Fatigue and Static Strength of Notched and Unnotched Aluminum-alloy and Steel Specimens Principal objective of paper is t o present data on s o m e c o m m o n l y u s e d aircraft materials i n a f o r m so t h a t variations in t h e fatigue strength with elastic stress-concentration factor can be shown
by Richard E. Whaley
.This p a p e r describes a method of presentation of fatigue d a t a on three commonly ~ ~ s aircraft ed materials, 2024 '1'3 a n d 7075-'l'fi aluminurn ~ ~ l l o ya ns d normalized S;2E -ll:IO steel, suc.11 th;it variations in f'atiguc: s t r e n g t h with siress-conc.enl~.niir)nfactor can he shown. ( : o n j):irisons of the fatigue sLrengt1-i~o f 2024-'I':I anti 7075-'I'6 aluininiim a r e m a d e for the most useful range of stresx~ : o n c e n t r Lion a factors. Static-strength results of not.clied a n d unnotcheci specimens of the three materials a r e presented to show how the s t r e n g t h varies w i t h some parameters of t h e stress concentration. Comparison of lhe d a t a w i t h one theory for the s t r e n g t h of cracked specimens was made. AHSTKACT
Symbols Neuber rnateriai c o n s t a n t , in. fatig~ie-strength-reductionfactor Neuber "practical" stress-concentration factor e h s t i c stress-coricenlratio~iFactor theore tical stress-concen tration factor for ul t i m a t e tensile s t r e n g t h notch-root radius, i n . niaxirnurn load divided b y initial net, sect,ional area, ksi ultimate tensile s t r e n g t h of unnotched specimen, ksi ultimate tensile s t r e n g t h of notched specimens, ksi S,,/S,, = n o t c h s t r e n g t h ralio n o t c h sensitivity relative stress gradient, in: 1
variations i n the fatig~lestrength witii elastic. stressconcentration factor can be sllown. I t is generally assumed t h a t thr static strength of rmterinls is litti? affected by stress c o n c e ~ i t r ~ ~ t i o n s such as ~ ~ o t c l ~holes e s , and illlets Anatller objective of this pa1)er is to s l ~ o wthe reL~tionshipof the static strength with some patnmeters of the s t r t'bb .-COIIC~IItrations
Description of Specimens The specimens used to obtain the notch tensilestrength results are shown in Figs. 1 and 2. Speciinells w e r e designed to obtaln the widest range of notch-root radii t h a t was practical for each value of stress-concentration factor. The spechimens were all nlacilir~cd from the snrnc sheet of 0.091-in. thick 2024-T3 alumirlum alloy. The static-strength properties are given inTable 1.
Introduction When aircraft structures are designed, or when fatigue failures occur, comparisoris are usually made with data from simple specime~ls. I t is often difficult to compare t h e d a t a , because comprehensive data on simple specirnens is not usually found for the elastic stress-concentration factor desired. One of the objectives of this paper is to prcsent data on some commonly used aircraft materials in a form so t h a t
Fig, 1-Configuration
of s h e e t specimens
TABLE 1-STATIC TENSILE-STRENGTH PROPERTIES OF 2024-T3 ALUMINUM-ALLOY SHEET SPECIMENS 1 0 00--Type of speclmen
Unnotched
2
1
1-5 00 1 . 0 0 0 RADIUS
C
Yield strength for 0.2% offset, psi
Ultimate strength, psi*
17.5
57,000
72,500
Notched ( K t = 2.0) r = 1.000 r = 0.100
...
...
69,300 73,300
N o t c h e d ( K t = 8.0) r = 0.050 r = 0.001
...
...
59,800 66,500
===i 0 5 0 RADIUS
...
...
...
...
*Average results of three specimens.
results was selected from t h e same lot. Notched fatigue-test specimens h a d a n e t width of 1.5 in.
2.0 RADIUS
1' Fig. 2-Configuration
Elongation in 2 in., %
Procedure Kt = 8 0
T h e fatigue tests, a s described in Refs. 1 t o 4, were conducted on Krouse direct repeated-stress machines. Procedures a n d tecllniques were generally t h e same for all tests reported. All stress values indicated are nominal net area stresses.
of sheet specimens
o OATA FOR UHHOTCHLO SPECIMENS
Results Discussion
SO = 1 3 . 0 0 0 PSI 2 0 2 4 . 1 3 A L U M I N U M ALLOY l o 7 CYCLES
2 0 2 4 - 1 3 A L U M I N U M ALLOY
KSI MEAN STRESS 30 20 10 0
\ Q ,
So 0
-
82 5 0 0 PSI 7 0 1 5 T6 A L U M I N U M ALLOY
7 0 7 5 . 7 6 ALUMIHUH, ALLOY
107 C Y C L E S KSI M E A H STRESS
9
--
0
1
2
Kt
3
4
5
S o ~ 1 1 7 . 0 0 0P S I "O loo
11
1
1 0 0 H O R M A L I Z E D SAE 4 1 3 0 S T E E l
NORMALIZE0 g A E 4 u o ST="' 104 C Y C L E S
~ O I
I
20 ' 0
1
2
3
4
5
0
107 c l c i t s
9
i-----
0
1
2
3
4
5
ELASTIC STRESS CONCENTRATION TACTOR. K t
Fig. 3-Stress variation with stress-concentration factor for constant mean-stress curves
T h e s1)ecimens and material for most reference test results are described in Refs. 1 to 4. All s1)ecimens were of sheet material which was 0.091-in. t11ic.k for the aluminum s1)erime11sa n d 0.075-in. thick for the steel specimens. All material for t h e fatigue test
Fntigue Test T h e fatigue-strength information given in Refs. 1 t o 4 is presented in Fig. 3 a s maximum stress vs elastic stress-concentratioll factor for values of cons t a n t meall stress and constant lifetime. T h e d a t a is pesemed iilc tluee ~uniei;als(2024-T2 a i d 7075TG nlurnjnum alloy arid normalized SAE 4130 steel) for u~rnotched sl)ecirnens and s p e ~ i m e n swith edgecut notcslles \vith constant over-all and net widths. 'The only geo~netrical ari,iblr., t h w e f o ~ e ,was t h e ~ m t c h - r o oradius, t I.. Examination of Pig. 3 reveals some interesting pl~enomena. T h e d a t a 1)oints for ummtched spccimens ( K , = 1P agree very well with t h e curves for t h e notched specimens extrapolated down t o I
