Issue34

B. Schramm et alii, Frattura ed Integrità Strutturale, 34 (2015) 280-289; DOI: 10.3221/IGF-ESIS.34.30 288 3D-Simulations with ADAPCRACK3D In further simulations the crack propagation within a fracture mechanical graded flanged shaft (Fig. 1) is analyzed. First, a rather sharp material transition between martensite and ferrite-perlite is considered with the martensitic width b (Fig. 11a). Subsequently the sharp transition is replaced by a bainitic intermediate layer [8]. The initial crack is a half-elliptical surface crack. Both illustrations in Fig. 11 show that this crack grows from the ferritic-perlitic material towards the martensitic microstructure. Upon reaching the martensite unstable crack propagation and thus the failure of the flanged shaft occur. The smoother transition with the bainitic intermediate layer enables a longer crack growth and therefore more load cycles until final failure. a) b) Figure 11: a) Unstable crack growth in the flanged shaft with sharp material transition between ferrite-perlite and martensite, b) unstable crack growth in the flanged shaft with smoother material transition by a bainitic intermediate layer The crack depth - load cycles - diagram in Fig. 12 illustrates the influence of a sharp or rather smooth transition between both microstructures. For the sharp transition unstable crack propagation occurs at the crack length a c = 10 mm and after approximately 2.500.000 load cycles. The use of a pronounced smoother transition with better fracture mechanical properties than the martensitic microstructure results in a higher critical crack depth a c  12 mm and increases the tolerated load cycles by the factor 1.6 compared to the sharp transition. This simulation with ADAPCRACK3D clarifies among other things that a good choice of material gradation might influence the lifetime of structure in a positive way [4, 9]. Figure 12: Interaction of crack depth a and load cycles N for sharp and smooth microstructural transition C ONCLUSION n general, it can be concluded, that fracture mechanical material gradations can have an impact on the range of stable fatigue crack growth, on the crack velocity, on the crack propagation direction as well as on the component life time. The knowledge gained from this work can be taken into account among other things directly during the production of fracture mechanical graded structures and components. Structures are often hardened to ensure a high wear protection at the component surface. Therefore, they possess a martensitic microstructure which is rather brittle and susceptible to I