Crack growth under thermo-mechanical and temperature gradient loads

The objective of this work is to examine the utility of so-called modified path-independent integrals, referred to as ''integral parameters'' in this paper, for the prediction of crack growth at elevated temperatures where the crack tip deformation involves plasticity of a large scale. In particular, this paper is concerned with the application of Blackburn's integral J* and Kishimoto ct cll.'s integral (J) over cap to thermo-mechanical fatigue loading and temperature gradient loading. Elastic-plastic finite element ana lyses are performed to simulate crack propagation, and the integral parameters are computed by post-processing the results. Numerical results show that these parameters are indeed path-independent for deformation conditions under investigation. While the isothermal results of previous studies yielded satisfactory correlations between the crack growth rates and these parameters, the quality of correlation for more complex loading conditions investigated here is found to be not always as good as the isothermal cases. However, the overall performance indicates that these parameters are worth investigating further. (C) 1997 Elsevier Science Ltd.X1110sciescopu

HOLD TIME CRACK-GROWTH ANALYSIS AT ELEVATED-TEMPERATURES

Integral parameters J* and ($) over cap J are used to predict crack growth under R(c) = O strain cycling with hold time at the maximum strain. Both nominally elastic and nominally plastic strain levels are investigated. Test data are obtained for single edge notch specimens of Alloy 718 at 593 and 649 degrees C with and without hold time. The da/dN - Delta J* or Delta ($) over cap J data for strain cycling without hold time, and the da/dt - J* or ($) over cap J data for the time-dependent crack growth under static control conditions obtained previously are used to predict the crack growth rate under strain cycling with hold time. The superposition method in which the predicted crack growth rates under strain cycling and under constant strain are directly added to obtain the total crack growth rate is found to be satisfactory only at low applied strain ranges. A modified superposition method suitable for both low and high applied strain ranges is proposed.X111sciescopu

TIME-DEPENDENT CRACK-GROWTH CHARACTERIZATION USING RATE INTEGRALS

This paper investigates the utility of the rate forms of Kishimoto et al.'s integral ((J) over cap) and Blackburn's integral (J*) as parameters for correlating time-dependent crack growth rates. These rate integrals are computed from the results of finite element analyses of crack growth in button-head single edge notch specimens of Alloy 718. The specimens are tested under constant strain and constant load conditions at 593 degrees C and 649 degrees C. The crack tip deformation includes large-scale plasticity, primary and secondary creep. The measured crack growth rates are correlated with the computed rate integrals. The results show that both integrals can consolidate the crack growth data very well.X112sciescopu

CHARACTERIZATION OF ELEVATED-TEMPERATURE CRACK-GROWTH IN HASTELLOY-X USING INTEGRAL PARAMETERS

Linear elastic fracture mechanics approaches are not suitable for prediction of fatigue crack growth in the nonlinear regime at elevated temperatures. The objective of this paper is to investigate the ability of the integral parameters by Blackburn (J*), by Kishimoto et al. (($) over cap J), and by Atluri et al. (Delta Tp*, Delta Tp) to correlate crack growth data of Hastelloy-X at elevated temperatures under nominally elastic and nominally plastic loading. Crack growth is analyzed using a finite element method, and the integral parameters are computed from the results of analysis. The experimental crack growth rates are correlated with these parameters. It is found that J*, ($) over cap J, and Delta Tp* can correlate crack growth data within an acceptable accuracy.X111sciescopu

The effect of ytterbium fluoride and barium sulphate nanoparticles on the reactivity and strength of a glass-ionomer cement

Objectives: While clinical advantages of glass-ionomers include fluoride release and radiopacity, disadvantages include low strength, slow initial setting times and opacity. The addition of nanoparticles, in particular those containing fluoride and cross-linkable ions, may mitigate the disadvantages while further improving the advantages. This investigation evaluated the effects of the addition of ytterbium fluoride (YbF3) and barium sulphate (BaSO4) on the strength and reactivity of a commercial glass-ionomer cement. Methods: YbF3 and BaSO4 nanoparticles were incorporated into the powder component of Riva SC (SDI Ltd., Bayswater, Australia) at 1, 2, 5, 10, 15, and 25% by weight. Capsules were assembled at a powder:liquid ratio of 2.9:1, activated and mixed, and the resultant pastes evaluated for working time, initial setting time, 24-h surface hardness and 24-h compressive strength. Results: Working and initial setting times were reduced with the addition of YbF3. Addition of BaSO4 at low concentrations reduced working and initial setting times, but further addition delayed the setting reaction. Compressive strength decreased with the addition of either YbF3 or BaSO4, while surface hardness was slightly but insignificantly higher at 1-2% nanoparticles and then decreased with increasing nanoparticle concentrations. Significance: Nanoparticles modified the setting characteristics, strength and surface hardness of a commercial glass-ionomer cement, and may be useful for refining the handling characteristics of these materials. Further improvements in powder blending may result in more significant improvements in mechanical properties. © 2005 Academy of Dental Materials.link_to_subscribed_fulltex

How Do Hunter-Gatherer Children Learn Subsistence Skills? A meta-ethnographic review

$\textbf{Purpose:}$ Hunting and gathering is, evolutionarily, the defining subsistence strategy of our species. Studying how children learn foraging skills can, therefore, provide us with key data to test theories about the evolution of human life history, cognition, and social behaviour. Modern foragers, with their vast cultural and environmental diversity, have mostly been studied individually. However, cross-cultural studies allow us to extrapolate forager-wide trends in how, when, and from whom hunter-gatherer children learn their subsistence skills. $\textbf{Method:}$ We perform a meta-ethnography, which allows us to systematically extract, summarize and compare both quantitative and qualitative literature. $\textbf{Results:}$ We found 58 publications focusing on learning subsistence skills. Learning subsistence skills begins early in infancy, when parents take children on foraging expeditions and give them toy versions of tools. In early and middle childhood, children transition into the multi-age playgroup, where they learn skills through play, observation, and participation. By the end of middle childhood, most children are proficient food collectors. However, it is not until adolescence that adults (not necessarily parents) begin directly teaching children complex skills like hunting and complex tool manufacture. Adolescents seek to learn innovations from adults, but do not innovate themselves. $\textbf{Conclusion:}$ These findings support predictive models that find social learning should occur before individual learning. Furthermore, these results show that teaching does indeed exist in hunter-gatherer societies. And, finally, though children are competent foragers by late childhood, learning to extract more complex resources, such as hunting large game, takes a lifetime.Cambridge International Trust, Social Sciences and Humanities Research Council of Canada (award number: 752-2016-0555), Gates Cambridge Trus