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Numerical calculations are becoming more and more efficient in estimating the lifetime of structures under thermomechanical loading. However, these life estimations cannot be reliable if the necessary parameters have not been correctly identified and measured and if all the causes of damage have not been considered. Disk testing under gas pressure is similar to oil bulging testing. However, disk testing can easily be used for the mechanical characterization of materials subject to more varied solicitations: monotone loading (biaxial rupture tests at strain rates from 10−6 to 100 s−1), constant loading under high stresses (sustained load) at elevated temperature (creep tests), cyclic loading (mechanical slow fatigue tests); the temperature may be chosen between 20 and 900 °C and the environment may be studied by comparing the results obtained with either an inert gas or reactive gas. Moreover, disk testing reveals light damage since crossing cracks through the thin membrane create leakages detected by a mass spectrometer. In this paper, we present an original method of calculation developed to determine the true mechanical properties of the pressurized disk; the method of calculation is validated because its numerical results are identical to the measured tensile properties. In addition, the range of uniform deformation is correctly determined; this property is needed to establish sheet formability which is not clearly determined by oil bulging. Of course, the mechanical behaviour can be determined within the whole ranges of temperature and strain rates; such wide ranges cannot be tested by other techniques such as tensile testing or oil bulging. As disk edges are not stressed during testing, the results are very reproducible at any temperature and at any strain rate while the machining or cutting defects initiate very scattered ruptures of tensile specimens tested at high temperature or at high strain rate. The disk and its loading simulate real applications with thin walls embedded by thick parts such as thermal exchangers or spatial engines. The analytical method of calculation may be used for identifying the needed parameters of thermomechanical modelling; it will be optimized by finite elements methods and it would allow a rational quantification of hydrogen embrittlement. [ABSTRACT FROM AUTHOR]

The fatigue of single thermoplastic fibres has been well documented to occur in a reproducible manner when they are subjected to certain cyclic loading conditions. The fatigue fracture morphologies of these fibres are very distinctive and differ markedly from other types of failure. This type of behaviour, which is clearly seen with the unambiguous tests on single fibres, must reflect behaviour of fibres in more complex structures which are subjected to cyclic loading. Only limited numbers of reports have, however, shown similar fracture morphologies with fibres extracted from fibre bundles embedded in a matrix material such as rubber. Usually the fractured ends of fibres taken from structures are seen to be shorter than those obtained in single fibre tests and also they show more complex and confused crack growth. The present study reveals that the low thermal conductivity of the fibres, exacerbated when they are embedded in a rubber matrix, leads to very high temperature rises, which is not the case in single fibre tests and under these conditions, crack initiation occurs across the fibre section instead of being restricted to the near surface region. Tests on single fibres at temperatures up to and beyond the glass transition temperature have shown how the fracture morphologies become modified. The fatigue process has been seen to become generalised throughout the fibre and failure occurs due to the coalescence of several cracks, some of which are initiated in the core of the fibre. In all cases, the cracks can be seen to have been initiated by solid inclusions in the fibres. [ABSTRACT FROM AUTHOR]

Al2O3–ZrO2 composites were sintered using low cost extended arc thermal plasma reactor and conventional heating. Composites prepared in a wide range of composition were studied in terms of their density, shrinkage, hardness, structure, microstructure and dielectric response. Experimental parameter such as sintering time, sintering temperature and plasma power were optimized to achieve higher sintered end product. Highly dense sintered products were obtained by plasma heating route within short sintering time compared with conventional sintered method. Interesting development pertaining to structure and phase evolution, structure and dielectric response are analyzed. It is found that compositional variation in this composite produces structural phase separation at different sintering conditions, which is more in plasma heating product than conventional heated product. Plasma sintered product always shows less dielectric constant as compared to conventional sintered sample. [ABSTRACT FROM AUTHOR]