A study of creep crack growth in 2219-T851
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INTRODUCTION
NICKELbase
superalloy parts in jet engines, some stainless steel assemblies in nuclear and conventional power plants, and titanium and aluminum alloy components used in hot sections of aircraft structures are all maintained in service at temperatures well within the creep regime (i.e., T/Tm > 0.4).* *A table of symbols is provided at the end of the paper.
It has been found that a single crack can often propagate at high temperatures under a sustained load, mainly under the influence of creep damage and/or environment-induced damage. 1-5 In the case where creep damage only is present, it is now well established that a crack propagates by nucleation, growth and coalescence of intergranular cavities on grain boundaries lying ahead of the crack tip. 1.6-13Whether the role of an aggressive environment is to accelerate one of these stages or to cause a damage of a completely different nature is still not clear in all cases. 13Thus, we define Creep Crack Growth as "the propagation of a single macroscopic crack under sustained load at temperatures well within the creep regime."
I. CREEP BRITTLE v s CREEP DUCTILE BEHAVIORS Materials susceptible to creep crack growth (CCG) can be said to be either creep brittle or creep ductile. Creep brittle
materials fail by CCG with almost no bulk creep deformation, while creep ductile materials fail by CCG with extensive bulk creep deformation, even under initial smallscale yielding loading conditions. For example, it has been shown that nickel base superalloys are creep brittle at temperatures as high as 760 ~ and 304 stainless steel is creep ductile at temperatures as low as 538 ~ The distinction between these two extreme behaviors can be rationalized to a certain extent by using concepts of fracture mechanics of creeping solids which are reviewed below.
A. Fracture Mechanics of Creeping Solids In a creeping stressed body, total strains are the sum of time independent elastic (eel) and plastic (epl) strains and of time dependent creep (eer) strains. Around the tip of a crack in such a solid, elastic and plastic strains develop instantaneously as the load is applied, and, as time increases, creep strains build up, in particular close to the crack tip where stresses, and thus creep strain rates, are very high. In the case of a stationary sharp crack loaded in Mode I, the stresses ahead of the crack tip can be approximately calculated as a function of time and distance from the crack tip as f o l l o w s . 16'17J8 Upon loading (at t = 0), in the region where the elastic strains are dominant, the stresses for initial small-scale yielding loading conditions are well approximated by the usual singular field: 19
K,
~j = 2 ~ / ~ f , j (0) PHILIPPE L. BENSUSSAN, formerly Graduate Student, Massachusetts Institute of Technology, is Ingenieur de l'Armement, ETCA, 94114 Arcueil Cedex, France. DAVID A. JABLONSKI is Senior Material Scientist, Instron Corporation, Research and Applications Laboratory, Canton, MA 02021. REGIS M. PELLOUX is Professor, Massachusetts Institute of Tech
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