It is proposed in this paper to identify some of the defects and failures in duralumin most frequently encountered by the aircraft industry with a view to indicate their importance. The defects and failures in duralumin may be classified into the following groups: 1) defects produced during manufacture; 2) defects produced during fabrication; 3) corrosion and erosion; and 4) fatigue failures. Only the first two will be covered in this report

Nelson, WM

English

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FILE COpy 
NO. 2-W 
TECEiH CAL 1,IEl 'IGRil.~1Dl~ZS 
NATIDNAL ADVISOEY COrD.UTTEL FOR AZRONAUT l OS 
No . 443 
DURALu-::J:IN - DEFECTS AND FA ILUHES 
By Lieut . Oomdr . Wm . IJel son (00), U. S. N. 
From "Aviation: II August 29 , 192 7 
y 
Washin::-ton 
D 8oombe.r;..1927 
https://ntrs.nasa.gov/search.jsp?R=19930090807 2020-06-17T03:54:17+00:00Z
" NAT IONAL ADVISORY COr\~UTTEE FOR AERONAUTICS. 
TECHNIOAL EE.!ICR!\NDmJI NO. 443. 
DURALD::AIN - DEF'EOTS .AND FAILURES . * 
By Li eut. Oomdr. Wm . Nelson (CO), U. S.N. 
One of the a~vantage s of metal over wood as an aircraft 
materi al is the uniformity of the Qetal. There aYe no shakes , 
checks, knots, sp iral grain, etc ., in metal; and, as a conse-
quence, aircraft bui lders more or less loo ked forward to the 
use of clural umin wit h relief. Tha.t their hopes were not en-
t irely c0l1firmed a.t first resulted from the inh erent property 
of duralumin of having more apparent defects than real ones 
and f r om the novel ties in the features of the metal. Experi-
enc e with this alum inum alloy s ince its introduction to air-
craft has tended to set the housc in order though, so that at 
the p resent time the advantage ment ioned is reco gn ized as a 
facto r of importance. 
I t i s proposed in th i s paper to identify some of the de-
f ects and fa ilurec in dural umin most frequently encountered 
by the aircraft ind.ust r y with a view to indicate their impor-
tanc e. Numerous cases have been called to the wr iter' s atten-
tion where the defects have been superficial, whereas, on the 
other hand, casual inspec tion has l'evealed m2.terial not f itted 
f or use as st ructural memb ers in aircraft. To distinguish one 
* From IIAviation, II August 29, 1927. 
~.y • .A. C. A. Techn i cal lLei~lOrCtndum Iiio . ,143 2 : 
fr om the ot11er i s not a1ways qu ite as s i mple as it seGrrls. 
The d efects and fa ilure s in Quralurain may !J e classified 
into the followi ng groups : 
a) D ef E.C ts 1)1'oduc ed during ;:1anufac ture of the raw rna terial. 
b) Def ect s produced durin g fab r ica.t ion of the ma.t erial into 
parts. 
c) Corrosion and eros i on . 
d) Fati &.,ue failures . 
Only the f ir st two will oe covered in th i s art icle' since each 
of the others is far too important to oe g i-'.ren other than de-
tailed attention . 
The met hods of manufacturing 6.u r alu:;J.in have been so stand-
ard i zed a no. so ri g idly supervised by the p roducers of this met-
a l that abnormal defects t ne rein are exceptional. Extraordi-
nar y effort s by the oroccucers in r es earch and in inves t iga t ions 
have been reflected in the mat eria l p r oduced. so that at the 
present t i nl e t he causes of ·~ost of the d efects occurr ing are 
kn own, and corrective measures are insti tuteci before the mat er-
ial i s d i st :dbuted.. 110wever, cluraluJ i n i s not alJ. that aircraft 
designers and constructors would like it to b e, nor is the prod-
uct always to be clas s ified e.S good, for in spite of unifonn 
production and h i gh g rade i nspection the conSUii1e r is sometimes 
required to make dec isions regarding II run of the mil l " material. 
As is known, du r a l um i n i s available in rolled, wrough~ or 
ext ruded forr:1, and in various c ond.i tions of thermal treatment. 
H. A. C. A. Techn ica l t:er.] oTandum No . ,~A·3 3 
The roll ed du r alumin usual ly l eaves the Dill n it h Cl p lan i shed 
surfac e, such a su r f ace be i ng produced by the rolling op e rations . 
