## 2 DIC Used in Impact Studies to Measure Target Deformation
A noise floor test demonstrated that their system had a noise floor of 0.6% of their maximum expected deflection [14]. The physical quantity that properly expresses the capacity to do work on tissue and cause blunt trauma from blunt impact is kinetic energy [18]. The ARAMIS software (GOM GmbH, Braunschweig, Germany) was able to provide instantaneous velocity of the back face of the helmet, allowing them to calculate the kinetic energy that the back face could impart onto the skull when it deforms to the helmet standoff distance [14]. Furthermore, 3D DIC has also been used to characterise the response of low velocity impacts on thin plates [19]. The purpose of the research conducted by [PERSON] et al. [19] was to assist in the design of explosive reactive armour (ERA). ERA consists of an explosive material sandwiched between two metal plates. When impacted, the shock waves generated can lead to the detonation of the confined explosive, pushing the plates apart and deflecting the projectile. By using chrome steel spheres fired at approximately 70 m/s at thin plates of copper and mild steel sheets, the authors were able to examine the early stages of penetration. The authors show that the early stages of penetration are typically dominated by ductile hole enlargement, where the target material is displaced laterally or backwards. Due to the significant plastic deformation in the plates, the authors reported that the paint coating with the speckle pattern was often damaged by forming radial cracks or spalling off between the radial cracks, highlighting the importance of choice of paint for high strain rate events. A sub-image size of \(32\times 32\) pixels and step size of 8 pixels gave a typical error that the authors show was approximately \(\pm 0.1\) mm for in-plane deflections and \(\pm 0.05\) mm for out-of-plane deflections. Given the low numbers of frames that some high-speed cameras can capture, the triggering mechanism is very important. [PERSON] et al. attempted to use two thin layers of electrically insulated brass on the point of impact as a trigger, but found that it had a significant effect on the plate response, highlighting the importance of non-invasive camera triggering mechanisms. The authors did report that while larger sub-image sizes gave smoother results, due to better signal-to-noise ratio and larger overlap between adjacent sub-images, it did reduce the spatial resolution, leading to under- or over-predicted displacements where the strain is non-uniform across the sub-image [19]. [PERSON] et al. [16] from the US ARL used a pair of Photron SA2 cameras (Photron USA, Inc., San Diego, CA, USA) to obtain 3D DIC measurements of the back face displacement of hybridized and thermoplastic laminates when impacted with various projectiles. The authors used the ARAMIS software (GOM GmbH, Braunschweig, Germany) for the processing of the high-speed imagery to obtain the deflection fields. This particular study was aimed at the reduction of back face deformation, through the use of hybridized materials or the clever use of architecture and fibre orientation. A baseline of laminates with an orientation of [0\({}^{\circ}\)/90\({}^{\circ}\)] fibres of UHMWPE (specifically SpectraShield II 3130 and Dyneema) was used, with new versions of laminates laid up in a quasi-isotropic manner, with every two plies rotating clockwise \(22.5^{\circ}\) (referred to as multi-orientation). [PERSON] et al. did ensure that all of the panels had the same areal density (10.74 kg/m\({}^{2}\)). The panels were impacted with a 5.56 mm diameter 440C steel ball bearing, at a velocity of 405.7 \(\pm\) 6.7 m/s and 9 mm diameter, and a 124-grain full metal jacket round at a velocity of \(473.13\pm 3.6\) m/s. Using the 3D DIC methodology, the authors show that the back-face deformation was reduced significantly with the multi-orientationpanels, but it also reduced the panel impact resistance.