Browsing by Author "Iremonger, Michael J."

Now showing 1 - 5 of 5
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    Crack softening damage model for ceramic impact and its application within a hydrocode
    (American Institute of Physics (AIP), 1997-12-31T00:00:00Z) Hazell, Paul J.; Iremonger, Michael J.
    A physically based crack softening damage model has been developed and used in a non-linear transient dynamic computer code (AUTODYN-2D). It is assumed that there is a finite number of orientated pre-existing flaws within the ceramic target. The mode I and mode II stress intensity factors are calculated in compression and tension and the strain energy release rate is then estimated and compared to a critical dynamic strain energy release rate. At initiation, a tension crack propagates at a velocity dependent on the mode I stress intensity factor and failure occurs in a computational cell when two neighbouring microcracks coalesce. The model was used to simulate two different plate impact experiments of alumina on alumina with encouraging results. The model was also used to analyze the impact of a steel sphere on alumina and shows strong correlation between experimental and predicted results.
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    The Effects of fibre pre-stressing on the impact performance of composite laminates
    (2011-09-19) Jevons, Matthew Paul; Iremonger, Michael J.
    This thesis has presented the results and findings of a study carried out into the effects of fibre pre-stressing on the impact performance of composite laminates. Fibre prestress has been explained as a way of mechanically altering the internal residual stress state of a composite, which typically is a result of thermal, moisture and chemical expansions. It has been suggested that pre-stressing can offer potential benefits to composites by reducing or reversing the hygro-thermal stresses in a composite. It has also been suggested that the impact performance could be improved through fibre prestressing, which has given rise to this study. In this study panels have been made with various levels of pre-stress. A special system was developed to apply pre-stress to the laminates and the produced laminates were tested under low- and high-velocity impact regimes. To apply these regimes, an instrumented falling weight and a gas gun were used respectively. A short finite element study was carried out to supplement the experimental study and offer further insight into the failure mechanics. The main findings of the study were that although pre-stressing had no discernable effect on the high-velocity impact performance of the composite laminate considered, there was a noted effect on the low-velocity impact performance. Under low-velocity impacts the laminate showed an improved impact performance for increase levels of pre-stress, except at one critical pre-stress level (60 MPa), where the laminate absorbed less energy per damage area compared with unpre-stressed laminates.
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    From capability to concept: Fusion of systems analysis techniques for derivation of future
    (Cranfield University, 2007-03-27T09:11:22Z) Sparks, Emma S.; Couldrick, Christopher A.; Iremonger, Michael J.; Allsop, Derek F.
    The intent of this thesis is to define a set of processes for use within UK Government dismounted soldier systems research that will provide stakeholders with auditable and traceable information to understand gaps in military capability and justify future procurement decisions. The need for this approach is linked to organisational shifts within the UK Ministry of Defence, and more specifically Government research with the move towards procurement of capability rather than equipment. In conjunction with reducing defence budgets and increased scrutiny, there is a need to prioritise spending to those areas that will provide the most significant enhancement to operational effectiveness. The proposed process suite provides underpinning data to support Government decisions, from definition of military need through to concept design and prioritisation of future research activities. The approach is grounded in the field of systems thinking and systems engineering providing the logical and systematic constructs required for highly complex systems where the human is a central focus. A novel fusion of existing systems tools and techniques enables both subjective data from domain experts and objective data in the form of operational analysis and field trials to be utilised for analysis across the five NATO capability domains, with output defining the relative importance of survivability, sustainability, mobility, lethality and C4I in the context of operational and strategic level military goals as well as wider challenges represented by the doctrinal defence lines of development. Future developments should include alignment with developing pan-MoD initiatives in the form of MODAF, if required by the customer organisation. This would enable generic versions of the process suite to be applied to any defence domain and problem.
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    The numerical analysis of dynamically loaded ceramic: a crack softening approach
    (Wiley, 2000-12-31T00:00:00Z) Hazell, Paul J.; Iremonger, Michael J.
    A physically based crack softening approach to modelling the failure of brittle materials that have been subjected to dynamic loading is presented and applied to a two-dimensional non-linear transient dynamic hydrocode. It is assumed that there are a number of evenly distributed and orientated micro flaws within the brittle material that are activated by a dynamically applied stress. Modes I and II stress intensity factors are calculated and compared to critical values, at which point the cracks grow at a velocity dependent on mode I stress intensity factor. The strength of the ceramic is degraded according to the length of the cracks. A simulation of a steel sphere impacting and penetrating a ceramic target at 1500 m/s is presented. Comparisons are drawn from experimental data.
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    A systems approach to the design of personal armour for explosive ordnance disposal
    (Cranfield University, 2004-11) Couldrick, Christopher A.; Iremonger, Michael J.; Gotts, Philip L.; John, Philip
    A qualitative description of the personal armour design system is elicited by comparing armour throughout the ages. Inputs that 'shape' designs are the materials technology, threat, wearer, task and environment. The emergent properties of protection, ergonomic effectiveness and financial cost form the basis of trade-offs to select final solutions. Work on the protection subsystem refines the key positive emergent property of personal armour. Existing quantifications of protection effectiveness are rejected in favour of a novel measure named the Usefulness Factor, UF. This is the first measure that accounts for the real benefit of armour. A five-stage model is proposed for the assessment of protection. Two feedback loops - due to making tasks as safe as possible and the ergonomic penalty of armour are evident. These must be considered in order to assess protection correctly. Casualty reduction analysis software (CASPER) is used to produce 'approach plots' and 'zones of usefulness' in order to make tasks safer and map the benefit of armour. This approach is demonstrated with the UK's Lightweight Combat EOD Suit against L2A2 and No. 36 Mills grenades, an HB876 area denial mine, a BL 755 sub-munition and a 105mm artillery shell. Assessment of secondary fragmentation from antipersonnel (AP) blast mines defines a threat input that is specific to Explosive Ordnance Disposal (EOD). Trials are carried out with explosive charges of 50g to 500g, buried under 5 or 10cm of stones and sand at a range of 1m. The threat is defined in terms of the probabilities of (a) being hit, (b) a hit perforating armour and (c) a hit incapacitating an unarmoured person. The chances of being hit close to the ground decrease to approximately 15% of the value when directly above the mine. Secondary fragmentation is not likely to perforate armour that protects against primary fragments. However, it is likely to incapacitate an unarmoured person. Protection is traded-off against proxies for ergonomic and financial cost effectiveness by using quantitative optimisation of personal armour. This introduces the concept of a 'protection optimisation envelope', which defines the bounds of possibility rather than a single solution. CASPER is adapted to produce weight and cost as well as incapacitation parameters. This provides a model that generates both benefits and constraints of armour. Hence, the foundations are laid for the world's first fully integrated personal armour design tools. The ergonomic effectiveness subsystem is the primary constraint of personal armour. Visor demisting for the UK's Mk 5 EOD Suit provides a simple example. Existing methods of assessment of the ergonomic penalty of armour are considered. A novel development of biomechanics computational models is proposed to predict both the mechanical and thermal burdens of armour.