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Improvement of endurance limitof alloy steel by electroplating
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Authors Quirós, José Mario, 1938-
Publisher The University of Arizona.
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IMPROVEMENT OF ENDURANCE LIMIT
OF ALLOY STEEL BY ELECTROPLATING
' by : :■
Jose M. Quiros
A Thesis Submitted to the Faculty of the .
DEPARTMENT OF; MECHANICAL'ENGINEERING
In P a r tia l FuHilhment of ,the Requirem entsF or the Degree of - \
MASTER OF SCIENCE J
In the Graduate College
THE UNIVERSITY OF ARIZONA
> ! 1962
STATEMENT BY AUTHOR
This thesis has been submitted in p a rtia l fulfillm ent of r e quirem ents for an advanced degree at The University of Arizona and is deposited in The University L ibrary to be made available to borrow ers under rules of the L ibrary .
Brief quotations from this thesis a re allowable without special perm ission , provided that accurate acknowledgment of source is made. Requests for perm ission for extended quotation from or reproduction of this m anuscript in whole or in part may be granted by the head of the m ajor departm ent or the Dean of the Graduate College when in their judgment the proposed use of the m ate ria l is in the in te rests of scholarship. In a ll other instances, however, perm ission m ust be obtained from the author.
APPROVAL BY THESIS DIRECTOR
This thesis has been approved on the date shown below:
Dr. B. S. MESICK DateP ro fesso r of M echanical Engineering
ABSTRACT
This thesis investigates effects on the fatigue life of an alloy
steel, " Ketos" , by electroplating it with various com m ercial e lec tro -
deposits--cadm ium , chromium, copper, nickel, s ilver, and zinc.
Tests for determining the endurance lim its of the electroplated steel
A : - ■ v . ' : - C . U - ' ; y : ; : :a re conducted by subjecting specim ens with these electrodeposits to
fatigue te s ts ; p resen t theories on the m echanism of fatigue a re reviewed
along with the principal s tre ss theo ries.
Results of the te s ts show that some types of electroplatings
■ increase the endurance ■limit of the base, m etal, w hereas other types
. decrease it. The moduli of e lasticity of the electro deposited m etals
and the resulting endurance lim its 'a re compared. Specimens plated
with m etals having the highest moduli of e lasticity had the, lowest fatigue
streng ths--n ickel being an exception.
R esults of the te s ts a re found to be in agreem ent with existing
data from previously conducted investigatibns.
a c k n o w l e d g e m e n t ;
The author wishes to exp re s s his gratitude to
Dr. B. S. Me sick,-w ho suggested the- problem and under
whose guidance this work was accom plished.
iv
SYMBOLS, ABBREVIATIONS, AND NOMENCLATURE
Symbol Term ..
' A A rea of cross-section, (square■inches)c . Distance from centroid to outerm ost fiber (inches) ■ .D ' ’ D iam eter (inches) : V-E . ; Moduiias. of . E lastic ity in tension or com pression (Ibs. /incbA) I Moment of Inertia (inch^)Kc Oompensator spring of. te s te r (lbs, per inch)M Mass of oscilla tor assem bly of teste r, ( lb s .)N Number of cycles ;•P Load (lbs. ) .R S tress ra tio . ■Rc Rockwell C Hardness .
. S S tre ss , norm al (lbs. / inch^)Ss S tress , shear (lbs. / inch.2)T - Torque (lbs. — ft. ) • ̂ \t Time (seconds)Q C ircular frequency (cyc les/sec . )
ABBREVIATIONS
AISI A m erican Iron and Steel-Institute • ^ASA Ame ridan.,Standards As ebci^tipA^ASM A m erican Society for M etals ; 1 ■.
' ASME A m erican Society of M echanical Engineers ■ ASTM A m erican Society for Testing M aterials cpm Cycles per minute . ■ y" : ' yUcps Cycles per second .psi Pounds per square inch '.. •.SAE Society. of Automotive Engineers
NOMENCLATURE
■ FATIGUE LIMIT (or ' ENDURANCE LIMIT), Se-- The lim iting value of the s tre s s below which a m a te ria l can presum ably endure an infinite number of s tre ss cycles. . '
FATIGUE STRENGTH, Sn--The g rea tes t s tre ss which can he sustained by a piece fo r a given number of s tre s s cycles without frac tu re .
NOMINAL STRESS, S i--T he s tre ss calculated on the net. section by sim ple theory such as S - P / A . or . S = M c/l or Ss = T c /j
X v: without taWiAg intb account the varia tion in s tre s s conditions ' causdd by geometrica-1-discontinuities such as holes, grooves,
fille ts , etc. .
RANGE OF STRESS^ Sr. - -The algebraic difference between.the m ax- • imum and mihimum s tre ss in one cycle, that is ,
Sr = Smax - Smin. . , : X-X X; .
FATIGUE LIFE, N .--T he number of s tre s s cycles which can be su stained fo r a given te s t condition.
STRESS CYCLES ENDURED, n.-r-The number of . cycles which a sp e cimen has endured a t any stage of fatigue te s t.
S-N DIAGRAM. - -A plot of s tre ss against number of cycles to fa ilu re
sttfdxif& ujsr S t r e s s
Figure showing Nomenclature
ABSTRACT -
TABLE OF CONTENTS
Page----------------------------------------------------------________ m
ACKNOWLEDGEMENT : - - — — — - — iv
SYMBOLS, ABBREVIATIONS, and NOMENCLATURE--- v
LIST OF T A B L E S -------------------------------- -------------__________ ix
LIST OF FIGURES - - r - - - - - - - - - - - - - - -------- x
Chapter . ■ .1. INTRODUCTION - - - - - - - --------- 1
• 1- 1
. . i : z
• Xiit r o diiicti. ojd. — — — — — ■ —■ 1
Statement of the P roblem - - - - ----1
1. 3 Uses of E lectrodeposited M e ta ls - - - - - - - - - 2
1.4 H is to r y -------- - - r - - --------------- ---------------------- 2
2. BACKGROUND - - ___________ _ — ___ 5
: ; . L: Intr o duct ion — — — — —— — — — — — ̂ — — — — — — — — . 5
2 M echanism of Fatigue — ------------ 5
2.3 P rinc ipa l -S tress Theories- ------8
2 .4 V ariables Affecting Fatigue P ro p e rtie s -- - 11
2 o 4 e 1 0 o id IVo r king - ~ 11
2 e 4a 2 'G rain Size - ——* — — — — — — — •— — — — — — — — — — 11
2 a 4. 3; Hydrogen E m brittlem ent - - - - - - - - - 1 1
Chapter " Page2. 4. 4 Re'sidual S tresses' --------------------- 12
2. 4, 5 S tress R a i s e r - s - V . - --------------------12
2. 4. 6 : Resonant V ib ra tio n s---------------------- 12
2.4 . 7 T emp e r atur e ~ - - - ■—- - - — — ~ — — 13
2 . 4. 8 C r e e p ----- ---------------------------------------- 13
2 . 4. 9 Size ----------- --------------------------------------13
2o 4. 10 Speed of Testing- ------------— - - —-? 14
2. 4e 11 R est Periods — ~ ~ ,— 14
2 . 4. 12 E lectrodeposits ---------- --------- ig
2. 5 H air-L ine Cracks in E lectrodeposits - - - - - - 18
,3c EXPERIMENTAL PROCEDURE — - - - - - - - - - - - - - 20
3. -o 1 Int roduct i on—11—- - — - -- — — 2 0
3 . 2 Types of Fatigue Testing M achines - - - - - - - 20
3. 3 Operation of the Fatigue Testing Machine - - 21
3. 4 P rep ara tio n of Specimens - -----------------------24
3. 5 P rocedure Us ed in Testing -—- - - -— - 26
4c' ANALYSIS AND CORRELATION > - - - — - - - - - - 29
5c CONC LUSIONS - - - - - - - - - - - - - - - - -------- - -> ----------- 41
Experim ental Data — - * - ------ - - - - — - - - ----------------------43
Specifications for Fatigue T ester -------------- ------------------45
v i i i
LIST OF TABLES
Table ^ . \ \ ' /' / '' ' " " ̂ Page
I Range of Residual S tresses in E lectrodeposits ----------------------------------- 17
II TMckness and Modulus of E lastic ity of E lectroplates ----------------------- 25
III Conlparison of Moduli of E lastic ity and Endurance Lim its ------ 31
IV Compared Slope/ S tress Ehdured5 Change in Endurance L im its-- - 32
V Experim ental R e s u l t s - ------ - ---"----------------------------------------------- - -4 3
LIST OF FIGURES , , ; "
Zo 1 • Combined Stres ses ̂ — 44 - ~ ~ T ~ —-- — —~ — — —10
