Fracture Toughness of Layered Materials with a Hardened Surface Layer

The peculiarities of crack growth inhibition during the fracture of laminated composite materials obtained by different methods are presented. During the fracture of materials with a hardened surface layer, crack retardation occurs by the mechanism of branching of crack trajectory on the boundary of the hardened layer, and during the fracture of layered deposited compositions by the mechanism of formation of internal delaminations on the boundaries of the layers. For the quantitative assessment of the fracture toughness of layered materials there are known recommendations concerning the determination of the stress intensity factor for the stage of crack retardation (K Ia or K IIc ), which allows a more correct estimation of the fracture toughness of laminated materials. However, these recommendations have not been widely used. Taking into consideration these recommendations, a method of determining the stress intensity factor under dynamic load at the stage of crack retardation K ID a , fracture diagrams in the force-time coordinates has been proposed. Tests and calculations of specimens with a hardened surface layer on tool steels (90KhF, R6M5) and layered deposited materials have shown that their crack resistance is much higher than the integral parameters K ID a . The increase in crack resistance is caused by additional energy expenditures in overcoming the boundaries between the hardened and deposited layers by the crack.