Study of creep crack growth in 2618 and 8009 aluminum alloys
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I.
INTRODUCTION
C R E E P crack growth (CCG) commonly occurs in structural alloys at elevated temperatures. Creep crack growth behavior can be classified as creep ductile or creep brittle. Creep ductile refers to extensive creep deformation during crack growth, whereas creep brittle behavior involves little creep deformation during crack growth in the bulk material. The mechanics of CCG have been summarized by Saxena m and Riedel. ~21 According to the fracture mechanics approach, the C* integral, analogous to the J integral for ductile fracture, is an appropriate load parameter for CCG when the material is under steady-state (extensive) creep throughout the ligament. The term C is an alternative to C* for small-scale creep and transient creep, where the creep zone at the crack tip expands with time. Both C* and C, have successfully described CCG in creep ductile materials (e.g., alloy steels), 13,4j but do not correlate with CCG in creep brittle materials (e.g., Ni-based superalloys t2j or Ti alloys).ls] In such materials, better correlations between the CCG rates and the stress intensity factor, K, as well as the J integral, were reported. The micromechanisms of CCG are complex and have not been extensively explored. Possible mechanisms include microstructural degradation, environmental attack (stress corrosion), and creep deformation-induced crack growth. In this sense, CCG is more appropriately called "time-dependent crack growth under sustained load" until a specific mechanism is clarified. The CCG behavior in high-strength aluminum alloys has received increasing attention, t6-~31 because of these YANG LENG, Assistant Professor, is with the Department of Mechanical Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Sai Kung, Hong Kong. Manuscript submitted January 21, 1994. METALLURGICAL AND MATERIALS TRANSACTIONS A
alloys' potential applications in supersonic aircraft structures, where service temperatures can cause creep deformation at the crack tip. Nevertheless, the current understanding of CCG in aluminum alloys is limited. Mechanistically, investigators have attempted to characterize CCG in aluminum alloys with several fracture load parameters. Kauffman et al. t6] correlated the CCG rate with stress intensity factor, K, in a 2219-T851 AI alloy. They observed CCG at 40 pct of K~c at 149 ~ Krishnan and McEvily t71 found that the CCG rate in a 6061 A1 alloy at 220 ~ correlated with a parameter [dV/dt (P/Po)m], where dV/dt is the load line deflection rate, P is applied load, and P0 and the exponent m are constants. Nikbin and Webster tm found that the C* integral was more relevant for CCG in an RR58 (2618) AI alloy in the temperature range from 100 ~ to 200 ~ Later, Bensussan et al. t9t studied CCG of 2219-T851 again at 175 ~ and they concluded the crack growth rate was correlated with K. Sadananda et al. tim correlated the CCG rate with K and J* (J* defined in Reference 10 is similar to C*) in two A1-Li alloys at 150 ~ Jata found good correlation of da/dt and K in a 2091 A1-Li a
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