Characterisation of tack for automated tape laying

Crossley, Richard James (2011) Characterisation of tack for automated tape laying. PhD thesis, University of Nottingham.

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Automated Tape Laying (ATL) trials using low cost wind energy suitable material and mould tools have been conducted. New materials proved problematic during ATL lay-up and observations of the ATL process show that the prepreg tack and stiffness properties significantly affect lay-up performance. Prepreg tack has not been widely researched within the composites industry due to the absence of a standardised method for characterisation. A new tack and stiffness test has therefore been developed which is representative of the ATL process. The new test was used to investigate the response to process and material variables. Two failure modes were observed and compared to those found in Pressure Sensitive Adhesives (PSA). Failure modes are associated with the viscoelastic stiffness of the resin. High stiffness appears to result in interfacial failure turning to cohesive failure when stiffness is reduced. A peak in tack is observed to correspond with the transition in failure mode leading to the conclusion that prepreg tack is the result of a chain system rather than a single property. The chain system consists of an interface and bulk components each having individual time and physical variable dependant properties.

Tack and stiffness is shown to conform to the Williams-Landel-Ferry (WLF) time-temperature superposition principle for both cohesive and interfacial failure modes. Cohesive viscoelastic and surface energy interface failure mechanisms may be theoretically linked via the Lennard-Jones energy well with molecular jumps triggered by thermal vibrations. This analogy allows both failure phenomenon to simultaneously follow the time temperature superposition principle and is typically demonstrated in LJ dynamic mechanical modelling. The theoretical analogy is used in the explanation of experimental results where tack is essentially thought of as a low energy non-covalent molecular bond or reaction.

The experimental technique developed here could allow for the standardisation of tack and stiffness specification for manufacturers. The application of results to ATL production is explored and demonstrated using ATL equipment. The results show that optimum lay-up conditions may be explored offline using the new tack and stiffness test. Results also show promising signs that the WLF relationship could be exploited to greatly Increase lay-up speed and consistency, increasing the attractiveness of the process to wind turbine blade manufacturers. A theoretical results curve is also presented which may allow manufacturers to determine the effect of changes in surface conditions and resin properties on tack.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Schubel, P.
Warrior, N.A.
Keywords: Wind turbine blades, Tack, Stiffness, Materials testing
Subjects: T Technology > TJ Mechanical engineering and machinery
Faculties/Schools: UK Campuses > Faculty of Engineering > Department of Mechanical, Materials and Manufacturing Engineering
Item ID: 13983
Depositing User: EP, Services
Date Deposited: 11 Feb 2014 09:51
Last Modified: 28 Apr 2018 00:50

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