FATIGUE STUDIES OF 24-S-T AND 24S-T ALCLAD
SHEET WITH VARIOUS SUEFACE CONDITIONS
A THESIS
Presented to
the Faculty of the Division of Graduate Studies
Georgia School of Technology
In Partial Fulfillment
of the Requirements for the Degree
Master of Science in Aeronautical Engineering
by
Aleck Gonstantine Bond
February 1948
FATIGUE STUDIES OF 24S-T AHD 24S-T ALCIAD
SHEET WITH VARIOUS SURFACE COMDITIONS
Approved:
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ACKNÖÄLEDGMENTS
The author wishes to thank Professor G. K. Williams for his
valuable criticisms and guidance throughout the preparation of this
thesis and for his assistance in determining the particular phase of
study. The author is also indebted to Mr. R. L. Templin, Chief Engineer of Tests of the Aiuminum Company of America, who very graciousiy forwarded desired fatigue data and encouraged the investigation.
Thanks are also due to Mr. H. W. S. Lavier, who was also
instrumental in the choice of the topic; to Mr. ¥. C. Slocum, Mr.
0. ¥. Rogers and other members of the Aeronautical Departments shop
for their assistance and Cooperation in making the test specimens;
to Mr. J. E. Garrett, photographer at the State Engineering Experiment Station; and to Professor H. V. Grubb of the Chemical Engineering
Department for his assistance in determining the depths of Scratches
on the specimens.
iv
TABLE OF CONTENTS
PAGE
Approval Sheet
ü
Acknowledgments • • • • • • • • • • • • •
iii
List of Tables
v
List of Figures
• • • • • •
vi
Suramary • • • • • • • • • • • • • • • • •
1
Introduction
1
• • • • • • • • • • • • • •
Material
• • • • •
4
The Fatigue Testing Machine
The Fatigue Specimens • •
•
5
•••
9
Preparation. • • • • • • • • • • • •
Grain Direction
10
•
10
Scratches* • • • • • • • • • • • • •
11
Depths of Scratches* • • • • • • • •
13
Test Procedure* • • • • • • • • • • • • •
14
D i s c u s s i o n of R e s u l t s • • • • • • • • • •
15
F a t i g u e R e s u l t s of 24S-T
15
F a t i g u e R e s u l t s of 24S-T A l c l a d . • •
17
Stress Concentration • • • • • • • •
19
Size E f f e c t
21
A p p l i c a t i o n of Data t o Design. • • •
22
Conclusions • • • • • • • • • • • • • • •
24
BIBLIOGRAFHY
25
APPENDIX I , Tables
27
APPENDIX I I , F i g u r e s
31
V
LIST OF TABIES
PAGE
Table I
Table I I
Mechanical F r o p e r t i e s of 24S-T and
24S-T A l c l a d S h e e t s Used i n F a t i g u e T e s t s
28
Average Depth of S c r a t c h f o r Varioias
Abrasives on 24S-T and 24S-T Alclad
28
Table I I I Eepeated F l e x u r e F a t i g u e S t r e n g t h s of
0.040 Inch 24S-T Sheet with T r a n s v e r s e
S-urface S c r a t c h e s Uue t o S e v e r a l A b r a s i v e s
and t h e Corresponding S t r e s s C o n c e n t r a t i o n
Factors
Table IV
Eepeated F l e x u r e Fatig-ue S t r e n g t h s of
0.040 Inch 24S-T A l c l a d Sheet with T r a n s v e r s e Surface S c r a t c h e s Due t o S e v e r a l
Abrasives and t h e Corresponding S t r e s s
Concentration Factors
29
30
vi
LIST OF FIGUEES
PAGE
Figure 1
S t r e s s - S t r a i n Curve of 0,040 Inch 24S-T Sheet
M a t e r i a l Used i n F l e x u r e F a t i g u e T e s t s
32
S t r e s s - S t r a i n Curve of 0,040 Inch 24S-T A l c l a d
Sheet M a t e r i a l Used i n F l e x u r e F a t i g u e T e s t s
33
Figure 3
Sonntag F l e x u r e F a t i g u e Mach i n e , Model SF-2
34
Figure 4
Sonntag F l e x u r e F a t i g u e Machine, Model SF-2
35
Figure 5
View Showing Speciicen Loa&ed i n F a t i g u e Machine
36
Figure 6
Specimen and Mounting D e t a i l s
37
Figure 7
F r a c t u r e d 24S-T Specimens
38
Figure 8
F r a c t u r e d 24S-T A l c l a d Specimens
39
Figure 9
24S-T Specimens Before F r a c t u r e
40
F i g u r e 10
34S-T Alclad Specimens Before F r a c t u r e
41
F i g u r e 11
V a r i a t i o n of Average Depth of S c r a t c h with G r i t
Number of Abrasive i n 24S-T and 24S-T Alclad
42
Eepeated F l e x u r e F a t i g u e , 0.040 Inch 24S-T
S h e e t , Specimens Scratched with Crocus Cloth
43
Eepeated F l e x u r e F a t i g u e , 0,040 Inch 24S-T
S h e e t , Specimens S c r a t c h e d with No. 240 A l o x i t e
44
Eepeated F l e x u r e F a t i g u e , 0,040 Inch 24S-T
S h e e t , Specimens S c r a t c h e d with No, 180 A l o x i t e
45
Eepeated F l e x u r e F a t i g u e , 0,040 Inch 24S-T
S h e e t , Specimens S c r a t c h e d with No. 100 A l o x i t e
46
Eepeated F l e x u r e F a t i g u e , 0,040 Inch 24S-?
