Metallurgical factors affecting fracture toughness of aluminum alloys
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T H E t o u g h n e s s l e v e l s of m e t a l s g e n e r a l l y d e c r e a s e as the s t r e n g t h l e v e l is r a i s e d by a l l o y i n g and heat t r e a t m e n t . T h i s t r e n d is i l l u s t r a t e d in Fig. 1 f o r a l u m i n u m a l l o y s . At the s a m e t i m e , the m i n i m u m f r a c t u r e toughn e s s r e q u i r e m e n t s b e c o m e m o r e s t r i n g e n t as the y i el d s t r e n g t h is r a i s e d . * T h e s e opposing t r e n d s p l a c e a *The toughnessrequirementfor a particular component depends on lhe sensitivity of the flawdetectionprocedures,as wellas on the safety factor (the ratio a/ay, the designstress to the yieldstress), the cyclicload and environmentalhistories, and on the sizeand geometryof the part. At the present time,good NDI practices can reliablydetect surface cracks 2ao/> 1.27 mm-long(0.05 in.-long). The ratio O/Oy = 0.8 is frequentlyused in aircraft design. For the case of a deep surface flaworientednormalto the stress axisKIc = o ~X/c~, wherea* is the critical flawsize.The toughnessrequirementthen takes the form: Klc ~>0.8 oy x/(0.64 mm).zra*/ao. c e i l i n g on the y i e ld s t r e n g t h l e v e l that can be e m p l o y e d s a f e l y by the d e s i g n e r . Two h y p o t h e t i c a l t o u g h n e s s r e q u i r e m e n t s a r e i l l u s t r a t e d in Fig, 1.* F o r s e r v i c e i n *Toughnessrequirementsare specifiedin Fig. 1 for 2 hypotheticalsituations: Requirement 1. No stable crack growth:a* =ao, and (Kic/Oy)~ 2.5 mmy~cyclic loadingor environmentsthat produce stable crack growth. Requirement 2. Extensivestable crackgrowth: a* = 5no and (Klc/Oy)>~1.1 mmy2. lids requirementis appropriate when cyclicloadingand/or stress corrosion produce a 5-foldincreasein the natural flawsizeduring the servicelife. G. T. HAHN and A. R. ROSENFIELD are Manager and Associate Manager, respectively, Metal Science Section, Battelle, Columbus Laboratories, Columbus, Ohio 43201. This paper is based on an invited presentation made at a symposium on "Advances in the Physical Metallurgy of Aluminum Alloys" held at the Spring Meetings of TMSIMD in Philadelphia, Pennsylvania, on May 29 to June 1, 1973. The symposium was co-sponsored by the Physical Metallurgy Committee and the Non-Ferrous Metals Committee of TMS-IMD METALLURGICAL TRANSACTIONS A
v o l v i n g e x t e n s i v e fatigue o r the s t r e s s c o r r o s i o n type of s t a b l e c r a c k growth ( R e q u i r e m e n t 2), a m a x i m u m y i el d s t r e n g t h of r o u g h l y a y ~ 350 MNm -z (~ 50 ksi) is i n d i c a t e d . * U t i l i z a t i o n of h i g h e r s t r e n g t h l e v e l s and the *In practice, maximumstrength levelsare based on minimumguranteedKicvalues, rather than typicalor averagevaluesas are presented in Fig. 1. attendant o p p o r t u n i t i e s f o r i n c r e a s i n g s t r u c t u r a l e f f i c i e n c y depend on i m p r o v i n g the f r a c t u r e t o u g h n e s s or the c r a c k d e t e c t i o n p r a c t i c e . T h i s p a p e r f o c u s e s on
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