In order to determine the influence of ductility on the fatigue crack growth rate of aluminum alloys, fatigue tests were carried out on central notched specimens of 2024-T3 and 2024-T8 sheet material. The 2024-T8 material was obtained by an additional heat treatment applied on 2024-T3 (18 hours at 192 C), which increased the static yield strength from 43.6 to 48.9 kgf/sq mm. A change in the ultimate strength was not observed. Fatigue tests were carried out on both materials in humid air and in high vacuum. According to a new crack propagation model, crack extension is supported to be caused by a slip-related process and debonding triggered by the environment. This model predicts an effect of the ductility on the crack growth rate which should be smaller in vacuum than in humid air; however, this was not confirmed. In humid air the crack-growth rate in 2024-T8 was about 2 times faster than in 2024-T3, while in vacuum the ratio was about 2.5. Crack closure measurements gave no indications that crack closure played a significant role in both materials. Some speculative explanations are briefly discussed

Louwaard, E. P.

English

NASA Technical Reports Server

_ _._T_ _-_,:,-_i_..."___:_?': __:_ 7:r ,._ _
)
,8
NASA TECHNICAL EMORANDU_ ri_f;; : _............. _ _i_ NASA TM-77984
illlilri]iiiriJ?i?i;ii rllrij ..................
i' ') NASA-TM-77984 198600121863 1176 01432 2557 _ ,),/INL
EFFECT OF ADDITIONAL HEAT TREATMENT OF 2024-T3 ON THE GROWTH OF
FATIGUE CRACK IN AIR AND IN VACUUM
E.P. Louwaard
(N,%S,%-TH-7798q)EFFF.C_O_ ADDI31¢N,%LHP.A_ N86-21657
TRE_TBENT OF 20_@-_3 CN _HE G_O_IB C£
_&T£GUE CRACK I_ Al_ A_D IN VA_UU_ _National
`%eronauticsand S_ece Administration) 19 p Onclas
HC ,%02/BF ,%01 CSCL l IF G3/26 05792
Translation of "Invloed van het naveredelen van 2024-T3 op
het Groelen van Vermoellngsscheueren in Lucht en in Vacuum,"
Technlsche Hogeschool Delft, Luchtvaart en Rulmtevaarttechnlek
(Delft Technical University, Dept. of Aerospace Engineering),
Report No. LR-237, Delft Netherlands, December 1976, pages
1-16, (N78-15255).
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
WASHINGTON D.C. 20546 FEBRUARY 1986
https://ntrs.nasa.gov/search.jsp?R=19860012186 2020-03-20T14:59:36+00:00Z
fW••,w.::m~!"r'''l~~~~\:f~fi:I.r,';;''[:~12;T~~~::l~Yip!~~':~Fr~T,·,'!C~·i:';,".'c",·,:..~)t~~~·"";.~,,:~,,~··
f· ..·· .' ,'ORIGINAL PAGE IS
Of POOR QUAUTY
"
· I
\
!
l
I
"I
,
lI!
I
\
I
11
Ii. ...
" loG I
,. __,H.. TM -7798412. 0.....-... A............ ,. .ul,I_'·. C",'.. H••NASA
<III. Ttt', M4 s..... ,.. S• ••,••, 0.,•.
',EFFECT OF ADDITIONAL HEAT TREATMENT OF 2024-T3 February
1986
ION THE GROWTH OF FATIGUE CRACK IN AIR AND IN VACUllJ t. , .. ,..... ,". 0 •••"' ..1111' Col. "
r-
I 7. "".".0<" •• , .. '.....". 0........11.11 ••,WI H••E.P. Louwaard lO. w... U..11 H••
'.
. " ""'.",,,",0.,.; ..,j.1I N......4 A44,...
n. C,"""", C•.,., N..
SCITRAN
HAS", 4004
.
Box 5456 lJ"T". i' It.,.,. •• ,,,114 c••••e4
c: ........ R......... _- reA Q,' na Tuulat10Q
u. s"'......"."!."l' H_• •• AU....
• tlona .rooaut1c. aDd Space AdaiDiatratloQ
Wun1D&toQ, D.C. ~OS4' ,.. ........... A....., c:...