S h e e t , Specimens S c r a t c h e d with No, 60 Sandpaper
47
Eepeated F l e x u r e F a t i g u e Curves Showing t h e
E e l a t i v e E f f e c t s of Various A b r a s i v e s on t h e
F a t i g u e Life of 0.040 Inch 24S-T Sheet
48
Eepeated F l e x u r e F a t i g u e , 0,040 Inch 24S-T Alclad
S h e e t , Specimens Scratched with Crocus Cloth
49
Figure 2
F i g u r e 12
F i g u r e 13
F i g u r e 14
Fagure 15
F i g u r e 16
F i g u r e 17
F i g u r e 18
vii
PAGE
Figure 19
Figure 20
Figure 21
Figure 22
Figure 23
Figure 24
Repeated Flexure Fatigue, 0,040 Inch 34S-T Alclad
Sheet, Specimens Scratched with No. 340 Aloxite
50
Repeated Flexure Fatigue, 0 # 040 Inch 24S-T Alclad
Sheet, Specimens Scratched with No. 180 Aloxite
51
Repeated Flexure Fatigue, 0*040 Inch 24S-T Alclad
Sheet, Specimens Scratched with No. 100 Aloxite
52
Repeated Flexure Fatigue, 0.040 Inch 24S-T Alclad
Sheet, Specimens Scratched with No. 60 Sandpaper
53
Repeated Flexure Fatigue Curves Showing the
Relative Effects of Various Abrasives on the
Fatigne Life of 0.040 Inch 24S-T Alclad Sheet
54
Variation of Average Stress Concentration Factor
with Average Depth of Surface Scratches i n 0.040
Inch 24S-T and 24S-T Alclad Sheet in Repeated
Flexure Fatigue
55
1
FATIGUE STUDIES QF 24S-T AND 24S-T ALCLAD
SHEET 3HTH VARIOUS SURFACE COi\IDITIONS
SIMfiARY
Fatigue studies of two common structural aircraft materials,
24S-T and 24S-T Alclad, with respect to various surface conditions
are presented in this investigation. Only one gage of the two materials, 0*040 inch, was used throughout the investigation. Various
abrasives, crocus cioth, numbers 240, 180 and 100 Aloxite Finishing
Cloth, and number 60 sandpaper were used to impart Scratches of different depths to the surfaces of the specimens#
From the fatigue results obtained average stress concentration factors resulting from each Scratch pattern imparted by the
various abrasives were calculated, The repeated flexure fatigue
strengths of the alloys for surfaces scratched with the several abrasives are presented and also the curves of the relation between the
average depths of Scratch and concentration factor for repeated flexure fatigue are included.
INTRODUCTION
Even though the various components of aircraft are designed
to withstand certain calculated loads, the limits of which are assumed
never to be exceeded in the normal specified Operation of the aircraft,
2
failures of these components may occur due to fatigue. These fatigue
failures may be brought on by any number of reasons, one of the major
causes with respect to sheet components being scratched surfaces, as
a result of handling and fabrication.
Surface Scratches and varying degrees of finish have been of
great concern to de signers and a great deal of virork has been carried
out in this respect on various sbeels. However, the available fatigue
data on present day aluminum alloys is indeed meager.
It has been
shown in numerous tests by Horger , Hankins , Lea3, Moore and Kommers4,
Thomas^ and a number of other investigators that surface Scratches
are very deleterious to the endurance strength of various steels.
Matthaes^ has shovm by tests made on an early aluminum alloy
0. J# Horger, "Fatigue Strength of Members Influenced by Surface Conditions," Product Engineering, 11:490, December 1940.
2
G. A. Hankins, M. L. Becker and H. R. Mills, "Further Experiments on the Effect of Surface Conditions on the Fatigue Resistance
of Steels," Journal of the Iron and Steel Institute, 133:399-425,
February 1936.
F« C. Lea, "Effect of Discontinuities and Surface Conditions
on Failure Under Repeated Stress," Engineering, 144:87-90 and 140-144,
July 1937.
H« F. Moore and J. B. Kommers, "An Investigation of the Fatigue
of Metals", University of Illinois Engineering Experiment Station Bulletin, 124, 1921.
% . N. Thomas, "Effect of Scratches and of Various Workshop
Finishes Upon the Fatigue Strength of Steel", Reports and Memoranda No.
860, Aeronautical Research Committee, 2:542-568, March 1923.
6
K. Matthaes, "Fatigue Strength of Airplane and Engine Materials",
Technical Memorandum 743, N.A.C.A., p. 12, April 1934.
3
having file Scratches deoidediy reduced the fatigue strength of the material«
Igarash and Fukai? mention in their report that a 9 to 24
percent increase in fatigue strength was obtained by very light polishing of duralumin specimens«
The data on early duralumin alloys with surface discontinuities are of significance in that they show that Scratches are very
detrimental to the life of structural components under repeated loading.
These data, however, are of very little use to the present day
designer of aircraft«
Even the up to date fatigue data of the present
day alloys cannot in most cases be used directly in design since the
endurance limits are arrived at through the testing of polished specimens free from all "stress raisers" such as notches, holes, surface
Scratches, etc., whereas the materials used in fabrication are generally
used in the "as received" condition. Aluminum sheet materials in the
"as received11 condition are far from being highly polished and very frequently have Scratches of varying intensities, If the designer is concerned with fatigue in a particular design he must approximate the
effect of the surface irregulärities which may be present in the "as
received" material, His approximations may be too conservative or may
be unsafe, since up to this time no data on the effects of surface
Scratches on fatigue strength of aluminum alloys have been published,
UTith this thought in mind the idea for this investigation was conceived,
Fatigue failures generally originate at a point on the surface
I» Igarash and S. Fulcai, "On the Fatigue Test of Light Alloy
Sheets", Transactions of the Society of Mechanical Bngineers, Japan,
6:S-3, February 1940,
4
of a member, because usually, with normal stress distribution, the
stress is greatest at the surface. Furthermore, stress concentrations
are present at notches, fillets, Scratches and similar irregularities,
thus creating highly stressed focal points for the propagation of fatigue fractures. Although microscopical irregularities and other surface conditions usually have little influence on static strength, they
often form the nucleus of fatigue failure.
It has been attempted in
this investigation to determine the intensity of concentration of Stresses about Scratches of various depths and to relate this Information in
such a form that it may be useful for design purposes. It is hoped that
the presentation of such fatigue data will be of benefit to the designer in maicing more accurate calculations with regard to some surface irregularities in 24S-T and 24S-T Ale lad,
MATERIAL
The two materials used in this investigation are the well known
aircraft structural alloys, 24S-T, Army-Navy Specification AN-A-12, and
24S-T Alclad, Army-Navy Specif'ication AN-A-13.
Both materials were in
the form of sheet and only one Standard thiciaiess, 0.04Q inch, was used
for the tests. The nominal chemical composition of the 24S-T is as follows:
4.4 percent copper, 1.5 percent magnesium, 0.6 percent manganese
and the balance aluminum»
The nominal chemical composition of the 24S-T
Alclad is the same as that of the 24S-T for the core material, upon
which a surface coating of pure aluminum is rolled, comprising approximately 10 percent of the total thickness of the sheet or 5 percent on
esch surface. Representative Stress-strain curves of the two sheets of
5
material used are presented in Figures 1 and 2, The data reported are
the averages of two tests for each material. The specimens used were
the Standard American Society of Testing Materials' Tension Text Specialen, described by Davis^ in his handbook on materials testing. The
tests were conducted on a Riehle Universal Hydraulic Testing Machine,
a Huggenberger type extensometer being used for measuring -che elongations.
