Brueggeman, W C

Mayer, M , Jr

English

NASA Technical Reports Server

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N.4TIONAL ADVISORY COMMITTEE
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FOR AERONAUTICS
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GZI13ES
“{ TECHNICAL NOTE
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No. 93i
3’OR
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FAT IGI?E TESTS.
By
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03’ THIN 6E#XW-METAL SPECIMENS
Brueggtm8n and M- Mayer, Jr.
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Washington _
April 1944
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By 11.’”0.Bruoggonan and
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Guid.o fixtures are .aoscribd
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M. ~ayer, Jr. ‘
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by moans of irhich
axial fatigue .loa?s including ”’oompresti%dn may bg +ppliod .
to a thin shoct+metal speoimen, Tests indicate “that the
guides prevented buckling withcnz.t bypass~ng the load or
inj%r,i,~gthe spocirnen. “ “ ., ...---—-- -.._<____ ~
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: in connection w“$th invcstigation~ spons~;od .iiythe- “-–
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National Advisory Uommittea for Aeronautics it was dc-
sirbd t-o mako axial fat Lgue tests op aluqin,um-al~;y 24ST
sheet-metal s~ecirncms 0.03”2,inc~ a~d Q,l O~. i~c-h thick.
In general , alternating loads wero %’j‘h~ applied; that is, ““” “
the load+ wo”uld -vary siquso idally frem. a rn~ximum co”mpr6s-
sive load (ninimum 10ad) to an equal maximuin tensile load
(maximum load), the mean load being zero. , ..: ,-- ““‘--- ,- -
.
To attain uniformity’ of stress and for okhar ‘,p~ac~i-
cal reasons ,“it is necessary that th6”-length of an &xial
fatiguo specin9n be relatively great in relation to the
wi-dth, yorhaps of the order of 10: 3.. A66uming that thti
minimum width is equal “to tha$ of the standard Shaat-
metal static tonsilb spooimen, 0.5 inch, , the dimensions
of tho thinnest specimen would” be” 0.0!32 by 0.5 inch by
5.0 inches. Unless this specimen is supported laterally
it is not possible to apply a, compressive Load” of th~
magnitud. o nooossary for fa%”i@e ‘test in& as buckling would.
occur at a Lowor load. :
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RESTR IGNED
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2 N+CA ?~ 80. .931 ._
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Sovoral moans .for :uppoy.tipg,.sh~et-met+ spscimene
against -.~pQYllhg .imder Stat ic loads h’a+e’’%eon used (rof—
oroncos 1 to 4). The most promising of theso for fatigue
loads ap~oarod to .bo that us9~ at thq:,‘Nationa~ Bur~au .of
Standards by Aitchison and Miller “(’r’ef&ence 4), which
consists of restraining the specimen between stiff lubri-
catod guidas. This is an ~dap, tation,.Qf.the Montgomery
f ixt:q:o ~~-q~a~dnc.e .3), &he rollers hav-iiig--beenr eplaced
by ‘~oli”dbars~- Light-weight gu,id~ were developed for
fatiguo uso employing this principle ”.-‘.”-
3ofor,c confidence could be’placed .“ln%hoso guides
it was necessary to verify that they prevented buckling
without bypassing load around >ho specimen and wiihout
injurious offocts on the specimen, sych as abrasion.
Exporimontal evidence of the adequacy of the @ido$
is proscntod in this report togother”w>t~ a doscripti Qn
of the. testing technique.
,.,
.Machincsk- ‘Tosbs .wero mado in “two axial t~nsion-co~re~~
fatiguo machines. In machino A (fig. 1). tho load is
applied to tho spocimon by means of a crank+lrivon lever
at tho rato of 1000 cycle-s”por mina:to; in machino 3,
(f~. 2) by noans of an eccdntric at tho rato of 2000 cycles
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por minute. The capacity of both machines is about +1500 ,.
pounds. . , i-.-
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.:jPoci.uoa9-”Roduced~&’~,c’tion sheg&sot& fat igu spobi-
moiis.&s.”shot/ni’tifiguro 3 ‘wore used in: both machines. ““
Tho tliihknos? was 0J032 inch for machiao A and 0.102,,inc~”;
for machifia B; ~‘ ‘
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Gu.5dos--- Guidos similar to .thos~. shown in figure 4’ “
wero used in nach~no ~ to make possihlo strain me.asur-
monts on tho spo.cimqn; th~s,o shown i:ti-figuro 5 wore usot -.
in machino 2 for that p,urposo-~-IFoi’routine testing strain.
is not ndssur,od; “tho guidbs shoti ih figures 6 and 7 ,are .
used. The, parts c.onsis~ 03?.A (fig. 7) thb specimen; ?,- .
a pair’ of stool ,bar~ Iight oned. %y milling”to an “I—section;
C, sido pla$cs whi:~ hold $h~ ba.@$.in place; ‘D, paper .: -
saturat~d, with lub~ican~; Jjl;l)oltg; F,-washo~s; G; ,nuts. .,
T’ho spocitil side. plates ,H havo an opening for B ~uckormqn,
gago l Thb Bol’t,’bolos.in tho side’ plates are” elongated to
permit .adjusttiont. , ~~
,., .,
.,. .,,