3.1 Schematic R epresentation of the S ystem - - - - - - - - - - - - - - - - - - - - - - - 2 3
3. 2 Photograph of. Actual System - - - - - - - - - - - - - - - - - - - - - - - - - - - - -------23
3 o 3 Ge om etry of Spe cim en— - ̂ « - - - - -— - - - - - ~ - - - - —̂ — - —— « -----2 6
3o 4 Bending F ixture ~ -r ~ ̂ - 7- - ~ ------ - —--------'- -28
' . ■ - ■ ■ . . \4; 1 Nominal S tress vs. Cycles to F ailu re (Unplated.Specimens)------------34
4 .2 Nominal S tress vs./'C ycles to F ailu re (Zinc E lec tro p la tes)-----.--35
4. 3 • Nominal S tress vs'. Cycles to F a ilu re (Cadmium E lec trop la tes)--36
4. 4 Nominal S tress vs. Cycles to F ailu re (Nickel E lectroplates)------—37
4. 5 Nominal S tress vs. Cycles to F a ilu re (Copper E lec tro p la tes)- ---38
4. 6 : - Nominal S tress vs. Cycles' to F ailu re (Silver E lec tro p la tes) -—--39
4. 7 . Nominal S tress vs. Cycles to F ailu re (Chromium "Electroplates)--40
C H A R T ER 1
INTRODUCTION
lo 1 INTRODUCTION. E lectrodeposited coatings have become
of great im portance in the engineering industry for providing corrosion
resistance to m eta ls5 for modifying surface cohditiohs such as prom o
ting w ear by increasing surface hardness, or for building up worn or
over machined com ponents. On the other hand it is we if .known that
m ateria ls subjected to repeated cycles; of s tre s s frac tu re in the ordin
ary static tension te s ts 0 The 'problem of fatigue of m etals has also
become im portant to the engineer3 since alm ost every machine part is
subjected to repeated loadings. ,
,f v I - 2 : STATEMENT OF THE PROBLEM. It is known that some /
types of electroplating tend to in crease the endurance lim it of steel while
others tend to decrease it. This thesis proposes to make an investi
gation to determ ine which of severa l com m ercial electroplatings will
im prove the endurance lim it of an alloy steel. y
The method of trea tm en t consisted of the following:
(1) L ibrary re se a rc h of available data.
(2 ) P rep ara tio n of specim ens of alloy steely K etoss with six
types of com m ercial platings. V
2
(3) D eterm ination of endurance lim its for each type of plating„
(4) Analysis of re su lts ,
1, 3 USES OF ELECTRODEPOSITED METALS, E lectrodepo- .
sits used for this repo rt a re shown below, along with some of their
m ajor uses, ' ■ ■ ■ / ; ■■ " . /
Cadmium: Aircraft,, m arine, and m ilita ry outdoor use; radio, television
and electronic chasiSo
Chromium: Bearing applications; build up worn or m ism achined s u r
faces; corrosion resistance; im parting w ear resistance to components.
Copper: Therm al and e lec trica l conducting coatings; electroform ing; ,
to im prove adhesion and protective ability of subsequent e lectroplates.
Nickel: Chemical and atm ospheric corrosion resistance; electroform ing
build up of worn or m ism achined p a r ts ; decorative applications,
S ilver: Decorative and ta rn ish resistance; e lec trica l contacts; chem ical
resistance; re flecto rs an d .m irro rs ,
-Zinc: Electrogalvanizing; corrosion resistance ,
1, 4 HISTORY, . August Wohler is generally given credit as being
the father of fatigue testing although previous work had been ca rried
out by other investigators. The m ain im pulse which s ta rted a period
of experim entation was due to the problem of fatigue fa ilu res of locom o
tive axles. The f i r s t period of in te re s t in the fatigue of- m etals centered
around Gqrihan laboratories such as W ohler-s. His m ain work was
ca rried out for the N iederschlesisch.~M arki6hchen Railway^ M th the
objective: of remedying, the occurence of frac tu res of these axles.
; Wfohler discovered^ as a resu lt of his tests^ that s tre ss ra ise rs
such ds ho les, filletsg abrupt changes in section, etc. 5 would consi
derably lower thevendurance lim it of any machine elem ent. He showed
that for -a given load, the number of cycles required to produce frac tu re %
dim inished as the range of s tre ss was increased,, 'Conclusions also
stated w ere that in instances where the g rea ter p a rt of th e :maximum s tre s s
is produced by static loads, a much higher allowable s tre s s can bb us ed - ‘
than in axles or piston rods, where there a re rev e rsa ls of s tre ss .
, The beginning ‘of the s econd period of in te re s t in the study of. ,
fatigue problem s was centered around English and ^m erican lab p ra -; ‘ \
to ries around 1900 when J. A. Ew ing/ W. Rosenhain, and 'J . .C., W.
Humfrey used the optical m icros cope to examine m etals under repeated
s tre s se s . , : • ; - , : y: ,, , ‘ s " - - .
.Jv Up to this tim e .the,,,idea was generally accepted that repetition o f .
s tre ss caused a m ysterious change in a m etal. It was believed that a
m etal 'ic ry s ta llized 11 and becam e b rittle - '-th a t it becam e "fatigued14 and
lost its toughness.y It was sdon learned that this idea of "crystaliizatiqn" ;
was wrong. M etals w ere always crystalline and under heavy repetition
of s tre sse s the crysta ls w ere broken up into thin lam inae which slid
over each other. This sliding s ta rted cracks which spread across the
s truc tu re and if the loading was repeated enough tim es, frac tu re took place
Many attem pts have been made to find out if any relations exist
between the endurance lim it and other m echanical p ro p erties of a m a
te r ia l which a re determ ined by static te s ts . These efforts have m et
with little sucess, although it has been found that the endurance lim it
of fe rrous m etals subjected to complete rev ersa ls of s tre s s is approx
im ately equal to 0.4 to 0.55 tim es the ultim ate strength of the m etal.
: BACKGROUND
2„ 1 INTRODUCTION* A considerable amount of work has- been
done in the past on the fatigue of m eta ls. Although sev era l theories
have been proposed to describe the phenomenon of fatigue, the theories *
may not be co rre la ted in detail, as varying assum ptions w ere made for
each theory„ In this repo rt the author w ill review some of the existing
theories, on. the m echanism of fatigue and w ill consider briefly the „
p rin c ip a l-s tre ss theories used in design p ractice .
2. 2 MECHANISM OF FATIGUE. The ASTM S tandards1 contain
the'foTlowing definition of fatigue: nA general te rm used to describe
the behavior of m ateria ls under repeated cycles of s tre s s or s tra in
which cause a deterio ration of the m a te ria l that resu lts in a p rogressive
f r a c t u r e T h r e e m ain features by which.fatigue is distinguished a re : .
C H A P T E R 2.
i a loss of strength, .'a Tosa of ductility, and :an increased uncertainty, in
both strength and serv ice life of the m a te ria l (1 )^,
A m erican Society for Testing M aterials Standards designation (D671-51T). P a r t 9 , p, .262, 1958.
■ The..numbe^s/in Phhenthe:sis' re fe r '.tQ;-the'list.oi references appended to this repo f t . See page 46. /
5
, 6 . .
Fatigue fa ilu res -can usually be recognized from the two distinct
zones of a f ra c tu re : the smooth, polished a rea of the orig inal crack and
the granular a rea of the final frac tu re . Taking as example a shaft that '
has broken; due to fatigtie conditions, there is a sm all a rea of the shaft
th a t'lo o k s‘very smooth,while the .rem aining area appears very rough.
While the crack is developing, the adjoining faces rub against each
other, producing the smooth, polished surface. Eventually, the rem ain
ing sound m etal can no longer: sustain the load and the m em ber br eaks e
A b rittle j 'cleavage-type frac tu re of granular appearance finally result^ :
due to the restra in ing action of the crack.
