Static and Dynamic Raman Spectroscopic Studies of Woven Body Armour

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Abstract

This thesis, by David Viita, submitted for the degree of Doctor of Philosophy to the University of Manchester in September 2013, entitled “Static and Dynamic Raman Spectroscopic Studies of Woven Body Armour”, studies the behaviour under strain of the high strength para-aramid polymer fibre, Twaron® and fabrics woven from it using Raman spectroscopy. These fabrics are widely used in armour and the project aimed to understand how strain developed in them during penetration. Three key conclusions were reached. Firstly, as well as shifting near-linearly with strain, the 1610 cm-1 spectral peak of Twaron broadens reversibly with strain. The broadening follows an exponential curve. Others have attributed this broadening to either molecular-level shear lag or misorientation of the pleated crystallites causing unequal loading. However, analysis of the data herein contradicts both hypotheses.Secondly, in order to understand the shear lag that occurs as fibres are strained over metal surfaces, Twaron yarns and filaments were stretched over metal rods to replicate and extend previous work by Kuo. Reasonable agreement with Kuo was found. Fabric were then strained step-wise by indenting the fabric with a bullet-shaped head. The strain in these fabrics was mapped from yarn and sub-yarn (microscale) levels through Raman spectroscopy, using the dependencies measured earlier in the thesis. The strain along yarns was found to decay with distance from the bullet, due to friction with transverse yarns. This data was initially fitted with 2D Gaussian and Voigt curves. A theoretical underpinning was then developed presenting a similar, yet distinct, behaviour to an exponential decay. Typically, the strain decayed to 36% of its peak value by 15 yarns distance (1.8 cm) from the bullet apex. The strain was also mapped in two-layer fabrics which had a misorientation between the layers. These two-layer fabrics had a more “circular” strain profile than single-layer fabrics.Finally, the measurement of strain in real time through Raman spectroscopy was considered (“dynamic strain” tests). A spectrum simulator was built in Excel® to predict how Raman spectra would appear when taken from samples which changed strain state during the measurement. This simulation produced realistic-looking spectra and were analysed in the same manner as real data. The main difference between spectra taken at a single strain and those taken with changing strain was an increase in the apparent width of the peaks due to their shifting during the measurement. Noise was added to the simulated spectra through analysis of experimental data taken at different exposure times.The simulation was then used to predict the minimum change in strain that could be measured given the signal noise. This is essential groundwork to interpreting spectra taken during a shooting test on a fabric panel. It was predicted that static curves could be distinguished from dynamic ones (0% to 3.4% strain) in spectra taken in 50 ms or less. Longer exposure times enabled slower strain-changes to be detected.The simulation was validated by collecting dynamic spectra from systems that were strained by hand turning a test rig. This data compared well to the simulation. Attempts to get better-controlled straining using the DTMA were unsuccessful, though an attachment to combine the DTMA and Raman spectrometer was successfully developed and tested.

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