PSI - Issue 2_B

Richard H. A. et al. / Procedia Structural Integrity 2 (2016) 1821–1828 Richard H. A. and Eberlein A. / Structural Integrity Procedia 00 (2016) 000 – 000

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Moreover, it should be noted that the greatest deviations of the 3D-criterion by R ICHARD occur in a range of low mode II-parts between 0 2 0 1 n II . . K   . Here always smaller crack kinking angles are calculated. However, the greatest measuring differences at 1   -criterion by S CHÖLLMANN et al. for loading combinations of mode II and mode III are in an area of higher n II K -parts with n II K > 0.6. All in all, the average deviation of the crack kinking angle, measured by Schirmeisen (2012), is around ± 7°. Within this experiments no significant crack kinking angle at pure mode III-loading was measured. By comparison hereto, Schirmeisen (2012) measured for pure mode III-loading a crack kinking angle φ 0 of ca. 11°. Generally, for pure mode II-loading slightly smaller crack kinking angles, with a maximum deviation of ca. 7° and an average deviation of ca. 4° from the predicted value by criteria, were measured. The criterion by P OOK compared to the others exhibits the greatest discrepancies to the resulting values by the prediction of the crack kinking angle as well as crack twisting angle. A very good accordance of criteria by R ICHARD and by S CHÖLLMANN et al. arises by the forecast of the crack twisting angle ψ 0 . The average deviation to the measured crack deflection angles for both criteria is ca. 4° (Schirmeisen (2012)). Within this research the crack twisting angles for pure mode III-loading are around 3° smaller as the criteria predict. A comparison of the crack deflection angles between the 1   -criterion by S CHÖLLMANN et al. and FE-simulations is i. a. published by Kullmer et al. (2013). The measurement resp. determination of the crack twisting angle generally is very difficult. The crack twisting angle should be measured at the local crack initiation point as close as possible. Fatigue crack under spatial mixed mode-loadings with high mode III-part propagates by segmenting in several daughter-cracks along the crack front. Therefore many crack initiation points exist, where the cracks propagates at first facetted and then each daughter crack after further crack growth unify to one continuous crack front. Each daughter-crack or so-called facet is a new crack front with a corresponding crack twisting angle. In addition, it can be supposed that plastic deformations during rupturing the specimen at each facet occur. This plastic deformations distort, of course, the real crack twisting angles. Furthermore, some typical fractured surfaces for pure mode I-, pure mode II as well as pure mode III-loading developed within this investigations are pictured in Figure 6. Under pure mode I-loading the fatigue crack grows perpendicular to the loading direction. A pure mode II-loading leads the crack to a characteristic kinking of ca. 70°, while a crack under pure mode III-loading conditions twists by nearly 45° and propagates in all directions radially even back to the starter notch.

Mode I

Mode II

Mode III

Fig. 6. Typical fractured surfaces under pure mode I-, pure mode II- as well as pure mode III-loading

References

Campbell, J. P., Ritchie, R. O., 2000. Mixed-mode, high-cycle fatigue crack growth thresholds in Ti-6Al-4V: I. A comparison of large- and short crack behavior. Engineering Fracture Mechanics 67, pp. 209-227. Eberlein, A., 2016. Einfluss von Mixed-Mode-Beanspruchung auf das Ermüdungsrisswachstum in Bauteilen und Strukturen. Fortschritt-Berichte VDI: Reihe 18, Mechanik, Bruchmechanik, Band 344, VDI-Verlag, Düsseldorf. Kullmer, G., Richard, H. A, Wang, C., Eberlein, A., 2013. Numerische Untersuchungen zu Ermittlung der Rissablenkungs- und Rissverdrehungswinkel bei allgemeiner Mixed-Mode-Belastung. In: DVM-Berichte 245, Bruchmechanische Bauteilbewertung, Beanspruchungsanalyse, Prüfmethoden und Anwendungen, Deutscher Verband für Materialforschung und –prüfung e.V., Berlin, pp. 59-68. Nalla, R. K., Campbell, J. P., Ritchie, R. O., 2002. Mixed-mode, high-cycle fatigue crack growth thresholds in Ti-6Al-4V: Role of small cracks. International Journal of Fatigue 24, pp. 1047-1062. Pook, L. P., 1980. The significance of mode I branch cracks for combined mode failure. In: Radon, J. C. (Ed.). Fracture and Fatigue: elasto-plasticity, thin sheet and micromechanism problems. Pergamon Press, Oxford, pp. 143-153.