The wrought a nd extruded f OTlJls do not as a rule have t he p lan-
i shed aupea r ance , be i n g duller and. presenting some evidence of 
too l act ion . 
Perhaps one of the nlOst wi dely discussed p oints l'egarding 
du r a l urll i n shee t is t he quest i on of na tural colorine; . Th is mate r-
i a l vari es i n it s surfa ce a :.)p earance f rom a c lean si l very gr ay 
to a dirty gray with all colors of the rainbow super imposed . 
The cl ean si l very gr ay resem-oles a mach ined p iece in color 
whe rea s the dirty gray can be described as d i rt rubb ed into the 
metal. When looked a t f rom an an ~ 'l e hues can s ometimes be secn 
on the p l an i shed surface varyin g lJ eculiarly, c ente ring at time s 
i n a s -:)ot and wanderin g reckl css l y at other t i!11es . These hu es 
have been descr i bed as a c op p er glint. It has been defin itely 
a S\'J;.;rt Ll i ned by nur.1erou s t enG ile test s that :none of thes e color-
ings has Clny b earing on t h e phys i c2.1 p roperties of the mat erial 
nor i s the shade of gray a measure of i mpo rtant fLlcto r s . Ther e 
a re, howeve r, cases wh ere stains on duralum i n a re a ccompanied 
by i mpe r fections in the sur fac e . These imp erfections are usu-
a ll y the Tesul t of c ontact with SODE) forei gn substanc e af te r 
the materia l has left the mi l l. Duralumin s o ma r ked should not 
f ind it s way into a1 rcraf t unde r any c ircumst2.nc es if the sur-
fLl ce i mperf ections, oti,1er t han the color, are abnormal. 
.. 
K. A. C. A. Techni cal ~emo:rru1dum I-a . 443 4 
Raw IV1o.terial Inspections 
The imperfect ions in spect ed for on dur alumin in the raw 
st ate, as ide f r om mechan i cal p roperties u,xe laminations, cr a_cks , 
pits, dirt, scrn,tches , buckles, blisters, sl ivers, roll mo..rks . 
lost of these when smal l have little effect on the phys i c81 
proper t i es of the mate rial , but ar e not des irable for other rea-
sons , indi cat i ng as they do more serious h i dden defic i encies 
discerni ble only by the u se of a mi cro scope or the X-ray. 
Lmninations occurring in u part of a hee t or shape have 
been mo r e f r equent in the P2.st than is the case now. Faul ty 
rolling of sheets r esult ed in a condi tion where the f ini shed 
she e t could be separated fo r 0. cons i der able area i nto two thick~ 
nesses with no bond between the two. Fortunately th i s defect 
occurs i n the edge :vhe r e it is reo..dily di scovered i n inspection:; 
but thel'e a r e cQ.ses where the sheets have fitted toge ther so 
well that In:'1linations were not found unt il the pressing ope r a-
tion had been completed. LQrninat i ons have ~lso been fOlliid i n 
tubes , ae i ng due t o ne tal in the ext rusion block caxrying over 
to the tl8xt following ext r us i on . I t seems un ecessary to state 
that l L'J.1inated durnlmni rr- i s not su i table ,,,aterial for uircrc:tft 
u se . 
Cracks i n sheets and ot~er rolled s: ~pes are due to the 
temper of the j,1aterial or to pull i ng of the rolls.. Cracks i n 
the bends of rolled shape s may be due to the thickness radi us 
N. A. c. A. Technical M 8UOrD. ndui. No . 44·3 5 
of curvature ration being beyond the capacity of the material . 
. li: 
Cracks in extruded shapes are uGunl ly f.ound. at the ends and 
,".1' e caused by the us e of f:lOre of an ingot than is warrant ed . 
