FATIGUE CRACK PROPAGATION IN RAIL STEELS
112
REPORT NO. FRA/ORD-77/14 FATIGUE CRACK PROPAGATION IN RAIL STEELS C.E. Feddersen R.D. Buchheit D.· Broek BATTELLE COLUMBUS LABORATORIES 505 King Avenue OH 43201 JUNE 1977 INTERIM REPORT DOCUMENT IS AVAILABLE TO THE U.S. PUBLIC THROUGH THE NATIONAL TECHNICAL INFORMATION SERVICE. SPRINGFIELD, VIRGINIA 22161 Prepared for U.S. DEPARTMENT OF TRANSPORTATION FEDERAL RAILROAD ADMINISTRATION Research and Development Washington DC 20590 REPROCUCEC BY: NlJS. u.s. Depllrtment of Commerce Nalional Technlcallnronnalion SeNicl Springfield, Virginia 22161 PB272062 1111111111111111111111111111
Transcript of FATIGUE CRACK PROPAGATION IN RAIL STEELS
REPORT NO. FRA/ORD-77/14
FATIGUE CRACK PROPAGATION IN RAIL STEELS
C.E. FeddersenR.D. Buchheit
D.· Broek
BATTELLE COLUMBUS LABORATORIES505 King AvenueCol~mbus OH 43201
JUNE 1977INTERIM REPORT
DOCUMENT IS AVAILABLE TO THE U.S. PUBLICTHROUGH THE NATIONAL TECHNICALINFORMATION SERVICE. SPRINGFIELD,VIRGINIA 22161
Prepared for
U.S. DEPARTMENT OF TRANSPORTATIONFEDERAL RAILROAD ADMINISTRATION
Research and DevelopmentWashington DC 20590
REPROCUCEC BY: NlJS.u.s. Depllrtment of Commerce
Nalional Technlcallnronnalion SeNiclSpringfield, Virginia 22161
PB2720621111111111111111111111111111
NOTICE
This document is disseminated under the sponsorshipof the Department of Transportation in the interestof information exchange. The United States Government assumes no liability for its contents or usethereof.
NOTICE
The United States Government does not endorse products or manufacturers. Trade or manufacturers'names appear herein solely because they are considered essential to the object of this report.
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Technical Report Documentation Page
1. Report No. 2, Government Accession No_ 3. Recipient's Cololog No,
FRA/ORD-77/144. Tide and :Subtdle 5. Report Dote
FATIGUE CRACK PROPAGATION June 1977
IN RAIL STEELS 6. Performing Orgonizatlon Code
8, Performing Orgonl zotion Reporl No,7. Authorls;
Buchheit, D. Broek DOT-TSC-FRA-77-3G.E. Feddersen, R.D.
9. Performing Orgoni lotion Name and Addres.s 10. Wo,k Un,t No. (TRAISj
Battelle Columbus Laboratories* RR7l9/R732l505 King Avenue 11. Contracf or Granl No.
Columbus OH 43201 DOT-TSC-l07613. Type of Report and Period Covered
12. Sponsorir"lg Agency Nome and Address Interim ReportU.S. Department of Transportation July 1975 July 1976Federal Railroad Administration -
Research and Development 14, Sponsoring Agency Code
Washington DC 2059015. 5upplemenlory Noles U.~. Department 01 TransportatIon
*Under contract to: Transportation Systems Center
I Kendall SquareCambridge MA 02142
16. Abstract
In order to establish safe inspection periods of railroad railsinformation on fatigue crack growth rates is required. These datashould come from a sufficiently large sample of rails presently inservice. The reported research consisted of the generation and analy-sis of fatigue crack growth data of 66 rail samples taken from exist-ing track all over the United States. Additional information concernsmechanical properties, chemical composition, microstructure, andfractographic features.
A statistical analysis was performed to evaluate possible correIations with fatigue crack growth properties and microstructural para-meters. Weak correlations were found with carbon, manganese and oxygercontent, and wi th the fraction of pearlite.
A subsequent phase of this research program is laid out.
17. Key Words 18. Dislribution Statement
Rail, Cracks, Fatigue ,DOCUMENT IS AVAILABLE TO THE U.S. PUBLIC
Crack Propagation, Chemical Compo- THROUGH THE NATIONAL TECHNICAL
sition, ~lechanical Properties, INFORMATION SERVICE. SPRINGFIELD.VIRGINIA 22161
~Iicrostructure, Fractography
19. Security Clossif. (or this reporf) 20. Secu,i'y Clo •• if. (of th,. pogel 22. Price
Unclassified Unclassified~;~J\0(" - Ael
Form DOT F 1700.7 (8-72) Reproduction of completed poge authorized
',"
'~~f',, ',-,
PREFACE
This report presents lile results of the first phase of a program on
Rail Material Failure Characterization. It has been prepared by Battelle's
Columbus Laboratories (BCL) under Contract DOT-TSC-1076 for the Transportation
Systems Center (TSC) of the Department of Transportation. The work was conducted
under the technical direction of Mr. Roger Steele of TSC.
The results of this phase of the program are the basis for the layout
of the second phase. The objective of the second phase is the development of a
computational rail failure model. This model, in conjunction with the results
of ongoing studies on Engineering Stress Analysis of Rails and on Wheel-Rail
Loads when incorporated into a reliability analyses will enable establishment
of safe inspection schedules.
The cooperation of the American Association of Railroads (AAR) and
the various railroads (Boston & Maine Railroad Company, Chessie System, Denver
and Rio Grande Western Railroad Company, Penn Central Railroad Company, Southern
Pacific Transportation Company, and Union Pacific Railroad Company) in acquiring
rail samples is gratefully acknowledged. The cooperation and assistance of Mr.
Roger Steele of TSC, Mr. Ornar Deel and Mr. David Utah of BCL were of great value
to the program.
f~Preceding,page b.!ank "iii
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TABLE OF CONTENTS
Section
'-1.
2.
3.
INTRODUCTION .
RAIL MATERIALS: SAMPLE SOURCE AND DESCRIPTION •...•.....•.......
METALLOGRAPHIC CHARACTERIZATIONS .
1
2
2
3.1 Chemical Analyses.......... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23.2 Macrostructures............................................ 7.3.3 Microstructures............................................ 18
4. EXPERIMENTAL DETAILS . 21
4.1 Specimens....................... . . . . . . . . . . . . . . . . . . . . . . . . . . . 214.2 Testing Procedures......................................... 21
5.
6.
TEST RESULTS .....•...•...•.......••.•.....................•.....
DATA ANALySIS .........................•.........................
26
26
6.1 Analysis of Rate Data...................................... 346.2 Synthesis of Crack-Growth Data............................. 41
Synthesis Results................................... 42
6.3 Correlation of Rate Data with Other Properties .......•..... 42
6.3.16.3.26.3.36.3.46.3.5
General Approach .Automatic Interaction Detector (AID) Analysis ..•..•.Process of Analysis .Res ul ts 0 f Analys is .......................•.........Correlation Analysis •....... , .
4243464648
7. CATEGORIES FOR FURTHER RESEARCH ............................•.... 48
7.1 Selection of Categories.. 487.2 Microstructural Analysis of Three Categories. 51
7.2.17.2.27.2.37.2.4
Rail Samples Used .........••...•....................Grain-Size Measurements ...............•........•....Pearlite Interlamellar Spacing ..........••..........Other Microstructural Parameters ..•.................
51515253
I ,
'-J'l
8.
7.3 Fractography .7.4 Projected Experiments for Phase II ..
REFERENCES .............•......................•..•......•.......
5569
71
APPENDIX A BASELINE CRACK-GROWTH DATA .. '" A-I
APPENDIX B REPORT OF INVENTIONS........................... B-1
v
Figure 1.
