Gfj Hill

Finite element analysis of epitaxial lateral overgrown GaN: Voids at the coalescence boundary

We report on the finite element analysis of stress distribution at the coalescence boundary in epitaxial lateral overgrown (ELO) GaN related to voids. Different void geometries were considered in our model to investigate the influence of their size/shape on the stress distribution. Large compressive stress is localized in the vicinity of the voids, also an increased tensile stress is present at the corners of the SiN mask. Confocal micro-Raman mapping experiments confirm the presence of increased stress at the coalescence boundary of ELO GaN.

Design of composite helicopter rotor blades to meet given cross-sectional properties

This paper examines the design of a composite helicopter rotor blade to meet given cross-sectional properties. As with many real-world problems, the choice of objective and design variables can lead to a problem with a non-linear and/or non-convex objective function, which would require the use of stochastic optimisation methods to find an optimum. Since the objective function is evaluated from the results of a finite element analysis of the cross section, the computational expense of using stochastic methods would be prohibitive, It is shown that by choosing appropriate simplified design variables, the problem becomes convex with respect to those design variables. This allows deterministic optimisation methods to be used, which is considerably more computationally efficient than stochastic methods. It is also shown that the design variables can be chosen such that the response of each individual cross-sectional property can be closely modelled by a linear approximation, even though the response of a single objective function to many design parameters is non-linear. The design problem may therefore be reformulated into a number of simultaneous linear equations that are easily solved by matrix methods, thus allowing an optimum to be located with the minimum number of computationally expensive finite element analyses.

Analysis of anisotropic prismatic sections

The dynamic behaviour of rotor blades is often modelled using one-dimensional beam analysis with equivalent mass and stiffness properties to those of the full blade. Calculation of accurate elastic stiffness terms for these arbitrarily shaped sections with differing material properties is vital to this process. A method which produces these properties using standard finite element analysis codes is presented. The method is then compared with theoretical results for a simple rectangular section beam and case studies are performed on a composite laminate and box-section.