In considering the m echanism of fatigue, C. R ichards (1) proposes
the following theory: ; the .p rogress of sim ple fatigue in m etals can be
traced in a general way. Localized changes in atomic struc tu re begin
within the f ir s t few cycles at sca tte red points in the, m a te ria l. These
soon develop into submicros,copic cracks which grow, as the cycles of
loading continue, through m icroscopic size and 'eventually become yisi~
ble to the eye. Finally^ when the cracks have grown to some c ritica l /
size, the m em ber is so Weakened that it breaks. These th ree stages of
fatigue a re usually re fe rre d to as nucleatioh, crack growth, and* frac tu re
Fatigue in m etals begins with highly localized yielding. In poly-
crystalline m etals in sim ple tension, a few crystals w ill always be so
oriented that they will slip easily„■ v:,.
; As the load is, in c reased these !!w eakn
.c ry s ta lsW h ich a re surrounded by elastic m aterial,, yield f ir s t and at
overall s tre sse s that a re nominally within the elastic range of the
m ateria l. As cycling continues, new slip bands form , m ostly in the
same cry sta ls , so that deform ation rem ains highly localized. F u r th e r
m ore, as the groups of slip bands grow into s tr ia tio n s , the m ateria l
within them becomes progressively harder because of cum ulative.s tra in -
hardening. V . ' /
. At some point in.the process the 'm ateria l in the stria tions becom es
so hard that it cracks, bn a subm icroscopic scale. The -submicroscopic
cracks begin to form in the same direction as the slip bands. Individual
cracks grow, and groups of cracks a re joined until the whole reaches
m icrpscopic size. /
. : As each crack grows it is .preceded by a nucleation process sim ila r
to that by which it started , - Localized slip takes place because of s tre s s
concentrations at the crack tip, and subm icroscopic cracks develop in
the slip bands, causing a gradual extension of the crack. A notch effect-
accompanies the crack and increases its tendency to grow in the general
direction a t,righ t angles to the tensile s tre ss . When the remaining cross
sectional a rea becomes sm all enough, final frac tu re occurs; the notch
effect becoming the controlling factor. Thus, fa ilu re is b rittle in nature.
As mentioned e a r lie r , a m ajority of fatigue fa ilu res s ta rt at v is i
ble d iscontinuities, which act as s tre ss - ra is e rs „ In such Instances' the
8
in itia l yielding is - caused not by an unfavorably oriented c rysta l, but by
a local increase in s tre ss resulting from a s tre ss concentration. Once
the fatigue cracks s ta r t, propagation and frac tu re are the same as before*
E; Orowan (2) has suggested a m odel where a p lastic element in an
elastic shield undergoes s tra in hardening in decreasing amounts with the
number of cycles. As the p lastic elem ent s tra in hardens, the s tre ss
acting on the element in c reases . If the- s tre ss reaches the failure value,
a crack occurs and fatigue failu re follows due to self-propagation of the
crack. If the p lastic elem ent does not a ttain the c ritic a l fa ilu re value
before s tra in hardening ceases, fatigue fa ilu re w ill not occur»
THEORIES. A brief review of the p rin -
c ip a l-s tress theories (3) w ill be considered here. Since fatigue fa ilu res
s ta r t at a surface, .only biaxial s tre sse s need to be considered. ’
P rin c ip a l-S tress theory ; This theory is based on the idea that
fatigue failu re at a given number of cycles under combined s tre sse s w ill
occur whenever the maximum principal s tre s s in the combined s tre sse s *
reaches, a value that would cause fa ilu re in the sam e num ber of cycles
if the principal s tre ss w ere acting alone.
The magnitude of the equivalent s tre s s can be computed from the
equation; >: , , ,
0i> ==' V' -
CT7 ■ '+>:■5 . a %
where 0̂ is the equivalent p rincipal s t r e s s 9 and Qz a re direct
;stires.ses, a n d s h e a r i n g or to rsion s t r e s s . F igure 2 .1
illu s tra tes how the s tre sse s might occur.
For bending and shear s tre sse s alone, the equation becomes:
• v''; ^ " i r L 0* + _
: ' - . ;■■■ • .... 9
If no: to rs ion or shear, s tre sse s a re p resen t, the equation becom es:
^e= 0/ .
Maximum Shear-S tress Theory. This theory is based on the idea
that fatigue fa ilu re under combined s tre sse s will occur in a given number
of cycles when the maximum shear s tre ss is equal to that- required for
failure in bending or tension under the sam e number of cycles.
In th is case, the equivalent s tre ss is computed from the relation
For comhinations of bending and torsion ,
For to rsion or shear alone,
V-e : 2 ^ .
M aximum- D istortion - Ene r gy, The o ry . This theory is usually
called the ^o c tahed ra l-stress theory'/ , .or the "von M fses -Hencky
theory" ; However, the form given here is somewhat different from the
10
usual expression in order to avoid a num erical coefficient in computing
the equivalent s tre ss for sim ple tension or bending.
, This theory is based on the idea that failure w ill occur under com
bined s tre sse s when the d istortional energy (neglecting the energy of- 1 ' ■' " : " , ■ ' ' ' ■ ' ■ ' . ' ■ ' ■■■ ' ■■ ; = - ■
volume change) under combined s tre sse s is the same as that required
to produee a sim ilar d istqrtion energy in sim ple bending and tension.
The equivalent s tre ss is computed from the relation «
a? +• <rH 3
Under sim ple bending or tension, - 0% ,;
f
y- - '’F igure 2.1 COMBINED STRESSES ■
C ontrast between the theories can be sum m arized by saying:
1. The p rin c ip a l-s tre ss theory predicts that • r ' C T ^ , where (7̂
is the bending -fatigue strength and ' f is the to rsion-fatigue strength.
2. The maximum sh e a r-s tre s s theory predicts that 'V - ,
3. The m axim um -distortion-energy theory p red ic ts that
r e. ~ r <9. ^VS-. , ■. ■
11
2 A VARIABLES AFFECTING FATIGUE PROPERTIES, The
following section is devoted to some variab les encountered during te s t
ing and their effects on the fatigue strength of the m a te ria l being tested,
2 c.4„ 1, Cold Working: Since fatigue fa ilu res usually s ta r t as ex ternal
surface cracks, a p rocess which strengthens the su rface fibers of the
m etal generally tends to improve fatigue resistance . A cold working of
the surface la y e rs , such as produced by shot-peening, cold rolling, or
the use of special tools which can be used to exert a com pressive force
in notches, or keyways, resu lts in im proved fatigue strength.
O. J* H orger and H» R, N eifert (4) investigated the effects of shot-
peening specim ens 2 -1 /4 inches in diam eter of ho t-ro lled S. A, E. 1045
steel, norm alized at 1700° F. and tem pered at 1100° F. Results of
these tests showed that the endurance lim it increased from 3% to 19%
for plain specimens' and from 4% to 31% for filleted specim ens as com
pared to s im ila r polished specim ens.
2.4.2 G ra in ,S ize: Since strehgth and hardness decreases as grain
size in creases , one would expect the endurance lim it of m etals to de
crease with increasing grain size. This was shown to be true by R.
Cazaud (5).
2.4.3 Hydrogen Em brittlem ent:, High carbon stee ls (over 0 . 35% C)
have a great tendency to absorb hydrogen and become em brittled in the
pickling and cleaning steps employed in preparing the surfaces of the
12
stee l for electroplating. Difficulties with em brittlem ent increase with
increasing hardness of the steel. To relieve hydrogen em brittlem ent/
a ll specimens used for testing in this investigation were heated to a temp
era tu re of 3?5Q F. for a period of l - l /Z hours after electroplating (6 ).
2 .4 .4 Residual s t r e s s e s : Residual s tre sse s play an im portant
part in fatigue failu res by radically changing the mean s t r e s s . : If the
residual s tre sse s a re compressive^ they w ill improve fatigue life by
lowering the inean s tre ss toward the com pressive side, as found with
shot-peened su rfaces . Also see Stress R aisers section below.
2 .4 .5 S tress R aisers : P roper allowance for the effect of s tre ss
ra is e rs is the m ost im portant item in a design which is expected to
re s is t failure by fatigue. Almost all cracks that are responsible for
fatigue failu res s ta r t at discontinuities which act as .stress ra is e rs ,
such as holes, fille ts , keyways, or m acroscopic flaws in the m a te r ia l- -
inclusions, blowholes, or fabrication c ra ck s . The m a te ria l available
for consideration on this subject is too voluminous to be considered in
- detail, in a report such as this one. Results of tests on specim ens that
have no s tre ss ra ise rs give a m easure of the fatigue strength of a
. m etal under very favorable conditions; such data indicate the maximum
fatigue s tre ss that can be developed in a m ateria l.