The mechanical properties of the materials thus derived are
listed in Table I. These values were checked with those presented in the
q
ANC-5
and were found to agree closely.
The main purpose of these tests
was for the determination of the yield point for the particular sheets
of material and for defining the pertinent mechanical properties.
THE FATIGUE TESTING MACHINE
The tests to be reported here were run on a Sonntag Flexure Fatigue Machine, Model SF-2, with a capacity varying from 250,000 pounds
per Square inch on 0.025 inch sheet to 20,000 pounds per square inch on
0.250 inch sheet. The motive power is produced by a l/4 horsepower
synchronous motor operating at a constant speed of 1800 revolutions
per minute. Three photographs, Figures 3,4 and 5, show a sample loaded in the machine and indicate ciearly the main features of loading.
% . E. Davis, G. E. Troxell and C. T. Jiskocii, The Testing and
Inspection of Engineering Materials (New York: McGraw-Hill Book Company,
Inc., 1941;, p. 80, Fig. 48, Type B.
9
Anonymous, nStrength of Aircraft Elements", ANC-5, Army-NavyCivil Committee on Aircraft Design Criteria, Revised Edition, December 1942, Amendment-2, August 1946, p.5-6.
6
In the following discussion on the Operation of the machine these
photographs will be referred to in the mention of the various components«
The machine is a constant repeated force fatigue machine using
an eccentric mass, A, to generate the force, By adjusting the eccentricity of the mass the force output may be read directly fram the
scale, B.
The force is transmitted through rod, C, to load yoke, D.
The travel of rod, C, is limited to the vertical, the side forces of
the eccentric being absorbed by the pivot rod, E«
The specimen is
ciamped in the load yoke by means of the pivot bar, F, clamp bar, G,
and clamping bolts, H»
The fixed end of the specimen is rigidly
held in the pedestal, I, and ciamped by bar, J, and bolts, K« Pedestal, I, is adjustable for different length specimens.
The machine is equipped m t h a micro-switch, L, which automatically shuts off the motor when the specimen braaks. Also, the machine
is provided with counter, M, which registers the number of cycles to
failure in a ratio of 1000 : 1.
As previously noted the force is applied by a rotating eccentric mass and remains a constant for any fixed value of eccentricity.
A System of inertia compensation is used in order to maintain the
force applied on the specimen constant irresprective of amplitude,
This means of compensation absorbs all the inertia forces in the vibrating System so that the eccentric force alone acts on the specimen,
A mathematical proof of the method is presented in the operating man-
7
ual of the machine.^
However, stated simply, this method is as
follows: A spring, the tapered drive shaft, N, is used whose deflection constant is equai to the inertia forces of the vibrating System.
As the deflection of the System increases the inertia forces in turn
increase, but compensating this is the spring reaction which cancels
the inertia forces. This leaves only the eccentric force or a repeated force of constant maximum value applied on the specimen.
Since the system must be in resonance for the condition to hold, it
is only valid for a given frequency and a given mass System. The
synchronous motor, 0, maintains constant frequency for the system
and the variable poise weights, P, are provided in order to adjust
for differences in the mass of the system when different weight specimens are used.
As was noted in the foregoing paragraph the mass of the system
must be kept constant and for this reaxon it was necessary to calculate the poise setting for an 0.040 inch thick specimen in order that
the inachine be tuned to resonance. Using the calculated effective
weight of the specimen,
We =
«385 x d x t 1 1
where W e = effective weight in pounds
d
= density in pounds per cubic inch
and t
— thickness of material in inehes
u
Anonymous, "Instructions for Installation, Operation and Maintenance of Flexure Fatigue Testing Machine, Model SF-2, Serial No.
472875." Manual furnished by Sonntag Scientific Corporation, Greenwich,
Connecticutt, prepared July 1947, Appendix Print No» 90273-S.
Ibid., p. 90450-S, sheet 3.
and referring t h i s to graph No. 90452-S 12 the poise s e t t i n g for an
0.040 inch thick specimen was determined.
Graph No. 9G452-S i s a
curve which was determined a t the factory for the purpose of tuning
t h i s p a r t i c u l a r machine to resonance when using various thicknesses
of m a t e r i a i .
The l a s t adjustment i s t h a t for the amount of force to be applied t o give any desired s t r e s s .
Graph No, 90446-S
i s provided
as a c a l i b r a t i o n curve of specimen s t r e s s per poond of force developed by the eccentric mass, against the thickness of the m a t e r i a i .
This curve i s merely the adaptation of the familiär beam formula,
f
where
f
=
=
My14
I
unit normal s t r e s s in p s i .
M = bending moment on the cross-section in i n . - l b s .
I
= moment of i n e r t i a of section about i t s neutral axis in
inches 4
y
=
distance p a r a l l e l to the plane of bending between the
point under consideration and the n e u t r a l a x i s , or ortehalf the thickness in inehes for maximum s t r e s s , which
oecurs a t the surface.
This formula may be modified t o include the force of the eccentric mass
12
I b i d . , graph No. 90452-S.
15
I b i d . , graph No. 90446-S.
1
A. S. Niles and J. S. Newell, Airplane S t r u c t u r e s , Second Edit i o n , Vol. 1 (New York: John JäTiley and Sons, I n c . , 1938;, p . 143.
9
and may be written
i
where
P x 1 = M
P
= force of the eccentric mass in lbs.
1
= the distance in inehes from the ioad yoke to the point
in question on the test section of the speeimen.
The graph was found to be very useful in that it eliminated the making
of individual calcuiations for each speeimen each time the load was
varied.
Knowing the desired stress, it was only necessary to read the
speeimen stress per pound of force for a given thicjoiess from the
graph and divide this value into the desired stress for the machine
setting on the eccentric»
Important considerations in running any speeimen include: (l) adjusting and determining the values of the loads to be applied, and (2)
determining the weakest section or point of minimum thickness.
THE FATIGUE SPECMENS
A sketch of the speeimen used for all of the fatigue tests reported here is shown in Figure 6, giving complete dimensions and mounting details. As may easily be deduced from the layout of the speeimen,
it incorporates as the test section a beam with constant bending stress.
Since the bending stress is constant at any point on the test section
between the two points where the radii become tangent to the straight
sides, it is obvious that failure, as a result of fatigue stressing, may
take place anywhere between the two extremities noted above.
This fact
may be validated by Figures 7 and 8, which show speeimens of 24S-T and
10
24S-T Alclad, r e s p e c t i v e l y , fractured a t random locations along the
t e s t section.
Preparation:
In preparing the specimens i t was necessary a t
a l l times during the handling of the material to be extremely careful
so t h a t the specimens would not be marred or scratched.
