## 2 DIC Used in Impact Studies to Measure Target Deformation  
Many armours are designed with an extremely hard strike face to defeat an incoming projectile. These materials are usually technical ceramics, such as aluminium oxide, silicon carbide, boron carbide and tungsten carbide [11]. These hard materials tend to spall during impact, leading to the requirement of a backing plate constructed of a softer material; usually a composite material constructed with polymer fibres, such as ultra-high-molecular-weight-polyethylene (UHMWPE), encased in an epoxy resin. Some other armours are constructed solely of the UHMWPE composite materials, such as combat helmets. The large deformations/strain rates that these composite plates undergo during a ballistic impact event are ideal applications for high- and ultra-high-speed 3D DIC. In 2004, [PERSON] et al. [12] reported on several projects being undertaken utilizing 3D DIC in impact testing, as a part of the NASA return to flight initiative. One of the crucial aspects of the NASA return to flight initiative is the extensive impact testing of foam projectiles on various orbiter subsections. As a precursor, their initial testing was focused on gas gun testing of coupons of the composite materials (6-inch reinforced carbon/carbon), being impacted by foam projectiles. The authors used a frame rate of 26,900 fps and were able to capture the entire initial deflection and recovery period and it agreed well with their computational calculations. With this success, the authors moved onto impacts on the orbiter wing leading edge. Due to very limited space within the wing leading edge, the authors used mirrors within the leading edge to lengthen the effective working distance from the cameras to the interior surface. The researchers determined that so long as the mirrors moved as a rigid body, with no local deformations, the authors concluded it would be the same as if there was no mirror. The authors showed that the noise floor of their setup had an accuracy of approximately 50 \(\upmu\)m. With the mirrors, the authors were able to successfully capture the displacements from a local deformation at the impact point to a global buckling mode [12]. The US Army Research Laboratory (ARL) has had an interest in the use of 3D DIC in the impact of high velocity projectiles on armour materials for many years [13; 14; 15; 16; 17]. Early tests were conducted by striking a rigid polycarbonate and flexible poly(urethane urea) elastomer with spherical steel projectiles, with a diameter of 5.56 mm at a striking velocity of between 100 m/s and 200 m/s, which is below the ballistic limit of both materials [13]. By using Photron SA1 Cameras (Photron USA, Inc., San Diego, CA, USA), recording at 67,500 FPS and 256 \(\times\) 256 pixel resolution, the authors were able to measure the strain field of the impacted specimens. The authors used a commercial image processing package,ARAMIS (GOM GmbH, Braunschweig, Germany) to process the images, to give the full strain field. With this setup, the authors were able to produce a displacement accuracy to within \(\pm 25\) um, providing quantitative data that the authors could not obtain through other methods [13]. Another application of 3D DIC that has been examined is the back-face deformation of helmets to assess blunt force trauma. Modern helmets have become quite effective at defeating the penetration capabilities of ballistic threats, but often require large deformation to absorb the energy of the ballistic threat. If this back face deformation (BFD) is larger than the helmet standoff distance, this can impact a soldier's head, causing blunt force trauma [14]. [PERSON] et al. [14] developed an experimental methodology utilising 3D DIC at a frame rate of 50,000 FPS to produce a wide range of robust, accurate and repeatable time-dependent BFD data. As with the previous ARL study [13], the authors used the ARAMIS software (GOM GmbH, Braunschweig, Germany) to process the high-speed stereo photography of the BFD.