Accumulation of stress in constrained assemblies: novel Satoh test configuration

A common test used to study the response of a transforming material to external constraint is due to Satoh and involves the cooling of a rigidly constrained tensile specimen while monitoring the stress that accumulates. Such tests are currently common in the invention of welding alloys which on phase transformation lead to a reduction in residual stresses in the final assembly. The test suffers from the fact that the whole of the tensile specimen is not maintained at a uniform temperature, making it difficult to interpret the data. To eliminate this problem, the authors report here a novel Satoh test in which the material investigated is a part of a composite sample. It is demonstrated that this helps avoid some of the complications of the conventional tests and gives results which are consistent with independent tests

Microcracking in Electron-Beam Deposited Scandia-Stabilised Zirconia Electrolyte.

It is the aim of the present work to address some of the aspects of microcracking in electron beam deposited scandia-stabilised zirconia electrolyte applied for solid oxide fuel cells (SOFC) where a thin electrolyte layer is deposited on a relatively thick anode substrate. A model of microcracking for the electrolyte material is proposed which takes into account the statistical distribution of grain sizes, the stress redistribution due to failure of individual structural elements as well as the local criterion of grain fracture. The combination of electron microscopy research with model calculations permits both the specific energy of new surface creation in the electrolyte and critical parameters of the microcracking process to be determined. The annealing-induced electrolyte microcracking discussed in this work corresponds to localised microcracking, where each next structural element fails mainly at an existing microcrack tip. The features of localised microcracking in electron beam deposited scandia-stabilised zirconia electrolyte are analysed. (C) 2009 Elsevier B.V. All rights reserved

Comparative Study Of Static And Cyclic Fatigue Of Zrb2-Sic Ceramic Composites

Room temperature static and cyclic fatigue of ZrB2-32 vol% SiC and ZrB2-45 vol% SiC particulate ceramic composites has been studied. It was established that the presence of grain bridging plays an important role in the lifetime and time dependent mechanical performance of ZrB2-SiC composites. It was also established that the cohesive strength of grain boundaries of the composites was a determining factor if grain bridging would occur during crack growth, as the grain boundaries strength would determine the pathway of the moving crack. Grain bridging was limited in ZrB2-32 vol% SiC leading to the absence of a cyclic fatigue effect, while grain bridging indeed occurred in ZrB2-45 vol%SiC contributing to a cyclic fatigue effect which limits the lifetime of the composite. Such differences were responsible for the occurrence of R-curve behavior in ZrB2-SiC ceramic composites

Mechanical Properties And Residual Stresses In Zrb2-Sic Spark Plasma Sintered Ceramic Composites

ZrB2-17 vol% SiC, ZrB2-32 vol% SiC, and ZrB2-45 vol% SiC ultra-high temperature particulate ceramic composites were sintered using Spark Plasma Sintering (SPS). The mechanical performance of the ZrB2-SiC composites was investigated using 4-point bending to determine their instantaneous flexural strength and fracture toughness. Resonant Ultrasound Spectroscopy was used to determine the Young\u27s, shear, and bulk moduli as well as Poisson\u27s ratio of all examined composites. The distribution of thermal residual stresses and the effect of applied external load on their re-distribution was studied using micro-Raman spectroscopy. Piezospectroscopic coefficients were determined for all three ZrB2-SiC ceramic composites and their experimentally obtained values were compared with the piezospectroscopic coefficients both published in the literature and calculated theoretically. Finally an attempt was made to evaluate the redistribution of thermal residual stresses under external applied stress in order to estimate their contribution to the mechanical behavior of the material

Erratum: Corrigendum To “Mechanical Properties And Residual Stresses In Zrb2–Sic Spark Plasma Sintered Ceramic Composites” (J. Eur. Ceram. Soc. (2016) 36(7) (1527–1537))

The authors regret that the original online version of the above article contained the following errors: M. Lugovy\u27s affiliation was incorrectly listed. His affiliation should be with the University of Central Florida in Orlando. There should only be one email address for the corresponding author N. Orlovskaya: [email protected]. The Publisher decided to ‘resupply’ (repost and replace) the XML and online PDF of the article. Unfortunately the discovery of the errors was too late to correct the printed issue. The authors would like to apologize for any inconvenience caused. Elsevier regrets and apologizes for any inconvenience caused by posting a new version of this article online, but hopes that the reader will understand the reasons for doing so

A Further Insight Into Spherical Indentation: Ring Crack Formation In A Brittle La0.8Sr0.2Ga0.8Mg0.2O 3 Perovskite

