Imperfection-insensitive rapid tow sheared rocket launch structures

: a design, build, and test of an imperfection-insensitive composite cylinder

Abstract

Composites are utilised in the design of state-of-the-art aerospace and energy structures for weight and performance advantages. A good example of a structure where the conflicting requirements of weight and performance must be balanced is a thin-walled monocoque rocket launch structure: the architecture must be stable under axial compression while maintaining low mass to increase payload capacity. However, axially-compressed thin-walled monocoque cylinders are sensitive to manufacturing and loading imperfections and often buckle well before classical theory would predict, leading to either stiffened-shell designs that increase fabrication costs or the use of conservative safety factors that add mass to the structure. Hence, the dilemma of choosing between heavier conservative guidelines and costlier manufacturing complexity makes monocoque thin-walled composite structures an attractive option if the associated imperfection-sensitivity can be mitigated.

In the work herein, a design-build-test campaign of an imperfection insensitive cylinder is accomplished through the use of variable-angle tow composites manufactured using Rapid Tow Shearing (RTS). RTS allows fibres to be sheared in curvilinear paths instead of straight paths and therefore opens avenues towards stiffness blending and tailoring of load paths. RTS also exhibits a fibre-angle-thickness coupling that increases the local thickness of the tow when sheared, enabling the in-situ embedding of stiffening features. This thesis explores the viability of using the non-uniform stiffness field created by RTS as a means to alleviate imperfection sensitivity of axially-compressed thin-walled cylinders.

To achieve an imperfection-insensitive RTS design, the present work first explores the influence of design parameters on the mechanical response of RTS-designed cylinders under axial compression both with and without geometric imperfections. It is shown that the influence of shearing periodicity, angle, and direction can be significant both in terms of linear buckling load and imperfect-geometry nonlinear buckling load. Following the initial parametric study, a novel imperfect-geometry optimisation is described and implemented as a method to optimise towards an imperfection-insensitive cylinder. The optimisation aims to maximise the imperfect geometry nonlinear buckling load—the load achieved by the cylinder with nonlinear geometry active within the FE environment with geometric imperfections present—with imperfections derived from eigenmodes and includes axial stiffness constraints. It is found that some RTS cylinder designs are both more imperfection-insensitive and have a higher imperfect-geometry buckling load than a conventional straight-fibre design. A robust RTS cylinder design is chosen from the optimisation results and manufactured, along with a quasi-isotropic (QI) cylinder, to enable a comparison between the optimised RTS architecture and the theoretically best (perfect geometry) straight-fibre design. An axial-compression experiment is successfully conducted and nonlinear finite element predictions are within 3% and 4% of experimental results for the RTS and QI cylinder, respectively. Compared to the QI cylinder, the RTS cylinder has a 10% greater experimental buckling load and, when imperfections are equal across both cylinders, the RTS shell has an FE-predicted mass-normalised buckling load 10% greater on average than the QI cylinder. The thesis concludes with a probabilistic, reliability-based optimisation framework based on the first-order second moment method coupled to a genetic algorithm and includes axial stiffness constraints. The optimisation is applied to both straight-fibre and RTS designs and demonstrates further possible improvements in both imperfection-insensitivity and imperfect-geometry buckling loads. These additional straight-fibre and RTS designs provide ample opportunity for further experimental research.

Date of Award	21 Mar 2023
Original language	English
Awarding Institution	University of Bristol
Supervisor	Paul M Weaver (Supervisor), Alberto Pirrera (Supervisor) & Rainer Groh (Supervisor)