LIST OF FIGURES
Typical Coarse-Textured Macrostructure ofRails - Sample 027. • •••• 8
Figure 2. Typical Fine-Textured Macrostructure ofRails - Sample 019••••••
Figure 3. Macrostructure of Rail Sample 058
Figure 4. Macrostructure of a Heat-Treated RunningSurface - Rail Sample 059. •• •••
Figure 5. Macrostructure of a Repaired Running Surface Rail Sample 002 ••
Figure 6. Macrostructure of Rail Sample 001
Figure 7. Macrostructure of Rail Sample 061
Figure 8. Macrostructure of Rail Sample 062 •
Figure 9. Macrostructure of Rail Sample 063 •
Figure 10. Macrostructure of Rail Sample 003
Figure 11. Pearlitic Microstructure Typical of theMajority of Rails- Sample 051L
9
10
11
12
13
14
15
16
17
19
Figure 12. Ferrite Network in a Matrix of Pearlite
Figure 13. Heat-Treated Pearlitic Microstructure ofRail Sample 058L. • • • • • .• •
Figure 14. Internal Crack in Rail Sample DOlL.
Figure 15. Orientation of Specimens.
Sample 004. 19
20
20
22
Figure 16. Compact Tension Fatigue Crack Growth Specimen
Figure 17. Compact Tension Specimens Before and After Testing.
Figure 18. Compact Tension Specimen in Fatigue Machine
23
24
25
Figure 19. Typical Fatigue Crack Propagation Curves.
Figure 20. Variability of Fatigue Crack PropagationRate Behavior • •. ••••• •••
vi
33
36
,\ "
'I-
LIST OF FIGURES (Continued)
Page
Figure 21. Sample Graphical Output from Program AID. 45
~' Figure 22. Variation of Life With Leading Predictors 47'~I
' .. Figure 23. Typical Scanning Electron MicroscopeViews of Pearlite In Rail Samples 54
Figure 24. Fatigue Test Fracture Surfaces. . 56
Figure 25. Fracture Surface of Sample 004 at' the Notch Tip 60
Figure 26. Fracture Surface of Sample 002 at the Notch Tip . 61
Figure 27. Fracture Surface of Sample 030 at the Notch Tip 62
Figure 28. Fracture Surface of Sample 006 at the Notch Tip 63
Figure 29. Fracture Surface of Sample 001 at the Notch Tip 64
Figure 30. Fracture Surface of Sample 024 at the Notch Tip 65
Figure 31. Fracture Surface of Sample 002 0.17 Inch...,.".. from the Notch Tip. 66. · .
\-;,/Figure 32. Fracture Surface of Sample 024 0.56 Inch
from the Notch Tip. 67
Figure 33. Examples of Fracture Surface Striations 67
Figure 34. Cross-Hatched Line Pattern - Sample 024,1.21 Inches from Notch Tip. . · . 68
Figure 35. Cleavage Fracture - Sample 024, 1. 21Inches from Notch Tip . . · 68
LIST OF TABLES
'-I'
Table 1. Rail Materials Inventory ..
Table 2. Results of Chemical Analyses ofRail Samples 001 through 066
Table 3. Rail Samples Not Within Chemical Requirements.
vii
3
5
6
LIST OF TABLES (Continued)
Page
Table 4. Tension Test Results for 66 Rail Samples · . 27
Table 5. Charpy Impact Test Results for Category 1 Rails. · · 30
Table 6. Charpy Impact Test Results for Category 2 ,~,, -Rails (Medium Growth Rate) . . · · . . 31 "-
Table 7. Charpy Impact Test Results for Category 3Rails (Low Growth Rate). . . . 32
Table 8. Sample of Computer Printout of Basic DataAlong With First Stage of Rate Analysis. . · · . 37
Table 9. Summary of Crack Behavior Parameters forBaseline Rail Material Specimens · 38
Table 10. Results of Crack-Growth Synthesis. 42
Table 11. The Three Categories for Phase II. . 50
Table 12. Prior Austenite Grain-Size Measurements. · 52
Table 13. Pearlite Interlamellar Spacing . · 53
Table 14. Locations of Fractographic Studies 55;--,
'wITable 15. General Fracture-Surface Characteristics · . 57
Table 16. Samples Selected for Additional Testing. · 70
Table 17. Experiments in Phase III . . · . . 70
viii
,~
EXPLANATORY NOTE
This report conveys preliminary information on tqe crack growth behavior
of a sample of rail steels (66 rails) taken from the population currently
in use in the United States. Ultimately, this information will be used
to predict the flaw growth behavior of actual rails in service under
various loading and support conditions. A more comprehensive treatment
of the subject, with additional test data, will be available later in
1977. This interim report is being issued at this time to provide other
investigators working in the field with the results which have been
generated thus far.
ix/x
-.
1. INTRODUCTION
Fatigue cracks in railroad rails can be the cause o~ rail failures and
subsequent derailments. Prevention of these failures relies on timely detection
of fatigue cracks when they are still small and not likely to cause failures. In
order to establish safe inspection periods, data are required on the available
time for crack detection, i.e., the time it takes for a small detectable crack to
grow to a critical size that can cause rail failure. Therefore, the rate of fatigue
crack propagation has to be known.
One portion of the Federal Railroad Administration's (FRA) Track Perfor
mance Improvement Program is the development of a predictive rail failure model
that enables a determination of optimal inspection periods through a calculation
of fatigue-crack-propagation behavior. The research reported here concerns the
first phase of a program to develop this rail failure model.
In order to predict fatigue-crack growth and failures under a service
load environment, fatigue-crack-rate data are required. These data should come
from a sufficiently large sample of rails presently in service to properly evaluate
the statistical variability of fatigue-crack-growth properties. The first phase of
this program consisted of the generation and analysis of fatigue-crack-growth data
of 66 rail samples of various age, make, and weight. The samples were taken from
existing track from all sections of the United States.
This report presents the crack-growth data for the 66 rail samples. Also
presented are chemical compositions, mechanical properties, and some data on micro
structure and fractographic features. A statistical analysis was performed to
evaluate possible correlation between one or more of these parameters and the re
sistance to.fatigue-crack prop~gation.
On the basis of the present results,. the 66 samples were divided into
three broad categories of rate behavior. Further characterization of the three
categories will be conducted; i.e., the effect of parameters such as stress ratio,
temperature, and microstructural orientation be experimentally evaluated. The be
havior under variable amplitude loading also will be i~vestigated. Subsequently,
the computational failure model will be developed after which the results will be
reported.
1
,"...--
2. RAIL MATERIALS: SAMPLE SOURCE AND DESCRIPTION
At the outset of this program,' an effort was made to assemble a representa
give sampling of rail materials which are presently, and will continue to be, in
service on U. S. railroads. Variations of rail size, rail producer, and year of" "
production were the primary selection criteria. Eleven of the major railroad
organizations were contacted for contributions of rail samples. Directly or in
directly samples were received from the following organizations:
• Association of American Railroads
• Boston and Maine Railroad Company
• Chessie System
• Denver and Rio Grande Western Railroad Company
• Penn Central Railroad Company
• Southern Pacific Transportation Company
• Transportation Systems Center
• Union Pacific Railroad Company.
A total of 66 material samples were received representing sizes from 85 Ib/yd to
140 lb/yd, produced over a period from 1911 to 1975 in both U. S. and Japanese mills.
The samples were given identification numbers from 001 to 066. Basic information on
the samples is presented in Table 1. ~
".
3. METALLOGRAPHIC CHARACTERIZATIONS
3.1 CHEMICAL ANALYSES
Specifications for the chemical composition of rail steels vary slightly
with the rail size (expressed as the weight per yard of rail). The ASTM Standard
Specification for Carbon-Steel Rails, ASTM Designation: Al-68a, states the fol
lowing chemical requirements:
Element, Nominal Weight, lb/ydpercent 61-80 81-90 91-120 121 and Over
Carbon 0.55-0.68 0.64-0.77 0.67-0.80 0.69-0.82
Manganese 0.60-0.90 0.60-0.90 0.70-1.00 0.70-1. 00
Phosphorus, max 0.04 0.04 0.04 0.04
Silicon 0.10-0.23 0.10-0.23 0.10-0.23 0.10-0.23.