: 2.-4, 6 Resonant Vibrations : The fatigue testing m achine us ed in
conducting the te s ts for .this report included a means for balancing the
system 5 making it possible to operate the machine at a frequency well
below resonan'cer therefore^ this would act as a known variab le in the
testing. .■ . . - - . ■; . , - '
2. 4. 7 T em perature: At tem peratu res and s tre sse s below those ,
which cause continuing flow under a steady lo ad ,; the effects of speed
of testing at these tem peratu res is re la tively sm all. Above the "creep
tem p era tu re11 , the effect of frequency of cycles is , in general^ much
la rg er (7). If creep or yielding is involved in any way in a te s t, a spe
cimen tested at low speed would n ecessarily have these effects presen t
to a different degree than would a specim en tested at high speed.
2 . 4. 8 Creep: Creep of m etals is dependent on tim e, while the
fatigue of m etals is commonly regarded as a function of the number of
s tre ss cyclesV Creep produces its frac tu re by a load continued for a
length of tim e, while repeated s tre ss cycles develop a spreading damage
as some function of thd number of cycles of s tre ss applied.
■ 2 .4 .9 Size: Since fatigue frac tu re depends on a d istribution of
im perfections in the m ateria l, it is to be expected that a large .member,
having a la rger distribution of im perfections than a sm aller one, will
have m ore extrem e values in its distribution. The la rger m em ber,;
therefo re , is m ore likely to fa il at a lower s tre s s than the sm alle r one.
The actuaTvariation with size has hot been determined( 1).
2 ; 4„ 10 Speed of Testing: An increase of frequency produces two
sim ultaneous changes::
•, 1. .'Speed.of s tre ss applicatio .u increases. , ,
■ 2 . Time that specim en is allowed to dwell at low s tre s s e s
in creases . ' ■; T : . \ 1' :'l ''''l"":: / ' . " \ . / ' ; ’ ;
-: ' It has been found from a number of experim ents perform ed, that
the effect of frequency of application of s tre s s cycles to a specim en or. •
a s tru c tu ra l part is re latively sm all between the lim its of 200 to 7, 000
cycles per minute (7). Increasing ra tes of loading at speeds higher
than 10, 000 cpm w ill tend to increase the fatigue, limit., but even at 30, 000
cpm the fatigue lim it increases less than 10% over the value obtained
a t 1500. cpm. (12). For te s ts ca rried out at a s tre ss appreciably above
the endurance: lim it, high frequehcies cause heating of the specim en and. ■
prem ature fa ilu re re su lts . At very slow operating speeds or s tre ss
cycles for which the m ean s tre ss is not zero’, creep or re laxation w ill
affect the fatigue resu lt when, the, tem peratu re is high enough.for noti-
ceable creep or s tre ss relaxation to occur (1 1 ). V
' . ' 2 .4 .11 R est P eriods: A factor re la ted to frequency of cycling is
. that of re s t periods. C. S. Yen and J . T . Dolan (5) perform ed tests to
determ ine the effects of re s t periods on the cycles to fa ilu re for
7075-T 6 aluminum alloy. Testing was done on rotating beam specimens
at a s tre ss of 50, 000 psi. Results of the te s ts showed that for a .
sam ple size of 50, a probability of 4 /5 tha t re s t periods decreased , ■;
sca tte r , but had no signifioant effect on the average life of the specim ens.
N either the peak nor the m ean of the curves w ere affected. The re s t
periods used w ere at room tem peratu re for a period of .24 hours after
1 0 , 000 cycles of s tre s s . • -
,2 .4 .12 E lec trodeposits : The application of any coating on a m etal
w ill affect the fatigue lim it of that m etal: cleaning procedures may either
smooth or roughen the surface; the applicatibn p rocess may induce ten
sile or com pressive s tre sse s in the base m etal and in the coatings; the
basic fatigue strength of the coating m a te ria l and its soundness may be
im portant since fatigue cracks' form ed in the coating may propagage into
the base m etal; and the relative Young8s moduli of the bage m etal and
coatings are im portant in determ ining the ratio of the s tre sse s in the
coating to the s tre sse s in the base m etal under loading (5).
(a) H. L. Logan (8 ) conducted tests to determ ine the effects of
chrom ium plating bn S. A. E. X413Q and S. A. E . : 6130 steel. The fatigue
lim its of specim ens plated and subsequently ground to remove a part.of ■
the plating were, equal to or g rea te r than those in itially plated to the same
thickness as the ground specim ens and tested as plated. This was p r i
m arily due to the rem oval of some of the tensile residual s tre s se s in
duced by plating. Cphclusibns in a ll cases found chromium plating to
reduce the fatigue lim it of the steels studied. .In general, reductions in '
the fatigue lim it increased with, increasing hardness of the s te e ls . , <
: 'v : ■ : ' - : : :':y ?'' ̂ : .. ' -(b) JV, O, AIrnen (5) conducted sim ila r tests to Logan?s and also
f ound that , chromium plated specimens; lost 2 0% of the endurance value
of polished speciinens, but that the endurance lim it slightly increased
by shot-peening the specim ens before plating.
- (c) Almen also conducted te sts using nickel e lec trodeposits„ The
endurance limit, of-a polished steel spe cimen was f ound to be 45,: 000 psi..
One procedure of electroplating developed tensile residual s tre sse s of
25, 000 psi in the coating and decreased the endurance strength from
45, 000 to 29) 000 psi. Another process of plating developed com pressive
residual s tre sse s of 6 , 000 psi in the coating and increased the endur-
ance lim it slightly. In other tests perform showed that while
another stee l had an endurance lim it of 47, 000 psi in the polished con
dition, nickel plating reduced this value to 2 0 , 000 psi, w hereas shot-
peening of the specim ens p rio r to plating increased this lim it to 55, 000
.psi" , ' . y . . ' C
(d) Dolan and Benninger (9) subjected zinc and cadmium plated
specimens to completely reversed cycles of s tre ss while in contact
with fre sh tap w ater. The zinc coated specim ens had somewhat higher
corrosion-fatigue endurance lim its than did the cadmium plated ones.
Both platings gave endurance lim its of about 45% of the uncoated m ate-
tested under the Same conditions (in contact with tap w ater). The
17
chief effect of the c:oa:tit^h;-seemeti to he-a lengthening of the number
of cycles of s tre ss required to cause fa ilu re of the coated specim ens. : - . ■ . ■ ■ ; ■ " ■ • ■ ' ■ • i ' . ■; - , - ■ '
in corrosion-fatigue.
(e) A. W.' H othersall (10) made a special study of residual s tre s se s
in electrodeposited m etals. ; The range of the magnitudes of the residual
s tre sse s found during his experim entation a re listed in Table I below.
■:vf : A XE lectr odeposited
' ; m etal ' V' - ,; .Residual s tre ss
Zinc . Com pressive (0-4, 000 psi) .
Cadmium . Com pressive (slight)
Copper Tensile or com pressive (0-2, 000 psi)
Silver Tensile (slight)
Nickel ; V ' Normally tensile (0-40, 000 psi)
Chr omium i Tensile (0 - 60, 000 psi)
RANGE OF RESIDUAL STRESSES IN ELECTRODEPOSITS
Table I
; (f) Logan (8 ) found that com pressive residual s tre s se s in a m etal
increased the fatigue lim it w hereas tensile residual s tre s se s decreased
it. From this statem ent, one would expect zinc electroplating to give
the highest fatigue strength, of the electroplatings lis ted in Table I, and
chromium the lowest. These electroplatings are listed in decreasing
order of: expected fatigue strengths of depositing them on m etals. '
(g) Fo; CL Lrea (13) found that a nickel electroplate lowered the
endurance lim it of m ild steel. This reduction was due to residual
. s tre sse s set up by the deposition or by the discontinuity between the
m etals. The absorption of gases during the plating operation Was
quoted as not being the cause of the reduction in fatigue strength.
(h) Swanger and F rance (14) found that the endurance lim its of
zinc electroplated specim ens were equal to or g rea te r than those of
unplated specimens, and accounted for th is on the basis that the e lec tro
plated zinc coatings had sufficient ductility to deform about the bottom
of the advancing crack, thus reducing the s tre ss concentration.
