Abstract
Accuracy of indirect fatigue crack length measurement by potential drop method or by compliance technique may be affected at low load ratio due to fracture surface contact, crack closure or mixed mode fracture. As an alternative, the maximum value of crack opening displacement, CODmax, from a clip gauge was utilized. Middle crack tension M(T) specimens were used to obtain conservative data at a low load ratio (R = 0.1). Thin sheet specimens (B = 3.2 mm) with different widths (100 mm ≤ W ≤ 400 mm) of AA6056-T4 were investigated in the mid-regime (Paris regime), which is of interest for damage tolerance analysis. The use of CODmax is found to provide crack lengths equivalent to those measured optically. Hence, the method is very suitable for indirect crack length measurement. Furthermore, small width specimens provided data equivalent to large width specimens. Insofar, the size effect is found to be absent, and fatigue crack propagation data can be acquired on small width specimens when material availability is limited.
Kurzfassung
Anwendung der maximalen Rissoffnungsverschiebungswerte fur die indirekte Risslangenmessung bei der Ermudungsrissausbreitung an mittig angerissenen Zugproben der luftfahrtspezifischen Aluminiumlegierung AA6056. Bei der Ermudungsrissausbreitung kann wegen Bruchflachenberuhrung oder Rissschliesung bei einem niedrigen Krafteverhaltnis sowie des Scherbruchs die Genauigkeit der indirekten Risslangenmessung durch die Potenzialmethode oder die Nachgiebigkeit (Compliance) beeinflusst werden. Als Alternative wird der Maximalwert der Rissoffnungsverschiebung, CODmax, genutzt. Es wurden mittig angerissene Zugproben, M(T)-Proben, verwendet, um konservative Rissausbreitungsdaten bei einem niedrigen Kraftverhaltnis (R = 0,1) zu erhalten. Dunne Blechproben (B = 3,2 mm) mit unterschiedlichen Breiten (100 mm ≥ W ≥ 400 mm) aus AA6056-T4 wurden im mittleren Rissausbreitungsbereich (Paris- Bereich), der fur die Schadenstoleranzanalyse von Interesse ist, untersucht. Der Gebrauch von CODmax. Werten lieferte indirekte Risslangen, die nahrungsweise identisch mit den optisch-gemessenen Risslangen waren. Weiterhin wurde festgestellt, dass die gelieferten Daten der Proben mit kleinen Breiten denen der grosen Probenweiten entsprachen. Insofern ist der Groseneffekt vernachlassigbar und die Rissfortschrittsdaten von den kleinen Probenbreiten konnen ebenso genutzt werden, wenn nicht genugend Material fur die grosen Proben zur Verfugung steht.
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