The speci-
mens were f i r s t s t a c k - d r i l l e d and then cut s l i g h t l y oversize on a DoAll Metal Saw, using a tempiate as a guide.
Next, the specimens were
stacked with dowel pins in the d r i l l holes and were l'iled almost to
size on a toolmaker's mechanical f i l e .
The finishing of the edges
and f i n a l cutting down to size were done by hand, using No. 240 Aloxi t e Finishing Cloth and then polishing with crocus c l o t h .
Finally
the specimens were each polished with a liquid aluminum polish on the
surfaces and edges.
All specimens were carefully inspected for nicks
or Scratches on the surfaces and finished edges.
The specimens of
24S-T were p r a c t i c a i l y free from a l l surface Scratches after the final
polishing.
However, the 24S-T Alclad specimens usually contained
s l i g h t Scratches or imperfections as a r e s u l t of the handling, due to
the softer surface of the pure aluminum coating»
I t was necessary to check with a micrometer the thickness of
each specimen, since there was a considerable Variation in thickness
of the finished specimens in spite of the fact that they were a l l cut
from the same sheet.
The 24S-T specimens varied from a thickness of
0.0382 inch to a thickness of Q.Q392 inch and likewise the 24S-T Alclad specimens varied from a thickness of 0.040 inch to a thickness of
0.0412 inch.
Grain Direction:
All specimens were cut with the c e n t e r l i n e par-
11
allel to the direction of rolling of the sheet in order to give higher
values of cycleir and also to provide uniformity of tests. Brick and
Phillips15 have investigated the effect of grain direction on 24S-T
and 24S-T Ale lad and arrived at the following conciusions:
(l) At
5 x 107 cycles for 24S-T samples cut parallel to the direction of
rolling, a vaLue of 20,500 + 1000 psi was indicated and for samples
cut perpendicular to the direction of rolling, *a vaiue of 18,500 +
2000 psi was indicated.
(2) For 24S-T Alclad at 5 x 10 7 cycles for
speeimens cut parallel, 15,000 + 1000 psi was indicated and for speeimens cut perpendicular, 11,500 + 1000 psi was indicated, As can
be noted frora these results it was necessary to seiect a given grain
direction with respect to the speeimen in order to maintain uniformity and also since the investigation was rather extensive in its entirety it was necessary to limit the study to only one grain directiom.
Scratches;
Five different roughnesses of cornmercial abrasives
were used to Scratch the speeimens, namely, crocus cloth, Numbers 240,
180 and 100 Aloxite Finishing Cloth and Number 60 Jewel Garnet Sandpaper. Both sides were uniformly roughened over the entire test section, on both sides, with a given abrasive, applying the force by hand•
All Scratches were made transverse to the direction of stress in order
to obtain the greatest reduetions in fatigue strength. Figures 9 and
10 show unbroken polished and scratched samples of both materials used.
R. M. Brick and A. Phillips, "Fatigue and Damping Studies of
Aircraft Sheet Materials«. Duralumin, Alloy 24S-T, Alclad 24S-T and Several 18:8 Type S^ainless steels", Transactions, American Society for
Metals, 29:441, June 1941.
12
Eorger-^ states that surface finish marks in planes transverse
to the direction of stress have detriraental effects on fatigue strength.
He also cites the service Performance of Springs"and axles where longitudinal cracks were found to have iittie or no infiuence on the bending fatigue strength. Transverse Scratches have a considerably more
detrimental effect on fatigue life than do iongitudinal Scratches because, in bending, the normal stresses are perpendicular to the transverse Scratches. As a Scratch, nick or any other type of discontinuity
provides nuclei for stress concentrations, the entire width described
by the transverse Scratches tends to upset or retard the flow or normal stresses and hence stress concentrations are present all along the
transverse Scratches.
In the case of longitudianl Scratches the effect
is relatively small, since the Scratches are in iine with the normal
stresses and hence do not tend to retard the flow to such a degree as
do the transverse Scratches.
Since the entire test section of each specimen was thoroughly
roughened, a great number of Scratches were imparted in a small area,
thus making the Scratches lie side by side or overiap.
For this rea-
son it is not proposed that the stress concenbration factors, obtained from the tests reported, are the highe st for a given depth of
Scratch, but merely average stress concentration factors, presented
for the condition where Scratches are -very close together. Roark-1-7
explains that a Single isolated "stress raiser" has a worse effect
16
Horger, op. cit., p. 492.
17
R. J. Roark, Formulas for Stress and Strain, Second Edition
(New York: McGraw-Rill Book Company, Inc., 1943J, p. 32.
13
than do a number of similar "stress raisers" grouped close together.
In other words, one "stress raiser" closely adjacent to another
"stress raiser" would tend to relieve, to a certain degree, the
stressee on the other "stress raiser" and vice versa. Thus it is
reasonabie to say that tests run with specimens having only a Single transverse Scratch will provide stress concentration factors of
soxnewhat greater magnitude than those presented in this paper.
Depths of Scratches: Measurement of the depth of Scratch resulting from the various abrasives m s done by means of a Baush and
Lomb Research Metallograph. A magnigication of 2175 times was found
to give a clear and distinct view of the grooves and ridges. By
rirst focusing on a ridge and then focusing on the adjacent groove it
was possible to determine the depth of Scratch through the differential
of focal length.
The microscope was provided with a calibrated ad-
justing knob, thus simplifying the caiculation of the depth of Scratch
to merely taking the difference in readings i'or the ridge and groove.
As the Scratches imparted to the surface were not uniform in depth,
ten readings were taicen on a given scratched specimen and the average
was taken as being representative of the Scratches on the entire
surface of the specimen. Generally, each Single reading of depth
varied from about 5 to 15 percent of the average.
In a few cases,
with the coarser abrasives, the difference was as much as 25 percent.
This Variation may be attributed to the method of applying the Scratches.
The average Scratch depths for the various abrasives are presented in
Table II. The contention that the average value of depth is valid may
also be justified as being reasonabie by the fact that the vaiues report-
14
ed provide a smooth curve when plotted against the grit number of the
abrasive.
This graph is shown in Figure 11.
TEST PROCBDURE
The specimen is placed in the machine in a horizontal position
as shown in Figures 5 and 6.
One end of the specimen is fixed rigid-
ly and the other end ciamped in the load yoice. The desired ioad is
adjusted by means of the eccentric mass, and once the machine is
started it will continue until faiiure occurs and the limit switch
automatically stops the motor,
The tests for each particular curve were begun with the high
stresses, gradualiy decreasing the stress in increments of about 2 to
3 thousand psi for each succeeding specimen.
