University of Cambridge
Thermo-mechanical characterisation of energetic materials for additive manufacture
Abstract
dc:description.abstractThis thesis presents an investigation of the additive manufacture of energetic materials such as polymer bonded explosives and composite propellant. The work focuses on the use of extrusion methods to print the materials as uncured pastes comprising a high volume fraction of solid energetic particles dispersed in a liquid polymer resin. Initially, a new framework for considering the "printability" of energetic materials is proposed, in which different physical and rheological properties of the material are assessed against different unit processes used in additive manufacturing techniques. A series of experiments are then described in which methods have been developed to investigate some of these properties. These methods were then applied to various energetic and model materials. The first piece of experimental work presented describes the design of an apparatus for measuring the extrusion properties of an energetic paste using the Benbow-Bridgwater model for paste extrusion. This apparatus and method were then applied to a developmental energetic formulation in collaboration with the Defence Science and Technology Laboratory. These experiments yielded a parameterised extrusion model for the formulation as well as a number of other useful observations. A second series of experiments is then discussed in which methods were developed to further investigate the extrusion process as a means of both rationalising the results of extrusion experiments and better understanding how the formulation of materials might affect the process. These experiments included tensile testing of individual printed strands, visualisation of static zones formed in the extrusion barrel and an investigation of particle size effects on flow through the extrusion nozzle. X-ray micro computed tomography of the extrudate structure was also undertaken. Finally, an experimental investigation of surface energy in polymer binders for energetic formulations is discussed. Surface energy is an important parameter in energetic formulations generally as well as in the printability framework presented. The method developed uses a Wilhelmy plate with a discrete time step approach to better investigate the rate dependency observed in the experiment. Following the presentation and discussion of the experimental work undertaken, overall conclusions are presented and discussed with a particular emphasis on the implications for additive manufacturing techniques applied to energetics. Future avenues of research are also recommended. The key findings and achievements of this research include: - The introduction of a framework to describe the interaction between the material properties of energetics and additive manufacturing via material extrusion. To the author’s knowledge, this is the first such framework proposed and represents an important step in understanding material printability and the factors affecting it. - The development of an approach to extrusion analysis as relevant to the additive manufacturing of energetics. Again, this is believed to be the first such work in this field. It resulted in an extrusion model for a new high explosive formulation as well as a number of qualitative insights into the extrusion process. - The expansion of the extrusion analysis to include simple 3D prints, allowing samples to be produced with a full extrusion history. This capability was demonstrated by producing single printed strands of an inert material for tensile testing, allowing comparison of extrusion speed, extrusion pressure and tensile strength. The results suggested that tensile strength decreased with increasing print speed. - The development of a new visualisation technique to directly observe static zones formed during extrusion, demonstrated with an inert material. - The observation, using extrusion experiments and x-ray computed tomography, of a particle size effect on the pressure drop in barrel-die extrusion, separate to the effect of particle packing. A layered flow structure in the particles close to the die wall was also observed and characterised. Based on these observations, the idea of an apparent maximum extrudable solid volume fraction, related to flow structure, was introduced and discussed. This is of particular importance in the formulation of energetic materials for additive manufacturing applications. - Measurement of the surface energy in different polymer resins using in energetic formulations for additive manufacturing, including their polar and dispersive components which had not been previously reported. The developed method also provided new insight into how the apparent surface energy measured varies with rate in these experiments.
Degree
thesis:*- Name dc:type.qualificationname
- Doctor of Philosophy (PhD)
- Level dc:type.qualificationlevel
- Doctoral
- Grantor dc:publisher.institution
- University of Cambridge
- Year dc:date.issued
- 2023
Author and committee
dc:creator, dc:contributor.*- Author dc:creator
-
- O'Donnell, Michael
- Advisor dc:contributor.advisor
-
- Williamson, David
Subjects
dc:subject × 8Rights
dc:rightsIdentifiers
dc:identifier.*- DOI dc:identifier.doi
- https://doi.org/10.17863/CAM.109543
- OAI identifier oai:identifier
- oai:www.repository.cam.ac.uk:1810/369953