■ 2. 5 HAIR-LINE CRACKS IN ELECTRODEPOSITS. It has been
suggested (1 0) that the m echanism of the development of the hair-line
cracks in certain electrodeposits is described by the following sim ple
explanation: the cohesive forces which operate across the boundary
of the deposit and the paren t alloy are atom ic or m olecular in character
this means that the boundary would be the site of quite considerable
short range s tre sse s due to m isfit of the two crysta l la ttices involved.
Thq deposit and the im m ediate m etal contains the m ajority of the
s tre sse s brought into play by the differences in p roperties of the two
m etals . ,It seem s likely that when a certa in thickness of deposit has
growth the ,ma;croscopic: tensile s tre ss developed would be sufficient
to rupture the deposit and relieve this local s tre ss by the form ation of
18
a. tra'c^Lx'^n' iiicreased curr^nt den$ity foliows from the .crack edges ’ ;; ;
which may rapidly sea l over the crack by 1 u rther depositiph„: ’ Some
evidence to support this m echanism is the fact that the density of h a ir
line cracks .increases when chromium deposits a re stress, relieved.
The effect of heat treatm ent upon the residual s tre s s of chromium v
deposits, a re sum m arized as follow s:: ’ . , : . . • / , ;•
; ',.v 1. for room tem perature to. 300° C., ran g e--in c rease tensile s tre s s
due to perm anent contraction of the chromium
2 . for the, 300° to 500° range- -p rogressive s tre s s re lief ac com -
panied by grain growth of- chromium : . ^
v 3. on cooling from .500:° G„ - cleyelopmeht of cornpressive s tre ss
due to differential contraction . ; : '' ' :
4. on cooling from above 520° C. --the com pressive s tre ss in the
chromium may induce tensile s tre ss in the steel sufficient to initiate
fatigue failu re of the steel. > '• y - * :
, M ost of the experim ental evidence concerned with the fatigue strength,
of electroplated m etals points conelusively to the dominant effect of the
residual s tre ss fields generated as a re su lt of the electrbdeposition.
The fact that there a re nickel plating baths capable of inducing controlled /
levels of residual com pressive s tre s s^with m arked .beneficial effects on \ :
the fatigue strength may serve .as ind irect pyidence in ’ siipport of the re - ' ;
sidual s tre s s theory . ; • . ' i . .
' CHAPTER 3
EXPERIMENTAL PROCEDURE
3, 1 INTRODUCTION. Tests to determ ine the fatigue life of m achine
components a re usually carried out on s cale m odels, on the actual p arts “ .
themselves*, or on te s t specimens designed to accomodate a specific type
of loading. The usual procedure used in conducting fatigue tests is to
subject the specim en to varying s tre s se s by means of mechariical or m ag
netic devices „ The s tre sse s imposed may vary between equal positive
and negative amplitudes or between any - combination of positive and nega- ,,
five am plitudes. Whatever method of s tressing is 'used, the s tre ss ra tio J
should be given in order to compare the resu lts with other tests that have
been perform ed. The s tre ss ratio , R> is the algebraic ra tio of the m in i
mum s tre ss and the maximum s tre ss in one cycle.
' 3i 2 TYPES OE EATIGUE,TESTING MACHINES. The m ost general '
types of fatigue testing m achines a re of the constant load type and the con
stant displacem ent type. In the constant load type, the applied load or /
amplitude of loading is constant throughout the te s t and, after the fatigue
crack in itiates its ra te of propagation usually in c reases . In the constant
displacem eht type the applied deform ation or amplitude of deform ation
21
is constant throughout the te s t aud the load on the specim en is reduced
after fhtigue crack in itiates and the r a te ” of propagation of the crack is
usually re tarded (1 1).
The machine used for this thesis is of the constant load type and
the essen tia l parts found oh it a re :
'•:v-y 1; A drive for applying repeated cycles of s tre ss to the specimen.
'2 . A m eans, of m easuring the maximum and m inim um stre ss
applied to .the specimen. - i ,
3. A counter for indicating the num ber of cycles of s tre ss applied
to the specimen. . ; •
4. A device for stopping the testing •machine autom atically when
a Specimen b re a k s . 1 u . '-"V”
.3.3 OPERATION OF THE FATIGUE TESTING MACHINE. The
function of the testing machine used for this investigation is to apply a
v e rtica l vibratory, force to any specim en o r struc tu re fastened between
the ‘top p lat p and vib rating cag e. :See f igur e 3.2. ’ •
The dynamic or v ibratory s tre ss applied to an elastic specimen or .
struc tu re is completely reversed and sinusoidal. It is produced by m eans
of a m echanical oscillator operating at a speed of 1800 rpm* and well
below the natural frequency of the equipment. N orm ally, in such v ib ra -
ting system s the force ih the specim en is the sum of the force produced
by the oscillator plus the inertia force of the oscillator fram e vibrating
with the specim en. This inertia force is an unknown fac to r/ as the r ig i
dity of the specim en is not usually accurately known. F urtherm ore , if
the rigidity of the specim en changes: during the test* i t w ill cause h: ’ '
change of s tre ss amplitude in the specim en which is undesirab le .
This fatigue te s te r incorporates a spring known as an ^inertia
force com pensator sp ring 11 , whose function is to absorb all unknown
inertia forces so that the force in the specim en is and rem ains equal
to that produced by the o s c i l l a t o r , irresp ectiv e of the rigidity of the • :
specimen. * t h ; :
The system , shown schem atically in figure 3.1, is a single degree
of freedom system . The vibrating oscilla tor m ass M is connected elas -
tic ally to the rigid W s e thr ough the spe cime n and com pensator spring- - ■
the' twp b eihg - in pa r allel. ’
/-; , The 'iig id ity of tlie compensatof sp ring , Kc (lbs. per inch), is ..
designed to be M W , where M is the m ass of the oscilla to r assem bly, '
and OJ is- the c ircu lar .frequency (synchrohous) of the oscillator force.
With the specim en out, the vibrating; system m ass M and spring Kc
has w as its n a tu ra l f requency (15)„ ’ i J h
The instruction m anual for this machine p resen ts a m athem atical \
explanation, which w ill be om itted in th is report, to show that irre sp ec tiv e
of the rigidity of the specim en, the fo rce in the specim en is equal to. the ■ *
force produced by the o scilla to r. ' *
/ / / / / / / / s' /J23
-Specimen Ks
Mass M
Sin u)t
Com pensatorSpring Kc
Figure 3.1
SCHEMATIC REPRESENTATION OF THE SYSTEM
Figure 3.2
PHOTOGRAPH OF ACTUAL SYSTEM
. ,3.4 PREPARATION OF SPECIMENS; Since fatigue frac tu res b e
gin at surfaces, the method used for preparing test specim ens has a
m arked effect on the. resu lts of the. te s ts . By preparing the specimens
under controlled conditions, many of the V ariables .encountered in their
p reparation can be controlled and varied one at a tim e in order to obtain
desired effects. ; ' ' .V:
The following procedure was used for the p reparation of the spe
cimens i the rad ii of the specimens w ere f i r s t m illed with a two-inch
diam eter cutter using a coolant at a speed of 95 rpm . Very light cuts
w ere taken in order to elim inate heating of the specim ens. All bu rrs
w ere rem oved after m illing and the rad ii w ere finished in successive
stages using em ery-cloth of finer grades; The specim ens w ere then
surface ground. Surface preparations such as degreasing, pickling,
etc. , w ere used and the specimens w ere then electroplated. The spe
cimens w ere then baked at a tem peratu re of 375° F. for a period of
1-1/2 hours in order to relieve hydrogen em brittlem ent resulting from
the plating, as explained e a rlie r in this report. Since a study of the .
effects of the platings was sought, a tru e r picture of the effects of the
platings was obtained by omitting the polishing of the surfaces of the
specim ens after electroplating. ' • . -
The types and thicknesses of the platings along with their re sp ec
tive moduli of e lastic ity a re given in the table below. 7
25
1 ; ‘ - ' ■ " ' ' ^
Electroplating Thickness M0dulus of E lastic ity
■■ \ 1 ■ V.. ' ■■ ■(inches) psi x 106
. '■ > ; ''Cadmium ; - ' 0. o o i i 5 -o,; O d l i 8 : . . 8 ■ " ■ ■'Y .