It is known that theoretical considerations of fracture under loading by a spherical indenter are based on the concept of pre-existing cracks. However, nucleation and growth of the critical crack could occur during indentation, as happens during microcracking. The goal of the presented research is to develop a new concept of fracture under spherical indentation in a brittle elastic material taking into account the possibility of critical crack nucleation and growth during loading. La0.8Sr0.2Ga0.8Mg 0.2O3 (LSGM) perovskite has been chosen as a polycrystalline elastic low fracture toughness ceramic to perform indentation using a tungsten carbide spherical indenter. Experimental measurements of ring crack radii for well-polished LSGM cannot be explained within the framework of the pre-existing crack hypothesis. The local risk calculated using the concept of pre-existing cracks gives a most probable range of ring crack radii that does not match the radii measured experimentally. However, the local risk calculated using the assumption of critical crack nucleation and formation during spherical indentation results in a most probable range of ring crack radii which exhibits good agreement with the experimental data. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved

Room Temperature Fatigue Of Zrb2-Sic Ceramic Composites

Room temperature time dependent properties of ZrB2-30 wt%SiC ceramic composite have been studied. Both static slow crack growth and cyclic fatigue deformation have been investigated. While static slow crack growth has been evaluated only in air, three different environments, water, air, and dry air, have been used to study the cyclic fatigue. It was established that under cyclic fatigue the environment plays an important role and humidity significantly facilitate crack growth in ZrB2-30wt%SiC. The fractography of selected ZrB2-30wt%SiC samples was performed and it was established that both defects introduced during machining as well as larger defects introduced during the processing served as fracture origins of ceramic composites. © 2013 Elsevier Ltd and Techna Group S.r.l

Time Dependent Mechanical Properties Of Zrb2-Sic Ceramic Composites: Room Temperature Fatigue Parameters

Room temperature fatigue of ZrB2-30 wt% SiC ceramic composite has been studied. The linear regression technique, proposed by Fett and Munz, was used to determine the static and cyclic fatigue parameters of ZrB2-30 wt% SiC ceramic composite tested in dry air, air with 40% humidity, and water at room temperature. It was found that cracks grow much faster under cyclic test conditions in comparison with constant static loading. This phenomenon can be attributed to (1) the degradation of bridging interactions between the two growing crack surfaces; and (2) a corresponding decrease in shielding stress intensity factor. When cyclic fatigue experiments were performed in water in comparison with the results obtained in air, it was also found that crack grows much faster under 100% of humidity. It was determined that water molecules increase the crack propagation rate in ZrB2-30 wt% SiC composite. The lifetime of the material decreases from 18 hours at σ = 580 MPa static loading in air to 5.5 minutes at Δσ = 1200 MPa cyclic loading in air and 0.4 seconds at Δσ= 1240 MPa cyclic loading in water.© 2014 by American Scientific Publishers

Residual Stress And Biaxial Strength In Sc2O3- Ceo2-Zro2/Y2O3-Zro2 Layered Electrolytes

Multi-layered (Y2O3)0.08(ZrO 2)0.92/(Sc2O3) 0.1(CeO2)0.01-(ZrO2) 0.89(YSZ/SCSZ) electrolytes have been designed, so that the inner SCSZ layers provided superior ionic conductivity and the outer YSZ skin layers maintained good chemical and phase stability. Due to the mismatch of coefficients of thermal expansion between layers of different compositions, the thermal residual stresses were generated. The theoretical residual stress and strain were calculated for different thickness ratios of the electrolytes. In order to study the residual stress effect on the mechanical properties, the biaxial flexure tests of electrolytes with various layered designs were performed via a ring-on-ring method at room temperature and 800°C. The maximum principal stress at the fracture indicated improved flexure strength in the electrolytes with layered designs at both temperatures. It is believed to be the result of the residual compressive stress in the outer YSZ layer. In addition, the Weibull statistics of the stress at the fracture at room temperature was studied, and the values of residual stress presented at the outer layer were well verified. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Mechanical Properties Of Zrb2-Sic Ceramic Composites: Room Temperature Instantaneous Behaviour

The mechanical properties of ZrB2-30 wt-%SiC ultra high temperature ceramic composites have been studied. The composite was processed by hot pressing at 2100°C, 30 MPa and 45 min dwell time to achieve a good densification. Young\u27s modulus, single edge V notch beam fracture toughness, hardness, stress-strain deformation, four-point bending strength and Weibull parameters were measured. Fractography and microstructure analyses of ZrB2-30 wt-%SiC ceramic composite were also performed. © 2013 Institute of Materials