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Fracture and environmentally assisted subcritical crackgrowth processes are examined in bulk Y-Si-Al-O-N oxynitride glasses with compositions typical of the grain boundary phase of silicon nitride ceramics. Both long-crack (in compact tension specimens) as well as short-crack behavior (using indentation techniques) were investigated to establish a reliable fracture toughness and to elucidate the anomalous densification behavior of the oxynitride glass. Environmentally assisted subcritical crack-growth processes were studied in inert, moist, and wet environments under both cyclic and static loading conditions. Behavior is discussed in terms of the interaction of the environment with the crack tip. Likely mechanisms for environmentally assisted crack growth are discussed and related to the subcritical crack-growth behavior of silicon nitride ceramics.

The determination of residual stress in engineering materials with large grains is a challenge when it comes to using diffraction techniques. Not only are the accuracies of the residual stresses themselves important but also the accurate evaluation of their uncertainties. An austenitic steel three-pass slot weld (NeT- TG4) with varying grain size high-lights the potential problems with the data evaluation when grain size is not taken into account whilst measuring strain. Neutron diffraction results are compared with each other (with combinations of slit and radial oscillating collimator beam defining optics) and with high energy synchrotron radiation results with a spiral slit gauge volume defining system. The impact of the grain size on the determination of residual stresses and their associated uncertainties when using diffraction techniques in engineering components is emphasized and discussed. A simple model to estimate the extra random uncertainty contribution due to the so-called grain size statistics is applied and verified. The benefit of continuous or stepwise oscillation to increase the number of detected grains on the detector is discussed together with how to optimize the time of a measurement. From the data obtained, best practice guidelines will be suggested on dealing with large grains when determining strain and stress with neutron diffraction

A nickel-base superalloy sub-scale disc has been studied using neutron diffraction to characterise the residual stress distribution following heat treatment. Strain has been calculated using measured neutron diffraction d-spacings in combination with d 0 reference values obtained using three distinct methods, namely, measurement of a stress-free cube, a sin 2 c characterisation (used independently and as a correction factor), and an axial stress balance. In addition, data obtained using the contour method have been included since it allows comparison of diffraction-based data to a technique that does not require a d 0 value. In terms of residual stress, agreement between all d 0 measurement techniques and the contour method was high, with the exception being those made using the sin 2 c technique. An offset of approximately 350 MPa was observed in these data, and a possible systematic error was ascribed to a unique diffraction characteristic of a g/g# nickel-base superalloy. A global d 0 value obtained using a stress balance or stress-free cube was found to be reliable throughout the sample bulk in cases where thermal processing was carried out at a slow rate, such as ageing. However, d 0 differences equivalent to 400 me were observed between central and surface regions when a global d 0 was applied to a water-quenched sample.