Silver ■ : 0 .00008-0. 00011 ' 11 '' L y : :
Zinc 0.00058-0. 00066 ■' 12
Copper • .: 0. 00031-0. 00040 16
Nickel ' \ 0.00910-0,00990 ,. '■ ' 30,, , ' ' ’ ' r
•Chromium 0.00034-0.00045 36 ■ '
THICKNESSES AND MODULUS OF ELASTICITY " OF ELECTROPLATINGS
Table II.
The m a te ria l used for the p reparation of the specim ens was a
non-deform able tool steel, "K etosu , with an analysis as follows:
• v , C arbon --------- 0.90% '. • : Chrom e--------------0. 50%
- , : M anganese------1„ 30%Tungsten------------ 0„ 50%
One reason for the selection of this m ateria l was its availability
in rectangular section in 12 foot lengths„ The m a te ria l was obtained in
the annealed condition, reducing the possib ility of in te rna l residual
s tre s se s . All specim ens w ere cut from one strip , thus increasing the
consistency in the m ateria l. Specimens of rectangular section are ideal
for tes ts involving pure bending loads - -the intent here being to impose
.'maximum s tre sse s on the surfaces of the m ateria l in o rder to study the:: ' ■ . ■ Vsurface effects of the e lectrodeposits„ ’ ! \ ?
■ : . A Rockwell "C" Hardness te s t was taken of the m a te ria l and was
• 'found to have a hardness of 31. , By comparing this num ber to a table .
of the relation of hardness numbers to .tensile strength, a correspond
ing tensile strength of approxim ately 140, 000 psi was obtained.
The geom etry of the specim en is given in figure 3.3 below.
o. u s 'm
T h ic k n e s s = 0, 1121 n
rigure3,3:(am bM ^
/■v v - Th e ' ' width of the spdcimeiis w ere. individually m easu red by means
of an optical com parator with a m agnification of 20 power.; The th ickt
nesses w ere.m easured using a vern ie r m icrom eter and the m easurem ents
w ere taken at the center of each p iece» M easurem ents of the th icknesses
of the platings were obtained using m agnetic methods by the Arizona
.Testing Laboratories (see page 48 in appendix). /
. 3.5 PROCEDURE USED IN TESTING. When i% ^
determ ine the effect of sL variable as. completely as possib le, the number
of d iscre te values of the variable tested should be at least five ,/su itab le
spacing to be used throughout the range of the variable (11).
The endurance lim it for each type of plating was determ ined by
subjecting six specim ens of each type of plating to different S tress le~
* velSo .The s tre sse s endured were then plotted in a diagram as the o r
dinates and the number of cycles (N) for frac tu re as the absc issas.
S tress levels of deer easing magnitudes we r e imp o s e d on the specimens
until a value was reached which did not produce fa ilu re a t approxim ately.... , . : : V - , v .y ;V . -
two m illion cycles. This was defined as the endurance lim it for that
set of specim ens for,the purpose of th is report.
To determ ine if the specimens that endured the two m illion
. cycles of s tre ss w ere damaged^ a higher s tre ss was im posed on the
specim ens and frac tu red . The frac tu re qf a specim en in the vicinity
of the curves established by the other specim ens5 indicated that the ,
specim en was not damaged and that the observed endurance lim it was
correctly defined. To indicate this sequence the point representing
failure was joined to the ebrrespqndihg "arrow ed" point by a dotted line.
A stress; ra tio , R, equal to -1 was used throughout the testing.
Stressing of the specimens was accom plished by placing the sp ec i
mens in the bending fixture of the machine and loaded,as shown in figure
3,4. The s tre sse s applied to the specim ens were calculated by analy ti
cal procedures s i. e, , . s tre s ses.w ere computed on the basis of the loads
applied and the dimensions of the specim ens, The equation used to ca l
culate the bending s tre sse s in the specim ens w as: , '
28
b = 3 P R . whereTS = maximum bending s tre ss (psi)
P = force produced by eccentric (lb s. )
R - bending leverage= 2U
b = specim en width at minimum test section
h = specim en thickness (inches).
F igure 3.4 BENDING FIXTURE
CHAPTER 4 - - , ' ''
ANALYSIS AND CORRELATION
Any surface coating w ill affect the fatigue life of a m etal. Re
sults of the te s ts conducted showed this to be true since every coating
had a different effect on the endurance lim it of the same base m etal.
The unplated specim ens w ere found to have an endurance lim it of -
40, 300 psi; the highest endurance lim it of 42, 000 psi was obtained from
the m etal electroplated with zinc; the low est value of approxim ately
35, 000 psi was obtained from the chromium plated specim ens. The .
re su lts of the te s ts for the chrom ium electrop lates indicate that this
value of 35, 000 p si was not the endurance lim it for this se t of speci
m ens, since the testing was incom plete.
Incompleteness was due to the ruining of a specim en during the
testing , thus leaving.insufficient specim ens to complete the te s t- - th e
testing was ended after the "specimens had endured 590, 000 cycles
instead of the two m illion which defined the endurance lim it. A copper
plated specim en was also ruined, but this occured early in the testing
and a fatigue lim it was determ ined; ■
Surface cracks w ere detected on some of the chromium and n i
ckel platings after the specim ehs had failed, w hereas they w ere not
found on any other plating. The cracks in the chromium electrop lates •
w ere deeper than those in the nickel* but the ones found here were
longer - - alm ost 1 /8 inch long. ^
- : When a crack is produced in the coatings of nickel or ’Chromium
deposits on strong, s te e ls5 .the in ternal s tre sse s w ill be relieved .locallyj
but the crack w ill act as a notch concentration--the effect of the notch
being to increase considerably the nominal s-tres's in the stee l in its .. „
vicinity. The crack w ill propagage into the steel when the s tre ss con
centration at the foot of the crack reaches the fatigue lim it of the steely
leading to fatigue fa ilu re » ‘
Cadmium and zinc electrodeposits a re very weak and ductile in
’ com parison w ith ,even low -strength s te e ls . ‘ Although fatigue cracks
w ill presum ably develop in such coatings/ the ready ‘p lastic deform ation
of these soft m ateria ls would prevent any effective s tre s s concentration
at the root of the cracks. It is unlikely therefore that such coatings '
would have a g reat r eduction on' the fatigue 'strength of the base rrietalo
This is confirm ed by the te s t re su lts .
If a g reat reduction in the fatigue strength should occur as a r e
sult of zinc or cadmium plating, this would very likely be due to p a rtia l
rem oval of residual com pressive s tre s se s (induced by machining) during
the p reparato ry pickling p rocess. A fu rther possibility would also be
due to hydrogen em brittlem ent of the steel, i t this was not relieved by
V ; . : . ; : 31
A striking s im ila rity was f ound between the unplated and the' copper
plated specim ens. The slopes and positions of the S-N curves drawn for ■
the two wbre Identical^ 'except for the lower fatigue lim it of the copper .
' :{see figures 1 fand: 4. 4)'f A/h yAA;' A : ■ -; f,
By comparing the moduli of e lastic ity of the electroplatings, it was -' : A. . . . . .A : A""' " ■' '.A- ; /A;A' 'A - A- - A,
. found: that:the platings with the lower values had the highest fatigue ,
strengths A-nickel being an exception. The nickel was electroplated onto
the m eta l after having been copper plated, -which accounts for this
behavior. •
VA E lectroplate
Unplated
A Zinc ...: '
■ Cadmiurn
- N ickel .
A- Uppper
A S ilv e r /
Chromium
Modulus of ' E lastic ity (psi x 1 0 6 )
/Endurance LimitA (psi)
30
12
■ 8.A: A '
30
u
...A-36A
40, 300 ■
4.2,000
:4lAT60 A
39, 800 A
39,000
36, 200
35,500
A’A E.-V-;
iCOMPARISON of MODULI of ELASTICIT Y and ENDURANCE : LIMITS/: :-'VL-:>A A:;:'
T a b le 111
• ;
The zinc electroplate increased the endurance lim it of the alloy „
Me el; by: 4; Ztfoi :vThe s lop e of the S -N curve was -0 .76 as compared to ;
-0; 83: fpr; the unpla^ ̂ position dhthe curve for the zinc
plated specim ens w a s ;iti the saidie position as that of the unplated spe
cimens , , except that by extrapolating the curve at 3/000 cycles of s t r e s s «,
the s tre ss endured by the zinc was 72, 000 psi as com pared to 75, 000 ;
psi for the unplated specim ens. V‘
Comparison of the resu lts of the other platings are sum m arized,
in Table IV below. \ : ‘
Ele c tr odep o sit Slope of S-N;'curve
i-V '-ck :.
