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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)

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The performance of the Acoustic Emission (AE) technique was investigated to establish its reliability in detecting and locating fatigue crack damage as well as distinguishing between different AE sources in potential SHM applications. Experiments were conducted to monitor the AE signals generated during fatigue crack growth in coupon 2014 T6 aluminium. The influence of stress ratio, stress range, sample geometry and whether or not the load spectrum was of constant or variable amplitude were all investigated. Timing filters were incorporated to eliminate extraneous AE signals produced from sources other than the fatigue crack. AE signals detected were correlated with values of applied cyclic load throughout the tests. Measurements of Time difference of arrival were taken for assessment of errors in location estimates obtained using time of flight algorithms with a 1D location setup. It was found that there was significant variability in AE Hit rates in otherwise identical samples and test conditions. However common trends characteristic of all samples could be observed. At the onset of crack growth high AE Hit rates were observed for the first few millimetres after which they rapidly declined to minimal values for an extended period of crack growth. Another peak and then decline in AE Hit rates was observed for subsequent crack growth before yet another increase as the sample approached final failure. The changes in AE signals with applied cyclic load provided great insights into the different AE processes occurring during crack growth. AE signals were seen to occur in the lower two-thirds of the maximum load in the first few millimetres of crack growth before occurring at progressively smaller values as the crack length increased. These emissions could be associated with crack closure. A separate set of AE signals were observed close to the maximum cyclic stress throughout the entire crack growth process. At the failure crack length AE signals were generated across the entire loading range. Novel metrics were developed to statistically characterise variability of AE generation with crack growth and at particular crack lengths across different samples. A novel approach for fatigue crack length estimation was developed based on monitoring applied loads to the sample corresponding with generated AE signals which extends the functionality of the AE technique in an area which was previously deficient. It is however limited by its sensitivity to changes in sample geometry. Experiments were also performed to validate the performance of the AE technique in detecting and locating fatigue crack in a representative wing-box structure. An acousto-ultrasonic method was used to calibrate the AE wave velocity in the structure which was used to successfully locate the 'hidden' fatigue crack. A novel observation was made in the series of tests conducted where the complex propagation paths in the structure could be exploited to perform wide area sensing coverage in certain regions using sensors mounted on different components of the structure. This also extends current knowledge on the capability of the AE technique.

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