U. .....'_.....,M....
Translation of "Invloed van het naverede1en van 2024-T30p het Groeien van
Vermoeiingsscheueren in Lucht en in Vacuum", Technische Hogeschool Delft,
Luchtvaart en Ruimtevaarttechniek (Pelft Technical University, Dept. of
Aerospace Engineering), Report No. LR-237, Delft Netherlands, December 1976,
pages 1-16, (N78-15255).
,t. Ahtt'Mt'
In order to determine the influence of ductility on the fatigue crackgrowth rate of aluminum alloys, fatigue tests were carried out on central
notched specimens of 2024-T3 and 2024-T8 sheet material. The 2024-T8
material was obtained by an additional heat treatment app1ied'on 2024-T3
(18 hours at 192 C), which increased the static yield strength from 43.6
to 48.9 kgf/sq mm. A change in the ultimate strength was not observed.
Fatigue tests were carried out on both materials in humid air and in high
vacuum. According to a new crack propagation model, crack extension is
supposed to be caused by a slip-related process and debonding triggered by
the environment. This model predicts an effect of the ductility on the
crack growth rate which should be smaller in vacuum than in humid air;
however, this was not confirmed. In humid air the crack~growth rate in2024-T8 was about 2 times faster than in 2024-T3, while in vacuum the
ratio was about 2.5. Crack closure measurements gave no indications that
crack closure played a significant role in both materials. Some
___ ,., .... ~ ..t3 exnlanations are briefly discussed.
'7. r., ..... (.....,.. " A_tNt(" 'I. 0. ..,..."_1.......
Unclas~ified and Unlimited
i
!
,t. k ..... " C,...H. l.f ........ Jauu.;;-- 111 ••io . I•• I...." C,...,f. eef .........
:I . UIlC1Ud,U," UQCI...1U..
Z?1o /\J :;21&5- 7;~
---~-------------------~-_.- ~~
[21
CONTENTS
page
Summary 1
I. Introduction 3
2. The test program 4
2.1. The material 4
2.2. The specimen type 4
2.3. The fatigue loading 4
2.4. The environment 5
3. The carrying out of the tests and the results 5
3.1. The crack growth tests 5
3.2. The crack closing measurements 7
3.3. Fractographic observations 7
4. Discussion 8
5. Conclusions 11
Bibliography 12
8 Illustrations 13
PRECEDINGPAGEBLANKNOTFLIJ,AF._
I.INTRODUCTION [3]*
In a study previously carried out on the material 2024-T3, a
growth rate of double was found for fatigue cracks as a
consequence of the raising of the elastic limit through the
"prestraining" of the material [ i]. The prestressing
was done by drawing the material 3% whereby _.2 increased from
43.6 kgf/mm2 to 48.9 kgf/mm2 . As a consequence of this higher
elastic limit less crack closure may be expected (the
Elber-mechanlsm, [2]. This was confirmed by
measurements, but only about a half of the faster crack growth
could be attributed to a lower crack closing tension (lower _cl
- higher _Keff). It was postulated that the other half could be
the result of higher tensions in the plastic crack tip zone.
According to the VTH-model for crack growth (biblio. 3), the
crack growth mechanism for this will be affected if growth of
the crack takes place in an environment that is either more or
less agressive than is found in the air of a laboratory.
When the fracture growth occurs in an inert atmosphere then the
mechanism is not (or not so greatly) affected.
The hypothesis outlined above formed the basis for the carrying
out of some tests orientated towards tracing the influence of an
elevated elastic limit on crack growth taking place in an inert
environment. The raising of the elastic limit of the 2024-T3
material was,however,not brought about by stretching but rather
by additional heat treatment of the T3 state to the T8 state. In
* Numbers in margin indicateforeign pagination
3
i
t
this report the results of some comparative tests are recorded
and analyzed further in the discussion.
2. THE TEST PROGRAM
2.1.Thematerial
We startedwith2024-T3Alcladplatematerialwitha thickness
of 2.5 mm. The additional heat treatment was carried out on the
materialby heatingit for 18 hrsat 192°C.The tensioncurves
forthe treatedand untreatedmaterislaregivenin illustration
I.
