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GALCIT REPORT NO.
REVERSED BENDING FATIGUE PROPERTI"
OF 25 S-T, 75 S-T AND 76 S-T ALUMINUM ALLOYS
Thesis by
James S. Cooley, Lt» Comdr. U.r.
1949
PASADENA, CALIFORNIA
FORM A4.-3 6M 2-49
3
Q duth to acknowledge Ipful g 18 end
ofiorta in the preparation of this report as follows* £rs.
Sechler, D. S. vlark and P, F.. Hudson iidanoe; F. *. Crandell,
Milton Woods for photo, llliaia Boworip C, a. Bartsch and the
GALCIT Machine Shop orew for equipment and repairs; Betty Ernst for
presentation of data; and Dor 1m Cook for stenography.
The author is further indebted to Dr. Loo Shapiro and The Douglas
Aircraft Co. for ehot-peenin^ work, and to jir. Joe Smith of Cooperative
1 Tunnel for photographs.
1092
iii
SlEJLiARY
The problem of this investigation was to determine the effects
of surfaoe roughness and surface stressing on the reversed b-inding
fatigue properties of 25 5-T, 75 S-T and 76 S-T aluminum alloys.
Tests were conducted in a stress range from 37,700 psi. to that
stress giving a fatigue life of 500,000,000 cycles
Surface roughness was varied from five microinches to 400 micro-
inches. Two different machining tools were used to obtain the various
de roes of surface roughness t l/8" radius tool, ond a Siiarp pointed
tool.
Surface stressing was obtained by shot peening with OJ028 dianeter
shot at .010/.012 A-2 intensity, and oold rolling at 100 lbs. and 200 lbs,
pressure.
It was determined that fetirue life decreased as surface roughness
increased in a similar manner for specimens machined with both types
of tool j but that endurance limit wps not affected by the sharp tool,
whereas it was decreased by the l/B" radius tool.
Shot peening increased fntigue life of 25 S-T by about 500%i it
had but slight effect on 76 S-T.
Cold rolling increased fatigue life of 25 S-T and 76 S-T by about 2500,.
75 S-T was neither shot peened nor oold rolled.
This work was carried out by the author at the Guggenheim Aero-
nautical Laboratory, California Institute of Technology under t he
suDervision of if. E. #• Seohler.
XV
nrLE paoe
Acknowledgemanta ii
Summary iii
Taula of Contents iv
List, of Figures v
Figure* 1-5 1
I Introduction 5
II I^uipasnt and Prooeduro - 7
III Roaulti and Discussion 11
IV Conclusions 19
V RoTerences 21
VI Sanple Calculations 22
VII Figures 6-23 2.3
LIS-
- f- * _ .r ice
r
11-22 -v.G of Results
frr.ai. r r.
-/-
1
?
3-
-4"
i. lira ctiok
Oil the preoedim;; pages. Pig. l-5pone can readily see the
importance and danger of fatigue failures. The installation
shown is part of the Cooperative uind Tunnel, Pasadena, California.
These figures show the damage after fatigue failure of one of the
propeller blades during operation* The resulting damage a -mounted
to about ?270,000 in material, not Including valuable research ''ime
and money lost while repairs were made.
This is only one example of many such failures which ocour
throughout industry; in the aircraft industry, especially, fatigue
is of primary concern whore the lightest possible parts must be
made to attain the necessary requirements of low oost and high per-
formance.
It is interesting to note here & statement of A. G Pugsley in
Ref • 1 to the effect that while the Metallurgical Science is giving
us stronger and stronger alloys, there is little or no change in
fatigue strength of these alloys. Therefore, it behooves the dosi-ner
to pay careful attention indeed to the problem of vibration in his
structure and the fatigue strength of the material with which he is
workint •
One very important consideration in fatigue studies is surface
finish, and much research has been conducted on the effects of surface
notches on fatigue strength. It was discovered that the propeller,
Fig. 1., which was fabricated from 26 S-T aluminum alloy, failed at a
fillet having a surface roughness of t 100 microinohes.
The neoossity for more fatigue data on aircraft materials at once
became apparent, especially with regard to the effects of various
ioaohining tools and finishing techniques.
