Issue34

S. Henschel et alii, Frattura ed Integrità Strutturale, 34 (2015) 326-333; DOI: 10.3221/IGF-ESIS.34.35 328 Figure 1: Microstructure of the investigated steel: tempered martensite. The inclusion distribution was analyzed for polished cross sections utilizing the software Particle Inspector (Olympus). Small inclusions located in a group were considered as one inclusion cluster. Details of the mathematical procedure are given in a previous paper [15]. The inclusion size distribution is given in Tab. 3. Size class (µm) Cast A Cast B Cast C 2–5 988 1635 773 5–10 180 159 136 10–20 51 77 74 20–60 47 89 52 > 60 6 9 0 Table 3: Inclusion size distribution (inclusions/mm 2 ). Fracture mechanics tests The materials resistance against crack initiation and growth under quasi-static loading conditions ( /sm MPa 2  K  ) was determined according to ISO 12135 [16]. The single specimen unloading compliance technique was applied. Pre-cracked and side-grooved specimens ( a 0 / W = 0.5, B N / B = 0.8) were tested in a servo hydraulic universal testing machine. Single edge-notched bend specimens ( L  W  B = 120  20  10 mm 3 ) were loaded in three-point bending ( S / W = 4). Periodical unloadings during the test enabled the calculation of the unloading compliance from the crack opening displacement. Hence, the crack length was determined for each unloading step. The plastic part of the energy U p was used to determine the J integral at the different stable crack extensions  a :               ) (2 1 ) ( 2 ) 1( 0 N p 2 2 I aW a aWB U E K J a  (1) Tests under dynamic loading conditions ( s/m MPa 109 8 4     K ) were performed in an instrumented Charpy impact- testing machine. The low-blow technique was applied. Multiple specimens ( L  W  B = 55  10  10 mm 3 ) were subjected to different initial velocities of the impact tup. Hence, different amounts of stable crack extension  a were achieved. The force signal was derived from the elastic deformation of the instrumented tup. The velocity and the deflection of the sample were determined by a laser system (Polytec OFV-525). This system utilizes the Doppler Effect, i.e. the frequency and phase modulation of the laser light by the velocity and the displacement, respectively, of the sample in laser beam direction. The laser beam direction was equivalent to the load line. The J integral was calculated with Eq. (1).