b Fig. 9. Stages of crack arresting in layered specimen:(a) crack is arrested in the tensile layer; (b)crack is arrested in the compressive layer Let us consider some features of the crack arrest in our experiments. Open circle A in Fig. 9 designates the initial state of testing: initial notch without loading. Open circle B corresponds to the crack growth onset at some critical applied stress. Open circles depict the initial notch with length ao. Filled circle C corresponds to the onset of unloading. Note that the applied stress increases permanently during the loading stage( from A to C in Fig 9), while the crack starts to grow only if the applied stress intensity factor exceeds the apparent fracture toughness(from b to c in Fig. 9). The crack growth under unloading can vary depending on the rate of the applied stress decreasing Schematically it is shown in Fig. 9 by two paths of crack development(C-D-E-F, Fig 9a; and C-D'-E-F, Fig. 9b) Curve C-D'-E-Fcorresponds to a higher rate of the applied stress reduction. Filled circles D and D'characterize current positions of moving crack tip under unloading. Filled circles E and e designate the crack arresting when applied crack intensity factor becomes less than the apparent fracture toughness. Filled circles F and F'depict the final state of crack with length af(or af) after full unloading. One can see from Fig 9 that different unloading conditions can result in various distance passed by the crack. If C-D-E-F path is realized, the crack is arrested in the next tensile layer(Fig. 9a). If C-D'-E - path is realized the crack is arrested in the nearest compressive layer (Fig. 9b). In the general case, unloading conditions can result in a number of various final positions of the crack tip. It can be either in the layer with the initial notch tip or in any more remote layer. Revisiting our experimental data we note that C-D-E-F path rather than C-D'-E'-F'is realized in the laminate specimens In such a way, we have two stages of the loading process and three stages of crack development. The first stage of loading process is the applied stress increasing to some maximum value( from A to C in Fig. 9a). The second stage of loading process is the applied stress decreasing to zero(from C to F in Fig. 9a). The first stage of crack development is the absence of crack growth until applied stress intensity factor is less than apparent fracture toughness(from A to B in Fig. 9a). The second stage of crack development is the crack growth(from B to E in Fig. 9a) The third stage of crack development is the absence of crack growth if the applied stress intensity factor is less than the apparent fracture toughness again(from E to F in Fig. 9a) The condition for stable crack growth in the residually stressed layers can be obtained from(26). The stable crack growth can occur when Kapp (a) a<<dKapp (a) da. As the condition is satisfied, the load decreasing results in indispensable crack arresting. The crack arrest under stable growth conditions differs from features of crack arresting described in our work. This is due to the fact that unstable crack growth was observed for the layered specimens investigated. Crack arrest does not depend on the unloading rate under conditions of stable crack growth A crack will be arrested in any case if the applied load would not increase. At the same time, crack arrest for unstable crack growth depends strongly on the stress decrease rate. The unloading rate is determined mainly by the stiffness of 301Let us consider some features of the crack arrest in our experiments. Open circle A in Fig. 9 designates the initial state of testing: initial notch without loading. Open circle B corresponds to the crack growth onset at some critical applied stress. Open circles depict the initial notch with length a0 . Filled circle C corresponds to the onset of unloading. Note that the applied stress increases permanently during the loading stage (from A to C in Fig. 9), while the crack starts to grow only if the applied stress intensity factor exceeds the apparent fracture toughness (from B to C in Fig. 9). The crack growth under unloading can vary depending on the rate of the applied stress decreasing. Schematically it is shown in Fig. 9 by two paths of crack development (C–D–E–F, Fig. 9a; and C–D′–E ′–F ′, Fig. 9b). Curve C–D′–E ′–F ′ corresponds to a higher rate of the applied stress reduction. Filled circles D and D′ characterize current positions of moving crack tip under unloading. Filled circles E and E ′ designate the crack arresting when applied crack intensity factor becomes less than the apparent fracture toughness. Filled circles F and F ′ depict the final state of crack with length a f (or a ′ f ) after full unloading. One can see from Fig. 9 that different unloading conditions can result in various distance passed by the crack. If C–D–E–F path is realized, the crack is arrested in the next tensile layer (Fig. 9a). If C–D′–E ′–F ′ path is realized the crack is arrested in the nearest compressive layer (Fig. 9b). In the general case, unloading conditions can result in a number of various final positions of the crack tip. It can be either in the layer with the initial notch tip or in any more remote layer. Revisiting our experimental data we note that C–D–E–F path rather than C–D′–E ′–F ′ is realized in the laminate specimens. In such a way, we have two stages of the loading process and three stages of crack development. The first stage of loading process is the applied stress increasing to some maximum value (from A to C in Fig. 9a). The second stage of loading process is the applied stress decreasing to zero (from C to F in Fig. 9a). The first stage of crack development is the absence of crack growth until applied stress intensity factor is less than apparent fracture toughness (from A to B in Fig. 9a). The second stage of crack development is the crack growth (from B to E in Fig. 9a). The third stage of crack development is the absence of crack growth if the applied stress intensity factor is less than the apparent fracture toughness again (from E to F in Fig. 9a). The condition for stable crack growth in the residually stressed layers can be obtained from (26). The stable crack growth can occur when K aa app ( ~) ~ < < dK a da app ( ~) ~. As the condition is satisfied, the load decreasing results in indispensable crack arresting. The crack arrest under stable growth conditions differs from features of crack arresting described in our work. This is due to the fact that unstable crack growth was observed for the layered specimens investigated. Crack arrest does not depend on the unloading rate under conditions of stable crack growth. A crack will be arrested in any case if the applied load would not increase. At the same time, crack arrest for unstable crack growth depends strongly on the stress decrease rate. The unloading rate is determined mainly by the stiffness of 301 Fig. 9. Stages of crack arresting in layered specimen: (a) crack is arrested in the tensile layer; (b) crack is arrested in the compressive layer. a b