The growth of small fatigue cracks in A286 steel
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I.
INTRODUCTION
THIN-walled tubes o f modified A286 steels have been used in the structures o f high-field superconducting m a g nets and are candidates for use in future superconducting magnet systems. [1[ The specific application is in the conduit o f a "cable-in-conduit conductor." The superconducting cable is enclosed in a thin-walled tube that serves as a path f o r forced flow o f liquid helium coolant. The conduit also serves as a distributed structural elem e n t . Since magnets operate at the temperature o f liquid helium, 4.2 K, and are often subject to cyclic magnetic fields, fatigue resistance at cryogenic temperatures is a critical issue for the magnet design. Since the conduits are thin-walled structures, it is particularly important to understand fatigue c r a c k growth in the conduit material in the small-crack regime. The research reported here was originally undertaken to address that problem. The long-crack fatigue crack propagation rates in A286 steel have been determined in both the Paris [2J and the near-threshold t3~ regimes from tests on standard compact tension specimens. However, recent results on the propagation o f surface fatigue cracks [4,5,6[ show that the longcrack constitutive relations may break down at small crack sizes; small cracks can propagate at rates that are very different from those o f the corresponding long cracks at the same n o m i n a l driving force. Small cracks sometimes grow at cyclic stress intensities b e l o w the long-crack threshold with growth rates much faster than would be predicted by extrapolating large-crack results. Particularly f o r thin-walled structures, fatigue life predictions based on the long-crack growth rate data may be misleading. Several mechanisms for the anomalous growth o f small Z. MEI, Associate Specialist, is with the Department of Materials Science and Mineral Engineering, University of California-Berkeley, Berkeley, CA 94720. J.W. MORRIS, Jr., Professor of Metallurgy, Department of Materials Science and Mineral Engineering, University of California-Berkeley, is also Program Leader, Structural Materials, Center for Advanced Materials, Lawrence Berkeley Laboratory, Berkeley, CA 94720. Manuscript submitted March 1 9 , 1992. METALLURGICAL TRANSACTIONS A
cracks are described in recent reviews. [7,8,9] (1) The c r a c k may be "physically short." At very small c r a c k sizes, the c r a c k closure that lowers the driving force for longc r a c k propagation is absent o r reduced. (2) The c r a c k may be "mechanically short." When c r a c k length becomes comparable to the scale o f l o c a l plasticity, the small-scale yield assumption that underlies the application o f linear elastic fracture mechanics becomes invalid. (3) The c r a c k may be "microstructurally short." When the c r a c k length is small compared to the relevant microstructural dimensions, such as the grain size, the assumption that the crack grows in a homogeneous, isotropic continuum is no longer valid. (4) The c r a c k may be "chemically short." Short cracks are
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