2
TABLE 1. RAIL MATERIALS INVENTORY
,eL w,. "Sequence iltIceipt Source Section Controlled Hill Year Month 5alllple~ulll.o.r 0.. ~e Soun:e ~umber N'ul:Iber Type Cool Brand Rolled Rolled !Anith _ a.marlLI
001 10/10/75 TSe _IS DO 85CO 1929 II ]4-718 Stu hon O?flin Heu ttl. L"1ed. "W,nl!l. Ht. 8)530 AREA
00' '" '5 1911 14 ~ryland ASCE001 '" DO 1929 11 37-1/8 Steeltcn Open Hel!lrth "lad, .'1an8. H,. 81366 AREA
c/o 00_ 100 85 BSCO 1920 ,. Steelton 0\Mn ~arth ASCE005 198 DO L929 )5-]/8 Stulton Open Hearth ~ed, M.anl!l. HL 81692 A.R!.A
'.. 00' VD-I 115 " 1974 )5·1/2 Vacuum Oeluud, S:tdney VT Rai.l , New liS Ib A&'1
'1 a07 VD-2 lIS " L97~ 36-1/8 Vacuum Degoill!lsed, Sydney II! Rall, New 115 Ib Ae.M
008 535 85 1924 )5·5/8 L.cka"'anna Open Heanh. I\SCE000 -., LJO 1929 . 36-1/8 SCllulton Open !1eanh "led. ~ng, H, , E1nliQ010 \l0 85 1919 )6·1/4 L.acuIoIann& Hr. asa ASCE
Oil 10/14115 MA UP·)·4 lJ]O " y" CF&I 1965 II 6)-1/2012 UP-I·1 lJJa " CF&1 1955 l' 41-1/2013 PC·l~l 127DH lLl1noiB 1954 1 60·1/201':' Up·l·14 13]0 BE y" CFli.I -1955 il 4.'IS UP-I·20 1])0 " y" CFli.I 1949 , 41-1/2
'" lJP~ZA~9 III v" CFli.I 1957 5 50-1/2017 l1P·2A,·8 III CFli.I 1957 I 48018 L'P~2A·2 1))0 A£ V" CFli.I 195) 4 _0019- L:P·)·S 1))0 BE y" CFli.l 1965 11 1.0~)/"
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0.4 124 Nippon 1075 7 J6 Heat ,\·39262 D~2
065 1'4 Nippon 1975 7 35 lieu A·39780~D~5
0.' 12' Nippon 1975 1 36 l:Ieet A·39376 C-7
, .\:'"
3
No specification for the sulfur content is given by the ASTM Standard,
but it states "that thoroughly deoxidized steel will be furnished and that, in
every stage of manufacture, strict adherence to the standards of best practice
of the individual mill will be observed". On this basis, it is reasonable to
assume that the sulfur content of rail steels should be controlled by the mill
to a maximum of about 0.050 weight percent.
Chemical analyses of each of the 66 rail samples were made for total
carbon, manganese, silicon, and sulfur in percent by weight, and for hydrogen and
oxygen in parts per million (ppm). The results of the analyses are presented in
Table 2. Duplicate and, in some instances, triplicate analyses were made for
hydrogen and oxygen and these are shown individually in the table.
Four rail steels, Samples 001, 003, 005, and 009, were designated by
the suppliers as medium manganese steels. The manganese contents of three of
these steels (Samples 001, 005, and 009) were within a range, 1.36 to 1.48 percent,
normally associated with medium manganese steels. However, the manganese content
of Sample 003, 0.76 weight percent, was within the standard chemical requirements
for its rail size. A fourth rail steel, Sample 038, contained a manganese content
of 1.48 weight percent, which means that it is a medium manganese steel also.
Since the chemical requirements for the medium manganese steels were not
available for rail steels, an assessment of these values in the total range of
compositional variation cannot be made.
An analysis of the composition data presented in Table 2 indicates that
the compositions of several rail samples, excluding the medium manganese steels,
do not meet the chemical requirements contained in the ASTM Standard and the assumed
maximum sulfur content. Table 3 lists the samples which do not meet the require
ments and the manner in which they deviate from the requirements.
With the exception of Sample 053, the hydrogen content determined in
each of the 66 rails was between 0.2 and 1.1 ppm. The hydrogen content of Sample
053 was reported to be 6.1 and 6.5 ppm in two determinations. The concentration
of hydrogen in all other rails was characteristic of residual levels of hydrogen
concentrations present in steels. Since hydrogen will effuse from steel at
ambient temperatures over a period of time, it would be expected that rails of
eqrly vintage that may have had high hydrogen contents when placed into service
would now contain only residual amounts.
The oxygen contents of the 66 rails were generally well below 100 ppm.
The only exceptions were rail Samples 004 and 045 which contained averages of 538
and 333 ppm of oxygen, respectively. These oxygen contents are considerably
higher than normal for silicon deoxidized rail steels.
4
,
,
TABLE 2. RESULTS OF CHEMICAL ANALYS~S OF RAIL SAMPLES 001 THROUGH 066
Elemental Content,Rail Size, weight percent Hydrogen, Oxygen,
._'1' ...... Sample 1b/yd C Mn Si S ppm ppm
~j
001 0.63 1.48 0.21 0.022 0.8, 1.0 100, 96130002 85 0.74 0.61 0.07 0.154 0.8, 0.9 46, 48003 130 0.77 0.76 0.20 0.036 0.4, 0.5 71, 69004 85 0.67 0.62 0.30 O. 052 0.7, 0.5 519, 435, 659005 130 0.63 1. 36 0.21 0.033 0.6, 0.8 52, 54006 115 0.72 0.97 0.10 0.028 0.4, 0.4 23, 25007 115 0.73 0.93 0.18 0.037 0.4, 0.3 24, 26008 85 0.66 0.94 0.20 0.029 0.8, 0.8 57, 61009 130 0.61 1.46 0.29 0.039 0.7, 0.7 56, 59010 85 0.63 0.74 0.14 0.028 1.1, 0.9 132, 138011 133 0.73 0.81 0.19 0.028 0.4, 0.4 57, 51, 56012 133 0.79 0.84 0.18 0.029 0.8, 0.7 54, 58013 127 0.74 0.89 0.24 0.028 0.8, 1.0 51, 47014 133 0.78 0.74 0.17 0.014 0.8, 0.8 86, 84015 133 0.76 0.82 0.19 0.033 0.6, 0.6 54, 54016 133 0.81 0.93 0.17 0.044 0.6, 0.8 39, 43017 133 0.79 0.85 0.26 0.048 0.9, 1.0 44, 43
~~- .. 018 133 0.75 0.89 0.17 0.046 0.7, 0.6 45, 43
~1019 133 0.74 0.88 0.21 0.038 0.4, 0.4 38, 36020 119 0.75 0.83 0.15 0.033 0.8, 0.7 34, 32021 133 0.79 0.90 0.21 0.024 0.7, 0.6 41, 45022 133 0.78 0.87 0.20 0.028 0.4, 0.5 46, 47023 133 0.79 0.92 0.21 0.040 0.6, 0.7 39, 35, 46024 133 0.81 0.83 0.12 0;030 1. 0, 0.7 26, 28025 133 0.80 0.91 0.23 0.016 0.7, 0.7 29, 27026 133 0.78 0.94 0.17 0.050 0.5, 0.5 47, 46027 133 0.78 0.87 0.23 0.022 0.7, 0.6 45, 45028 133 0.71 0.90 0.17 0.022 0.7, 1.0 79, 53, 69029 119 0.72 0.89 0.19 O. 046 0.5, 0.6 45, 43030 119 0.80 0.90 0.16 0.028 0.5, 0.7 52, 54031 133 0.79 0.76 0.15 0.022 0.5, 0.4 53, 49032 133 0.80 0.94 0.18 0.035 0.5, 0.5 63, 61033 133 0.78 0.92 0.23 0.025 0.6, 0.5 37, 35034 119 0.77 1.04 0.17 0.023 0.5, 0.7 38, 38035 115 0.76 0.80 0.23 0.028 0.5, 0.4 27, 27036 112 0.75 0.81 0.18 0.016 0.4, 0.5 57, 54
.;.-r 037 115 0.72 0.93 0.25 0.017 0.4, 0.5 86, 67, 61038 112 0.57 1.48 0.16 0.029 0.3, 0.3 78, 82...) 039 90 0.71 0.81 0.17 0.028 0.3, 0.3 81, 107, 168040 100 0.58 0.64 0.08 0.030 0.4, 0.4 39, 34041 115 0.77 0.81 0.21 0.043 0.4, 0.3 91, 93042 100 0.63 0.71 0.08 0.026 0.3, 0.4 49, 36, 64043 90 0.75 0.81 0.15 0.032 0.6, 0.4 84, 85
5
TABLE 2. (Continued)
Elemental Content,Rail Size, weight percent Hydrogen, Oxygen,
Sample 1b/yd C Mn Si S ppm ppm
044 110 0.78 0.88 0.20 0.016 0.3, 0.3 84 86045 110 0.65 0.65 0.21 0.027 0.6, 0.5 342, 286, 372046 133 0.78 0.90 0.20 0.027 0.2, 0.3 49, 48047 130 0.76 0.46 0.11 0.044 1.1, 0.7 43, 41 "'-.
