Issue34

B. Schramm et alii, Frattura ed Integrità Strutturale, 34 (2015) 280-289; DOI: 10.3221/IGF-ESIS.34.30 282 consequence is the existence of two potential kinking angles: the stress induced kinking angle  0,MTS and the gradation angle  M itself. a) b) Figure 2: Fracture mechanical graded structure with potential crack propagation directions: a) crack tip with distance to material transition, b) crack tip within material transition. Influence on the limits of fatigue crack growth and on the crack velocity In the following, the influence of a fracture mechanical material transition is considered for a structure with the gradation angle  M = 90°, which is loaded cyclically with  (t) resulting in a pure Mode I loading situation. Fig. 3 shows the threshold value curve  K I,th and the cyclic fracture toughness curve  K IC for a crack growing from the fracture mechanical worse material M1 into material M2 (  K th,M1 <  K th,M2 ;  K C,M1 <  K C,M2 ). A homogeneous and isotropic structure consisting only of material M1 would fail at stress level A (cf. point A 1 ), whereas the considered graded structure doesn’t fail until reaching point A 2 . At stress level B stable crack growth occurs in both considered microstructures, i.e. the crack grows stable until either the cyclic fracture toughness  K IC,M2 of material M2 or the edge of the structure are reached. For the lower stress level C the crack grows until reaching the material transition (point C 1 ). The occurring cyclic stress intensity  K I is smaller than the treshold value  K I,th,M2 of material M2, so that the crack is not able to propagate and stops within the material transition. Figure 3:   - a -diagram for a sharp material transition from M1 to M2. It should be noted that the crack growth from a region with worse fracture mechanical material properties into a region with better properties might lead to crack arrest, if the cyclic stress intensity factor  K I is at transition smaller than the