13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- material produce a local failure, which develops into a dynamically propagating cleavage crack. The local “initiators” may be precipitates, inclusions or grain boundaries, acting alone or in combination. An example of a typical cleavage fracture initiation process is presented schematically in Fig. 2. The first step involves the cracking of a precipitate or inclusion (sometimes, it may also be a grain boundary or grain triple point). The second step consists of the carbide size micro-crack propagating into the surrounding matrix and the third step consists of the grain-size crack propagating into neighboring grains. The two first steps are mainly affected by the particle size and the local stress and strain at the initiation site. The third step, however, is also affected by the stress gradients in the vicinity of the initiation site, since the third step covers a larger material volume. σ, ε σ, ε σ, ε σ, ε σ, ε σ, ε Local stress and strain produces a dislocation pileup which impinges on a grain boundary or carbide. Cracking of the carbide or the grain boundary introduces a microcrack which propagates into the matrix. The advancing microcrack encounters the first large angle boundary. Figure 2. Schematic presentation of the necessary steps for cleavage fracture initiation [1]. Depending on loading geometry, temperature, loading rate and material, different steps are more likely to be most critical. For structural steels at lower shelf temperatures and ceramics, in the case of cracks where the stress distribution is very steep, steps II and III are more difficult than initiation and they tend to control the fracture toughness. At higher temperatures, where the steepness of the stress distribution is smaller, propagation becomes easier in relation to initiation and step I becomes more and more dominant for the fracture process. The temperature region where step I dominates is usually referred to as the transition region. On the fracture surface of a specimen with a fatigue crack this is usually seen as a difference in the number of initiation sites visible on the fracture surface (Fig. 3). At lower shelf temperatures, numerous initiation sites are visible, whereas at higher temperatures, corresponding to the transition region, only one or two initiation sites are seen. In the case of notched or plain specimens, only a few initiation sites are seen even on the lower shelf. This is due to that, for cracks, the peak stresses are very high virtually from the beginning of loading, whereas for notched and plain specimens, the peak stresses increase gradually during loading. The fracture surface appearance is an effective tool in the decision if the material is on the lower shelf or in the transition region. The region has an implication on the macroscopic statistical behavior of cleavage fracture and is therefore also affecting the application of the fracture toughness test results. TRANSITION REGION LOWER SHELF Crack propagation Figure 3. Typical cleavage fracture surfaces for fatigue pre-cracked specimens indicating differences in transition region and lower shelf behavior [1].
RkJQdWJsaXNoZXIy MjM0NDE=