2.2. The specimen type [4]
The tests were carried out on flat specimens with a central
notch,see illustration 2. In the specimens tested in a vacuum
the notches were created by means of sparks. In the remaining
specimens they were created with the help of a jewellers saw.
2.3. The fatigue loading
The crack growth tests were carried out with a loading at a
constant amplitude of _m ± _ = 11 ± 4 kgf/mm2. A high value of
%in was chosen in the hope that crack closing would then not
take place or be as minimal as possible. Without crack closing
2_Kef =_K. A possible effect of the ductility (T8 in comparison
with T3) or environment will thus be not to have an indirect
influence because of a different crack opening tension.
2.4. The environment
the tests were carried out in an inert atmosphere (vacuum < 10-6
Torr) and a non-inert atmosphere (air, relative humidity.50%,
temp._+ 20_C). As has already previously been mentioned in
the introduction it was anticipated that the ductility (T8
compared to T3), which had such an unfavorable effect on the
crack growth rate in the laboratory atmosphere, would be much
reduced in an inert atmosphere.
3. THE CARRYINGOUT OF THE TESTS AND THE RESULTS
3.1. The crack growthtests
The testswere carriedout on the 20 ton electro-hydraulic
Amslermaterialtestingmachineof the departmentb2, see
illustration 3. For the tests that were carriedout in a
vacuum,usewas made of the vacuumcell developedin the b2
laboratory. The emptyingpumps of this vacuumcell can
achieveas low a pressureas 10-7 Tort with for instancea
rotaryprepump(Beltzer)and an oil diffusionpump (Beltzer).
The fatiguecracksof the specimenplatesto be testedin the
vacuumwere startedin laboratoryair in viewof the very long
initialtimelengthin a vacuum. The crack expansionwas [5]
measuredas a functionof the total changeswith the aid of a
stereoscopicmicroscope. The performanceresultswere recorded
in a special report. (illustration6.)
5
From this data the crack growth speed was calculated from
8 - a
i+l i-i
(da/dN) =
a=ai
Ni+1 - Ni_1
I
/ in which
i = observation number
a = half the crack length
N - number of changes
For the calculation of/_K for working out the final width of the
specimen plate, use is made of the Federson correction factor
where:
AK =&S . /_m-r C
Fedd
where:
AS =_2x tension amplitude
CFedd= / sec =a2b
6
in which
b = halftheplatewidth
The resultsare setout in theillustrations4 and 5. The
averagedcurvesaregivenin illustration6.
3.2. The crack closing measurements.The crack closing
measurements were carried out with a COD meter (illstration 1,4)
with a measuringlength of 4 mm. This was placed about 2 mm
behind the crack front. The principle of the measurement is [6]
outlined in illustration 7. The dynamometer of the
Amsler-machine provides the signal for the tension _ of the
specimen plate. The tension level at which crack closing begins
(at reduced _) or at which crack opening is complete (at
increased o0 is determined as being the transition point of the
non-llnear part of the recording to the fully linear part
(illustration 7).
Although in other test series the crack closing tension was
easily perceptible in the method outlined above, the
measurements here gave only weak and not completely clear
indications of the occurrences of crack closing between _ and
min
This was equally true for the T3 material as well as foro" .
max
the T8 material. The indications suggest that crack closing did
not occur between W . and _' or closeby.mln max
The indications are in either case insufficient to consider
theseas a definiteexplanation,for on the other hand there is
the differencebetweenthe resultsin air and in a vacuum.
3.3. Fractographic observations
There were only macroscopic observations made. Therefore it was
obviously difficult to report a transition point for crack
growth in the T8 material in air, because the macroscopic
turning of the fracture front was preceded by multiple shear.
In the T3 material it was definitely possible to talk of a
transition point (illustration 8). The general tendencies for
the transition points are shown in illustration 6.
From the tests carried out in the vacuum it did not appear
possible to obtain a transition point, because right from the
start of the crack widening in the vacuum (after initiation in
air) the crack lines clearly showed multiple shear. A clear
transition into crack widening in the single- (or double-) shear
mode was therefore not obtainable either (illustration 8).
