No description available.

No description available.

No description available.

No description available.

This work describes the effect of microstructure on the behaviour of small fatigue cracks in the two Al-Li-Cu-Mg-Zr alloys 8090 and 8091. The slip distribution in these alloys was varied through thermo-mechanical processing. Although slip distribution affected tensile, cyclic and long fatigue crack behaviour, it did not affect small fatigue crack behaviour due to low levels of closure and reduced slip reversibility near to a free surface. Similarly, underageing and overageing had no effect on small fatigue crack behaviour. Therefore, small fatigue crack growth is concluded to be insensitive to precipitate size and distribution. Due to differences in grain size, small fatique cracks in 8091 were observed to propagate twice as slowly as in 8090. Small fatigue cracks grew more rapidly than long fatigue cracks under the same nominal K and displayed no threshold behaviour. This was shown to be due to the combined effects of the low closure levels, high applied stresses and proximity to the surface. Corrections to K to account for these factors were shown to be less effective than using J as a correlating parameter. None of these factors affected the suitability of K for characterizing microstructural effects. Small and long fatigue crack data were comparable when plotted versus calculated plastic zone size. Low cycle fatigue behaviour was shown to be related to small fatigue crack behaviour and similar microstructural effects were observed. Total rather than plastic strain was identified as the critical parameter when comparing small fatigue crack and LCF behaviour. Small fatigue crack growth behaviour was shown to be inconsistent with the plastic work accumulation criterion for the exhaustion mechanism of fatigue. Finally, because of low K's and the lack of closure, fatigue mechanisms may be easier to study in small cracks than in long cracks. (D91005)

No description available.

No description available.

Mémoire numérisé par la Direction des bibliothèques de l'Université de Montréal.

Mémoire numérisé par la Direction des bibliothèques de l'Université de Montréal.

The air transport markets affected by global economic climate and regional demand characteristics are evolving fairly dynamically. To cope with the evolving demand and to penetrate desired market segments, operators' usage of aircraft has been getting more and more dynamic with increased deviation from the originally designed mission objectives. The objective of this research is to investigate the effects of in-service rrusslon variation on aircraft structural performance so as to provide airlines with a more realistic approach toward better fleet management. During the research, the scenarios and phenomena causing fleet in-service mission variations were investigated. A survey on the utilisation of world-wide aircraft fleets was conducted and presented. The in-service missions of aircraft fleets were found to vary significantly in terms of mission profile. Furthermore, the utilisation patterns of individual aircraft in the same fleet of a major airline are also being thoroughly analysed. Tremendous variations of mission mixes are found among individual aircraft. In order to analyse the effects of the in-service mission variation, methodologies and models based on fatigue test results have been developed by the author. Actual service data are being input for the effect analysis. It is found that the mission variation has a considerable engineering influence on the aircraft's structural performance either in terms of mission airworthiness or fatigue life span. Finally, a conceptual model, the 'Integrated Airline Fleet Management Model' (IAFM), has been developed as a blueprint for practical application. By implementing the IAFM, airline will be able to obtain a realistic picture of the health of its aircraft. With a more reliable basis for maintenance planning, improved inspection accuracy, reduced maintenance cost and better structural airworthiness can be achieved.