The Small Fatigue Crack Growth Behavior of an AM60 Magnesium Alloy
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UCTION
WITH increasing fuel costs and global environmental awareness, magnesium (Mg) alloys are gaining increased interest because of their potential in automobile applications where weight reduction is preferred.[1,2] A better understanding of the strengthening mechanisms is essential for the development of Mg alloys with higher strength. It has been shown in a recent study[3] that a significant improvement in the tensile and fatigue properties can be obtained by applying thermomechanical processing and subsequent heat treatment to a Thixomolded AM60 Mg alloy. Characterization of the fatigue crack growth (FCG) behavior is, however, required to provide a mechanistic basis for the improvement in fatigue lives of Thixomolded microstructures. The FCG rate for long cracks has been characterized based on linear elastic fracture mechanics (LEFM). One of the well-recognized empirical relationships is the ‘‘Paris Law’’[4] where the crack extension per loading cycle, da/dN, is characterized by the stress intensity factor range, DK, in the following form: da=dN ¼ CðDKÞm
½1
where C and m are empirically determined constants for a given material in a given testing condition. The above relationship can only be applied to a fixed load ZHE CHEN, Ph.D. Student, and CARL J. BOEHLERT, Associate Professor, are with the Department of Chemical Engineering and Materials Science, Michigan State University, 2527 Engineering Building, East Lansing, MI 48824-1226. Contact e-mail: chenzhezju@ gmail.com AMIT SHYAM, R & D Staff, is with the Oak Ridge National Laboratory, P.O. Box 2008, MS6069, Oak Ridge, TN 378316069. JACK HUANG, Technical Director, RAY F. DECKER, Chief Technical Officer, and STEVE E. LEBEAU, President, are with the NanoMAG, LLC., 620 Technology Drive, Ann Arbor, MI 48108. Manuscript submitted February 24, 2012. Article published online October 3, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
ratio (R value), and as such the constants C and m need to be determined, as the R value is changed. Other relationships have been proposed to incorporate the effect of load ratio. A simple relationship was proposed by Walker,[5] in the form: da=dN ¼ CðKmax Þp ðDKÞn
½2
where the maximum stress intensity factor, Kmax, can be related to R and DK by the relationship: Kmax ¼ DK=ð1 RÞ
½3
However, it has been shown that the empirical relationships based on LEFM are generally inappropriate for small cracks, which can be described as (1) microstructurally small (when their length is comparable with the characteristic microstructural dimensions, e.g., grain size), (2) mechanically small (when their length is comparable with the scale of local plasticity), or simply (3) physically small (e.g., smaller than 1 mm).[6] The ‘‘small crack effect’’ was first discovered in aluminum (Al) alloys,[7] and this effect refers to the phenomenon where small cracks can propagate faster than long cracks at an equivalent LEFM driving force (DK). Thus, crack growth data obtained from long crack growth experiments may lead to a non-conservative lifetime predicti
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