High resolution depth and area measurements of low velocity impact damage in carbon fiber laminates via an ultrasonic technique

This research presents a method using high frequency ultrasonics to quantify the location, extent, and depth of damage in a carbon fiber laminated composite subjected to a low velocity impact (LVI) producing a Barely Visible Impact Damage (BVID) zone. Carbon fiber laminated composites are used extensively in a wide variety of industries from aerospace to athletic gear. Laminated composites provide a favorable strength to weight ratio but due to their layered structure an interior lamina can be damaged with little to no externally observed damage. This paper analyzes four in-house fabricated carbon fiber laminated composites with a custom high-resolution ultrasonic immersion C-scan system, that have been subjected to increasing levels of low velocity impacts (LVI's) to produce Barely Visible Impact Damage (BVID). Two mathematical techniques are used in analyzing C-scan data to obtain high image contrast and an unbiased ply-by-ply technique is used to create internal 3D profiles of laminate damage due to LVI's. These regions of damage are substantially larger than anticipated based off of surface profile measurements, thus significantly underestimating the reduction in the load carrying capacity of the damaged system. Finite element results of the reduction in load carrying capacity of simple plaques with a damage region obtained exclusively from surface observations of the damage zone indicate in each scenario investigated a reduction of 55-60% in load carrying capacity. Conversely, studies using the actual 3D damage zone from ultrasonic measurements indicate reduction in overall load carrying capacity for the same plaques a 65-75% reduction from that of the undamaged composite.