Digital image analysis of ASB assisted dynamic fracture under impact loading

International audienceExperimental observation of fracture mechanisms is a key point to understand and further describe the physical phenomena involved in structural material failure. Crack propagation under high loading rate is a complex and strongly coupled thermo-mechanical problem involving large deformations, high strain rates and (quasi) adiabatic conditions. The work presented aims at giving a new insight into the process of dynamic fracture of structural materials under impact loading for further model development and calibration. Kalthoff and Winkler-type impact tests, see [1], were carried out on double notched plates consisted of a high strength steel (the protection steel ARMOX500T). Above a critical impact velocity, the plate is seen to fail prematurely under adiabatic shear banding, see e.g. [2]. The chronology of the plate failure mechanisms was observed thanks to the use of an ultra-high speed and high spatial resolution camera (1Mfps, 312x216 pixel²). Three stages were observed: first the homogeneous then weakly heterogeneous deformation of the plate, second the propagation from the notch tip of an (adiabatic) shear band (ASB) of localised deformation, and finally the propagation of a crack within the band. The digital images were then analysed in order to describe the aforementioned mechanisms of deformation and failure from a kinematic point of view. Two types of results were then obtained: first the evolution of the notch tip (shearing/opening) displacement/velocity and ASB tip velocity, and second the displacement (and further strain and strain rate) fields of points located along lines (quasi) perpendicular to the ASB/crack propagation path (line tracking method). References: [1] Kalthoff J.F., Winkler S., Failure mode transition of high rates of shear loading. In: Chiem C.Y., Kunze H.-D., Meyer L.W., editors. Proc. of the Int. Conf. on Impact Loading and Dynamic Behavior of Materials, Germany: Deutsche Gesellschaft für Metallkunde, DGM-Verlag, Oberursel, pp.185–95, 1987. [2] Longère P., Dragon A., Deprince X., Numerical study of impact penetration shearing employing finite strain viscoplasticity model incorporating adiabatic shear banding, J. Eng. Mat. Tech., ASME, 131, pp.011105.1-14, 2009