In this way the general
trend of the curves was obtained, and it enabled the Operator to predict approximately the iife of a specimen at a given iower stress,
The high stresses for all tests were determined as those necessary to give only a few thousand reversals of stress.
In all cases
these stresses neTver exceeded the yield point of the material, Specimens
stressed in the lower ränge were generally loaded so that the life
would not exceed 10 miiiion cycles, Almost all the tests were continued
to faiiure, however, some few specimens were removed even though faiiure had not occured after 10 to 15 miiiion cycles of stress. Such
points are indicated on the plots with the conventional horizontal arrow at the point where the test was discontinued,
The curves of the poiished specimens were run first, in order to
determine the "par value" and also as a check to insure correct Operation of the machine. Thäse curves agree closely with similar curves
15
as reported by Bricic and Phillips18 for polished 24S-T and 24S-T Ale lad,
with the difference that the values reported in this investigation are
siightly higher in all cases. Since 1941, when Brick and Phillips
carried out their investigation, there have been notable improvements
in the processing of aluminum sheet, thus bettering the mechanical
properties of the sheet, Hence it is reasonable to assume that
simiiar improvements have resulted with respect to fatigue strength.
Next, the curves of the scratched specimens were run in successive order with respect to the Scratch depth, the speciraens with
the lightest Scratches being run i'irst»
DISCÜSSIQN OP KESULTS
The results of all fatigue tests were plotted in the form of
S-N diagrams, stress versus number of cycles. Semi-iogarithmic paper
was used for piotting the results in keeping with conventional methods
of presenting fatigue results,
Fatigue Results of 24S-T; Figures 12, lö, 14, ±5 and 16 show
the results of fatigue tests on 24S-T specimens with transverse Scratches made by the various abrasives. The experiraental points obtained are
clearly indicated on the graphs and the curves are faired through thenu
In addition, the fatigue curve of polished specimens is included on
each graph for purpose of comparison«
Figure 17, which is merely a com-
pilation of all the fatigue curves for the 24S-T specimens resulting
from the investigation, is also presented in order to show tho relative
effects of the various abrasives on the fatigue strength,
Brick and Phillips, op. cit., p. 440.
16
There is a considerable degree of scatter in the experimental
points in all the curves, however, in comparison it should be noted that
other experimental fatigue tests show a similar scatter.
In fact, some
investigators are of the opinion that fatigue results should be plotted
as a scatter band and not as a Single curve. There are several possibie
reasons for the scatter of points, some being as follows:
(l) slight
errors in setting required ioading, (2) non-uniformity of Scratches,
(3) slight errors in machining and finishing, (4) possibie error in
reading caiibration curve of machine, (5) the effect of work hardening,
(6) metallurgical differences in the samples, and (7) the actual
nature of the fatigue phenomenon.
Figure 17, the graph showing the relative effects of the various
abrasives, indicates clearly that there is a distinct progressive reduction in fatigue life for any predetermined stress with the increase
in Scratch depth. Also, the greater the roughness imposed, the greater the reduction in fatigue strength. The specimens scratched with
crocus cloth showed the least reduction, since the Scratches produced
v/ere very slight, and as might be supposed, the greatest reductions
were obtained from specimens scratched with the No. 60 sandpaper, the
coarsest abrasive used.
In all the tests run there was no indication of an endurance
limit for either of the materials. By referring to Figure 17 it is
seen that there is no ieveling-off of the curves as in the case of steel.
Instead there is a definite downward slope. Many other investigators
have cited this fact with respect to aluminum and its alloys.
17
Fatigue Results of 24S-T Alclad:
Figures 18, 19, 20, 21 and 22
show the results of fatigue tests on 24S-T Alclad specimens with
transversa Scratches made by the various abrasives. Here liieewise,
the experimental points are indicated on the graphs and the curves
are faired through them. Again the curve of the poiished specimens
is included with each graph in order to illustrate the reduction in
fatigue strength due to the surface rouglmess. Figure 23, similar to
Figure 17 for the 24S-T, shows the curves of all the 24S-T Alclad specimens and the relative effects of the various abrasives on the fatigue strength.
The degree of scatter of points for the 24S-T Alclad specimens
is less than that of the 24S-T specimens. This is probably an indication that surface Scratches are not so detri:mental to the fatigue
strength of alclad material as to the unclad material. Eowever, the
other considerations for the scatter of points in the unclad material
also apply to the alclad material.
Referring to Figure 23, the graph showing the relative effects
of the various abrasives, it is seen that there is a distinct progressive reduction in fatigue life with successive cycles of stress, but
not so great as in the case of the unclad material. Likewise the more
rougher abrasive produced greater reductions in fatigue strength but
in much smaller increments than those on the 24S-T, even though the
average depths of Scratches were greater. The specimens scratched with
crocus cloth showed the least reduction and, as might be expected, the
specimens scratched with the No. 60 sandpaper showed the greatest redetet ions.
18
The f a c t t h a t g r e a t e r r e d u c t i o n s i n f a t i g u e s t r e n g t h were obt a i n e d for 24S-T t h a n f o r 24S-T A l c a l d does n o t imply t h a t t h e a l c l a d
materlal is the b e t t e r .
24S-T i s s u p e r i o r t o 24S-T A l c l a d where s t a t i c
s t r e n g t h i s concerned and Che same happens t o be t r u e i n t h e case of
f a t i g u e s t r e n g t h i n s p i t e of the g r e a t e r r e d u c t i o n i n s t r e n g t h of t h e
former.
As an example, compare the e f f e c t of a given S c r a t c h depth
on t h e f a t i g u e p r o p e r t i e s of b o t h m a t e r i a l s .
R e f e r r i n g t o Table I I
i t i s seen t h a t No. 60 sandpaper produces approximately t h e same d e p t h
S c r a t c h on 24S-T a s does No. 100: A l o x i t e on 24S-T A l c l a d .
Now r e f e r r i n g
t o Table I I I i t i s seen t h a t for No. 60 sandpaper on 24S-T a t 10 ^ c y c l e s ,
a s t r e n g t h of 16,000 p s i i s r e p o r t e d .
In c o n t r a s t t o t h i s , i t i s seen
t h a t a value of 13,300 p s i i s r e p o r t e d for No. 100 A l o x i t e on 24S-T A l c l a d
a t 10^ c y c l e s i n Table IV.
The s t r e s s c o n c e n t r a t i o n f a c t o r s f o r 24S-T
a r e h i g h e r , b u t t h e a c t u a l f a t i g u e s t r e n g t h i s a l s o h i g h e r , because
of t h e i n i t i a l l y h i g h e r f a t i g u e s t r e n g t h of t h e 24S-T.