Chatterino; of t~8 die or nn imperfection in the di e used for ex-
trus ion causes a series of crac~cs readily discernible in the fin-
ished product. Although these defects are not admissible in 
dura1Uillin in gencral, cracks i n tUbing should be culled from 
aircraft mater i a l with exact i tude. Pits and inclusions can be 
seen with little d i fficulty in an oti.1erv!ise clear dura1umin 
sheet. I t is practically imp ossible to avoid some pits and in-
c l usions i n rolled duralumin. Dirt, due to dross or foreign 
sUbstances p i cked up by the rolls subsequently rolled into the 
metal, will appear from time to time and unles s the amount is 
unusual no c.ire results ne ed be expected. Corrosion does not 
localize i n t ~'. ese p it s no r at the inclusion unless it happ ens 
.,~ s·· 
to be fo reign metal, in wh ich case the corrosion of the duralu-
min mayor may not be hastened . As a rule pits a nd inclusions 
are no t caUG$ for rejection unless the quantity or f requency 
war r an t s such ac t ion. 
Scratches come through handling of the material. They are 
not d eep enough as a r ule t o do other than mar the surface , but 
they have a bearing on corrosion and fa tigue failure. Corrosion 
will concent rat e a lon g a scratch if the pie ce of dura1umin is 
exposed, so it is to the advantage of buil ders to avoid whenever 
pract icable all scratching in duralumin. Manufacturers of 
J 
fLA. . C.A . Technical Memorandum No . 4-4·3 6 
aluminum alloys now go to grGut l ength to reduce mecbanical 
abra~ion caused by carelessness in handling; and it is well if 
storekeepers take equal care to retain the planished surface. 
Buckles Appear in Sheet St ock 
Buckl es appear i n sheet stock in quenching after heat 
treating. The se buckles are wiped out by the st reng thening 
roll which is the l ast mill operation prior to packi ng. Buck-
les in plates involve difficulties in fabricatin g parts there-
r rom asicle from t 11e eff ect on appearances. 
Slivers may make their appearance as entrained dross, 
rolled in flakes of metal, or as a thin shell on the sheet or 
rod. Dross ca rried over to the rolling operation is drawn out 
in the d irection of rolling and adhered rat her firml y to the 
sheet . The s ame is true of rolled i n flakes of aluminum or 
duralumin which might be picked up by the rolls. Exfoliation 
of the ingot on account of the worl<:in g of the die results in a 
s liver. Mater i al left in the chamber of an extrusion press by 
a previous ingot makes its appearance on the extruded shape as 
a s liver or a thin shell . All types of slivers reduce the value 
of the p roduct fo r g eneral us e and in case they are extensive 
no part of the sheet or s hape is acceptable. The complete eradi-
cat ion of slivers by brushi ng or grinding requires the removal 
of considerable material resulting in a rather diminished dimen-
sion fo r the part affected . 
'-- -.----.~-
) 
1.A . C .. A. Te chn ic c..l l,1e i'i101'o..no.um .To. 443 7 
I t is beli eved that blisters should be segr egated into two 
groups; those resulting from normal ~eat treatment and those 
resulting from overheating the metal . Blisters on dUTalumin: in 
the first classification have no effect on the mechanic~ prop-
ert i es of the iUQ.ter ial. T~1ese blisters ar e small and not fre-
quest . Blisters that :'M1ke their l1ppearance on overheated mate -
rial ar e Q certain indicat ion of change s in the constituents 
and inferior physical properti es accompany such burned metal . 
Occluded Gas Forms Blisters 
I n the course of the ii1anufacture of duralumin i t can hc:.rdly 
be expected that all occluded gas in the ingot is driven out . 
Natur t111y then, a certain portion of that gas will find its way 
to the surface or near the surface in a she et or shCLpe . Upon 
heat ing the dur alumin fo r heat treatment this g CLS will tend to 
expand 2nd in cases form blisters. Heat treCLting the metal at 
5100C will bring them out and it is only by subsequent rolling 
that the smoothness of the sheet is restored . It is ~ op inion 
that sheets displaying Q. tendency to blister can be culled first 
prior to the final rolling. Samples of badly blistered sheet 
have been subj ected to induced COTTO sion and. the indicat ions 
c.re thllt no concen trCLt ion of corro sion c.t the bl i st ers is ef-
fected . 
On the other hand, i f duralumin is heated to a temperature 
near 5400C, there is apparently a change in the constitution 
---------------------------
f 
~·~ . A . C . A . Techn ic0..1 Ee;;10r2.J1dum ro . 443 8 
of the const it uents indicating very strongly the beg inning of 
the mel t i ng e Thi s change according to the theory expressed 
m2..Y be accompanied by the formo..tion of melting blisters which 
in themselves may not be harmful, but they do point out ch~nged 
physico..l properties . The u8u~1 result of overheating i s 2.. per-
ceptible drop in the elongation . 