4. DISCUSSION [7]
From illustration 6 two tendencies can be clearly seen:
i. The crack growth speed in the 2024-T8 material is about twice
as high as in the.2024-T3 material. In air it is somewhat
less than twice and in the vacuum somewhat more than twice.
The result for air is in agreement with the data provided by
8
Broek [ 5].
2. The crack growth rate in air is clearly higher than in a
vacuum. This is true for both materials. This influence of
the surroundings is smaller with a high value of_K, which
agrees with the general tendencies, such as can be found in
different places in the bibliography.
The results did not confirm the conjectures expressed in the
introduction, that the greater or lesser degree of ductility of
the material in an inert atmosphere would be of less
significance. The difference between the T3 and T8 material is
in the vacuum itself somewhat greater. The conjecture was based
on the idea that at the tip of the crack in the T8 material it
was true that a higher stress tension was created, but because
of the absence of an agressive environment this would not be
able to lead to a greater part in the crack widening. It is
obvious that there is more to it than meets the eye.
_rom the results obtained little of a concrete nature can be
said; however some more speculative arguments can be mentioned:
I. The crack closing measurements at present do not form a basis
on which to make a statement, because crack closing could not
genuinely be demonstrated to have taken place. That is not
to say that it may not have occurred to some very slight
degree, either in the air or in the vacuum. Then the T3
material would be at greatest advantage due to its greater
ductility which would give rise sooner to crack closing.
2. In the VTH model the function of the pulling tension in the
area of the crack tip in an agressive environment (i.e. air)
[
9
is associated with the increasing of the crack growth through
"debonding". Crack growth through "debonding" and through
slipping are complementarily assumed. The translation of [8]
slip motion into crack widening should be reinforced through
the pulling tension in an inert environment.
3. Finally the difficult correlation between the size of the
plastic zone (dependent upon the ductility) and the
distribution of the plastic deformation in that zone must be
mentioned (as well as dependency on ductility). Thereby
"cracktip-blu.ting" could play a role; likewise then the T8
material in the vacuum would remain a disadvantage. It
cannot be excluded that the crack tip geometry is different
in air and in a vacuum. The contribution of the slips to
fracture widening should itself be fundamentally different in
air and in a vacuum.
The specializedtests still leave obviousroom for
reinterpretation.In order for furtherdefiningof the
mechanismto be made,itappearsthat the followingare
necessary:
(a) There must be somethingwith more certaintysaid about
crack closing,because Keff is still a first base for
makingcomparisons.
(b) With more refinedprogramsof loading,moreconcise
informationabout what happensat the fracturetip can be
compiled.
(c) fractographicresearchat the micro-levelis essential.The
lO
characterof the growthlines (brittleor ductile
striations)shouldbe mentionedin this context. [!
5. CONCLUSIONS
The less ductilebehaviorof the materialAluminium2024-T8
with respectto the more ductile2024-T3materialboth in
laboratoryair as well as under vacuumconditionsgives rise to
a highercrack tip growth rate as a consequenceof a higher
level of tensionin the directsurroundingof the crack tip. A
clear image of why thisalso leads to a highercrack growthrate
demandsstill more accuratefractographicobservationsand crack
closingmeasurements.
s_
!l
11
BIBLIOGRAPHY /9
(I) J. Schijve--The effect of pre-straln on fatigue crack growth and crack
closure. Delft University of Technology, Report VTH-203, July 1975.
(2) W. Elber--Fatigue crack closure under cyclic tension. Eng. Fracture
Mechanics, 1970, Vol. 2, p. 37.
(3) L. B. Vogelesang--Some aspects of the environmental effect on the fatigue
mechanism of a high strength aluminum alloy. Delft University of
Technology, Report VTH-200, June 1975. Also: Proc. 8th ICAF Symp.,
Lausanne, June 1975.
(4) W. J. Arkema--Crack closure during fatigue and the effects of sheet
thickness and environment on crack closure behavior (in Dutch).
Delft University of Technology, Thesis Aerospace Department, Dec. 1975.
(5) D. Broek--Crack propagation properties of 2024-T8 sheet under static
and dynamic loads. Nat. Aerospace Laboratory, Amsterdam, Tech. Rep.
