Fracture resistance and heat treatment in 7075 aluminum alloy
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Table I. Composition of 7075 Aluminum Alloy
Element
Wt Pct
Zn Mg Cu Fe Si Cr Ti Mn AI
5.1 2.1 1.2 0.7 0.5 0.2 0.2 0.3 Remainder
Fracture Resistance and Heat Treatment in 7075 Aluminum Alloy RICHARD A. QUEENEY The mechanical response of high-strength metallic alloys (yield s t r e s s e s in excess of E/150, where E is the Young's modulus of the solid) containing sharp flaws has been rigorously analyzed I and test methods devised for measuring their notch sensitivity. 1'2 More recently, efforts have been made to enhance alloy f r a c ture resistance, while maintaining acceptable yield strengths, by m l c r o s t r u c t u r a l manipulation. Kirman 3 thoroughly examined the possibility of Increasing the resistance of 7075 aluminum alloy to critical flaw extension through various heat treatment regimens. Kirman s chose the Kahn tear test 4 as a means of measuring fracture resistance. Although the tear test has been applied extensively to aluminum alloy sheet materlals, 4 it overemphasizes the role of material ductility during fracture, as it involves forcing a stable crack through a specimen configuration that exhibits large amounts of plastic flow at the tip of the advancing flaw. The concept of fracture toughness ( K I c for plane strain, K c for plane stress) applies more directly to critical flaw extension I and structural design practice. The present study characterized the fracture r e s i s tance of 7075 aluminum alloy, as it depends on heat treatment, in t e r m s of the plane s t r e s s fracture toughness K C. The alloy composition is given in Table I. Double edge-notched specimens 0.050 in. thick, 4 to a given microstate, were solution treated for one h in salt at 480~ ice water quenched, and aged as indicated by the data in Fig. I. All K C testing was carried out with specimen geometry and methodology according to accepted ASTM practice, 5 with thin-sheet plane s t r e s s specimens rather than the plane strain configurations described in Ref. 5. The results of the K c tests are shown in Fig. 1 for all heat-treatment states. The present results differ significantly from those previously reported by Kirman, 3 in that the unit propagation energy (UPE), the l a t t e r ' s measure of fracture resistance, decreased for all aging corresponding to increasing time at aging, except for extensively overaged alloy at 177~ The present work Indicates that the fracture toughness measure increases, to a maximum value, for conventional 8 aging schedules at 120~ decreasing after achieving that value. Similar behavior
Kc
308-VOLUME 5, JANUARY 1974
-5/2
50
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" ~ C
20-
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i
! I0
1.0 Aging
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hours
Fig. 1 - - F r a c t u r e t o u g h n e s s KC of 7075 A l u m i n u m alloy f o r v a r i o u s aging t e m p e r a t u r e s and t i m e s .
Kc
" 312
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510 RICHARD A. QUEENEY is Associate Professor of Engineering Mechanics, The Pennsylvania State University, University Park, Pc. 16802. Manuscript submitted August 2, 1973.
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