It is not p racticable to distinguish be t ween the two kinds 
of blisters by a visual i nspection, so in case an abnormal 
u~ount of blistering is occurring it is best to suspect the tem-
pero..ture of heat treatment and i f that is correct it is a ~atter 
for t~1e .11akers of the raW mate rial. It is i.l0t to the best int-
erests of duralumin to lower the heat treating temperature to 
reduce blistering effect . 
Roll £E2..l'ks De stroy Uniformity 
Roll marks are depressions in the di rection of rolling 
that meW 01' ;',1ay not be accompanied by t rC:Ulsver se scratches . 
These roll m2Xks destroy the continuity and uniformity of the 
part and should not be unusu2..1 in ~flOunt. They ~re caused by 
improper design of rolls or by defects in the roll surfaces. 
Tool marks and scratches in duralumirr co~e primurily f rom 
the methods of fabrication used. , Careful handling and workman-
ship will avoid an excess of both . 
Coiled sheet is sometimes received with a line of imper-
fections down the center . These imperfections re semble corro-
sion pits and are due to the method of heat treating such durnl -
N • A . C. A • Toe hn i co.. 1 )1 G.J 01'2.- nd u:n H o . !~-4-3 9 
umin coils . The co i led s h e e t s are readily heat treated in t h e 
sal t bath by 11av i ng 2..n a lurn i num wire separate the various layers 
of the coil. This C'luil1 i num wire a ')parently causes more or le ss 
p itt ing of the sheet w11 il e it i s i n the bath . Improved methods 
have obv i ated t h i s defect . 
~-i_ ~_v ets B.re manufactured from b oth d rawn ane. ext ruded wi r e. 
I n cert'" in cases one suits where t h e other does not. The mos t 
c ommon o.efec t in the manufactu red rivet as rece i ved is cracking 
the he-:ld . vhere cracke~ heads are few and far between it is 
n ea rly an impossib ility to avoid getting SOr.1e into the fin i shedl. 
ai rc raft , f or as a rule the c r acks are so fine t hat the great-
est care i s necessary to be sure there is a defect . I n pract i-
ca lly all r i vets as ~,tb:ey are now mad e t here is a small area in 
the ceIl ter of the _ ead Nh ich i s rou gh . This rough spo t mayo r 
may n ot b e a sour ce of trouble; but it has been noted in induced 
c orro s ion t e s ts that this po i nt i s u sually more active than the 
ot h er parts of t~1e rivet . I t is d.es irable for the rivet rnanu-
f acturers to correct as is the question of cl'acked ri v et heads. 
The most comr.lOn se riou s defect in du r a l urn i n being f abri-
cat ed i nto parts is a c r ack . Thes e cracks are prir.1arily due 
to st ress ing the materia l by working it too long, too cold, or 
to o hard . They are f requent on parts drawn or pressed or. shap ed 
and extended i n s ize f rom very mi nute cracks t o complete part i ng 
o f the r.1a terial. Sheet bent on a very small r adius in propor-
t ion to the t h ickness of the mate r ial wil l tend to crack inside 
.. ------------~~~~~----~---~-~------ -----
iLA . C. A. Technical ECi!!or2.nduj.l No . t-43 10 
and outside . The s ize of the cracks or their frequency deter-
mines whether or not t he mate rial can be used for the purpose 
intended. In sor,le l'are cases the const itution of the material 
is responsibl e f01' the cI'acks but in most cases it is due to 
the desi gn or to the shop proce s s. Sheets usually show cracks 
if at all a t sh8.rp bends. Forgings may show craCKS at sharp 
b ends or in any part that has been worked too cold. Metal pro--
pellers should b e i nspec ted with a magnifying glass to be as-
sured that no cracks exist. Etching dural~nin in a caustic 
soda sol ution will tend to make cracks more a pparent and is a 
procedure l'esorted to for inspection purposes i n forgings. 
Scratches, buckles, blisters and tool marks in duralurnin 
are other defects that occu r in the fab rication of parts. 
