On fatigue crack propagation of titanium alloys under dwell time conditions
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On Fatigue Crack Propagation of Titanium Alloys under Dwell Time Conditions A.W. SOMMER and D. EYLON The results published by Stubbington and Pearson ~ on fatigue crack growth rate (FCGR) in titanium alloys under dwell time conditions are in marked contrast with those reported by Bania and Eylon. 2 Stubbington and Pearson ~ reported FCGR in an annealed bar of highly basal transverse textured Ti-6A1-4V (tensile axis normal to the basal plane) was increased by one or more factors of 10 under dwell of 5 or 45 minutes at maximum load. In contrast, Bania and Eylon2 did not detect any FCGR acceleration in similar material, orientation, and test conditions. In other material conditions, they even reported lower FCGR under dwell. The major differences between the two works were: Bania and Eylon employed a triangular waveform at 6 Hz with a 5 minute dwell, while Stubbington and Pearson used 0.3 Hz frequency with 5 or 45 minute dwell periods. In addition, while Bania and Eylon obtained most of their data below AK = 20 Ksi × in -~/2 (21.98 MPaX/--mm), Stubbington and Pearson observed the dwell related FCGR acceleration above AK = 25 Ksi × in -1/2 (27.47 MPaX/--mm).The objective of this work was to conduct dwell FCGR tests in the low and high AK levels at 0.3 Hz and 6 Hz to provide additional data with respect to the above described disagreement. A displacement controlled constant stress intensity method was employed in this work. This method allowed accurate FCGR measurements over long crack growth lengths. It can also detect the crack growth (da/dt) during the dwell period, which is not an easy task in a conventional FCGR compact tension testing set-up. FCGR testing was accomplished by controlling the deflection amplitude of notched, rectangular-cross sectioned, simply supported beam in three point bending. The load amplitude resulting from the applied deflection amplitude was continuously monitored autographically using a conventional strain gauge A.W. SOMMER is with Del West Engineering, Inc., 9440 Irondale Avenue, Chatsworth, CA 91311. D. EYLON is with Metcut Research, Materials Research Group, P. O. 33511, Wright-Patterson Air Force Base, OH 45433. Manuscript submitted December 31, 1982. 2178--VOLUME 14A,OCTOBER 1983
bridge load cell. Crack formation and growth during such a constant deflection amplitude test causes the load amplitude to decrease since the sample's compliance increases. Creager and Sommer (References 3 and 4) have shown that if one matches the initial compliance and geometry of the bend bar to that of the test frame, it is possible to create a crack path length duration on the order of 0.080 inch (2.08 mm) in the center of the bend bar where K remains constant for constant deflection conditions as the crack grows through it (see Figure 1). Moreover, they have also shown that one.can unequivocally determine the rate of crack growth through the constant K region of the sample from a knowledge of the relation between load and cycles or time, depending upon whether one has applied a cyclic or a static defle
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