S tress endured at 3, (D00 cycies (psi)
Change in, Endurance L im it;
Unplated ; ■ -0. 81 . 7 75, 000/
. Zinc ■■■■■ .■ -0.76 72,000 ; + 4 .2
Cadmium ;-0 . 76 73,000 3,5 ' ;- ; ' / :
Nickel . ■ -0. 65 : : 69,000 ' - - 1- 2 .. - A ' :
Copper ' -Q.81 75, 000 ■ ' ' ,-- 3. 2 .. . "
Silver > , ' -p .8 8 ';V : —10.2 v
Ciir dipitorl;' i : :''',-ov94 75, 000 : .y . ; l • ~ 1Z' 0 . : (inconclusive)
COMPARISON of SLOPE, .STRESS ENDURED,
and CHANGE IN ENDURANCE LIMITS
/ / /%. - Table IV : ̂ ' A;
33
Gomparison of the S-N curves shows that although th e , chromium
plated specim ens had th e :lowest endurance lim it, they endured the sam.e■ . . ■ v V V ; ; v : - : : :
number of; cycles as the unplated specim ens at high s tre sse s (70, 000
psi and above). \ ' /: 'Y . . ... ' ,
The nickel electroplates withstood the least num ber of stress, cycles
at high s tre s se s . The S-N curves for the zinc and the cadmium e le c tro
plates had the same slope and position except that the zinc had a slightly
higher endurance lim it. Silver electroplates withstood about 2, 0,00 psi
hi ghe r s t r e s s than the unp lat e d: sp e ciimc hS’; up t o 40 ̂ 30 0 p s i - - th e " e ndu r ah c e
lim it of the uriplated specim ens. y ' :
NO
MIN
AL
ST
RE
SS (
SMA
X) f
?S.I.
8 0 , 0 0 0 -
7 5 ,0 0 0
7 0 , 0 0 0 -
6 5 ,0 0 0
6 0 .0 0 0 *
5 5 ,0 0 0
5 0 , 0 0 0
4 5 ,0 0 0
4 0 0 0 0
3 5 , 0 0 0
UNPLATED SPECIMENS
R e p r e s e n t a t io n _ of No Failure
2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 | 2 3 4 5 6 7 8 9 | 2 3 4 5 6 7 SQ |104' 10J 106 107
NUMBER OF CYCLES TO FAILURE(N )
F i g u r e 4.1 NOMINAL STRESS v s . CYCLES TO FA IL U R E ( Unplated S p e c im e n s )
NO
MIN
AL
ST
RE
SS (
SMA
X) R
S.I.
ZINC ELECTROPLATED SPECIMENS8 0 , 0 0 0 -
7 5 ,0 0 0
7 0 , 0 0 0
6 5 ,0 0 0
6 0 ,0 0 0 -
5 5 ,0 0 0
5 0 ,0 0 0
4 5 ,0 0 0
4 0 , 0 0 0
R e p r e s e n t a t io n _ of No Failure
3 5 , 0 0 0
3 4 5 6 7 8 93 4 5 6 7 8 9 3 4 5 6 7 8 9
NUMBER OF CYCLES TO FAILURE(N)
F i g u r e 4.2 NOMINAL STRESS v s . C YC LES TO FA IL U R E (Zinc E le c t r o p la t e s )
NO
MIN
AL
ST
RE
SS (
SMA
X) P
S.I.
8 0 ,0 0 0 CADMIUM ELECTROPLATED SPECIMENS
7 5 ,0 0 0
7 0 , 0 0 0
6 0 ,0 0 0 -
5 5 ,0 0 0
5 0 , 0 0 0
4 5 ,0 0 0
4 0 , 0 0 0
R e p r e s e n t a t io n _ of No Failure
3 5 , 0 0 0
3 4 5 6 7 8 93 4 5 6 7 8 9 3 4 5 6 7 8 9
NUMBER OF CYCLES TO FAILURE(N )
F i g u r e 4 .3 N OMINAL STRESS v s . CYC LES TO FA IL U R E (C adm ium E l e c t r o p l a t e s )
NO
MIN
AL
ST
RE
SS (
SMA
X) P
S.I.
8 0 , 0 0 0 -
7 5 ,0 0 0
7 0 . 0 0 0
6 5 . 0 0 0
6 0 .0 0 0 f
5 5 .0 0 0
5 0 . 0 0 0
4 5 ,0 0 0
4 0 . 0 0 0
3 5 . 0 0 0
NICKEL ELECTROPLATED SPECIMENS
2 3 4 5 6 7 8 9 2 3 4 5 6 7 8 9 2 3 4 5 6 7 8 9 2 3 4 5 6 7 89107
NUMBER OF CYCLES TO FAILURE(N )
F i g u r e 4 .4 NOMINAL STRESS v s . C YC LES TO FA ILUR E ( N ic k e l E le c t r o p la t e s )
NO
MIN
AL
ST
RE
SS (
SMA
X) P
S.I.
8 0 , 0 0 0COPPER ELECTROPLATED
SPECIMENS7 5 , 0 0 0
7 0 , 0 0 0
6 0 ,0 0 0 -
5 5 ,0 0 0
5 0 ,0 0 0
4 5 ,0 0 0
4 0 , 0 0 0
R e p r e s e n t a t io n _ of No Failure
3 5 , 0 0 0
3 4 5 6 7 8 93 4 5 6 7 8 9 3 4 5 6 7 8 9
NUMBER OF CYCLES TO FAILURE(N )
F i g u r e 4 .5 N OMINAL STRESS v s . CYC LES TO FA ILU R E (Copper E l e c t r o p l a t e s )
NO
MIN
AL
ST
RE
SS (
SMA
X) f
?S.I.
SILVER ELECTROPLATEDSPECIMENS
7 5 ,0 0 0
7 0 , 0 0 0
6 0 ,0 0 0 -
5 5 ,0 0 0
5 0 , 0 0 0
4 5 ,0 0 0
4 0 , 0 0 0
-— ^ R e p re s e n ta t io n _ of No Failure
3 5 , 0 0 0
3 4 5 6 7 8 93 4 5 6 7 8 9 3 4 5 6 7 8 9 3 4 5 6 7 89
NUMBER OF CYCLES TO FAILURE(N)
F i g u r e 4 .6 N OMINAL STRESS v s . C YC LES TO F A IL U R E (S i lv e r E l e c t r o p l a t e s )
NO
MIN
AL
ST
RE
SS (
SMA
X) R
S.I.
8 0 ,0 0 0 -CHROMIUM ELECTROPLATED
SPECIMENS7 5 ,0 0 0
7 0 , 0 0 0
6 5 ,0 0 0
6 0 ,0 0 0 -
5 5 ,0 0 0
5 0 , 0 0 0
4 5 ,0 0 0
4 0 , 0 0 0
R e p r e s e n t a t io n _ of No Failure
3 5 , 0 0 0
3 4 5 6 7 8 93 4 5 6 7 8 9 3 4 5 6 7 8 9
NUMBER OF CYCLES TO FAILURE(N)
F i g u r e 4 .7 NOMINAL STRESS v s . CYCLES TO F A I L U R E (C hrom iu m E l e c t r o p l a t e s )
. / .:: ̂ \ \ CHAPTER 5 ' . : . : :. :: /' ,
CONCLUSIONS
5o 1 SUMMARY. . The resu lts of this investigation confirm the
statem ent that the endurance lim it of an alloy steel can be increased by-
el e ct r op lat i ng.
As was expected^ zinc plating the base m etal increased its endur- ;
ance lim it by 4. 2% w hereas the cadmium plating increased it 3. 5%» The
silver and chromium platings induced the lowest endurance lim its on the
base m etal. Silver reduced it TO. 2% and chromium reduced it 12%. The
nickel and copper had, the least effect on the endurance lim it- -reductions
of 1. 2% and 3. 2% w ere obtained respectively .