This report is thus an attempt to furnish aircraft designers
with additional fatigue strength data on 25 S-T, 75 S-T, and 76 S-T
aluminum alloys, with particular emphasis on the effects of cj various
maohine surface finishes, b) shot peening, and o) cold rolli
The investigation was conducted by the author in the Cu?:--enheim
Aeronautical Laboratory, California Institute of Teohnolo?', Pasadena
California under the supervision of Dr. E. K. Sechler during the
period October 1948 to May 1949.
II. ! uir T
. 11 tost specimens SALCIT nochine shop
according to the standard 3,R 9 MOORE Specifications in Ref. 2.
T'?9 various surface finishes wars machined as fallows:
FfiiSD - ISCHiSS/^EVOLUTIOB
1/8" Hadius Tool Sharp Tool
25 S-T 75 S-T 76 S-T 25-75 S-T 76 S-T
5 U 3.0015 0.0015 0.0015 Finest
50 JU 0.0160 0.0145 O.X)26 0.00S8
100 /' 0.0240 0.0210 0.0030 0.0046
200 p 0.0420 0.0052
400 /( 0.0740 0.0076
Polishing with 600 Durite abrasive and levigated alumina
followed the machining; of 5// specimens.
M IJ
ft refers to surface roughness in micro inches. The surface
roughness of the specimens was ohecked in b Physicists Research Co.
Profilome^or , Type Q, &odol l tSerial No. 141,
The shot p?eninc~ was done by Douglas .Aircraft Co., with 0'.'028
diameter shot at 0.010/0.012 A 2 intonsit; .
In order to study the effects of surface rolling, a rolling
device was designed by the author j it was built by the OALCIT machine
shop and adapted to a Pratt rnd Whitney 15" Lathe, Model B#
Serial
No. 113 S3. See Figs. 6 and 7. Rolling pressures usod were 100
end 200 lbs. at "0012/revolution feed. A constant pressure was
maintained by the operator by maintaining a constant spring deflec-
tion as the rollers progressed along the test specimen. 'Sample
' **?
Calculations '
t section VI, show computations involved in c
and use of this dp paretui.
The specimens thus trsated were 23 S-I nnd 76 S-T, mpchined
and polished to 5 jj • minimum diameters of these s ?_ ens were
checked after rolling, and e final diameter v.-aa used in stress
oimputations,
25 S-7 specimens were mpde iron rem 3nta of the : from
which the propeller blade (Fir. 1) had been made. This for
fron the Chevrolet Transmission Co. end had the following properties!
Yield Strength 40003 psi
"Jit. Strength 600
Clon ation (2n
) 16.75£
lieces cut ti . ci .ens ion o : ^r* turned
nwi ih .-rain7
, t.iose out alon -- the short dimension ^ere teraed "^cross
3rain n.
75 S-T specimens case frar /looa ^S S-T6 red, 5/8* x 12 ft. end
thus were all "with grain •
76 S-* specie ^r.s trere ojtain^d fTc I jne ttest :i83es,
also nv?ith rein '.
The stnndrrd properties ; .£. specific . . of
25 S-I, 75 --* and 76 C-T ere list:
25 3-T 75 S-T
Yield Stror.^th 30000 psi 72000 psi psi
:it. Strer 55000 80000 psi .
ion (2') 16.:: 14£
Specimens were placed in an optics! gear tooth comparator end
measurements were mnde of the major s<;rf*ce irregularities. Figs.
8 and 9 show the profiles at 100 diameters* Horizontal measurements
ere or^st to crest; vertical mer surements sre crest to valley, verage
values of those measurements follow
t
v-
5 M
50 ju 0.0005 0.01480
loo m 0,00065 0.024X0
200 Jj 0.0035 0.03285
400 tf 0.00490 0.05000
1/3" Radius Tool Sharp Tool
Vertical Horizontal Vertical Horizontal
0.0003 0.0037
0.0007 . 30280
0.0008 0.0054
0.00140 0.00620
From these photographs it wps estimated that the "sharp" tool
used had a diameter at the tip of ebout 0,003 inches,
Tests were made in a set of four B.ALT SIX - E EE R.R. MOORE
FATiOUii ISS7D JiilNfiS, Serial Bos. 266, 268 , 270, end 271, running
at a nominal speed of 10000 rpci. Fig, 10. The lends veried from
50 lbs. do,mwerd until a load giving fatigue life of 500,000 ,000
oyclss ms reached. -^11 specimens ^ore ran to destru ith , .inor
exceptions in the range s'doy>? 100,000,000 cycles when it was felt
that more information could be obtained.
atoppin;: tud usin.; the
machine involved for other specimens.
