The digital image correlation method is extended to the study of transient deformations such as the one associated with a rapid growth of cracks in materials. A newly introduced rotating mirror type, multichannel digital high-speed camera is used in the investigation. Details of calibrating the imaging system are first described, and the methodology to estimate and correct inherent misalignments in the optical channels are outlined. A series of benchmark experiments are used to determined the accuracy of the measured displacements. A 2%-6% pixel accuracy in displacement measurements is achieved. Subsequently, the method is used to study crack growth in edge cracked beams subjected to impact loading. Decorated speckle patterns in the crack tip vicinity at rates of 225,000 frames per second are registered. Two sets of images are recorded, one before the impact and another after the impact. Using the image correlation algorithms developed for this work, the entire crack tip deformation history, from the time of impact to complete fracture, is mapped. The crack opening displacements are then analyzed to obtain the history of failure characterization parameter, namely, the dynamic stress intensity factor. The measurements are independently verified successfully by a complementary numerical analysis of the problem.

Compositionally graded glass-filled epoxy sheets with edge cracks initially along the gradient are studied under dynamic loading conditions. Specimens with monotonically varying volume fraction of reinforcement are subjected to mixed-mode loading by eccentric impact relative to the crack plane. The optical method of Coherent Gradient Sensing and high-speed photography are used to map transient crack tip deformations before and after crack initiation. Two configurations, one with a crack on the stiffer side of a graded sheet and the second with a crack on the compliant side, are examined experimentally. To elucidate the differences in fracture behavior due to functional grading, a homogeneous sample is also tested. The differences in both pre-and post-crack initiation behaviors are observed in terms of crack initiation time, crack path, crack speed and stress intensity factor histories. When a crack is situated on the compliant side of the sample, it kinks significantly less compared to when it is on the stiffer side. Crack tip mode mixity histories show small but positive values during crack growth from the stiffer side of the sample towards the compliant side whereas a small but negative mode mixity prevails for the opposite configuration.

Mixed-mode dynamic crack growth behavior in a compositionally graded particle filled polymer is studied experimentally and computationally. Beams with single edge cracks initially aligned in the direction of the compositional gradient and subjected to one-point eccentric impact loading are examined. Optical interferometry along with high-speed photography is used to measure surface deformations around the crack tip. Two configurations, one with a crack on the stiffer side of a graded sheet and the second with a crack on the compliant side, are tested. The observed crack paths are distinctly different for these two configurations. Furthermore, the crack speed and stress intensity factor variations between the two configurations show significant differences. The optical measurements are examined with the aid of crack-tip fields, which incorporate local elastic modulus variations. To understand the role of material gradation on the observed crack paths, finite element models with cohesive elements are developed. A user-defined element subroutine for cohesive elements based on a bilinear traction-separation law is developed and implemented in a structural analysis environment. The necessary spatial variation of material properties is introduced into the continuum elements by first performing a thermal analysis and then by prescribing material properties as temperature dependent quantities. The simulated crack paths and crack speeds are found to be in qualitative agreement with the observed ones. The simulations also reveal differences in the energy dissipation in the two functionally graded material (FGM) cases. T-stresses and hence the crack-tip constraint are significantly different. Prior to crack initiation, larger negative T-stresses near the crack tip are seen when the crack is situated on the compliant side of the FGM.

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