Residual s tre sse s play an im portant part in fatigue failu res by
radically changing the m ean s tre s s . E lectrodeposited chromium and
silver contain residual s t r e s s e s s which are tensile in nature in contrast' . ' ' ' r . - ' ' _ ,. y X ̂ ' ' ';; y
to electrodeposited zinc and cadmium in which these s tre s se s are com-'
p ressive . Copper and nickel deposits can have either tensile or cdm-
p ressive residual s tre sse s^ depending upon the method of deposition.
In this repo rt they,seem ed to be tensile since the endurance lim it of the
base m etal was lowered in both cases. :
; Hydrogen em brittlem ent was not considered to be, a factor in the
reduction of the endurance limits; found since steps w ere taken to relieve
thiso Even though the endurance strengths of the chrom ium electro- '
plated specimens w ere the lowest at low s tre sse s , they endured the
;same number of s tre ss cycles as the unplated specimens' at very high
s tre sse s . . " y.. , . . ' T ': \ 'y
A fu rther increase in the endurance lim its can be obtained by cold
workihg the surfaces by a treatm ent such as shot-peening. In general^ - •
it was found that the electroplatings with the highest modulus of elastic ity
induced.the lowest endurance lim it to the base m etak . ,
. 5.. 2 SUGGESTIONS FOR FURTHER STUDY. Data available on the
effects of ele'Gtroplatihgs on the fatigue life of m ateria ls is fa r from com -
plete. The author suggests the following topics for fu rther investigation:
(1) Effects of various thicknesses of electrodeposited m e ta ls ,on the
endurance lim it of a specific m aterial,. ,
(2) Effects of the sam e electroplating on various m a te ria ls .
(3) Effects of various s tre ss ra tios on the same electrop lated m a te r ia l .1
43
Table V
EXPERIMENTAL RESULTS
Type of Plating
SpecimenNumber
PlatingThickness
(inches)
Nominal S t r e s s
Smax . (psi)
. Number of . Cycles to F ailure
R estressed and Cycles to Failure
UNPLATED i. 74,000 ■ 4,0002. ' 68,700 7, 000 '3. 57, 000 > y . 42,000 '4. / S I ,800 - : 92,000 . ■ ' :5. 43, 100 323, 0006. 40,300 ' 1,836,000 S = 63,100
N = 17, 000
COPPER 1. 0.00031-0. 00040 74,000 ; 4, 000' - , ■ . ' 2. Specimen ruined during testing. : -■ - , ■ ;
3. 58,200 ■ 40, 0004. 52,100 84,0005. 45, 200 • 324, 0006. 39,000 1,968, 000 S = 63,900
N = 15,000
NICKEL 1. 0.00910-0. 00990 68,000 . 4, 000- ■ _ • . '
2. 58,000 ' 24, 0003, 52,500 76,0004. . 46,000 260,0005. 39, 800 . 1,486, 000 : S = 62,000
N - 13, 0006. : 38,200 1, 948, 000 S = 62,000
- N = 7,000
xaoie v v continue a;
EXPERIMENTAL RESULTS
Type of SpecimenPlating Number
P l a t i n gThickness
(inches)
Nominal S t r e s s
Sm a x( p s i )
Number of Cycles to Failure
R estressed and Cycles to Failure
CADMIUM 1.2.3.405.6.
0.00115-0.00118 74,300 62,700 55,900 50,600 45,000 41,750
3, 00014.00051.000
145.000250.000
1,949,000 S = 62,250 N = 16,000
SILVER I.2.3.4,5.6,
0.00008-0.00011 71, 500 58,200 50,000 44,900 .39,500 36, 200
7,00045,000
103.000 431, 000638.000
1,972, 000 S = 66,900 N = 8,000
ZINC 1.2..3.4.5.6.
0.00058-0.00066 69,90062,80058.00049.80045.80042.000
5, 00014.00036.000
124.000249.000
2,280, 000 S = 60,000 N = 24,000
CHROMIUM 1.2.3.4. Spec5.6. '
0. 00034-0". 00045
im en ruined during
70,100 61,000 52,900
; testing. 40,200 35,500
6, 000 21,000 60,000
» 339,000590,000
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45
REFERENCES
' 2 .
1. R ichards, Cedriq W;-=,' 'Engineering M aterials Science. San F ra n cisco:' -Wadsworth Publishing Co. Inc. , 1961. 546 pp.
Orowan, E. , "Theory 'of the Fatigue of M etals, " Proceedings of- the Royal Society of London, Yol. 17!A, pp. 79-105, 1939.
3. G rover, H. J. , S. A. Gordon, and L. R. Jackson, The Fatigue of M etals and S truc tu res. Washington, D. C. : U..S. Government P rin tingG ffice , NAYAER 00-25-534, 1954. 394 pp.
4. H drger, O. J . , and H. R. N eifert, "Improving Fatigue R esistance -by Shot^Peening, " E xperim ental S tress A nalysis, Yol. 2, ' .
' . - No. 1, pp. 178-189, May, 1944. : , ; ' .. . ' V ’
5. C; - Sines, George and J. L. . W aisman, M etal Fatigue. New York, To-■ " ' ronto, Lcmdon: M cGraw-HilI Company, Inc. , 1959. 415 pp. : .: 6-' - :
; 6. - Lang don, P . ' H. (pub. ), M etal Finishing Guidebook. ■Westwood, y. .N. J / : .Finishing Publications Ihc. , 1958. pp. 277-278.:
7. "Report of the Task Group on Effects of Speed of Testing on Fatigue Test R esu lts , " ASTM P roceed ings, Vol. 50, pp. 421 -424,
/ ■ i95o. ■; ' 1 . .... v ■ ' F'r-j:- ■ V' . ' .
8. '", Logan, H. L. , "The Effects of Chromium P la ting on the EnduranceLim it of Steel Used in A irc ra ft, " ASTM P roceed ings, Vol. 50, pp. 649-653, 1950. \ . / ; : ' '■ , : /-Tv A
9. : Dolan, T. J .., and H. H. B enn ihger,"T he Effects of P ro tective ‘, Coatings'on the C orrosion-Fatigue Strength of Steels, ". . ■ ■ •
ASTM Proceedings, Vol. 40, pp. 659-669, 1940. ' ' .
10. Harris^ W. J. 9 M etallic Fatigue. Oxford^ London9 New York/P a ris : Per'gamon P re ss / pp. 285-304* 1961.
■ . ^ ■ : : : ■ ■ ■ v : ■■■■. I . . ,11. A m erican Society for Testing M aterials* Manual of Fatigue T esting . \ ^
ASTM Special Technical Publication No. 915 Philadelphia: • \, A m erican Society for Testing M aterials* 1949. 82 pp. , '
46
47
12. K eyser, C. A. , Basic Engineering M etallurgy, Englewood Cliffs,' N. J. : P ren tice Hall, 1,952o 384 pp,
13. Eea, R. G. , "The Penetration of Hydrogen into M etal Cathodes andIts Effect upon-the Tensile P roperties of M etals and Their Resistance to Repeated S tresses; with a Note on the Effects of Non E lectrolytic Baths and Nickel P lating on These P r o p ertie s , n Proceedings of the Royal Society of London, Vol. 123A, pp. 171-185 , 1929.
14. Swanger, W. H. , and R. D. F rance, "Effects of Zinc Coatings onthe Endurance P ro p erties of Steel, " ASTM Proceedings, Vol. 32, 1932. pp. 4 3 0 -4 4 8 .
15. Baldwin-Lim a-Ham ilton Corp. , Instruction Manual SF-01 -U-2,Fatigue Testing M achine. Waltham, M assachuse tts: 1955.
16. G ensam er, Maxwell, Strength of M etals Under Combined Stre s se s .Cleveland: A m erican Society.for M etals, 1953.
17. Timoshenko, S. P . , H istory of Strength of M ate ria ls . New York,Toronto, London: M cGraw-Hill Company, Inc. , 1953
18. Moore, H. F. , "Fatigue of M etals, A Backward Glance, " M etalsand A lloys, Vol. 3, No. 9, p. 195, Sept. 1932.
19. Hammond, R. A. F . , and C. W illiams, "The Effect of E lec tro plating on Fatigue Strength," M etallurgical Reviews,Vol. 5, No. 18, I960,
20. Battelle M em orial Institute, P revention of the Fatigue of MetalsUnder Repeated S tre ss . New York: John Wiley and Sons, Inc. , 1941.
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