From the data thus obtained standard (T-if curves wore plotted.
The possible sources of inaccuracies or deviations from oonstant
results are as follows
t
a) Vibration of rotating parts.
'Xj-
b) Boating of speoi e to *' 'jrntion and friction,
o) Unavoidable impacts on specimen diirir. • lo<dir- •• i_
• " s by he] «,
d) Lag between loading and cycle oounter setting,
e) Variation in niachinin .
f) *!on-horaor;eneit; 7 of metal.
».«, JL =
III. R .SULT, SIOK
Or utmost importance in the analysis or use of fatigue data,
such as CT-i,T curves, is the fact that for one metal no definite
0--K curve can be obtained] instead, an area, or upper and lower
bound, containing the fatigue properties is obtained* The possible
errors outlined in the Introduction give adequate reasons for this
situation. It is, therefore, a problem for the designer to take into
acoount this possible variation in fatigue life of a metal at a ^iven
load.
The results of these tests have been plotted in Fi^.s. 11 through
22 from test data obtained. On the figures where actual tests points
are plotted, a dashed curve has been drsrwn to indicate the average
0~ -N line. The aotuel spread in the' data can be seen from these
figures. On figures -where a group of solid curves appears, the
solid curves are replots of the dashed curves of average values, /ll
test data has been plotted.
Figures 11, 12, 13, 14,#end 21 she--* all tests of 25 S-Tj Fig, 15
shows 75 S-Tj Fif> 16, 17, and 22 show 76 S-T. The remaining curves
Fi£S. 19, and 20 are comparisons of the three metals tested.
lu general 25 5-1 exhibited t irl\
Host •peox.i»sus broke near the center of the t s1 1 ha brei
perpendicular tcy the axial center lit:.. >' typicsl fr le el
in Fig. 23.
The endurance limit for 25 S-T appears to be ebout 18, Tn si.
based on a liie oi' 6OO fOOOt0O0 cycles. Ihii arrees with specifications,
From Fig. 11A it can be seen that increasing rou>~hnejs free. 5/f ~o
400/f on spec lineno machined with the 1/8" Radius tool end cut "with
grain'1 docreesed the endurance limit fror ebout 18,300 psi. to some-
thing under 10,000 psi. This result was to be expected.
However, when 25 S— r
i' "with grain" wn3 machined with a sharp tool
an entirely different result wag obtained as sho'.vn by Pig. 12 .. In
this oase, increasing roughness decreased fatigue life in the high
stress range but hat very little effect on endurance Unit; a value of
about 13,000 psi* held for all surface roughness.
/ similar result was obtained from 25 S-T ''cross grain" tents
illustrated in Figs. 13 and 14. . The l/3'' R. Tool ajc-ain produced a
decrease in endurance limit while the sharp tool had little effect.
A vsry interesting result was obtained from the tests on 25 S-T,
"oross grain11 share tool. See Fir:* 14a. No attempt was made to draw
curves since all d&ta fell in ono bend, apparently the effect of the
grain direction counterbalanced the effect of surface finish.
Fig* 142 gives a comparison of 25 S-T, 5// , in the four conditions
tested.
if 71 <"-".. ' with t ' machine :;
' sharp
tool only. This metal g*"*e very uniform results with little scatter
ir the data, ses Figs. 15, Ag in the cap' o r' 25 S-T rly pc v i-
all curves converged or as enduranoa limit of abou* 10,010 pal.
Fractures were not as regular as t>oao of 25 S-T, Tee Fir, 23, In
the upper stres3 'range 73 S-T had about three tines the fatigue li">
of 25 S-T, and about ono and one-half timea that of 70 S-T. For
comparison see Pig* 20*
76 S-T
VJhereas 25 S-T and 75 S-T broke with fairly clean fractures,
76 S-T had a comparatively apeotaoular rupture , The breaks were very
irregular, large pieces were thrown off tho specimen, and there was an
accompanying loud noise when rupture occurred. Pig. 23 shows a typical
fracture.
