Fatigue crack propagation in 99.99 + and 1100 aluminum at 298 and 77 k
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seems unlikely in view of the small "activation energy," 0.03 to 0.04 eV, calculated from the slope of the log(dc/dN) vs 1 / T line. In a previous paper written by Liaw, Fine, Kiritani, and Ono, 9 the large decrease in the fatigue crack growth rate on cooling from 298 to 77 K in aluminum was shown not to be due to environmental effects. The slower d c / d N at 77 K was attributed to the large plastic energy required to propagate the crack. While dislocation cells were formed during cycling at 298 K, tangles were formed at low temperatures. For the intermediate range of the d c / d N vs AK plot, where the Paris relationship holds, the following equation has been empirically established, 1~
dc --=A dU
AK 4 - o'y21~U
Ill
where A is a dimensionless constant, AK is the stress intensity range, O'y is the 0.2 pct offset cyclic yield strength,/z is the shear modulus, and U is the plastic work to create a unit area of fatigue crack. This equation was derived in many theoretical treatments, n21 The parameter U was first measured by Ikeda, Izumi, and Fine 22 using foil strain-gages. Weertman's recent theoretical work 19,2~predicts that U is independent of AK when the Paris exponent on AK is 4 but U varies as (AK) 2 when the Paris exponent is 2. This has been confirmed by experiment. 23 The previous measurements of U were on relatively high-strength aluminum alloys and steels. 11,12,22-24 One of the purposes of this paper was to test the validity of Eq, [1] in lower strength 99.99 + A1 and 1 100 At at room temperature. Measurements of U at 77 K were attempted without success but the dc/cIN results at 77 K are discussed using Eq. [1]. Additionally, the plastic zone ahead of the crack tip resulting from the determination of U was mapped and the size of the plastic zone compared with theoretical predictions. Cyclic mechanical properties are more valid parameters to characterize the fatigue crack propagation rate than static mechanical properties since the material at the fatigue crack tip goes through cyclic deformation.
ISSN 0360-2133/81/1111-1927500.75/0 METALLURGICALTRANSACTIONSA 9 1981 AMERICAN SOCIETY FOR METALS AND VOLUME 12A, NOVEMBER 1981--1927 THE METALLURGICALSOCIETY OF AIME
Relatively little research has been done in correlating the fatigue crack propagation rate with cyclic stressstrain response at cryogenic temperatures because there is little such data. In the present study, strain controlled stress-strain relations were also determined at 77 and 298 K to assist in understanding the effect of temperature on fatigue crack propagation rate. EXPERIMENTAL PROCEDURES A. Fatigue Crack Propagation and Cyclic Stress-Strain Tests The fatigue crack propagation study was done in pull-pull at 298 and 77 K on aluminum of 99.99 + pct purity and commercially pure aluminum (1100). The chemical composition of 1100 A1 in wt pct, which was obtained by a quantometer using spark-spectra analysis 2s and furnished to us by ALCOA, is Si, 0.09; Fe, 0.55; Cu, 0.15; Mn, 0.01; Zn, 0.02. The fatigue crack propagation rates were measured
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