## 2 DIC Used in Impact Studies to Measure Target Deformation
[22] examined the dynamic response of a panel composed of triangular prisms made from alumina ceramic, encased in aluminium with an outer casing of UHMWPE fibre reinforced laminates, shown in Figure 3. Schematic diagram of the target used in [PERSON] et al. [22] To measure the back face deflection, the authors removed a section of the UHMWPE fibre reinforced laminate \(100\times 100\) mm and covered this region of bare aluminium in a speckle pattern, to allow for 3D DIC. The authors used a pair of SA-X2 Photron cameras (Photron USA, Inc., San Diego, CA, USA) to obtain the high-speed imagery and the ARAMIS software (GOM GmbH, Braunschweig, Germany) to process the images. For the DIC testing, the authors used a powder gun, which was capable of launching the 12.7 mm diameter spherical projectile at speeds of up to 2.3 km/s. For the prism base impact, the authors show that when impacted by the projectile at 1.71 km/s, a small bulge formed 4 us from impact, but that the bulge was longer in the longitudinal direction (along the prism long axis) than the transverse direction. Around the time of peak deflection, the dimensions of the bulge and its velocity increased, and a y-oriented tear in the face sheet started to develop below the node of the centrally impacted cell. The authors measured the peak deflection at around 6 mm before ejecta exited the tear. Using the high-speed imagery, the authors were able to determine the plume of debris had a blunt front and was about 44 mm wide in the y-direction, consistent with the dimension of the face sheet tear opening. The authors were able to use the DIC to determine the velocity at the most deflected location, reaching 0.30 km/s for the 1.71 km/s projectile speed and 0.45 km/s for the 2.29 km/s projectile speed. For apex impacts, the authors reported that a pair of bulges occurred on either side of the base of the apex of the ceramic prisms. By 10 us, the peaks had merged into a single 20 mm wide transverse front. From this bulge, two y-oriented cracks formed, allowing a portion of the rear face sheet under the impacted prism to be torn away. The authors measured the residual velocity and found it to be approximately double that of the prism base impact [22]. Further research into the deformation of UHMWPE composites with the aim of preventing behind helmet blunt force trauma was conducted by [PERSON] et al. [23] The authors undertook this study due to UHMWPE fibres exhibiting large strains to arrest projectiles and a need to characterise the ballistic response of different UHMWPE fibres. The authors recorded the BFD with two Photron SA5 cameras (Photron USA, Inc., San Diego, CA, USA) recording at between 40,000 and 50,000 FPS and using the ARAMIS software (GOM GmbH, Braunschweig, Germany) to perform the DIC image processing. The authors examined several different UHMWPE fibres, Dyneema HB25 and HB80, and Spectra Shield II SR-3136 fibres, and used a woven aramid K705 laminate panel as a baseline. All of the panels were made to have the same areal density, 7.8 kg/m\({}^{2}\). To determine the V50 of the panels, the authors used a 0.22 caliber fragment simulating projectile and used a 12.7 mm steel sphere projectile to determine BFD (with a lower velocity to prevent perforation). Their results show that the ARL X hybrid panels constructed with Dyneema HB25 fibres exhibited a 10% drop in resistance to penetration, but a greater than 45% decrease in BFD. In comparison, their experiments using the Spectra Shield II SR-3136 fibres exhibited no decrease in resistance to penetration, but still retained a 40% decrease in BFD. Using the 3D DIC, the authors show that the peak BFD occurs after the initial impact, and is larger than the final BFD, indicating some elastic recovery. The ARL X hybrid panels exhibited larger lateral spread of deformation and delamination, but smaller out-of-plane deformation [23]. [PERSON] et al.