S c r a tches and tool Glarks are largely avoidable by proper care 
in handling . No particular advantag o accrues to rubbing out 
these defocts wi th emery cloth or steel wooL 
Buckles Co n Be Prevented 
Buckles can be prevented if dcs ir.ed by quench ing the part 
i n an air blast ins tead '-of water or oil. Buckles will be the 
most severe i n the case of n ost rapid cooling which in prac tice 
is by quenching in cold water. The working out of buckles 
wi thin 3. short time after the heat t reCltin g operation is a com-
mon practice and is not known to be injurious to the material. 
Some method of stI'ctching the sheet into place on aircraft sur-
N.A . C. A. Techn ical MCY'.1Ornndum · 11 
faces would be of great advantage from the appearance point of 
view. Vii th the pres ent methods in us e, the buc kl es in the 
sheets on surfaces are s o open to vibration and a pp earance ob-
j ections t hat fabric maintains its posi tion as a wing covering. 
I n d iscuss ing the question of rivets further it might be 
mentioned that defective rivets are, so fa r as the aircraft in-
dustry is concerned, up to the riveter. Each rivet passes 
through the hands of the riveting crew and is open to the ir in-
spection both before and after being driven. Cracked heads 
and cracked p oints are readily discernible to this crew and 
their whole- hearted supp ort is nec essary to insure good joints . 
There i s another factor connected with rivets which demands 
attention in a s sembly work and that is the question of lining 
up the rivet hol es . Correct bolting-up and reaming are essen-
t ial s. The complet e d riving of the rivet s o as to f ill the 
ho l e i s a matter worth de tailed attention. The flow obta ined 
wi th properly h eat~tr.eated rivets is c onsidered to be good . 
In g ener a l , practically anything within reason can b e done with 
clu.ra'lumin rivets but a clean job is usually the result of t h e 
care that should be taken to this phase of a ssembly work. 
A defect readily p roduced in duralumin by the anodic oxi-
dation treatment is that of burning the metal by using too 
g reat a current density. Duralumin which has oeen so burned 
is very weak and will not serve any us eful purpose. The pre--
ventra n is dependent entirel y on the current control durin g 
1: • A. C. 11.. T echnical !'U;i'n ora~l('l:Ll;i1 4 1-3 12 
the t rca t ing op el'at ion . 
Propel' Heat Treating Es sential 
When tbe a irc raft desi gn 31' sp ecifies duralumin for a part 
it is d one 1.'lith a vievJ to b.cw i n g mater i al that comes up to 
certain phys ical or mechanical values. It is therefore incum-
bent on t~10se '\'lOrk ing to that C.csign to pr oduce the final 
piece with all operations l eadinc toward that ultimate. Per-
haps -~he eas ie st p1a ce to make er r ors that will alter the dO"-
sired results ma terially is in not 1 aving properly h eat-treated 
duralu1'nin in t h e final a ss embly . Annealed and cold-worked 
duralumin will b oth find thei r ray in as h eat-treated mater i-
al unle s s care i s taken by rig id insp ection. The s h~plest 
arro..ng emen t to ch eck the parts i s by means of the scleroscope. 
Properl y heat-tl'Gllted. a nd a. ged du r alumin will g ive a 8clero-
scope hardness of a b out 28 whereas the a nnealed material wi ll 
give a val uo cons iderably lower, and col d-worked slightly higher. 
To be more certa in than the nardness values will i ndicate, 
bend tests should be comb ined ",it h tl e hardness determinations . 
Duralu:ni n V'hich has been 'cUTi'l ed in the t l ermal treatment 
is readily d. isc ernible to the eye . Extrek e b ri t t lenes s is a 
noteworthy feature in :nater i al wh i ch has b ee,n too hot. There 
is no recovel'y for a b-Ll_rned piece of duralu.::.'l in. 
I n conclusion it ca n be sta ted t hat the ClnOU:lt of duralum in 
r ej ect ed due to d ef ect s e nd f a ilures is not g roo.. t compared to 
N. A. C. A. Tech.n icD..l Jcmorandum No . 443 13 
the quant ities thrown out i n other a ircraf t n1a teriul; bu t 
there i s continually the p ossibil i ty t hQ..t a n undesirable pieo o 
will f i nd it s way int o the finished airplaho GO it i s e ss en-
t i a l tha t duc dil i g enc e be exerc i sed by tho c e in control of 
thi s f cutur e . 
J 
Inclusions in a Jar~lumin sheet . Photogr~ph i s slightly ~gn 1f 1ea 



Duralumin - Defects and Failures

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