M 2161, 1966.
(6) E. P. Louwaard--Measuz=ment results of crack growth tests on 2024-T3
and 2024-T8 plate material carried out in air and in a vacuum.
B2-doc., 1976.
12
ORIGINALPAGEIS
OF POORQUALrrY
/lO
e-q
Y
1600 212_-__T3] [2024 -TS]
11_00 __
1200 \\
lOO0
8o0
600 pull curves of
2024-T3 and 2024-T8
&00
200
i ._1 ! l I ! ] I
8 12 16 20 2_ 28
_- extension [mm]
GO.2 O'br 61%]
[kg/mm2] [kg/mm 2] starting , :50ramzength,"
T3 33.5 ! _5.3 18.9
Ta....._o16.....4s 9 12.4
Figure I. Effect of the heat treatment on the mechanical properties
ORIGINALPAGE,.IS
OF POOR O,UALFTY
/ll
kgf/_2 __is}o o o l ,I__.--",,R:0._7_7
fatigue loading freq, 20Hz
3
• , =_t
o
to Q ]
N
/ 2024-T3 Alciod 202t,-T8 Alclod/
environment
:no. of tests
Vocuum('_Sxl0-7torr} 2(Llr L21 21Ki, K21
0 0 0 air (R.H~50%) 2(G1,G2) 2(H1.H21
100
t = 2,5 mm
Figure 2. Summary of the crack growth tests
Figure 3. The 20-ton Amsler testing machine with a vacuum cell,
illumination and binocular microscope.
14
ORIGINALPAGEIS
OF POORQUALITY
/z3
5
aterial : 202_-T3 (G1 :o ; G2:o) i " )
" 202f'-T8 { Hl: e ; H2:==) i " _A( /loading : 11__f.kg/mm2,R =O.&7 =Yfrequency: 20 Hz • ,envlronment: laboratory alr j _
T " N u 'emp,: 20 C • I JY _-
' ' ' .Y _,Z',
1, , , ,/_ ,£ ,,
_°/_" t'1 _#D
0,5
• |202t,-TS,/_- - "
• _ • 0 "
o, oZ
0.05,
0.01
0.005
20 30 /.0 50 60 70 80
, • =_&K[kg/mm3/2]f
Figure 4. Crack growth rates in alr
ORIGINAL PAGE |S
OF POORQUALITY
/14
5
I I
material: 202/.-T3 (LI: o1L2 :e)
. : 202/.-TSl[K1: elK2:1 }
. loading : 11±/. kg/, mm,2eR =0.47
frequency : 20 Hz "
environment: ,oc,S_I0-_o_','. .*/_ ,
" I Temp'_20°C /1
da/dn _//lmm/kc] •
OS ......!'x
202_-T8/
. ¢/ o
0,0S
0,01 _ __e _0,005
20 30 /.0 50 60 70 80
5K [ kg/mm3/2 |
Figure 5. Crack growth rates in the vacuum
16
PAGE IS
POORQUALrrY
Imat_rlal 202/,-T3.202_-T8
]loadin_ 1,11__/.kgf/mm2:R=0.47
Jfrequency : 20 HZ // //
environment: lab air'/vacuum5x10-7|orr, // i s
I Temp. 20_C
, I ,1//i
doldn I / / /
[mm/kc] transltlon/ //_,,
....transition
/
202t.-T8
lab air
0.1 202&-T:
lab a ir
0,05.
202_ -T31VaCUUm 1
I
o.ool
i
._O,OOS
20" 30 /.0 50 60 70 80
= &k [kg fimm3/2]
Figure 6. Crack growth rates _n air and in the vacuum
17
q
D
ORIGINALPAGEiS
OF POORQUALITY
----,-COD
Figure 7. Principle for the determination of the tension level at
which crack closure occurs
TS}.... T vacuum- 3
-TI lab air
t
central notch .................
Figure 8. Fractures caused in 2024-T3 and 2024-T8 in a vacuum and
in alr (l.Sx magnification)
18
Irrl_iiiiii_i_i
3 1176 01432 2557
.'1


Effect of additional heat treatment of 2024-T3 on the growth of fatigue crack in air and in vacuum

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server