The results of tests on 76 S-T machined with l/8" Radius Tool
showed fairly consistent values. See Fig. 16. It is felt that the
50 U curve will converge at higher values of "12". Tims preventec"
further investigation.
The endurance limit for 76 S-T machined with I/O" Radius Tool
appears to be between 19,000 psi. and 20,000 psi., or roughly 10^
highar than 25 S-T and 75 S-T.
As can be seen in Fi^s. 17, machining 76 S-T with a sharp tool
produced rather irregular results. However, there was noted a sharp
rise in the endurance limit to a value of & iout 24,000 psi. This
I
reproc .• >r S»T and 75
3 B ra hica I
Despite ita irregularitye a 7. . - , rrp tool, ganre higher
flati; ue itroc , an 7j S-T§ l/8
u1 Tool, throughout the entire
range. See Fig. 17...
15
L'G
It has loi that compression of the outer fibers of
a structural member would increase its fatigue strength in reversed
bending, Shot-peening seems to be about the simplest way oj ec; ora-
plishing this effect,
25 3-T vU "wit}.- grain" specimens responded readily to shot-peer.
:
with results as shown in Fi". 21, The increase in fatigue life in the
hi^h stress range w»s pbout 500$, although the endurance limit may
not have "been changed; &n insufficient number of specimens were tested
to determine this,
Shot-peenin^ of 70 &-T specimens was carried out at the sp"»e
intensity as that used on 25 S-T, itowever, there was no improvement
in fatigue life over the unstressed 76 S-T, See Fi •, 22, Tne increased
surface roughness caused by shot-peening apparently offset any improve-
ment in fatigue life due to surface stressing*
COLD ROLLING
The increased fatigue strength due to surface rolling wes even
more pronounced than that due to shot-pcening. See Figs. 18, 21, and
22, For 25 S-T rolled at 100 lbs, pressure the increase over untreated
specimens amounted to 2500>S in the high stress range. It was not
determined what the effect was on endurance limit,
A pressure of 100 lbs, was decidedly .-nor© effective than 200 lbs,
C\ It is felt that 100 lbs. is about the optimum pressure for 26 S-T, for
^. the roller design indicated in "Sample Calculations'*, Section VI.
Viihen 76 S-T was rolled at 100 lbs, a similar improvement was
noted in fatigue life. See Fi; ;. 22. This amounted to about 3000/^
16
Tests of cold rolled . -ns not only oho - reater improve-
ment in fctipue life than did those of shot peei oes, trot also
produced nore uniform results. Furthermore, the design of rolling
devices 13 not difficult. Therefore it would seem advisable to use
oold rolling whenever the shape of th? pert lends itself to this
method of surface stressing.
-17-
OV 1 ESIKO
An etternpt wrs mp'v~> to deter r '. \gn
on fati^u« strength. Fi % 11F> ind3 I is. ?
25 S-T, 5 with jrrsin were rim nt 55 ,^T psi. .-' 30,00' si.
respectively for 50,000 cycles. Both specimens "erere then run to
destruction st 25,000 psi. The totrl i c; oles ntil fsilure
occurred brought the points onto the normal curve well within
scatter band*
These results, while not conclusive, precluded further invest-
igation along this line.
They furthermore do not substantiate .'-inar's Equation*
UNDSRSTRBSSI3G
Severel specimens which hed been run et a low stress for a
large number of cycles were re-run at a higher stress. The data
thus obtained vss plotted 'end found to lie -well within the scatter
bsnd for the particular meterial end test condition. These particular
speoirr.ens are as follows
i
. simen Metal F i5»
12, 12a 25 S-T 11 u
10, 11 25 S-T 11 E
15, 13a 25 S-T 11 I
3, 5 76 S-T 16 B
5, 6 7G S-T 17 D
-18-
There wis no indication te cycles *t a low atross had eny
eiYect on fati ue life at e. higher stress. Ir other wordf 25 S-T
and 76 S-T were nut susceptible to "coroci:. , :>r vra.3 tli«r
f tl ;u€ c jtad d.j to low 8tra»s operntion, - -3 furt
refutrtion or Ulnar's Equation,
-1
id
rk i
1} ?he ?:;d*;rrnce limit oi 2L S-Y was verified tc be IS,000 psi.
vi',': l/en
r-"'.
