This paper discusses the nature of the damage which can cause unanticipated, early fleet failures in legacy military aircraft. This damage often takes a form quite unlike the notional cracking which is used in structural integrity design, but can lead to high costs, or reduced fleet availability, by requiring additional costly inspections and recovery programs in response to discovery of the damage. Using examples from Australian fleets the paper demonstrates that a lack of diagnostic and prognostic tools contributed to the impact of the damage on the fleet. Development of even simple prognostic and diagnostic tools could reduce the fleet impact and cost of discovered damage, and would ultimately allow the non-crack damage to be incorporated into design to achieve a much more global level of damage tolerance. The paper discusses the key differences between these non-crack damage forms and the more traditional crack-like defect which is used in current damage tolerance based structural integrity management approaches for these aircraft. These differences are associated principally with damage variability and damage location, and they challenge some aspects of our existing structural integrity design methods such as reliance on testing and analysis of supposedly 'representative' example of aircraft structure. The larger challenge is to fully exploit the principles of damage tolerant structural design and management, and the paper argues that to achieve this we need to maintain a move towards a broader, risk-based approach to structural integrity management. This longer-term goal will also involve a reappraisal of the nature and distribution of damage, and a fundamental shift in our crack-centric view of structural integrity, additional diagnostic and prognostic tools for such damage would be essential for developing this transition to a more global risk-based damage management approach.

We report an investigation of the incorporation mechanism of iron (Fe) as a dopant in GaN grown by MOVPE. A series of Fe doped GaN structures were studied by secondary ion mass spectrometry (SIMS), atomic force microscopy (AFM) and X-ray photo-electron spectroscopy (XPS). A model is presented which describes the SIMS concentration profiles with excellent agreement, and, it is shown that Fe incorporation into GaN occurs via a surface segregation mechanism. This model is supported by direct measurement of a highly Fe-rich layer on the surface of Fe doped GaN by XPS. Furthermore, we find that the presence of Fe on the GaN surface promotes a transition from 2D to 3D GaN growth which is confirmed by AFM measurements of RMS roughness with increasing Fe flux. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Specular x-ray reflectivity probes morphological changes in a crosslinkable coumarin photoalignment polymer film resulting from ultraviolet irradiation. An ordered surface layer with density oscillations compatible with planar side-chain alignment is obtained before irradiation. The ordering is enhanced in the early stages of crosslinking. This is attributed to the photoinduced increase of mobility of the side-chains resulting from the creation of free volume by the crosslinking process. The expansion of the thin film confirms that free volume is created. The surface ordering decreases with prolonged ultraviolet irradiation because of increased material viscosity resulting from a high crosslinked density. The implications of surface ordering on liquid crystal photoalignment are discussed.

Specular X-ray reflectivity measurements are used to examine photoinduced surface roughness of side-chain coumarin polymers that form liquid crystal photoalignment layers. Thin films were irradiated with polarised or unpolarised ultraviolet light and the X-ray reflectivity at grazing incidence was measured for samples irradiated with different ultraviolet fluences. Kiessig fringes are observed over a large dynamic range and a comparison of the measured and calculated reflectivity gives a value for the surface roughness. The photoinduced roughness increases with ultraviolet fluence, from 5.5 to 20 Angstrom. Negligible photoinduced roughness is detected for a film, which was previously irradiated using conditions that give strong planar liquid crystal alignment. Specular reflectivity data taken with the plane of incidence of the X-ray parallel and perpendicular to the polarisation direction of the ultraviolet light are identical within experimental uncertainty. Hence in-plane anisotropic surface roughness is not observed. (C) 2002 Elsevier Science B.V. All rights reserved.

A doped coumarin based photoalignment polymer is irradiated with polarised UV light and used to macroscopically orient a light-emitting photoreactive mesogen. This is subsequently crosslinked to form an electroluminescent nematic polymer network. The photoluminescence order parameter of the nemtatic is discussed in terms of the photoalignment conditions.

A surface anisotropy has been shown previously to be induced in thin films of photoreactive coumarin side-chain polymers by polarized UV illumination. Consequently, the resultant cross-linked polymer layers can be used as photoalignment layers for liquid crystal displays. Homogeneous alignment of a nematic liquid crystal in contact with a layer of a model coumarin side chain polymer is obtained with the director parallel or perpendicular to the UV polarization axis depending on the incident fluence. Spectroscopic analysis of the alignment layer now confirms that both photodegradation and cross-linking occur with different dependencies on fluence. Low UV fluences give parallel photoalignment and high cross-linking reactivity. However, the residual, unreacted polymer side chains show negligible anisotropy because of their freedom to move in an isotropic fashion. Hence, parallel liquid crystal alignment is attributed to a steric interaction between the liquid crystal and syndimerized side chains of the cross-linked polymer. A switch of the photoalignment direction accompanies the subsequent development of anisotropy of the intact, unreacted polymer side chains. The side-chain anisotropy and hence perpendicular liquid crystal alignment is ascribed to photodegradation rather than cross-linking.

Polarized electroluminescence from a uniformly aligned nematic network, formed by photopolymerization of a LC monofluorene with diene end-groups (see Figure), is reported. It is shown that macroscopic orientation of the chromophore is achieved with a photoalignment layer doped to allow hole transport. Thus standard photolithography can be used to make polarized, patterned, multilayer organic electroluminescent displays.

In this paper, a range of polymethacrylate derivatives of hydroxycoumarin are investigated as photoinduced alignment layers for liquid crystals, and their performance is compared with polyvinyl cinnamate. For al the coumarin- containing polymers, the liquid crystal alignment direction is parallel to the polarization direction of the incident UV light at low fluences. At a critical fluence threshold, a sharp change to perpendicular alignment is found. Molecular models and spectroscopy are used to explain this phenomenon as well as the fact that only perpendicular alignment is observed for polyvinyl cinnamate. The azimuthal anchoring energies of the alignment layers are measured and values greater than 6 by 10-5 J m-2 are found. The incorporation of a flexible spacer into the coumarin sidechain results in stronger anchoring at low fluences. Exposure of the coumarin-containing alignment layers at oblique incidence gives pretilted alignment with pretilt angles up to 5.1 degrees.

The synthesis of polymethacrylate derivatives of 6- and 7-hydroxycoumarin is described and their use as photoinduced alignment layers for liquid crystals is discussed. Strong anchoring is found with azimuthal anchoring energies > 6.9 x 10(-5) J m(-2). The liquid crystal alignment direction is parallel to the polarisation direction of the incident UV light when low fluences are incident. At a critical fluence threshold, a sharp change to perpendicular alignment is found. The incorporation of a flexible spacer into the coumarin sidechain results in stronger anchoring at low fluences. These results are discussed in terms of a molecular model.