Multiaxial mixed-mode cracking – small crack initiation and propagation*

Manuel de Freitas; Luis Reis; Bin Li

doi:10.3139/120.100710

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Multiaxial mixed-mode cracking – small crack initiation and propagation*

Manuel de Freitas , Luis Reis und Bin Li

Veröffentlicht/Copyright: 28. Mai 2013

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Abstract

Both the fatigue crack path and fatigue life of CK45 steel and 42CrMo4 steel under various multiaxial loading paths are studied in this paper. The replica method was applied to monitor the crack initiation and small crack growth, the fractographic analyses were carried out on the fracture surface and the crack initiation angle was measured. The effects of non-proportional loading on both the crack path and fatigue life were studied, and the flattening of asperities on the crack surface due to compressive normal stress was also observed. An improved model is proposed based on correcting the strain range parameter of the ASME code approach, taking into account the additional hardening caused by the non-proportional loading path, which can improve the predictions of the fatigue lives for various non-proportional loading paths and provide an easy way to overcome the drawbacks of the current ASME code approach for non-proportional fatigue. Based on these corrected strain range parameters, a strain intensity factor range is used to correlate with the experimental results of small crack growth rates. It is concluded that the orientation of the early crack growth can be predicted well by the critical damage plane, but the fatigue life can not be predicted accurately using only the parameters on the critical plane, since the damage on all the planes contributes to fatigue damage as stated by the integral approaches.

Kurzfassung

Für den vorliegenden Beitrag wurden sowohl der Ermüdungsrisspfad als auch die Lebensdauer der Stähle CK45 und 42CrMo4 unter verschiedenen multiaxialen Belastungsrichtungen untersucht. Es wurde das Verfahren von Replica angewendet, um die Rissinitiierung und das Wachstum kleiner Risse aufzuzeigen. Es wurde eine fraktographische Analyse der Bruchoberfläche vorgenommen und es wurde der Risswinkel während der Initiierung gemessen. Es wurden die Auswirkungen der nichtproportionalen Beanspruchungen auf den Risspfad sowie auf die Lebensdauer untersucht und es wurde außerdem eine Abflachung der rauhen und unebenen Oberfläche infolge der Druckspannungen in Normalrichtung beobachtet. Es wird ein verbessertes Modell gezeigt, dass auf korrigierten Dehnungsparametern nach dem ASME Code beruht und das zusätzliche Verfestigung infolge der nicht proportionalen Beanspruchungsrichtung berücksichtigt. Das Modell kann somit Lebensdauervorhersagen bei verschiedenen nicht proportionalen Beanspruchungsrichtungen verbessern und stellt damit einen einfachen Weg dar, um die Schwächen des bisherigen Ansatzes nach dem ASME Code für nicht proportionale Ermüdung zu überwinden. Basierend auf den so korrigierten Dehnungsparametern wurde ein Bereich des Spannungsintensitätsfaktors verwendet, um die experimentellen Ergebnisse für geringe Risswachstumsraten zu korrelieren. Damit kann die Orientierung des Risses während eines frühen Wachstumsstadiums sehr gut mit der kritischen Schädigungsebene exakt vorhergesagt werden. Die Lebensdauer hingegen kann nicht akkurat vorhergesagt, wenn nur die Parameter der kritischen Ebene herangezogen werden, da entsprechend der Aussage der Integralansätze schließlich die Schädigungen in allen Ebenen zum gesamten Ermüdungsschaden beitragen.

Dr. Bin Li, Ph. D. in Mechanical Engineering from Technical University of Lisbon, Portugal, M. S. and B. S. in Mechanical and Aeronautical Engineering from Northwestern Polytechnic University, Xián, China. Presently Research Fellow at ICEMS (Institute of Material and Surface Science and Engineering, Portugal), and Assistant Professorat ISEL (Lisbon Institute of Engineering). Main research areas: Multiaxial Fatigue, FEM based simulation of structural integrity, Structural Optimization.

Professor Manual de Freitas, Diploma in Mechanical Engineering from Instituto Superior Técnico, Technical University Lisbon, Doctor in Mechanical Engineering, from Université de Technologie de Compiégne, France. Presently Full Professor of Mechanical Design at the Mechanical Engineering Department of Instituto Superior Técnico, Technical University of Lisabon, Researcher at ICEMS (Institute of Material and Surface Science and Engineering, Lisbon, Portugal). Main research areas: multiaxial fatigue, mixed-mode crack growth, mixed-mode fracture of laminated composites.

Dr. Luis Reis, Ph. D. in Mechanical Engineering from Technical University of Lisbon, Portugal. Currently Research Fellowat ICEMS (Institute of Material and Surface Science and Engineering, Portugal) and Assistant Professor at IST (Instituto Superior Técnico), Technical University of Lisbon. Main research areas: Mechanical Design, Multiaxial Fatigue, Fractographic studies, FEM-based simulation of structural integrity.

Contribution to the 7^th International Conference on Biaxial/Multiaxial Fatigue and Fracture (7ICBMFF)

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Published Online: 2013-05-28

Published in Print: 2006-02-01

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https://doi.org/10.3139/120.100710