Issue34

B. Schramm et alii, Frattura ed Integrità Strutturale, 34 (2015) 280-289; DOI: 10.3221/IGF-ESIS.34.30 286 da/dN of approximately 6.3  10 -5 mm/load cycles. This behavior is comparable to that of homogeneous and isotropic materials. However, a change of the crack velocity occurs at the crack length a  20 mm. A linear increase about an area of 4 mm is registered and clarifies the change of the fracture mechanical properties and therefore the existence of a fracture mechanical material transition. At the crack length a  24 mm the crack propagation takes place with a constant crack velocity. The crack has reached a region with isotropic and homogeneous material properties again. Figure 7: Crack velocity in fracture mechanically graded compact tension specimens with the gradation angle  M =90°. Influence on the crack propagation direction Fig. 8 shows a compact tension specimen with the gradation angle  M = 30° and a crack growing from the martensitic microstructure towards the ferritic-perlitic base material. While for homogeneous and isotropic materials and the global stress situation Mode I a crack propagation within the initial plane of the crack is expected, the application of the TSSR- concept leads to the theoretical kinking angle  TSSR =  M = 30° for an appropriate graded specimen. Fig. 8a shows the predicted crack propagation by the TSSR-concept. a) b) Figure 8: a) Gradation angle  M and theoretical kinking angle  TSSR = 30° determined by TSSR-concept, b) experimental determined kinking angle  0  23° The experimentally examined graded CT-specimens with gradation angles of  M  30° show kinking angles  0 of approximately 23°. The deviation between the experimental kinking angle  0 and the predicted kinking angle  TSSR is 7° and can be explained by minor differences in the material transitions caused by heat treatment and sample taking. Whereas the application of the TSSR-concept assumes a gradation angle of  M = 30° as well as a sharp material transition, the gradation angle and the material transition within the tested specimens may vary slightly. Finally, it can be stated that the experimental investigations confirm that the crack propagation is not only affected by the local stress field, but also by the fracture mechanical material gradation.