. s tool severely re ' endurrnoe
lir.lt of 25 S-7 -r surface roughness is increased.
5) 2!echinii S-T wit,'; e i *j e* ded tool gives effacta ppr-
allel to those obtained with I/O* radius tool until a stress
of about 20,000 psi. is reuoiied. The endurrr.ee limit remains
at 16,000 psi. for til surface conditions obtained with the
sharp 'cool*
4) Shot-pooning of 25 S-T with .013/.012 A-2 intensity increases
i'auigue life about 500^ in the stress range between 40,000 psi.
and 20,000 psi.
6) Cold rolling of 25 S-T et 100 lbs. increusos fatigue life
about 2500j£ in the stress range between 40,000,
; si. and
25,000 psi.
6) The endurance limit of 75 S-T is about 13,000 psi.
7) 75 S-T machined with a sharp tool has about four times the
fatigue strength of 25 S-T, similarly machined, in the high
stress range j out it has the sane endurance limit, 13,000 psi.
8) 76 S-T has a fatigue strength bet-ween 75 S-T and 2b S-T in
the high stress range j but it has a higher endurance limit*
1L,000 - 20,000 psi. for 1/8;
' R. Pool, 24,000 psi, for sharp
tool.
- ) The effect of snot-paenin^ 73 S-I with .010 .012 A 2 intensity
10) Cold rolling 76 S-T at 100 lbs. increased fatigue li.?s
~jfo in the rese range.
It is roeoisnended that future tests with these machines be
conducted with the machines mounted on individual stands and in
cushioned mountings • Ihe present set-u-> -lloivs vibrations from one
machine to be transmitted to ?11 the others.
It is further recommended that more tests be oonducted as follows/
1) 75 S-T machined with l/d* radius tool,
2) 76 S-T shot peensd and cold rolled,
3) Complete 25 S-T and 70 S-T shot peened and cold rolled to
determine effect on endurance limit*
4) Vary intensity of shot peeninf, and rolling pressure to
determine optimum values for each metal,
5) Further investigate the effects of understressin? and
overstressing on fatigue life.
V. RSFi&EHCBS
•
1*n3ehaviour of Structures under Repe&tod Loads" , Pugsley, A. G.,
J.R.A.S., Vol. 51, p. 715.
2. "Metals Handbook", 1948, p. 120, Fig. 3a.
3 # "Aeronautical Uaterlal Specifications", 41502, 4137, 4154C.
4. J, Apl. Meoh. Vol. 67, June 1935, p. A 69-71.
5. "Sxperijnental Verification of Cumulative Fatigue Damage*1, I:dinor, M. A.
Auto, and Aviation Ind. Vol. 93 t Dec. 1, 1S45.
VI. SAiSPLE calcui^ ,
1. Stress calculation*
33"
= 10 lb*. (T*re)->- Ad J.ec
= 755Vi p.s.i. 1* = 4 !"
2. Design of Rolling; Pevioea (?>on Ref, 4.)
Roller© x Specimen
I 0.8 0.53 OTIS 9.75
= 1,25 -r- 3 + 6.66 -r 0.102
= 11.01
R - 0.0909"
This value of R allows the use of the Boussinesq Formula*
r = .
gsF
A 7 " 2V7 r- *yt ?•••!•
P s Force lbs.
? =s Depth of penetration - inches
tT = I'oisscn's Ratio
Say,
i2 = £ radius of specimen
= i x 0.15 = 0"0376
? = iLZ^l' - 87T (Q.375) 2T^C*00r>4 -£
T^Tir - 6?o*
25 S«T 75 S-T 76 S-T
yt 30,000 66,000 60,000
r 15,000 33,000 30,000
p 81 178 1G2
VII. FIGURES 6-215
-<?4
FIG. 6. ROLLI/VG DEVICE
FIG. 7. ROLLING DEVICE AS used
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3 2768 002 09396 5DUDLEY KNOX LIBRARY
