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The sintering behaviour of silicon nitride ceramics has been investigated by preparing powder mixtures containing controlled amounts of yttria and alumina. The fired density of the compacts, having a fixed quantity of yttria, has been observed to increase with increasing amounts of alumina added to the powder. There exists, however, a maximum limit to the beneficial effect of AI203, because an excess of alumina in the powder mixture does not produce a further increase in density. Sintering of green compacts, obtained by uniaxial pressing, has been carried out at various temperatures and pressures in order to determine the relationship between fired density and the processing variables. The results obtained, after subjecting a selection of specimens to different sinter-HIP cycles, clearly show that the final density is very sensitive to the applied pressure and the time at which it is applied. The range of microstructures produced by different processing routes has also been followed by electron microscopy. From the results obtained by X-ray diffractometry it is shown that the additions of yttria and alumina not only produce denser specimens than pure silicon nitride, but also promote the ~-fl transformation.

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The growth rate of fatigue cracks in two single crystal nickel base superalloys, CMSX-6 and SRR99, along the (001) planes are presented and rationalised in terms of two interacting crack propagation mechanisms: one attributed to crack tip plastic blunting and the other attributed to the brittle failure of the oxide scales. The role of the oxide scale is twofold as it also wedges the crack and modifies the blunting term through a crack closure effect. On the other hand, a positive effective stress intensity range is required to fracture the oxide scale. Fatigue tests were carried out at different temperatures (500 to lO5O0C), frequencies (0.001 to 20Hz), cycle waveforms and load ratios (0 to 0.9), with starter crack lengths of abut 100 pm. The model predictions match the crack growth rates obtained for both materials. Even though both materials are nickel base superalloys, they have very different oxidation behaviours. CMSX-6 has an improved oxidation resistance over SRR99, however, because of the twofold nature of the oxidation process, which material provides the better life expectancy depends on the applied test temperature and loading cycle. NOMENCLATURE a = size of the major defect in the oxide scale d = twice the compacted oxide scale thickness A, B, C, = constants (da/dN) = crack growth per cycle (da/dN),*, =fatigue component of crack growth per cycle (da/dN)oi&tion = oxidation component of crack growth per cycle (da/dN),,, = creep component of crack growth per cycle (da/dt),,,,, = oxidation crack growth rate Do = constant in oxide growth law e = oxide scale thickness E = Young's modulus f = cycle frequency K = stress intensity factor KCL = stress intensity factor at crack closure K E = oxide fracture toughness K-, K,, = maximum and minimum stress intensity factors n =hardening exponent no = constant in fatigue crack growth law n* =blunting coefficient Q = activation energy in oxide growth law R = minimum to maximum load ratio (also gas constant = 8.314 J/mol K) t = time = time to failure of the oxide scale t , = cycle period t* = estimate of tfd, T = absolute temperature a = constant in oxide growth law 6 = crack opening displacement 771 112 J. M. MART~NEZ-ESNAOLA et al. AK, AKeR = stress intensity factor range and effective stress intensity factor range v = Poisson's ratio rrWc = cyclic yield stress u,, = yield stress

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RESUMENEn el presente artículo se intenta definir una propuesta de clasificación de los episodios de temporal de lluvia y viento en Galicia atendiendo a su intensidad y potencial peligrosidad, en la que se tendrá en cuenta la combinación de tres factores: precipitación media diaria del episodio, valor medio de las rachas máximas de viento y duración del evento. Asimismo, se analiza la evolución cronológica de los episodios de temporal en los últimos 40 años (período 1961-2001).Palabras clave: Temporal de lluvia y viento, intensidad, peligrosidad, Galicia. RÉSUMÉDans ce travail on prétende definir une classification des épisodes de têmpete de pluie et vent en Galice, selon leur intensité et leur dangereux; nous avons combiné trois facteurs: volume moyen de pluie du episode, valeur moyen des rafales maximals du vent et duration du episode. On analyse aussi l´évolution cronologique des épisodes de têmpete dans les derniers 40 ans .Mots clés: têmpete de pluie et vent, intensité, peligrosité, Galice. ABSTRACTIn this paper we try to define a classification of storms of rain and wind in Galicia with regard to their intensity and potential dangerousness, bearing in mind the combination of three factors: daily average precipitation in the episode, average value of the greatest gusts of wind, and duration of the event. Also, we focus on the chronological evolution of these events the latest 40 years (period: 1961-2001).