In a n a l y z i n g
f a t i g u e s t r e n g t h s of t h e two m a t e r i a l s , one must a l s o bear i n mind t h a t
t h e a l c l a d m a t e r i a l i s more s u s c e p t i b l e t o S c r a t c h e s t h a n i s t h e unclad
material.
During t h e t e s t i n g of t h e p o l i s h e d a l c l a d specimens i t was noted
t h a t a f t e r a c e r t t . i n t i m e , depending upon t h e s t r e s s , a l a r g e n\3mber of
c r a c k s i n t h e weak aluminurn c o a t i n g began t o a p p e a r .
These c r a c k s p r o -
pagated i n t o t h e a i l o y core and hence produced f a i l u r e .
No d o u b t ,
since such c r a c k s a r e i n h e r e n t t o f a t i g u e d a l c l a d , t h e a d d i t i o n of
S c r a t c h e s w i t h t h e a b r a s i v e s merely hastened a p r o c e s s which was t o o c cur anyway.
For t h i s r e a s o n t h e e f f e c t of t h e v a r i o u s a b r a s i v e s was
not a s g r e a t on t h e a l c l a d m a t e r i a l a s on t h e u n c l a d .
In a l l c a s e s t h e
d e p t h of S c r a t c h never p e n e t r a t e d through t h e aluminurn c o a t i n g of t h e
19
m a t e r i a l , the greatest average depth of Scratch being 0.O0O1981 inch
(see Table I I ) , and the thickness of the aluminum coating being approximately 0.002 inch.
Stress Concentration:
When there is an abrupt change in cross
section a considerable disturbance in s t r e s s d i s t r i b u t i o n r e s u l t s and
the maximum Stresses encountered a t the discontinuity are usually much
greater than would be indicated by the change in cross section.
Any
type of discontinuity such as a Scratch, notch, hole, e t c . , may cause
the s t r e s s e s in a member to be raagnified l o c a l l y , thus producing what
i s known as s t r e s s concentration.
If f i s the caiculated s t r e s s in the
section without a d i s c o n t i n u i t y , as determined by conventional methods,
and fjQBY the s t r e s s a t the point affected by the d i s c o n t i n u i t y , then
t h i s maxinium s t r e s s may be represented as a function of the caiculated
s t r e s s , or
' «
-
K f l 9
where K is the stress concentration factor, and is dependent upon the
character and relative size of 'the discontinuity.
Many investigators, concerned with stress concentration, have devised both theoretical and special experimentai methods for determining
stress concentration factors for numerous configurations of discontinuities. However, no such method is feasible in the case of Scratches
since there is no uniformity in their configuration.
Since, in all cases investigated, the scratched specimens failed
at lower numbers of cycies than did the poiished specimens at the same
S. Timoshenko, Strength of Materials, Part II, Advanced Theory
and Problems, Second Edition (New York: D. Van NoStrand Company, Inc.,
1941), p. 537.
20
stress, it is obvious that there is a definite concentration of Stresses
at the Scratches. Assuming that the scratched specimens are operating
at a stress greater than that obtained from the simple beam formula
for a given load, it is evident that there exists a factor of concentration such that,
f
max
=
Kf x f
where f.-«- is the stress at which the scratched specimens are operating,
f is the stress as obtained by the simple beam formula, for a given
load, and Kf is the stress concentration factor for repeated flexure fatigue. iience it may now be said that for a given number of cycles a
scratched specimen was operating at the same stress as was a given polished specimen, and that the stress concentration factor, K\p, must be
applied to the simple beam formula in order to obtain the actual operating stress.
Under the foregoing assumptions it was possible to calculate the
stress concentration factors for the Scratches imposed by the various
abrasives.
T&ble III gives the repeated flexure fatigue strengths of
0.040 inch 24S-T with transverse surface Scratches due to the several
abrasives and the corresponding stress concentration factors. Table IV
gives similar data for 24S-T Alclad.
These stress concentration factors
were calculated by dividing the stress of a scratched specimen, at a given
number of cycles, into the stress of the poiished specimen, at the same
number of cycles. These factors are calculated for six different locations on the curve, thus obtaining six factors which in most cases
are fairly constant for a given abrasive. The factors obtained for low
numbers of cycles are generally higher than those obtained for high
numbers of cycles. However, the six values were averaged and presented
21
as an average stress concentratlon factor.
By doing this the average
stress concentration factors will be more conservative in the lower
stress ränge. Figure 24 shows a plot of these average stress concentration factors against the average depth of Scratch as produced by
the several abrasives. From these curves one may conclude that a given Scratch depth has less effect on the fatigue life of 24S-T Alclad than a similar Scratch depth has on 24S-T. However, if the
Scratch were of such magnitude as to penetrace the pure aluminum eoating of the alclad material, it is likely that the effect would be more
in order with that of the unclad material•
Size Effect:
The effect of size i s a very important considera-
t i o n when applying fatigue data obtained from small specimens to the
design of large components.
I t has been noted by numerous investiga-cors^O
in size effect studies of metal bars and shafts, both notched and unnotched, t h a t fatigue strengths obtained as a r e s u l t of t e s t i n g small
specimens are greater than fatigue strengths obtained through the t e s t ing of large specimens, and t h a t notch s e n s i t i v i t y increases as the
actual size increases.
The actual fatigue data of l i g h t alloy sheet
with respect to size effect i s , indeed, meager.
Buchmann2•*• investigated
several l i g h t a l l o y s , Mg-Al6("Elektron AZM"), GMg-Al("Elektron A9V")
and Al-Cu-Mg("Igedur 26") with respect to size effect with the follow-
B a t t e l l e Memorial I n s t i t u t e : Prevention of the Failure of Met a l s Under Repeated S t r e s s , (New York: John Wiley and Sons, I n c . , 1941),
p . 123.
21
IT. Buchmann, "Influence of Cross-Sectional Area on Fatigue
Strength", Engineer's Digest, 5:137, March 1945.
22
ing results;
(a) There was a pronounced drop in the flexural fatigue
strength with increasing size, especialiy in the ränge of small crosssectional areas, ( 5 to 15 mm. dia. ).
(b) Beyond a certain linit, ( 30 mm. dia. ) , the rate of drop
in flexural fatigue strength is only slight; the curves tended asymptotically toward the fatigue strength due to reversed axial loads.
The excess strength on reversed flexure over the ssymptotic value is
explained by the stabilizing effect of the siightiy stressed inner
fibers on the highly stressed outer fibers. This effect obviously
depends on the stress gradient.
(c) The fatigue strength of unnotched samples due to reversed
axial loads is independent of scale factor, lith a notched sample,
however, there is a stress gradient and consequentiy, due to the stabilizing effect there is an influence of size on the fatigue strength.
