Crack Paths 2012

striations compared to the crack growth direction (Fig. 11) indicates faster crack

propagation in the flange surface with U F Gmicrostructure compared to the classical

work-hardened region. Though further investigations are required to confirm this aspect,

it would be plausible with respect to the lower resistance to fatigue crack growth which

is typical for U F Gmaterials [11, 12].

The secondary cracks in the U F Gregion (Fig 10, right) occur parallel to the flange

surface, e.g. along the strongly elongated grain interfaces of the pancake structures. The

tearing of these interfaces during the cyclic loading (even with the loading axis parallel

to the elongated pancakes), indicate a weakening of the grain boundaries due to the

linear flow splitting process. However, the secondary cracks exhibit no influence on the

fatigue crack growth, since the fatigue crack growths in perpendicular direction and thus

within the smallest grain dimension of this microstructure.

C O N C L U S I O N

The flanges from the material H480LAmanufactured by LFSare due to the presence of

a gradient structure with U F Gstructure suitable for light-weight application because of

their higher strength. The occurrence of overloads is nevertheless more damaging in

comparison with the as-received material. Considering also the higher scatter band in

the fatigue results in the specimens taken out from the flanges (Tab. 2), a more attentive

design of components where the flange material is used is required, especially if

frequent overloads are expected during service.

With respect to the fracture surface and crack path, the analyses reveal a strong

difference between the behavior of as-received and flange material state. Since these

gradient structures are relatively new, further studies are required in order to better

understand and to model the material behavior in presence of cracks.

A C K N O W L E D G M E N T

The work presented in this paper is supported by the German Research Foundation

(DFG). The authors thank the D F Gfor founding the subprojects C1 and C2 of the

Collaborative Research Centre 666 “Integral sheet metal design with higher order

bifurcations – Development, Production and Evaluation”.

R E F E R E N C E S

[1] Groche, P., Vucic, D., Jöckel, M., Journal of Materials Processing Technology

183 (2007) pp. 249–255.

[2] Bohn, T., Bruder, E., Müller, C., (2008) J. Mater. Sci. 43, pp. 7307-7312.

[3] Landersheim V., el Dsoki, C. et al., (2008) A S M E2008 International Design

Engineering Technical Conferences & Computers and Information in

Engineering Conference.

[4] Vinogradov, A., (2007) J. Mater. Sci. 42, pp. 1797-1808.

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