048 122 0.79 0.95 0.17 0.022 0.7, 0.6 58, 61 I...049 115 0.80 0.89 0.11 0.040 0.9, 1.1 48, 50050 133 0.75 0.91 0.20 0.036 0.5, 0.6 56, 56051 130 0.84 0.72 0.19 0.016 0.6, 0.5 47, 51052 100 0.72 0.90 0.19 0.021 0.4, 0.4 52, 54053 140 0.85 0.91 0.18 0.032 6.1, 6.5 44, 44054 131 0.78 0.76 0.20 0.021 1. 0, 0.6 36, 32055 131 0.78 0.90 0.17 0.028 0.8, 0.8 33, 35056 132 0.80 0.90 0.19 0.039 0.7, 0.7 44, 46057 140 0.77 0.94 0.16 0.028 0.7, 0.9 58, 46, 50058 140 0.83 0.84 0.18 0.048 0.4, 0.5 47, 44059 133 0.83 0.98 0.14 0.024 0.4, 0.3 22, 25060 124 0.80 0.90 0.12 0.013 0.5, 0.4 56, 36, 47061 124 0.80 0.91 0.12 0.015 0.4, 0.7 46, 46062 124 0.79 0.84 0.08 0.017 0.3, 0.6 45, 51, 48063 124 0.79 0.86 0.12 0.033 0.3, 0.3 49, 59, 64064 124 0.76 0.85 0.18 0.018 0.6, 0.6 43, 49, 54065 124 0.82 0.90 0.17 0.016 0.3, 0.3 41, 42066 124 0.75 0.90 0.18 0.019 0.4, 0.7 37, 36 ~.,
r
TABLE 3. RAIL SAMPLES NOT WITHIN CHEMICAL REQUIREMENTS
Rail High Low High Low High Low HighSample C C Mn Mn Si Si S
002 X X X004 X X008 X010 X013 X017 X034 X037 X040 X X X042 X X 1" .........
045 X X047 X051 X053 X058 X059 X062 X
6
3.2 MACROSTRUCTURES
Most of the 66 rail samples exhibited uniform macrostructures through
out their full cross sections. The principle variances in the macrostructures
among the rail samples were differences in fineness or coarseness. These dif
ferences may be related to the prior austenite grain size and/or the pearlite
colony size. Typical macrostructures observed are exempii~ied by the photomacro
graphs in Figures 1 and 2, Samples 027 and 019, respectively. Figure 1 shows a
typical coarse-textured macrostructure which was observed in 19 rail samples
(Samples 007,012, 014 through 018, 020 through 024,027 through 032, and 042).
Figure 2 shows a fine-textured macrostructure which was observed in the remaining
47 rail samples, except for Sample 058. Sample 058 had a macrostructure which
eXhibited very little of a structural pattern as shown in Figure 3.
The macrostructures of Samples 046 and 059 showed that the running sur
faces apparently had been heat treated. The heat-treated surface of Sample 059
is evident in Figure 4. The surface heat treatment suggested that these two
samples were from the ends of rails that were end-hardened, a process commonly
used to reduce wheel batter at the rail joint.
The macrostructure of Sample 002 showed that its running surface appar
ently had been repaired by the mechanical removal of surface damage and subsequent
deposition of weld metal. The repair weld in this sample is evident in Figure 5.
The macrostructure of Sample 001 showed evidence of a high inclusion
content and internal fissuring, both conditions being located primarily in the
web section and at the bottom of the head section. These conditions can be seen
in Figure 6.
Cracks were observed in the macrostructures of Samples 061, 062, and
063. The cracks in these three rails were located centrally in the web below the
rail head. All three cracks extended through the entire thickness (1 inch) of
the transverse cross sections. The cracks are believed to be the remains of
shrinkage porosity formed in the steels during solidification of the original
ingots. The cracks are visible in the photomacrographs of Samples. 061., 062, and
063 shown in Figures 7,8, and 9, respectively. Sample 062 eXhibited decarl:iuri
zation around the crack as indicated bi the white zone in· Figure 8.
Some chemical segregation in the central zone of the web rail section
was indicated by the macrostructures of Samples 003, 025, 040, 060, 061, 062, and
063. An example of this condition is shown by the photomacrograph of Sample 003
in Figure 10. Similar conditions of chemical segregation exist in Figures 7, 8,
and 9.
7
IX
FIGURE 1. TYPICAL COARSE-TEXTURED MACROSTRUCTURE OF RAILS - SAMPLE 027
8
lX
FIGURE 2. TYPICAL FINE-TEXTURED MACROSTRUCTURE OF RAILS - SAMPLE 019
9
IX
FIGURE 3. MACROSTRUCTURE OF RAIL SAMPLE 058
Note lack of any structural pattern.
10
lX
FIGURE 4. MACROSTRUCTURE OF A HEAT-TREATED RUNNING SURFACE - RAIL SAMPLE 059
11
lX
FIGURE s. MACROSTRUCTURE OF A REPAIRED RUNNING SURFACE - RAIL SAMPLE 002
12
•
..~-~-
".IX
FIGURE 6. MACROSTRUCTURE OF RAIL SAMPLE 001
Note internal fissures.
13
IX
FIGURE 7. MACROSTRUCTURE OF RAIL SAMPLE 061
Note crack in the web.
14
'.
r....IX
FIGURE 8. MACROSTRUCTURE OF RAIL SAMPLE 062
Note segregation and crack in the web.
15
lX
FIGURE 9. MACROSTRUCTURE OF RAIL SAMPLE 063
Note hairline crack in the centralarea of the web.
16
IX
FIGURE 10. MACROSTRUCTURE OF RAIL SAMPLE 003
Note segregation in the web.
17
3.3 MICROSTRUCTURES
Microscopic examinations of longitud nal. me tallographic specimens of
the rail samples showed that themicrostructur s ~f 48 rails consisted of essen
tially 100 percent ,fine p~arlite with very minor~mounts of free ferrite occur
ring adjacent to some mangau"ese sulfide inclusions or"-along a few prior austenite
grain boundaries. A typi6al micro.tructure is shown ~y the photomicrograph of
Sample 051 in Figure 11. ,Th7 ~i~rostructures of Sample~ 004, 010, 013, 028, 038,
041, 045, 047, and 052 consisted of 85 to 95 percent (visual estimates) fine
pearlite with the remainder being free f~~rite located primarily along prior
austenite grain boundaries. Rail Sample?" 004 and 045 contained the most free
ferrite in the form of a ferrite netwo~k *long prior austenite grain boundaries.
Figure 12 shows the microstructure of Sample 004. The remaining Rail Samples,
002, 036, 037, 043, 054, 058, 064, 065, and 066, had microstructures consisting
of about 96 to 99 percent (visual ~~timates) fine pearlite with the remainder
being free ferrite scattered along.~rioi austenite grain boundaries and adjacent
to some sulfide inclusions. The microstructure of Sample 058 (shown in Figure
13) had much finer pearlite and constderably smaller pearlite colonies than any
of the other rails. This type of microstructure was suggested already by its
fine macrostructure. The very small pearlite colony size is obvious by compari
son with the pearlite colony size in Figur~ 11. This fine structure suggests
Sample 058 was heat treated followirtg hot rolling.
