Effect of thermomechanical treatment on fatigue strength in 7075-Al
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TMT specimens were finished to 600 grit by hand grinding longitudinally. Tensile tests were conducted in air at room temperature at a strain rate of 1 x 10-3 S-l on smooth T6 and TMT specimens, and the results are given in Table 1. Fatigue tests were conducted in air on an MTS servohydraulic machine at room temperature and 20 Hz using a constant-load sawtooth waveform and axial loading in tension-tension with zero minimum load (R = 0). Relative humidity was not controlled but was usually about 30 pet. The resulting fatigue life curves for 7075-T6 and 7075-TMT are given in Fig. 2 (smooth samples) and Fig. 3 (notched samples). In both cases there is a marked improvement in fatigue strength as a result of TMT. The improvement is greatest above about 105 cycles to failure where an increase of 30 pet or more in fatigue strength occurs after TMT. Comparison of tensile strength data (Table I) with the fatigue data indicates that the improvement in fatigue strength does not arise from an increase in tensile strength alone. The ratio of fatigue strength at 106 cycles to ultimate tensile strength is 0.30 for 7075TMT compared to only 0.24 for 7075-T6. These results in tension-tension are in good agreement with those of Ostermann" in tension-compression for smooth samples, but differ from those of Reimann and Br-isbane" for notched samples. A direct comparison is difficult since factors such as the detailed procedure for TMT, specimen geometry, and test parameters used in this investigation differ from those employed previously. It seems likely that the fatigue strength can be improved by a variety of TMT procedures and that the optimum TMT has not yet been established. Nevertheless the underlying cause of
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METALLURGICAL TRANSACTIONS A
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Fig. I-Specimen geometry, dimensions in cm. a) T-6 spec.imens and notched TMT specimens, b) unnotched TMT specrmens prior to cold working during interrupted aging, c) unnotched TMT specimens: final geometry.
Table I. Tensile Properties of 7075-T6 and 7075-TMT (10 Pet CW\ Material
FARHAD MEHRPAY and W. 1. HAWORTH are Graduate Student and Assistant Professor, respectively, Department of Chemical and Metallurgical Engineering, Wayne State University, Detroit, MI 48202. DAVID 1. KUDSIN, formerly Undergraduate Student in the Department of Chemical and Metallurgical Engineering, Wayne State University, is now with the Babcock and Wilcox Company, Lynchburg, VA. Manuscript submitted November 4, 1975.
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7075-T6 7075-TMT
Ef, Pct
546 565
ay ~
Yield strength.
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= Ultimate tensile strength.
574 595
18.0 15.5
Ef ~ Elongation to fracture.
VOLUME 7A, MAY 1976-761
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o 7075·T6
250
t. 7075·TMT (lOpet)
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~ 200
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Fig. 2-Fatigue life curves for 7075-T6 and 7075-TMT (unnotched) .
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