Experimental Study of Dynamic Crack Growth in Unidirectional Graphite/Epoxy Composites using Digital Image Correlation Method and High-speed Photography

In this work, fracture behavior of multilayered unidirectional graphite/epoxy composite (T800/3900-2) materials is investigated. Rectangular coupons with a single-edged notch are studied under geometrically symmetric loading configurations and impact loading conditions. The notch orientation parallel to or at an angle to the fiber orientation is considered to produce mode-I or mixed-mode (mode-I and -II) fracture. Feasibility of studying stress-wave induced crack initiation and rapid crack growth in fiber-reinforced composites using the digital image correlation method and high-speed photography is demonstrated. Analysis of photographed random speckles on specimen surface provides information pertaining to crack growth history as well as surface deformations in the crack-tip vicinity. Measured deformation fields are used to estimate mixed-mode fracture parameters and examine the effect of fiber orientation (beta) on crack initiation and growth behaviors. The samples show differences in fracture responses depending upon the orientation of fibers. The maximum crack speed observed is the highest for mode-I dominant conditions and it decreases with fiber orientation angle. With increasing fiber orientation angle, crack takes longer to attain the maximum speed upon initiation. Continuous reduction of dynamic stress intensity factors after crack initiation under mode-I conditions is attributed to crack bridging. The crack initiation toughness values decrease with the degree-of-anisotropy or increase with fiber orientation angle. A rather good agreement between crack initiation toughness values and the ones from previous investigations is observed. There is also a good experimental correlation between dynamic stress intensity factor and crack-tip velocity histories for shallow fiber orientations of beta = 0, 15, and 30 degrees.