Internal cracks in Sample 601, which were evident during macroscopic
observations, were clearly apparent during microscopic observations. Three prin
cipal cracks running generally ~arallel to ~helongitudinal direction of the rail
were observed in the longitudinal metallogra~hic,specimenexamined. An example" - L
of one of the cracks observed is shown in Figure. 14. The: cracks pr0I;>agated pri-
marily across pearlit~ colonie's', 'but 'also some propagation wil.'s <?bserved along.' ' -,
pearlite coiony interfaces. In the speCimen e~amin~d, the"~iacks'were located
below the r~nnirig.sui~ace':~hout~inch.andd~eper. The longe~"~ cr'ack observed
was approximately 200 mils. The cracks are believed to be the result of a high
hydrogen content in the steel when the rail was manufactured.
18
100X
FIGURE 11. PEARLITIC MICROSTRUCTURE TYPICAL OF THEMAJORITY OF RAILS - SAMPLE 0511
100X
FIGURE 12. FERRITE NETWORK IN A MATRIX OF PEARLITE SAMPLE 004
19
100X
FIGURE 13. HEAT-TREATED PEARLITIC MICROSTRUCTURE OFRAIL SAMPLE 058L
100X
•
,
FIGURE 14. INTERNAL CRACK IN RAIL SAMPLE 001L
20
4. EXPERIMENTAL DETAILS
4.1 SPECIMENS
One tensile specimen and one fatigue-cracK-growth specimen were machined
from each rail sample,. The orientation of the specimens is shown in Figure 15.
Charpy V specimens were taken from six rail samples - 023 and 030 which exhibited
a high rate of fatigue-crack growth, 019 and 031 with medium crack-growth rates,
and 001 and 036 with low growth rates. Forty-five Charpy specimens were made, 15
from each of the three growth-rate categories. From each category, five specimens
were taken in each of the three directions shown in Figure 15. The specimens were
taken from the center of the rail head.
The tensile specimens were standard ASTM 0.25-inch-diameter specimens.
Charpy specimens were also of standard dimensions; i.e., 2.l65-inch l6ng, 0.394
inch thick with a square cross section.
Fatigue-crack-growth specimens were of the compact tension (CT) type.
Their dimensions are shown in Figure 16. The specimens were provided with a 1.650
inch deep chevron notch (0.900 inch from the load line). Details of the notch can
best be observed in Figure 17 which shows two specimens, one before and one after
testing.
4.2 TESTING PROCEDURES
Tensile and Charpy tests were perfor'med in accordance with standard pro-
cedures.
To expedite the crack-growth tests, specimens were precracked in a
Krause fatigue machine. Crack-growth experiments were conducted in a 25-kip
capacity electrohydraulic servocontro1led fatigue machine. Figure 18 shows a
specimen mounted in the fatigue machine. The tests were performed at constant
amplitude, the load cycling between 0 and 2500 pounds, resulting in a stress ratio
of R = O. Cycling frequency was 40 Hz, but was reduced to 4 Hz toward the end of
a test to enable more accurate recording of the crack size giving final failure.
The laboratory air was ,kept at 68 Fand 50 percent relative humidity.
Crack growth was measuted visually, using a 30 power, traveling micro
scope. The cracks were allowed to grow in increments of 0.050 inch, after which
the test was stopped for an accurate crack size measurements. Crack size was
recorded as a function of the number of load cycles.
21
Charpy
Crack growth specimen
rIGURE15. ORIENTATION OF SPECIMENS
22
I1.80"
·/.80"
0750" die
0.825"
i+~
Thickness: 0.5"
~-----3.00"-------.l
~-------3.75"--------...1
FIGURE 16. COMPACT TENSION FATIGUE CRACK GROWTH SPECIMEN
23
FIGURE 17. COMPACT TENSION SPECIMENS BEFORE AND AFTER TESTING
24
FIGURE 18. COMPACT TENSION SPECIMEN IN FATIGUE MACHINE
25
5. TEST RESULTS
The tensile properties of the 66 rail samples are presented in Table 4.
With a few exceptions, the tensile ultimate strength (TUS) and the tensile yield
strength (TYS) are in the order of 130 ksi and 75 ksi, respectively. One heat
treated rail showed a high TUS of 188.3 ksi and a TYS of 127.3 ksi. Two tensile
specimens (030 and 045) contained longitudinal cracks as became apparent after
fracture, since the fracture path partly followed these cracks. This resulted
in the strength of those samples being low. It should be noted that these samples
were different from the ones reported cracked in Section 3.2.
The Charpy data are presented in Tables 5, 6, and 7. They show that in
the range of ambient temperatuIEs the Charpy energy is essentially the same for
all these steels. Transition temperatures and upper shelf behavior show some
variation, but these are of limited interest under operational conditions.
Some typical fatigue-crack-propagation curves are given in Figure 19.
The curves show that the number of cycles to grow a l-inch crack to failure showed
a wide variation for the rails from which the specimens were taken. This will be re
flected in the rate of growth, which is the basis on which the materials will be com
pared in the next section. Also the final crack size at failure showed quite a wide
variation which will be reflected in the toughness number. The raw test data (crack
size versus cycle number) of all specimens are given in Appendix A.
6. DATA ANALYSIS
In order to develop a failure model for track rail, one must identify
and quantify the damage processes, couple them appropriately. provide a means for
accumulating the damage (i.e., compile the crack growth), and establish the
criterion for failure or fracture. The first step in implementing these tasks is
the baseline effort of crack-growth characterization and metallurgical studies
previously described. In the following sections, the approach to interpretation,
quantification, and correlation of these data is discussed. In the next phase,
this will be broadened to consider additional variables.
26
•
TABL
E4
.TE
NSI
ON
TEST
RES
ULT
SFO
R66
RA
ILSA
MPL
ES
Tru
eT
rue
Ram
berg
-W
ork
Elo
ng
ati
on
Red
uct
ion
Fra
ctu
reF
ractu
reO
sgoo
dH
ard
enin
gR
ail
TU
S,T
YS,
in1
Inch
,in
Are
a,E
,S
tress
,S
train
,E
xp
on
ent,
Ex
po
nen
t,N
umbe
rk
sik
sip
erc
en
tp
erc
en
t1(
]3ks
ik
siE:
tn
l/n
-00
11
36
.47
6.5
13
.52
8.0
34
.01
71
.2.1
26
67
.8.1
28
002
13
4.4