IIJLCA!l?~No- 9“31” 3
%ho paper, D, has baqn found noccss~y to retain
tho lubricant. Early experiments in which the paper was
not usad resulted in abrasion of tho
working out of tho lubricant. . ,.
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When assmbl$ng~ a strip of the
placed on each side of the spocimon;
togothcr snugly. hut. not t,ightly; tha
firmly against tho Spoci’nori“by small
tho bolts arc tightenq& ud the clamps arq removed. When
propo~ly ELssenblo’d, %ho guiiio m“ay e’asily bu slid by hand
on tho spocinon ,and in a-vortiaal machine will quickly
work down on the “spocino~ tind~,rits own wotght, Tho gUidO
iS maintained in its nedial position by placing a fblt or
rubber pad botwoon tho .ond (or ends in. the caso of machino
specimen duo to
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s“atura~od” paper is
tho parts aro bbltod
bars aro claupod
+clan~s at OaCil Ond;
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Best results woro obt~inod With a greaso lubricant
of a typo used in ball baarings, This greaso is<desi~
nated Andok 11-275 and is narketod by the Standard Oil
Conpany of ITOW Jersey. A coat of tho greaso is applied
to both sides of the paper and ‘is allowed to soak in un-
til the pa~>or 35 saturatad.
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Tho paper was tgdcon fron a lot pr.oduce~ for a special
purpose in the oxporinontal paptir nill of’tho Bureau. It
is a watorloaf shoot apprex~~atoly 0.004 inch thick nado
fron a furnish of 50 porcont sulfito and 50 porcont soda
wood pU1.pO 1%s ch~mical reaction was practically neutral.
This paper differs fron Most connoii conngrciaZ papers in
that it is free frou rosin and alunj-pr—obably its con-
sistency is noro nearly that of blotting paper than of
any othor cozmorcial papor~ Little” exporiefi~~”-%as been
had with othor papers, ‘One celorod “papbr-tihich was tried
apparently had a slight corrosivo action on tho aluminum-
alloy spocinon+ -
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gho guide bare A shown’ in figure 5 wero used in con-
junction with special wiro gages nounted on tho spoci~on.
The gages consisted of a hairpin loop of resist+co wire
B 0.00L inc,h in dianeter conoi%a~ to each side of th~
speciment I?or insulation a strip of paper about 1/8
inch by 5 inches was comontod to the spocimon, the wire
loop was ceaonted to the paper, anothor strip of paper
was conentod” on top of tho loop. ‘Tho rosistanco, of each
gage was 90 ohns. The guido %ar & is Iightonod by m~ll-
ing to a channel-shape cross-”sec%iori and tho f.aco is”
groovod to clear the wire gage, Grea.se=-.saturatod papa
was used as- with the regular ‘guides.
2R.iK&.” Strain amplitude measurements wero nade’ for
several load amplitudes on-=pegimen-s--$n nachino A under
both static md dynamic conditions. a6 $0120WR: -.
1, Minimum load is O, guides off
3. Mean load is O, guides on
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I?or tho static coadit ion the crank of the machino was
turnafi S1OW1Y by hand. Yha strain amplitudo was dotc?-
mined hy noans of a pair of TuckorDan optical strain
gages nountod on the etigos of tho specimen as shown in
flgurb 4. .“Tho .aut-o’coll%natorwas oquippod with a dunb-
bell aotimllo.
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the wire gages was deto~nin&t for-the @pectmon which
was Zoa”dod in.machino B under conditions similar to
those obtaino~ for machine A. In addition a sotios of
compr Dssivo loads (max$nun ~oad.is 0) wera applied.
TIIO chango ifiresi.stanoo of th,e”.wiro gagoe was do-
terminod:hy’ means of an a,c. WhQatstone bridgo of which
a Campbell-Sbaoko~ton shioldod ratio box (roforoncc 5}
fornad two zwn8, a Xohlrau Boh s1$130 wire the” third, and
the wira gago th.o fourth, ~ho chango in rosistanca of
the gago lll~h load n.odulata~ a 1~~0-~yclo Carriox signal
applied to thg bridge.; tho mount o-f the modulation was
moasurod oa *ho screen of a cathode-ray oscillograph~
The change in resistance of the gago was deternin~d by
adjusting tho Kohlrausch. slido wfr,o t~ produco t~e sano
.. chango in.modulation.
parent gbotwoon the anplitudo of the strain or ~
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corresponding to a given- load anplitudo with tho guides
on and with the guides offs
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“.. . Z+ is ovideny in figuro 9 that the.‘strain amplitudo
ib “shout 6.5 percent groator when nachina A is r“unntng
than wllon *ho “ctiank is turned slowly by hand; Z%is
difforonco is caused by the inertia of th”o Lover; a
sinilar di.ffer.cmce hnq~ been obsorvod in anothar uachino
by others ‘(yo.f”orek”cg6) . ‘
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Fi,gurc,11 fails to show an approctable ‘idynanic
overthro~fn for nachino B, within obscrrvat ional errors
which voro as high a-s 6 per cant for one of tho points.
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Yhero v~as no effect of. the guides on the””strain Or on ~
