## 2 DIC Used in Impact Studies to Measure Target Deformation
In this paper, the authors only gave the out-of-plane displacement (2.13 mm) for the CF/epoxy panel, impacted by a projectile with a velocity of 46 m/s [32]. [PERSON] et al. [33] continued their work into the response of carbon fibre reinforced polymers to impact, by using a gas gun to launch both hard and soft projectiles at CFRP panels and recording the response with 3D DIC. Two high-speed cameras, Phantom Miro M/R/LC310 (Vision Research Inc., Wayne, NJ, USA), running at 39,000 FPS were used to capture the images of the BFD during the impact, with the ARAMIS software (GOM GmbH, Braunschweig, Germany) being used for 3D DIC image processing. The extension of impacts to soft projectiles was done to simulate an event like bird strike, where the projectile is highly deformable. [PERSON] and [PERSON] suggested that a gelatin projectile, with a length to width ratio of 2 and a density of 1.03 g/cm\({}^{3}\), would be suitable as a bird stimulant for small scale laboratory tests [34]. The projectiles [PERSON] et al. [33] used were a gelatin projectile with a mass of 20 g, a diameter of 24.9 mm and a length of 37 mm and a hemispherical nosed T7075--T6 aluminium projectile, with a diameter of 24.9 mm and a length of 31.4 mm. The projectile speeds the authors used ranged from 42 m/s to 100 m/s. Their DIC measurements indicated that the CF/PEEK exhibited a maximum out-of-plane BFD of approximately 3.7 mm, when impacted by the soft projectile at an impact velocity of 100 m/s. For the same impact velocity and projectile, the CF/epoxy combination exhibited a 10% greater maximum out-of-plane deformation and a slower rebound sequence. The authors attributed this increased out-of-plane deformation and slower rebound sequence to significant damage found on the CF/epoxy panel after impact. In comparison, the CF/PEEK panels exhibited no visible damage for an impact of the soft projectile at 100 m/s. For hard projectile impacts, a projectile velocity of 50 m/s was sufficient to perforate the CF/PEEK panel. In comparison, the CF/epoxy panel was also perforated at a projectile velocity of 50 m/s, but the area of damage on the panel was approximately three times larger. In addition, the damage area did not increase in size for projectile velocities above 50 m/s for either panel [34]. [PERSON] et al. [35] conducted an investigation into the impact of a 9 mm projectile impact on a combat helmet worn by a human head surrogate. The authors used 3D DIC to measure the front face deformation, while the human head surrogate had sections filled with bio-simulant clay (specifically Roma Plastilina No.1 oil-based modelling clay). The combat helmet the authors tested was a composite, consisting of aramid-based fibres with a phenolic-based resin matrix. The projectile was a full metal jacket 9 mm pistol round, with an initial velocity of 335 m/s. The 3D DIC images were taken at 10,000 FPS and a resolution of \(1024\times 800\) pixels. No information was given about the software package used to process the images or what cameras were used. The authors show that the front face experienced a maximum dynamic deformation of 21.4 mm and affected an oval domain of approximately 62 mm short axis. The final deformation was approximately 12 mm and affected a circular area of approximately 51 mm. No indentation was seen in the clay head form, indicating that there was no perforation and that the helmet shell deformation does not exceed the standoff distance of 20 mm. Using the 3D DIC, the authors also found that the maximum velocity of the front face increases sharply to a maximum of 21 m/s in the first 1.2 ms after impact [35]. [PERSON] et al. [36] conducted a study to investigate the BFD of a 7.62x39 mm oblique hit on panels made from an Aramid fibre composite used in combat helmets. Initially, the authors used perpendicular hits on the composite panels with a 9 mm lead core projectile, to calibrate the 3D DIC system. [PERSON] et al.