Issue34

B. Schramm et alii, Frattura ed Integrità Strutturale, 34 (2015) 280-289; DOI: 10.3221/IGF-ESIS.34.30 285 These stress intensity factors define cyclic stress functions which possess points of contact with the material function at the inserted polar coordinates. The minimal value of these factors describes the first point of contact between the stress function and the material function und furthermore the beginning of stable fatigue crack growth. At the same time the occurring crack propagation direction  TSSR arises out of the used polar coordinate  . In Fig. 6a this point can be found at the polar coordinate  = 30°. Due to the fact that the real cyclic stress function is larger than the function which is relevant for crack propagation the crack will propagate along the fracture mechanical material transition with  TSSR =  M = 30°. The crack extension by the increment  a is shown in Fig. 6b. Consequently, the crack propagation doesn’t depend only on the existent stress situation, but also on the fracture mechanical material gradation. The new crack position and the global Mode I loading now cause a Mixed Mode stress situation which has to be considered for the determination of the further crack propagation. a) b) Figure 6: Crack propagation in a fracture mechanical graded structure for Mode I with the gradation angle  M = 30°: a) application of TSSR-concept, b) resulting crack increment  a. For other sample applications (further crack propagation, occurrence of unstable crack growth, different gradation angles and loading situations, continuous material transitions, expansion of the TSSR-concept to three-dimensional problems) see [4]. E XPERIMENTAL I NVESTIGATIONS OF F RACTURE M ECHANICAL G RADED M ATERIALS ased on the theoretical considerations experimental investigations are carried out to confirm the different influences of a fracture mechanical material gradation on the crack propagation. The studies [4, 6] are carried out with the electro-dynamic test system Electro-Puls E10000 of the company Instron according to ASTM E647-08 [7]. Previously, the compact tension specimens (CT specimens) were heat-treated resulting in a fracture mechanical material gradation. Influence on the crack velocity In the following, experimental results of a CT-specimen with a fracture mechanical material gradation characterized by the gradation angle  M = 90° are presented. Here, the crack grows from a ferritic-perlitic microstructure towards martensitic microstructure of the heat-treatable steel 51CrV4. Fig. 7 shows the determined crack growth rate da/dN with proceeding crack growth and  K max remaining constant. At the beginning the crack grows with a rather constant crack velocity B