Thesis

Metallurgical and mechanical property assessment of advanced grid stiffened structures produced by a modified rolling process

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Awarding institution
  • University of Strathclyde
Date of award
  • 2023
Thesis identifier
  • T16560
Person Identifier (Local)
  • 201984089
Qualification Level
Qualification Name
Department, School or Faculty
Abstract
  • Advanced grid stiffened structures (AGS) are lightweight structures with a high strength to weight ratio, originally developed for the aerospace industry. Isogrid is a type of AGS which has been employed in a number of high-profile projects from Atlas rockets to the recent James Webb space telescope. The utilisation of AGS structures throughout wider industry is an opportunity to reduce the demand for raw materials and their associated environmental impact; however, the conventional method of AGS production by computer numerically controlled (CNC) milling is time consuming, costly, and creates a high level of material waste. To address this, a modified rolling process was conceived to manufacture Isogrid which is faster with low material waste. This thesis records the experimental development of the novel rolling method and reports the body of scientific data generated during the microstructural and mechanical assessment of its output. The microstructural evolution during this novel rolling process was characterised via a combination of optical microscopy, electron backscatter diffraction and microhardness measurement. The varying degrees of deformation required to form the Isogrid geometry produced areas of greater and lesser cold work, with commensurate levels of hardness. Rolling Isogrid’s asymmetrical geometry also generated shear deformation textures alongside standard rolling textures. Mechanical testing in tension and bending was performed to explore the impact of the microstructural findings. This work demonstrated that rolled Isogrid is stronger than its CNC milled equivalent in tension and bending and stronger than its billet material in bending. Rolled Isogrid was found to be isotropic in the thickness direction for out of plane loading and more isotropic than the CNC milled Isogrid for in plane loading. Comparisons of both sample groups in their as produced and post-annealed condition revealed the degree to which cold work had strengthened each sample. Fatigue testing was performed to assess the influence of rolled Isogrid’s microstructural features on its fatigue performance. Rolled Isogrid performed poorly in comparison to CNC milled Isogrid under fatigue loading. The rolled Isogrid performed poorly in fatigue relative to CNC Isogrid, which was attributed to microstructural inhomogeneity after SEM fractography and supplementary fatigue testing of rolled specimens in the post-annealed condition.
Advisor / supervisor
  • Galloway, Alexander
  • Toumpis, Athanasios
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Note
  • This thesis was previously held under moratorium from 25/04/2023 to 25/04/2025.
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