(d) With fatigue due to alternating torsion there is a distinct influence of the size of the test samples, even when unnotdhed.
It is obvious from the findings of Buchmann that the results
of this investigation are limited to the design of small components
and to sheet of the sarr:e gage tested uniess adequate size effect correction factors are applied.
Application of Data of Design; Fatigue data as obtained from
the laboratory through the testing of polished specimens are not directly applicable to design, but must be modified to meet the particular design conditions. The data herein presented is one step in
the direction of reducing the designer's probiem of modifying fatigue
23
data when the design condition involves surface irregularities of the
same nature as investigated in this paper.
;*hen
applying the data to
large components adequate size effect factors should be used to correct the data. Also, since the stress ränges encountered in design
are not usually of the order of completely reversed stress, the data
should be adjusted to meet the particular rarige of stress in question.
Methods such as the endurance diagram, as explained by Maleev^, may
be set up to find endurance strengths for vttrious stress ratios.
Assuming that the size effect is of the same order as for the
results of this investigation and that the ränge of stress is that of
complete reversals of stress, then the problem of predicting the life
of a 24S-T or 24S-T Ale lad component becomes very simple. Knpwing the
operating stress of the component and the surface condition, reference
to Figure 24 will give the stress concentration factor for the
particular surface condition.
The operating stress is muliiplied by
the factor in order to obcain the true operating stress of the component.
This true operating stress is now referred to an S-N curve
for polished speeimens and the corresponding number of cycles is the
life of the scratched component.
The stress concentration factors reported are believed to be reliable for application to design.
They should be applied to design
stresses in Computing the margins of safety.
V» L. Maleev, Machine Design, Second Edition, (Scranton, Pennsylvania: International Textbook Company, 1946), p. 47.
24
CONCLUSIONS
From t h e foregoing p r e s e n t a t i o n of t h e r e s u l t s of t h i s
investi-
g a t i o n t h e f o l l o w i n g conciusions a r e drawn:
1.
Surface Scratches d e f i n i t e l y reduce the flexure fatigue
strength of 24S-T and 24S-T Alclad sheet material«
The greater the
depth of surface Scratch the greater the reduction.
2.
Surface Scratches are more detrimental to the fatigue proper-
t i e s of 24S-T than to 24S-T Alclad, provided the Scratches are not
through the pure aluminum coating.
Sven though the reduction in fa-
tigue strength for a given Scratch depth i s greater for 24S-T than for
24S-T Alclad, 24S-T s t i l l exhibits higher vaiues of fatigue strength
in pounds per square inch.
3. Average stress concentration factors have been determined
for small surface Scratches in 0.040 inch 24S^T and 24S-T Alclad materials.
These concentration factors may be applied to design provided
the design conditions are similar to those of the investigation.
4.
The stress concentration factors should be applied to de-
sign loads in Computing margins of safety.
5. Further study should be made with heavier g&ge material
having surface Scratches in order to investigate the size effect phenomenon«
25
BIBLIOGRAPHY
Anonymous, "Instructions for Installation, Operation and Maintenance
of Flexure Fatigue Testing Machine, Model N 0 . SF-2, Serial No.
472875". Manual furnished by Sonntag Scientific Corporation,
Greenwich, Gonnecticutt, prepared July 1947.
Anonymous, "Progress Report on Fatigue and Impact Characteristics and
Notch Effect in Tension of Artificially-Aged Aluminum Alloys0.
iTar Metallurgy Committee, OSRD No. 3579, Serial No. M-216, April
18, 1944.
Anonymous, "Strength of Aircraft Elements". ANC-5, Army-Navy-Civil
Committee on Aircraft Design Criteria, U. S. Government iJrinting
Office, Revised Edition, December 1942, Amendment-2, August 1946.
Batteile Memorial Institute, Prevention of Failure of Metals Under Repeated Stress. A Handbook prepared for the Bureau of Aeronautics,
Navy Department. New York: John liley and Sons, Inc., 1941, 273 pp.
Brick, R. M. and A. Phillips, "Fatigue and Damping Studies of Aircraft
Sheet Materials; Duralumin Alloy 24S-T, Alclad 24S-T and Several
18:8 Type Stainless Steels", American Society for Metals, Transactions, 29:435-463, June 1941.
Buchmann, W., "Influence of Cross-Sectional Area on Fatigue Strength",
Engineer's Digest, 3:136-157, March 1945.
Davis, H. E., G. E. Troxeil and C. T. Wiskocil, The Testing and Inspection of Engineering Materials. New York: McGraw-Hill Book Company,
Inc., 1941, 372 pp.
Kankins, G. A., M. I*. Becker and II. R. Mills, "Further Experiments on
the Effect of Surface Conditions on the Fatigue Resistance of
Steels", Journal of the Iron and Steel Institute, 133:399-425,
February 1936.
Eartmann, E. C , "Fatigue Test Results, Their Use in Design Calculations",
Product Engineering, 12:74-78, February 1941.
Horger, 0« J., "Fatigue Strength of Members Influenced by Surface Conditions", Product Engineering, Part I, 11:490-493, November 1940,
Part II, 11:562-565, December 1940, Part III, 12:22-24, January 1941.
Igarash, I. and S. Fukai, "On the Fatigue Test of Light Alloy Sheets",
Transactions of the Society of Mechanical Engineers, Japan, Vol. 6,
Abstract Supplement p.S-3 in English, February 1940.
26
Lea, F. C , "Effect of Discontinuities and Surface Conditions on Failure Under Repeated Stress", Engineering, 144:87-90 and 140-144,
July 1937.
Maleev, V. L., Machine Design. Second Edition, Scranton, Pennsylvania:
International Textbook Company, 194-6, 581 pp.
Matthaes, K., "Fatigue Strength of Airplane and Engine Materials",
Technical Memorandum 743, N.A.C.A., April 1934,
Mechling, W. B. and S. S. Jack, "Some Mechanical Tests on Aluminum Alloys, 14S-T and 24S-T", Proceedings of the American Society for
Testing Materials, 39:769-778, June 1939.
Moore, H. F. and J. B. Kommers, "An Investigation of the Fatigue of Metals", UniversitA/- of Illinois Engineering Experiment Station Bulletin, T2TrT9"2TTY78 pp.
~
Files, A. S. and J. S. Newell, Airplane Structures, Second Edition, Vol.
1, New York: John Wiley and~^bniT~Inc., 1938,"451 pp.
Roark, R. J., Formulas for Stress and Strain, Second Edition, New York:
McGraw-Hill Boolc Company, Inc., 1943, 366pp.