74
.71
2.0
20
.63
0.8
15
9.4
.11
33
7.7
.13
0
003
13
7.4
73
.61
2.0
17
.73
0.3
16
0.1
.11
33
13
.1.0
76
004
11
6.0
59
.91
5.0
24
.02
8.6
14
4.6
.13
97
10
.4.0
96
005
13
4.8
76
.41
3.5
26
.03
1.8
15
4.9
.12
66
11
.5.0
81
006
13
5.0
71
.21
1.0
21
.23
0.2
16
1.9
.10
43
11
.5.0
87
007
13
5.8
70
.01
2.0
17
.63
0.3
15
6.9
.11
33
12
.5.0
80
008
12
5.1
67
.01
4.0
25
.030
.1-
15
5.9
.13
10
10
.8.0
93
009
13
9.8
81
.81
4.0
29
.43
2.0
18
0.0
.13
10
12
.0.0
83
N -.J
14
3.1
010
11
1.5
58
.71
7.0
27
.22
9.3
.15
70
9.8
.10
2
011
12
6.9
73
.21
2.5
20
.83
3.8
14
4.3
.11
77
10
.3.0
97
012
13
4.7
78
.31
0.5
17
.03
2.4
15
3.1
.09
98
8.4
.11
9
013
12
9.3
72
.81
2.5
29
.12
9.1
16
0.8
.11
77
7.9
.12
6
014
13
5.4
75
.91
2.0
18
.03
3.1
15
8.7
.11
33
7.5
.13
3
015
.13
1.6
71
.51
1.0
16
.53
0.6
15
0.0
.10
43
6.0
.16
7
016
13
8.6
75
.69
.51
5.0
28
.81
54
.4.0
90
76
.3.1
59
017
13
7.1
74
.41
0.0
19
.52
8.2
16
3.6
.09
53
6.4
.15
6
018
13
3.2
70
.61
1.0
19
.92
7.5
.10
43
019
13
1.2
73
.41
2.0
19
.23
4.5
15
2.8
.11
33
8.5
.11
8
020
13
1.4
72
.01
1.0
18
.43
0.4
15
2.6
.10
43
6.5
.15
4
021
13
2.3
77
.21
2.0
18
.43
2.6
15
3.9
.11
33
9.8
.10
2
022
13
0.7
76
.01
3.0
22
.73
1.7
15
7.9
.12
22
8.2
.12
2
023
13
5.1
77.3
10
.51
7.9
32
.21
55
.7.0
99
87
.7.1
30
TABL
E4
.(C
on
tin
ued
)
Tru
eT
rue
Ram
berg
-H
ork
Elo
ng
ati
on
Red
uct
ion
Fra
ctu
reF
ractu
reO
sgoo
dH
ard
enin
gR
ail
TU
S,T
YS,
inlI
nch
,in
Are
a,E
,S
tress
,S
train
,E
xp
on
ent,
Ex
po
nen
t,N
umbe
rk
sik
si
perc
en
tp
erc
en
t10
3k
sik
siE
tn
lin
024
13
6.7
74
.61
0.0
16
.23
2.4
15
8.7
.09
53
6.3
.15
902
51
41
.17
5.7
9.5
'1
8.8
'26~
51
64
.9.0
90
76
.3.1
59
026
13
5.0
,7
4.4
11
.017
.52
9.9
15
3.1
.10
43
8.2
.12
202
71
36
.46
9.4
10
.01
3.6
29
.01
50
.1.0
95
36
.2.1
61
028
12
9.1
70
.51
1.5
18
.93
1.8
11
9.8
.10
88
7.5
.13
302
91
25
.56
1.7
12
.01
9.9
29
.41
46
.6.1
13
36
.8.1
47
030
11
0.0
(a)
76
.8--
--2
8.2
----
7.1
.14
003
11
33
.47
5.6
11
.01
7.6
31
.61
49
.4.1
04
38
.6.1
16
N03
21
39
.51
65
.3.1
13
3C
D8
0.0
12
.01
9.5
34
.88
.0.1
25
033
13
5.0
73
.31
0.0
13
.92
8.6
--.0
95
303
41
37
.377
.31
0.5
20
.73
0.2
16
4.3
.09
98
6.0
.16
703
51
28
.16
9.3
12
.51
9.6
33
.61
54
.1.1
17
77
.2.1
39
036
13
2.1
74
.61
2.0
21
.431
.1
15
5.3
.11
33
10
.0.1
00
037
12
7.7
68
.61
6.0
25
.93
2.6
15
6.8
.14
84
9.4
.106
038
12
4.2
74
.917
.04
2.3
33
.71
85
.3.1
57
01
1.5
.08
703
91
30
.77
5.0
14
.521
.63
0.9
15
5.9
.13
54
7.5
.13
304
01
38
.88
3.3
9.5
15
.02
6.9
15
6.5
.09
07
7.7
.13
004
11
32
.07
3.6
11
.52
2.0
28
.61
56
.1.1
08
87
.7.1
30
042
13
3.0
74
.71
0.5
15
.92
9.6
15
1.1
.09
98
6.8
.14
704
31
33
.27
5.6
13
.02
0.5
32
.81
56
.9.1
22
26
.9.1
4504
41
39
.78
0.0
10
.01
5.3
29
.31
58
.7.0
95
31
1.5
.08
704
59
6.8
(a)
66
.08
.01
6.3
33
.89
8.0
.07
69
10
.2.0
98
046
13
0.6
75
.91
4.5
20
.62
8.9
16
0.
'i.1
35
42
5.0
.04
0
TAB
Lt:
4.
(Co
nti
nu
ed)
Tru
eT
rue
Ram
berg
-~"'ork
Elo
ng
atio
nR
edu
ctio
nF
ractu
reF
ractu
reO
sgoo
dH
ard
enin
gR
ail
TU
S,T
YS,
in1
Inch
,in
Are
a,E
,S
tress
,S
train
,E
xp
on
ent,
Ex
po
nen
t,N
umbe
rk
sik
sip
erc
en
tp
erc
en
t1<
fk
sik
si€
tn
l/n
047
12
3.8
60
.41
4.0
21
.02
9.2
15
0.1
.13
10
24
.0.0
41
048
13
2.4
75
.5U
.51
7.5
29
.91
52
.9.1
08
81
0.5
.09
5
049
13
2.0
72
.4U
.52
0.1
30
.51
57
.8.1
08
87
.2.1
39
050
13
2.4
73
.81
2.0
21.0
29
.91
57
.5.1
13
37
.8.1
28
051
14
1.5
81
.29
.51
3.3
31
.21
59
.1.0
90
71
1.8
.08
5
052
12
6.0
64
.01
3.5
21
.32
9.7
15
1.0
.12
66
14
.0.0
71
053
14
0.2
75
.89
.51
3.3
30
.31
59
.4.0
90
7.
8.9
.11
2
054
13
5.9
76
.51
2.0
18
.83
0.9
15
9.5
.11
33
9.2
.109
N05
51
37
.4n
.99
.01
4.2
29
.51
56
.1.0
86
17
.7\0
.13
0
056
13
6.0
72
.69
.51
3.2
29
.61
49
.6.0
90
79
.2.1
09
057
13
6.6
72
.91
0.5
18
.22
7.1
15
8.9
.09
98
8.2
.12
2
058
18
8.7
(b)
12
7.3
U.S
31
.72
9.6
23
9.4
.10
88
.3
0.
.03
3
059
13
7.2
79
.11
1.0
15
.42
8.3
.10
43
060
13
5.3
74
;21
2.0
16
.53
0.9
15
3.4
.UD
13
.0.o
n06
11
32
.57
0.7
11
.51
7.1
31
.21
54
.4.1
08
81
7.5
.057
062
14
1.3
76
.91
1.0
19
.33
2.0
16
7.5
.10
43
13
.5.0
74
063
13
5.6
73·.5
11
.01
8.8
29
.61
55
.3.1
04
31
4.0
.07
1
064
13
3.1
69
.11
3.0
21.1
30
.51
59
.4.1
22
21
4.8
.06
7
065
13
1.3
73
.31
1.0
17
.73
1.0
15
7.5
.10
43
4.4
.22
7
066
13
4.2
70
.01
2.0
20
.73
0.5
15
9.9
.11
33
13
.0.o
n
(a)
Lo
ng
itu
din
al
cra
ck
sin
spec
imen
.
(b)
Hea
ttr
eate
dra
il.
TABLE 5. CHARPY IMPACT TEST RESULTS FOR CATEGORY 1 RAILS(HIGH GROWTH RATE)
Specimen Temperature, Energy, Shear Area,Orientation F ft/lb percent
----
L 32 5 0
L RT 4 0
L RT 5 0
L 212 5.5 20
L 300 18.5 99
T 32 2 0
T RT 2 0
T in 2 0
T 212 2 -:'0
T 308 3 98
ST 32 3 0
ST RT 4 0
ST RT 4 0
ST 212 5 20
ST 300 11.5 95
30
TABLE 6. CHARPY IMPACT TEST RESULTS FOR CATEGORY 2RAILS (MEDIUM GROWTH RATE)
"pecimen Temperature, Energy, Shear Area,Orientation F ft/1b percent
L 32 3.5 0
L RT 4 0
L RT 4 0
L 212 10 10
L 300 13 45
T 32 2 0
T RT 2 0
T RT 2 0
T 212 3.5 5
T 300 6.5 45
ST 32 3.5 0
ST RT 3 0
ST RT 4 aST 212 7 25
ST 300 12 95
31
TABLE 7. CHARPY IMPACT TEST RESULTS FOR CATEGORY 3RAILS (LOW GROWTH RATE)
Specimen Temperature, Energy, Shear Area,Orientation F ft/lb percent
L 32 3 0
1.. RT 4 0
L R'l' 5.5 0
L 212 11 45
L 300 14 70
T 32 3 0
T RT 2 0
T RT 2 0
T 212 4.5 Q
T 300 10.5 65
ST 32 2 0
ST RT ·3 0
ST RT 3 0
ST 212 5.S IS
5T 300 13 95
32
"
'. 2.6Pmox=2500 Ibs
2.4 R=O8=40 Hz
2.2 Specimen numbersVI 045~
.s:::. 2.0u 042c
C 0031.8
.s:::.-C"c 1.6~
~
..x:u 1.4c~
u
1.2
1.0
0.80 500 1000 1500
Number of Kilocycles, N
FIGURE 19. TYPICAL FATIGUE CRACK PROPAGATION CURVES
33
6.1 ANALYSIS OF RATE DATA
The rate of fatigue-crack propagation can be expressed as a function of
the stress-intensity factor K. The stress-intensity factor (unit ks~in.) is a
measure for the stress singularity at the crack tip. If two cracks in the same
material but under entirely different circumstances are subjected to the same
stress intensity, their behavior will be the same. For theCT specimen used in
this investigation, the stress intensity can be given as
•
Kp
2BW1ja (1+a/W)(1-a/W)-s/2 [7.000-7.050(a/W)+4.275(a/W)2j (1)
in which P is the load .on the specimen, B is the specimen thickness, W is
the specimen width, and a is the crack size.
