Design considerations for buckling of composite cylindrical shells on elastic foundations

Cylindrical shell structures are widely used in aerospace engineering, and buckling considerations often drive their design. However, cylindrical shell structures under compression loading are highly sensitive to imperfections of different nature. Indeed, geometrical, material, loading and boundary imperfections can severely reduce their load-carrying capability.  Despite considerable research devoted to geometrical imperfections, relatively little attention has been given to the effects of boundary imperfections on the buckling load of cylindrical shell structures. However, boundary imperfections can reduce the buckling load by more than 50%. As a result, investigating both the effects of boundary imperfections and the stiffness of the cylinder's surrounding structure becomes important for more accurate predictions of their buckling loads. Therefore, this thesis addresses the effects of boundary conditions and elastic edge support on the buckling behaviour of composite cylindrical shells under axial compression. First, the buckling behaviour of a fully homogenised quasi-isotropic cylindrical shell structure under compression is investigated under several combinations of boundary conditions. Besides these considerations, three different knockdown expressions for axial, radial and tangential elastic foundations are developed to assist designers in estimating the critical buckling load of thin cylindrical shells with elastic foundations. In addition, the presence of bend/twist anisotropy effects in combination with boundary conditions can reduce the critical buckling load of quasi-isotropic composite cylindrical shells by 61%. As a result, a thorough investigation is conducted for laminates with low, medium and high levels of bend/twist anisotropy. Furthermore, new empirical formulae are devised for axial, radial, and tangential elastic foundations to compute the upper and lower bounds of the critical buckling load, representing various boundary conditions at the ends of the cylinder.  Finally, potting supports around cylindrical shells are widely used to apply a uniform compression load and avoid edge crippling during laboratory testing. Therefore, the effect of potting single-sided potting and double-sided potting was investigated and found to significantly alter the buckling response of cylinder under axial compression by changing the nature of the boundary conditions. </p

Effect of potting support design on compression buckling of composite cylindrical shells

The design of thin-walled cylindrical shells under compression loading is mostly driven by buckling considerations. However, accurate experimental evaluation of the buckling load is challenging due to small variations in boundary conditions due to manufacturing tolerances in support conditions. To test a cylindrical shell under compression loading, potting support is usually created around its bottom circumference to avoid edge crippling that could otherwise drastically reduce its critical buckling load. Therefore, investigating the effects of the potting support on the buckling response of cylindrical shell structures is important to mimic boundary conditions as close as possible to real structural constraints of the boundary. The main objective of this work is to investigate the effects of single and double-sided potting supports on the critical linear buckling load of composite cylindrical shells under compression loading. Then, this paper provides insights into understanding underlying reasons for the deviations of theoretical buckling loads from their corresponding experimental values due to boundary conditions, which can occur independently from, or in combination with, geometric imperfection sensitivity. Finally, robust linear buckling expressions that can help designers estimate the reduction due to support conditions are presented. These expressions can be used for evaluating initial safety margins for the potting support design used for testing  cylindrical shells. </p

Evaluation of offshore wind turbine leading edge protection coating failure mode under rain erosion

Offshore wind turbine blades are exposed to a wide range of environmental and loading conditions during operation. Rain droplet impact is one of the load cases that causes erosion of leading edge protection systems, which can have a detrimental effect on the performance and power output. Therefore, rain erosion is one of the major design considerations for wind turbine blades to improve the durability of leading edge protection coatings for continuous power generation and lower operational and maintenance costs. Rain droplet impact can result in several complex failure modes such as delamination of the interface between the coating and the substrate, which can significantly affect the rain erosion damage rate and the failure mode of leading edge protection. The objective of this work is to perform rain erosion testing on leading edge protection coupons in a whirling arm rain erosion test rig, CT-scan the failed coupons, and perform test correlations to develop numerical models to capture the failure modes. To do so, a single rain droplet FE parametric study will be used in this study to consider various rainfall conditions. In this research, a robust finite element modelling is developed for rain erosion that can capture the leading edge protection failure modes of wind turbine blades. The theoretical and experimental results reported in the literature are found to correlate well with the axisymmetric and 3D finite element models developed in this study. Finally, this baseline work can aid in the modelling of failure modes and analysing different coating designs for the development of more durable leading-edge protection coatings for wind turbine applications.</p