## 2 DIC Used in Impact Studies to Measure Target Deformation
In comparison, a non-contiguous layer of lead granules, together with about the same mass of 1.5 mm diameter particles, gave more acceptable speckles for DIC purposes. The X-ray impact recording was triggered by the steel ball shorting out a thin, flexible Mylar/copper printed circuit tape, bonded to the outside of the aluminium tube. The authors modified the delay time from the trigger for a number of repeats of the same experiment, to give a sequence of images throughout the impact event. Due to a lack of easy optimization of the speckle size and contrast, the authors decided to compare three different correlation techniques on the X-ray images. The three correlation methods were: (i) the Vic-2D (Correlated Solutions, Inc., Irmo, SC, USA) commercial software package, (ii) making use of a fast Fourier transform to change the data to the frequency domain, with sophisticated sub-pixel image shifting to optimize the overlap between sub-images and (iii) also making use of a fast Fourier transform to perform the correlation, but utilizinga simple bi-cubic fit to find the maximum correlation peak, to find the average displacement between sub-images [20]. [PERSON] et al. found that all three correlation methods produced some valid data from the sub-optimal speckle patterns around the projectile in the case of the solid sample, but method (ii) produced poor results around the impact region for the case with the 'burn' hole [20]. The aim to develop better materials for ballistic helmets is predominantly focused on weight reduction with the maintenance of ballistic protection performance. This reduction in weight often comes with thinner materials, which can lead to greater back face deformation, and potentially increase behind helmet blunt force trauma [21]. This leads to future helmet design needing to balance weight, ballistic performance and back face deflection. [PERSON] et al. [21] examined a set of seventeen different composite materials, measuring ballistic limit and dynamic back face deflection through the use of 3D DIC. The projectiles that the authors used were a 9 mm full metal jacket, 0.30 caliber fragment simulating projectile and a 0.22 caliber fragment simulating projectile, which form what is considered to be a traditional set of small arms or fragmentation threats. There were a number of characteristics that the authors obtained from the DIC measurements. These were: (i) dynamic deflection time history, (ii) back face velocity time history, (iii) strain time history, (iv) spatial distributions, and (v) back face shape during loading and unloading. The authors used a pair of Vision Research Phantom v710 cameras (Vision Research Inc., Wayne, NJ, USA), running at 30,000 FPS and the ARAMIS software (GOM GmbH, Braunschweig, Germany) for image processing. [PERSON] et al. reported on the full-time history of the back face deformation for four target materials; Dyneema HB80, Spectra Shield 3130, Spectra Shield 3124 and Tensylon HTBD. The authors reported that the Dyneema and two Spectra Shield targets respond to ballistic impact over a longer time frame in an elastic mode similar to that of fabric armour. The authors describe this response as taking the form of a single longitudinal displacement wave that propagates relatively slowly towards the boundaries of the target. In contrast, the Tensylon target exhibited a more complex waveform over an equivalent time frame. This more complex waveform is attributed to localized failure in the region of the impact site dampens the deflection wave. This has the effect of reducing the back-face deformation, but also lowering the V50 velocity as well. The deformation in the Tensylon back face is localized to the impact region and has a significantly faster transmission of the deflection wave form, which indicates a stiffer material [21]. The use of 3D DIC is not just isolated to the measurement of purely composite panels. [PERSON] et al.