The rate of crack growth is related to K through
dadN
f(liK,R) (2 )
where N is the cycle number, R is the ratio between minimum and maximum load in
a cycle, and t:$. is the range through which K varies during the cycle. Thus, liK
is found by substituting the load range liP into Equation (1). In the present
tests, the load varied between 0 and 2500 pounds so that liP ~ 2500 pounds and
R ~ O.
Over a wide range of growth rates in steels and for fixed R, Equation
(2) can be approximated by
da = C (liK) n (3)dN .
where C and n are constants for a given material. Hence, the various rail steels
can be compared on the basis of their C and n values.
Equation (3) implies that a plot of da/dN versus ~K on· double-log paper
is a straight line. In reality there will be an upswing in the rate of crack
growth towards the end of the test, because the failure conditions are being ap
proached. This is reflected in the following equation:
dadN
C (liK)n(l-R) K -liK
Ic(4)
Not only does this equation take into account the effect of the stress ratio R,
it also shows that the crack-growth rate becomes infinite if the stress intensity
34
at maximum load becomes equal to KIc ' The quantity KTc is the fracture toughness
of the material, which is the value of K at which fracture occurs. For the
special case of R = 0, the equation reduces to
dadN (5 )
Both Equations (3) and (5) were evaluated for their applicability to
the present data base. For this purpose, da/dN was calculated from the measured
crack-growth data through the weighted average incremental slope approximation,
The results were plotted as a function of 6K as determined by Equation (1). Subse
quently, curves were fitted through the data to give values for C and n. A special
computer program was used to find the best fit.
Examples of the resulting plots of da/dN versus 6K are given in Figure
20. An example of a computer printout giving the basic crack-growth data, crack
growth rate, and the stress-intensity factor, is shown in Table 8. The variability
of crack-growth rates in the 66 samples can be appreciated from Figure 20. The
heat-treated rail appeared to have the lowest crack-growth rates. It did fall to
the right of the scatter band containing all other samples. All the curve fitting
data, in terms of C, n, and the correlation parameter, R2, are presented in Table 9.
The correlation parameter is generally close to unity which is an indication of the
goodness of the fits. These results have been derived from the basic crack-growth
data listed in Appendix A.
Also presented in this table are the apparent toughness, defined as
the stress-intensity factor, determined by Expression (1), for the last recorded
crack measurement, and a life parameter,
r"( \L ¥-l)
'-1
Jwhich is a coupled function of C and n used to rank the growth rates.
Very few crack-growth data for rail steels have been reported in the
literature. The data reported in References 1 and 2* are useful for a comparison
with the present results. The British rail steel tested contained 0.56 percent C,
1.02 percent Mn, 0.13 percent Si, and less than 0.05 of P and S each. The steel
had a 0.1 percent yield strength of 67 ksi and an ultimate tensile strength of 121
ksi. Test results for center cracked panels showed a value of 4 for the exponent
n in Equation (3) for the case of R = 0 (Reference 1). Experiments at various R-
* References are listed on page 70.35
10-3,.. ---.
A 045 1m)o 042 (TI)o 003 (II• 058 (heat treated rail) o o
605020 30 40
Stress Intensity Factor Range, l::lK, ksi-in.I/ 2
10-7~ .......;a;,.. __L ..J.. .L...._ __J
10
Q.l10-4
U>.~~0.=z-"0......C"0
ai-ca::c 10-~0;cCJ'C0.0...a..
o .¥0euQ.l;::,CJ';CIJ..
10-6
FIGURE 20. VARIABILITY OF FATIGUE CRACK PROPAGATION RATE BEHAVIOR
36
TABLE 8. SAMPLE OF COMPUTER PRINTOUT OF BAS IC DATA ALONG WITHFIRST STAGE OF RATE ANALYSIS
SPECIME~ IDENTIFrC~TION a0~9
G~AIN DlkE~TlnN =LTSPECIM~N CUN~IbU~ATluN aCTTHICKN~S5 a .~~2 INC~
~IUTH = 3.~~ INCH"AXIMU~ LOAO • 2.~~ KIPS~OAO ~AT10 = .~~
TEST FkEUUtNCV = 4~.~~ HZ.TEST TlMPERATllwE = 70.~'" OEli~EE FCATE O~ ANALYSIS • 2 21 ~
O~ERAL~ WIDTH = 3.7~ INCHHEIGHT • J.2~ INCH
BASIC UATA..-•••.•........-- ~ATE CALCU~ATIONS----._...-._-.-.-.-..--------_._.-OUtAGE PARA •
C~AC"
LENGTH,A,INCh
(,VCU.COLHlT,
"l,Ke
TIoiOpnINTSLllPE:.
THREEPOINTSLOPE
K(MAll) De-LTAI(
I(SI-SC~T (INCH)..._-_.. ~...-._..--._..-._.-.--_._-_.---_._--....-_.-.----._.-.-._.....----._ .
1.14!1l
~50."0
Cl.41il. cl!Cl
1:1221. il~
IlJ7.~Hl
C13cs.J0
.l1l:1E-~5
.187E-05
.232E-"'"
.~3IlE-11I4
.417E-04
.235E-03
• 815E-Ir.It5
.106E-05
• 12'1E-03
37
27.85
41.12
21.43
27 .05
32 • .:19
3~.47
TABL
E9
.SU
MM
ARY
OF
CRAC
KBE
HA
VIO
RPA
RAM
ETER
SFO
RB
ASE
LIN
ER
AIL
MA
TERI
AL
SPEC
IMEN
S
Co
eff
icie
nt,
C·
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!I
5.l~~.!.e
~u~~er
CO
lI;;
ffic
len
t.C
Lin
ear
Exp
on
en
tfn
Mo
del C
ort
e:"
a.ti
un
Co
eff
icie
nt.
S·
Com
.put
edU
f.M
arg
ln
Hed
ifie
dV
near
Mod
.!l
-------"'=-'-"~.~
·=~C
-"or
~r"'
.";I
'-:-
.--:
:r71
o-n-
--;;
C--:
:o~-
p-u-
--:'
,---
."'d
Ex
po
n:n
t,C
oeff
lfle
nc.
Lif
en
RH
.lcg
tn
Ap
pare
nt
To
ug
hn
e.a
,
k.fl. t!
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LIf
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ara
mete
r.N
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cle
.
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Gro
1.:t
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ratios indicated that n = 2.69 in Equation (4) gave the best fit. It appears that
the material compares with the materials showing the lower growth rates in the
present investigation.
6.2 SYNTHESIS OF CRACK-GROWTH DATA
As validation of the rate analysis, the crack-growth curves were recon
structed by integrating the rate data according to both the linear (Equation (3)
and the modified-linear (Equation (5») fatigue-crack-propagation models. In
simple terms the integration can be expressed as
fdaa = - dNdN
d -1N = S2. da
dN
(7)
(8 )
Since the crack-growth model cannot g~nerally be integrated in closed
form, the solution of the above expressions is accomplished by a numerical inte
gration or summation procedure wherein the computational steps must be defined
in detail. Basic sources of error include experimental error, material anomalies,
and simplicity of the model.