fo’r’static and dynamic loadi.. I?i,gure 12 shows that ‘a
F
was very nearly equal .on %oth sides of the syecinen for
all typos of loading with the gu$des .o”n”.
.
E.xporicmc~ with the gui~es on specinens to which
as nany CS 10 CYC1OS of qtress were applied has not
shown any injurious abrasid~ or other effo.cts which “
could%o correlated with prenat”ure failuro of the speoi-
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nen. .
C#-nolusion. - A nothod is doscri’bed” for ‘laterally
sup~oyt,inG a thin shqe%-netal specia~n to prevent buck-
ling when axial cmpressi.ve fatigue loads are applied.
Tests in two fatigu~testing nachines on laterally
suppottod 245-Y sheet-metal specinons O,032 inch and
0.102 inch thick havo indicated an axial condition of
loading and negligible bypassing of the load by tho
guides. ITo noticeable abrasion or other in~urious of–
focts have been experienced when using the technique
descrtbod.
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Nat ional ~ur eau of Standards ,
Nashingt on, D .C., January 7, 1944,
6RXJI’ERENO%S
NACA TH HO. 931
10 Aitchison,C. S., and Tuckornan, L. 3.: Tho ‘Paok~l
‘Xothod for Compressive Tests of,Thfn Specimens of
.:
~ “liaterials used.in Thin~Wall Structures, NACA Rep,
No. 649, 1939. , -
2. Franks, Russell, and 33frtdar,W. O.: The Stress-Strain
Characteristics of Cold Ro.11.edAustenitic Stainless
Steels in Compression as Determined by the Cylinder
Yest ;iethod, FrOCa ASZH,-VOI. 41, 1941, p. 6290
3. Paul} D. A., Howell, E.-M., and Grieshaber, Ii, E,:
Oonparison of Stress-Strain Curves obtained by the
Single-Thickness and Pack Methods. ~AGA TN No, 8&9,
1941*
4. Anon.: Cornpypss$v.e Tes$s. of Sheet ?!etd with $olj&,.
Guides for Lateral Support. ‘Strut. Remo, No. 10,
Aoro., Navy Dept., IIov, 10, 1942.
,.’
5~ Behr$ L- and i’~illiams,,4, J:, Jr.:. The Campbell- ~
“.Shaci:eltan Shielded Ratio ~OX. Proc. Inst? Rad~o
va~- 20, 1932, p. 969s
6, Russell~ -H. W., Jackson, L. R., (hover, H. J., and
Beaver, W: W,: Fatigue Charaoteyisti~s of Spmot-
Welded 24S-T Aluminum Alloy. ,NAC.A ARR No. 3F16,
1943.
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Figure 1.- ~atigue teOting machine A.
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Figure 2.- Fatigue testing machine B.
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NACA Technical Note No. 931
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Figure 4.- Apparatus for me~uring the strain on the edges
of a
guided fatigue specimen.
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NAOA Technical Note No. 931
A
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Fig. 5
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Figure 5.- Special wire gage mounted on a fatigue specimen and
guide for this specimen.
NACA Technical Note No. 931 Fig. 6
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Guide bars- 2 reqtire& tool steel ~“
hardened and tempered to Ro&well
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Side plates- aluminum alloy
24ST 2 required.
Figure 6.- ?%.teralguide fixhxe.
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Figure 7.- Parta of guide amemblv.
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Strain, amplitude,.
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Cl MearJ load = O, static
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!zO 35 40 x m-4
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mgum 8.- Results for machine A.
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(~Minimum lti = O, guides on
A“~=o,H off
U Mean load u O, guides on
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0 100 200 300 400
Load amplitude, lb
500 600 7CQ
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Figure 9.- BynEuuic overthrow
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for machine A. .~
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Figure 10.- Re8ult5 for machine B.
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NACA Teckwical Note No. 931
Max@um I! = O; ‘1 on
Mean load = --F----”–0, guides on ;
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800 laoo “ u
Load amplitude, lb
Figure 11.- Dynamic overthrow of -chine B+
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NACA Technical Note No. 931
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Fig, l?
50
40
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Llliaximum ~ = 0, I! on
Q?Jean load = 0$ guides on
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10 20 30 40 x 10-~
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R one gage
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Figure 12.- Equality of &R/Ilfor both sides of the spec~en (~chine B)o - ._
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Guides for preventing buckling in axial fatigue tests of thin sheet-metal specimens

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