## 2 DIC Used in Impact Studies to Measure Target Deformation
Overall, the authors show that a 50%/50% combination of multi-orientation laminates on the strike face and conventional [0\({}^{\circ}\)/90\({}^{\circ}\)] laminates on the back face provided the best performance, with both lowest back face deformation and highest projectile resistance. [PERSON] et al. came to the conclusion that there is a trade-off between stiffness and penetration resistance [16]. [PERSON] et al. [17] continued their research on the relationship between ballistic and structural properties of lightweight thermoplastic, unidirectional composite laminates. The experiments the authors conducted consisted of impacting Dyneema HB26 and Spectra Shield II SR-3136 laminates with 75% of the mass of the panel (strike face) of [0\({}^{\circ}\)/90\({}^{\circ}\)] orientated fibres and the rear 25% laid up in a quasi-isotropic layup, where every two succeeding plies are rotated 22.5\({}^{\circ}\) (known as ARL X hybrid). As a baseline, the authors used Kevlar KM2 style 705 PVB phenolic woven aramid composites and Spectra Shield and Dyneema panels with [0\({}^{\circ}\)/90\({}^{\circ}\)] orientation layups (referred to as monolithic [0\({}^{\circ}\)/90\({}^{\circ}\)] panels). All of the panels were constructed to have the same areal density. The impact tests were conducted with 0.22 caliber fragment simulating projectiles (FSP) to determine V50 and 9 mm, 124 grain full metal jacket projectiles to measure back face deformation and for the 3D DIC studies. To capture the high-speed imagery, the authors used a pair of Photron SA5 cameras (Photron USA, Inc., San Diego, CA, USA) with a frame rate of between 40,000 and 50,000 FPS and used the ARAMIS software (GOM GmbH, Braunschweig, Germany) to process the imagery into deflection and strain fields. By doing this, the authors were able to calculate the energy on the back face of the panel during impact events using the instantaneous velocity of the panel back face, the effective area of the delamination and the areal density of the material participating in the impact event [17]. Their 3D DIC results demonstrated that the ARL X hybrid panels have a significantly higher interaction area than the monolithic [0\({}^{\circ}\)/90\({}^{\circ}\)] panels, due to the stress being spread out in a more circular pattern, due to the increased numbers of angles of fibre directions. Comparatively, the monolithic [0\({}^{\circ}\)/90\({}^{\circ}\)] panels exhibit delamination in the 0\({}^{\circ}\) and 90\({}^{\circ}\) directions. The higher interaction area in the ARL X hybrid panels allow them to dissipate more energy into the panel over time, as captured by the 3D DIC. The plots of the ratio of available energy to interaction area demonstrate that the energy in the ARL X hybrid panels is more diffuse, reducing the back face deformation [17]. Advances in X-ray technology have allowed for 3D DIC to be extended to tracking the motion within objects, by seeding them with particles of significantly higher density. One example of this is a study conducted by [PERSON] et al. [20], where the authors examined the displacement ahead of a spherical projectile being fired at a mock (non-reactive) motor casing. The mock motor the authors used was made of 6061 aluminium tubing, a Sylgard 184 insulation layer and a mock explosive (PBS 9501) on the inside. The authors describe two versions of the mock motor casing, one with a central round 'burn' hole and one which is solid. The projectile the authors used was an 8 mm diameter hardened steel ball bearing, fired with a velocity of 490 m/s. The X-rays were developed on film, and then scanned in as 8-bit greyscale digital images. To obtain speckles in the X-ray images, the mock explosive was seeded with lead particles, which gave a significantly larger contrast than the surrounding particles. The lead particles the authors used were sized at \(<\)1.5 mm in diameter. The authors show that using only the smaller particles gave crisp, but unacceptable large speckles.