening. BioglassH based scaffolds were dip coated into PVA and PVA/microfibrillated cellulose (MFC) aqueous solutions and tested in tensile load. Coated samples exhibited remarkably higher tensile strength than non-coated ones, which further raised with the increased amount of MFC. Contact angle θ and linear viscosity n measurements of PVA/MFCsolutions showedthatMFCcauseda reduction inh andadrastic increase in θ, indicating that a balance between these two effects must be achieved

Experimental and numerical investigation of crack initiation and propagation in silicon nitride ceramic under rolling and cyclic contact

The focus of the work was to investigate crack initiation and propagation mechanisms in silicon nitride undergoing non-conforming hybrid contact under various tribological conditions. In order to understand the prevailing modes of damage in silicon nitride, two distinct model experiments were proposed, namely, rolling contact and cyclic contact experiments. The rolling contact experiment was designed in order to mimic the contact conditions appearing in hybrid bearings at contact pressures ranging from 3 to 6 GPa. On the other hand, cyclic contact experiments with stresses ranging from 4 to 15 GPa under different media were carried out to study damage under localised stresses. In addition, the experimentally observed cracks were implemented in a finite element model to study the stress redistribution and correlate the generated stresses with the corresponding mechanisms. Crack propagation under rolling contact was attributed to two different mechanisms, namely, fatigue induced fracture and lubricant driven crack propagation. The numerical simulations shed light on the tensile stress driven surface and subsurface crack propagation mechanisms. On the other hand, the cyclic contact experiments showed delayed crack formation for lubricated cyclic contact. Ceramographic cross-sectional analysis showed crack patterns similar to Hertzian crack propagation under cyclic contact load

Damage progression in silicon nitride undergoing non-conforming hybrid cyclic contact

Bearings experience one of the most severe mechanical loading of all machine elements. The contact stresses engendered are highly localised and bound to a very small volume of the material. The aim of this study was to investigate how localised stresses influence the damage mechanism in hybrid contact. Cyclic contact loading of a gas pressure sintered silicon nitride (GPSN) was investigated. Silicon nitride disks and tungsten carbide (WC) indenters were tested under different media, initially at âapplication relevantâ low contact pressures (4â6 GPa) and further on, to accelerate damage, at high contact pressures (10â15 GPa). The low load experiments showed various forms of surface damage with no significant difference between dry and lubricated contact. Whereas, the high load experiments showed different damage behaviour under unlubricated and lubricated conditions. Unlubricated contact resulted in the formation of transfer layers and Hertzian cracks on the silicon nitride surface whereas, damage under lubricated contact was mainly dominated by grain removal and delayed crack formation. Finite element simulations were carried out to study the stress state under different loading conditions. The FEM results indicated that the combination of surface tensile and shear stresses predominantly influence the fatigue damage observed in the experiments rather than fluctuating tensile stresses only

The influence of processing variables on the structure and mechanical properties of nano-quasicrystalline reinforced aluminium alloys

Eleventh International Conference on Rapidly Quenched and Metastable MaterialsAluminiumalloy powder having a nominal composition of Al93Fe3Cr2Ti2 (at.%) and a mean size of 50 μm has been prepared using high pressure gas atomisation. The powder was sieved to provide starting materials with well defined size distributions and was consolidated using extrusion. Processing parameters including temperature and extrusion ratio have been investigated as a function of both particle size and initial structure. The influence of process variables on the mechanicalproperties of the extruded material has been investigated using microhardness and tensile tests at room temperature. The result of these preliminary investigations are presented and discussedEuropean Union funded Research Training Network Project on Manufacture and Characterisation of Nanostructured Al Alloys, contract no. HPRN-CT-2000-00038Peer reviewe