• B K Sarkar

      Articles written in Bulletin of Materials Science

    • Fatigue of brittle materials—A critical appraisal

      B K Sarkar

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      Recent demands for high performance ceramics and glass for various applications from bioceramics to cutting tools under fluctuating stress conditions has focussed attention of the scientific community towards fatigue behaviour of brittle solids. Attention to fatigue phenomena in alumina ceramics phenomenological to metals, having an endurance dependent on applied stress with a limit at around 50% of the single cycle fracture stress, was first drawn by the author in late sixties. Slip assisted fatigue process was not considered to be dominant in ceramic materials due to the absence of appreciable crack tip plasticity. With the background of this general survey of fatigue behaviour some fatigue studies based on mode of testing, theoretical and experimental analyses and fractographic evidence have been presented. Studies have shown that there is a dormant period between each successive crack advancement during which the residual stress and a plastic component is built up in a cumulative manner leading to eventual failure. During fatigue$$\sigma _{p_j } $$ (plastic) and$$\sigma _{p_j } $$ (residual stress) components are predominant for ductile metals and brittle glass/ceramics respectively.

      It is also apparent that dislocation assisted plastic component as a contributing factor in the failure of brittle materials under fatigue cannot be ruled out.

    • Static and impact fatigue behaviour of borosilicate glass

      Soumen Maity B K Sarkar

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      Failure of a borosilicate glass as a result of repeated impact has been studied. Impact fatigue study was conducted in an improved pendulum type repeated impact apparatus specially designed and fabricated for determining single and repeated impact strength. For elimination of the effect of humidity, repeated impact tests were carried out under liquid nitrogen. Quasi-static measurements were determined under four-point bending. Using a square waveform as applicable to the present impact tests and fracture mechanics interpretation, the number of cycles to failure during impact fatigue tests were predicted from quasi-static fatigue measurements. It has been shown that repeated impact loading has a deleterious effect on the failure cycles compared to slow stressing. The role of an added mechanical effect during repeated impacts has been suggested in controlling the cyclic fatigue behaviour.

    • Study on elastic-plastic component in indentation fatigue of sodalime glass

      Rajat Banerjee B K Sarkar

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      Fracture mechanics studies on glass by indentation has become predominant in recent times. Interest in it has become more prominent due to a large application of such materials in areas of engineering applications encountering fluctuating stresses induced thermally, mechanically or physically. However, glass subjected to repeated indentation at a point prior to crack initiation with subcritical loads phenomenological to metal fatigue has not been systematically investigated. Repeated indentation at a single point with different subcritical loads (0·1N, 0·15N, 0·25N, 0·50N, 1·0N) was performed till radial cracks occurred. The length of the diagonal was measured after each indentation, which was found to increase with indentation cycle eventually leading to crack initiation. This observation was analysed considering the elastic plastic component and the residual stress developed during each cycle. A mathematical model has been postulated to correlate the contribution of cumulative residual stress for crack initiation.

    • Estimation of composite strength by a modified rule of mixtures incorporating defects

      B K Sarkar

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      Inadequacy of the rule of mixtures to estimate the true values of composite properties having a large concentration of defects has led to numerous tests being performed costing time and money. Statistical average of the properties from the Weibulls distribution law has thus been relied upon so far for the design of composite structures. Yet, to estimate the properties of a fibre reinforced composite material having sufficient flaw densities due to the methodologies adopted for its processing, more so for fibres, has long been appreciated. To avoid this inadequacy a modified rule of mixtures is developed incorporating defect concentration in the fibre and matrix enabling to arrive at a more realistic value of the composite.

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