Visual Methods to Assess Strain Fields in Armour Materials Subjected to Dynamic Deformation—A Review

When impacted by a projectile, ballistic protection undergoes very large strain rates over very short periods of time. During these impact events, materials will undergo a very short region of elastic deformation, before undergoing significant plastic deformation. Due to the high levels of plastic deformation the samples undergo, strain gauges and other embedded sensors are often ineffective or become damaged before useful data can be obtained. Three-dimensional digital image correlation (3D DIC) is a non-invasive measurement method that uses two high-speed cameras, offset from each other by 15–45° to observe a speckle pattern on the sample material. As the material, and by extension the speckle pattern, deforms, the images taken throughout the deformation can be compared in sequence, to determine the motion and deformation of the sample. Recent advances in camera technology have allowed for frame rates in the hundreds of thousands of frames per-second, allowing for the measurement of very high-strain rate impact events. This paper will describe the premise of 3D DIC and provide a review of the current applications and research into high-speed impact testing using 3D DIC.