Thomas, W. N., "Effect of Scratches and of Various Workshop Finishes
Upon the Fatigue Strength of Steel", Reports and Memoranda No. 860,
Aeronautical Research Committee, 2:542-568, March 1923.
Timoshenko, S., Strength of Materials, Part II, Advanced Theory and
Problems, Second Edition, New York: D, "Van NosTrlmd ComplxnyT" Ine.,
T94l7"5l0 pp.
APPENDIX I , Tables
TABLE
MECHANICAL
24S-T ALCLAD
E,MODULUSOF
MATERIAL
ELASTICITY,PSI
10.2 XIO 6
24 S-T
24S-T ALCLAD
I
PROPERTIES
SHEETS
I07
TABLE
AVERAGE
28
USED
OF
IN
2 4 S-T
AND
FATIGUE TESTS
ULTIMATE
YIELD
TENSILE
STRENGTH, PS.I.
STRENGTH. PS.I.
51,500
6 8,000
49,500
63,000
H
DEPTH OF SCRATCH
FOR VARIOUS ABRASIVES ON 24S-T AND 24S-T ALCLAD
AVERAGE
MATERIAL
24S-T
,
2 4 S-T
24 S-T ALCLAD
<
24S-T ALCLAD
ABRASIVE
CROCUS
DEPTH
OF SCRATCH, INCH
CLOTH
.0000508
NO. 2 4 0 ALOXITE
.000072 1
NO. 180 ALOXITE
.000081 1
NO. 100 ALOXITE
.0001070
NO. 6 0 SANDPAPER
.0001573
CROCUS CLOTH
.0000551
NO. 2 4 0 ALOXITE
. 0000866
NO. 18 0 ALOXITE
0001102
NO. 1 0 0 ALOXITE
.0001495
NO. 6 0 SANDPAPER
.0001981
TABLE
HI
REPEATED FLEXURE FATIGUE STREN6THS
OF 0 . 0 4 0 INCH
2 4 S - T SHEET WITH TRANSVERSE SURFACE SCRATCHES DUE
TO SEVERAL ABRASIVES AND THE CORRESPONDING STRESS
CONCENTRATION FACTORS. STRESSES IN PS.l.
CONDITION
CYCLES
POLISHED
CROCUS CUOTH
OF
NO. 2 4 0
ALOXITE
SURFACE
NO. 180
. ALOXITE
NO. 100
ALOXITE
NO. 6 0
SANDPAPER
STRESS
STRESS FACTOR STRESS FACTOR STRESS FACTOR STRESS FACTOR STRESS FACTOR
5XI04
42,500
38,000
1.118
35,500
1.196
35,000
1.213
34,600
1.228
2 9,700
1.431
I05
36,000
32,800
1.097
32,100
1.121
31,100
1.157
30,350
1.186
26,800
1.343
27,500
1.0 9 4
26,600
1131
25,900
1.162
25,100
1. 198
21,700
1.387
5XI05
30,100
I06
2 8,500
26,000
1.096
2 5,200
1.131
2 4 , 4 0 0 1.168
23,700
1.202
20,400
1.396
5XI06
25,350
22,900
1.106
22,400
1.13 1
21,650
1.170
20,950
1.209
17,200
1.472
24,150
21,750
1 110
21,350
l.l 3 1
2 0 , 6 0 0 1.172
16,000
1.510
1.140
1.173
I07
AVERAGE
STRESS
C O N C E N T R A T I O N FACTOR
1.103
20,000 1.207
I.2Ö3
1.422
TABLE T2T
REPEATED
ALCLAD
FLEXURE
SHEET
SEVERAL
••
WITH
FATIGUE
STRENGTHS
TRANSVERSE
P O L I S H E D CROCUS CLOTH
CYCLES
SURFACE
OF
NO. 2 4 0
ALOXITE
24S-T
SCRATCHES DUE TO
ABRASIVES AND THE CORRESPONDING
STRESS
TRATION FACTORS.
STRESSES IN PS.I.
CONDITIO N
STRESS
OF 0 . 0 4 0 INCH
CONCEN-
SURFACE
NO. 1 8 0
ALOXITE
NO. 1 0 0
ALOXITE
NO. 6 0
SANDPAPER
STRESS FACTOR STRESS FACTOR STRESS FACTOR STRESS FACTOR STRESS FACTOR
5XI04
35,800
32,500
1.101
30,400
1.178
28,500
1.256
27,900
1.282
27,150
1.318
I05
28,9 00
2 6,200
1.102
2 4 , 4 0 0 1.183
23,250
1.242
22,600
1.278
22,100
1.307
19,200
17,800
1.078
17,100
1.122
I6J50
1.146
16,250
1.181
15,600
1.231
17, 6 5 0
16,600
1.063
15,800
l.l 1 7
15,600
1.131
15,100
l.l 6 9
14,350
1.230
15,900
14,900
1.067
1 4 , 2 5 0 1.116
1 4 , 0 0 0 l.l 3 6
13,750
1.156
12,850
1.237
15,300
14,350
1.066
13,750
13,550
13,300
1.150
5XI0
5
I06
5XI0
I07
6
AVERAOE
STRESS
CONCENTRATION FACTOR
1.079
1.112
1.13 9
1.129
1.172
1.202
12,400
1.233
1.259
APPENDIX I I , F i g u r e s
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a
F1GURE 3.
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SPECIMEN AND MOUNTING DETAILS
STATIONARY MOUNT
37
FREE SPAN
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CROSS SECTIONAL VIEW OF
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I HOLE
£ DRILL 2 HOL ES
SPECIMEN DETAILS
ALL
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i
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TYPE SURFACE
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(D) NO. 180 ALOXITE , (E) NO. 100 ALOXITE , (F)NO. 6 0 SANDPAPER
(B)
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I
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FIGURE
8.
FRACTURED
24S-T ALCLAD
SPECIMENS
TYPE SURFACE FINISH
(A) POLISHED, (B)CROCUS CLOTH , (C) NO. 240 ALOXITE,
(D) NO. 180 ALOXITE, (E ) NO. 100 ALOXITE , (F) NO. SANDPAPER
40
FIGURE
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SPECIMENS
TYPE
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CLOTH,
BEFORE
FRACTURE
FINISH
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(E) NO. 100 ALOXITE,
ALOXITE,
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FIGURE
10.
24S-T ALCLAD
TYPE
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SPECIMENS
SURFACE
BEFORE FRACTURE
FINISH
Iß) CROCUS GLOTH, (C) NO. 2 4 0
ALOXITE,
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GRIT NUMBER OF ABRASIVE
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GURE
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FIGURE 17
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