An incremental definition of Equation (8) can be expressed ask
N=L (dar lt.a (9)
i=l dNwhere da/dN
t.a
k
fCC, n, t,K)
(a f - ao)/k
number of increments, arbitarily set at 100.
Two alternative schemes of crack-growth prediction are being adopted
in the basic data analysis computer program. One scheme predicts the number of
cycles to grow the crack from a precrack length, a , to a final crack length, af;. athe other predicts the final crack length, af, which results from cycling the
precrack, ao ' Nf times. If the analysis as well as the data models provided a
perfect correlation, the results would, of course, agree perfectly with the
experiment. In reality, however, perfect correlation will not be achieved due
to experimental error, material variation and mere oversimplicity (i.e., inade
quacy) of the analysis. The contrast in the results of the two computational
schemes will provide further insight to the source and degree of errors.
The measure of the effectiveness of these two schemes of analysis is
expressed in the "cyclic life margin of safety". which is expressed as
NactualN ' .
computed
41
':" 1 (10)
and in the cyclic crack growth margin, which is expressed as
(11)- 1M.S.a
aactual"a .
computed
Positive values of either of these margins infer that the computed value is less
than the actual and, hence, conservative. The degree of conservatism (+) or
unconservatism (-) is reflected in the variation of margin from unity (1,0).
(Note at the present time, only the life margin, Expression (10), has been tabu
lated in Table 9.)
Synthesis Results
The preceding crack-growth-synthesis procedure was applied to the 66
baseline data sets to obtain a set of life margins which in turn were analyzed
statistically. These results are presented in Table 10.
TABLE 10. RESULTS OF CRACK-GROWTH SYNTHESIS
ModelPredictedMean Life
Predicted Life Margin StatisticsMean Life Margin Variance Standard Deviation
Linear
Modified Linear(Forman)
0.936
1.035
-0.064
+0.035
0.010
0.011
0.100
0.104
From these results, several interesting observations can be made. First,
it appears that the linear model tends to be unconservative in that it predicts, on
the average, a larger crack lifetime than was encountered in the test. This is
evidenced by the negative value of the mean life margin. In contrast, the modified
linear model provides a conservative estimate of life and for that reason may be a
more preferable model to use. Second, since the variance and standard deviation
are nearly equivalent for each model, it is judged that lifetime scatter about the
mean is not particularly affected by the model.
6.3 CORRELATION OF RATE DATA WITH OTHER PROPERTIES
6.3.1. General Approach
One of the basic objectives of this research program is to discern whether
the crack behavior of rail materials can be linked to more fundamental mechanical,
metallurgical, and processing variables. As a result, a key activity in data
analysis is the broad scale assessment and evaluation of rate data with respect to
other material properties. The following sections describe the initial efforts
which have been undertaken and the results which have been ascertained to date.
42
The detection and isolation of primary variables affecting crack be
havior would be a straightforward procedure if all of the variables were truly
independent. In reality, however, most of the mechanical, metallurgical, and
processing variables are not mutually independent and interact in a very complex
manner. As a result, the discrimination of the dominant factors and the deter
mination of their order of precedence requires a deliberate search and involves
considerable trial-and-error data scanning.
For the baseline fatigue-crack-propagation specimens of this program,
a broad matrix of data was assembled. This consisted of the background, mechani
cal property, metallurgical and derived crack-behavior variables determined for
each material sample. These were extensively examined by computerized analysis
as well as by more intuitive technical review (i.e., engineering judgments).
While some general trends were discerned, more in-depth probing, analysis, and
data generation will be necessary to strength and more positively identify the
trends. It appears that the broad scatter of the data will require more diligent
screening and examination of individual tests. The following discussion of pro
cedures and resul~ presents the current status of this effort.
6.3.2 Automatic Interaction Detector (AID) Analysis
The AID computer program is a statistical tool for assessing the rela
tive influence of a set of independent variables (termed predictors) on the
behavior of a specified dependent variable. The correlation (or lack thereof)
between the dependent variable and any given predictor is established by decom
posing the total variance of the dependent variable (fixed for a given body of
data) into a within-subset and a between-subset variance of successive splits
(i.e., two-part divisions) of the set of values of the dependent variable.
For each predictor, the set of values of the dependent variable are
ordered by either the order of the predictor (if a monotonic predictor) or the
order of the dependent variable (if a free predictor). The set of values of the
dependent variable is then divided (or split) successively into two subsets
along the domain of the predictor. At each split, the within-subset variance
and between-subset variance is computed. The split which produces the largest
ratio of between-subset variance to within-subset variance (i.e., F ratio or
signal-to-noise ratio) is considered the optimum split for that predictor. The
predictor which exhibits the largest ratio of between-subsets variance to total
variance is the dominant or primary predictor for the dependent variable.
43
The computational scheme is semiquantitative in that the independent
variables are linearly scaled and coded to integral values from 0 to 63. However,
since known correlations do not exist, a method that compares data on such a
normalized basis can provide a clearer discrimination of the dominance (if such
vxists) of the primary independent variables.
Once the optimum split of the primary variable has been defined, the
procedure is repeated for the two groups at each side of the split and so on. The
resulting cascade of splits which is generated in this repetiti~e procedure can be
graphically displayed in the AID "tree", a sample of which is shown in Figure 21.
Only the salient features of the analysis are included in this pictorial summary.
In this particular illustration, the influence of a range of composi
tional variables--the carbon equivalents (CE), later discussed--on the logarithm
of crack life (the dependent variable or criterion scale) is evaluated. The body
of data consisted of 57 specimens (selectively called from a total data set of 67
specimens). The primary variable, CEI, revealed an optimum split into two groups
of 37 and 20 at a life value of 5.90. These two resulting groups subsequently
split on predictors CE4 and CE6. The mean and standard deviation values are given
along with the coded predictor values. Subsequent splits and their related numer
ical details are also given. Note that the dependent variable is noted as the
common logarithm of the life parameter.
At the outset of this task, the widest variety of independent variables
was chosen and put into the AID "hopper" to see what would be sorted out. These
variables included
Mechanical Properties• Elongation
• Reduction of area
• Elastic modulus
• Hardness
• Carbon
• Manganese
• Silicon
• Sulfur Metallurgical
• Oxygen
• Hydrogen
• Pearlite.
• Rail weight IBackground• Year produced
• Tensile ultimate strength
• Tensile yield strength
These were then related to the life parameter, NL, as a dependent variable.
44
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n.3.3. Process of Analysis
The AID analysis proceeded through several stages. It became obvious at
the outset that the variables were not mutually independent. An interspersion of
various metallurgical and mechanical variables became apparent when all the vari
ables were considered. This inferred that the mechanical property variables were,
in essence, a restatement of the compositional variables (or vice-versa)--a not
too surprising result. This led to selective regrouping and fitting of the vari
ables to discern those that were most dominant.
6.3.4. Results of Analysis
The dominance of a particular independent variable may be expressed as
the percentage contribution of its BSS to the TSS of the dependent variable.
The results of the AID analysis of independent variables for leading contenders
can be suriunarized as follows:
Category
Mechanical Property
Me ta llurgical
Variable
1Tensile Ultimate Strength
Hardness
% Pearlite
% Carbon
% Oxygen
% Sulfur
% Manganese
Contributionto Variance,
percen t
13
14
19
8
8
5
4.
For the mechanical property category, the nearly equivalent dominance
of strength and hardness is not surprising because of their well-documented inter
relationship. However, the statistical impact is lessened when one then views
the graphical relationship of strength and life as shown in Figure 22(a).
A similar disillusionment is encountered when one observes the display
of percent peir1ite versus ljfe in Figure 22(b). The latter part of the above
tabulation suggested the consideration of a carbon equivalent (CE) which was
expressed as
% CE = % C + ry [% Mn-1.7 (% S))
The factor 1.7 is the ratio between the atomic weights of Mn and S. As a resu1~,
the term between brackets is the percentage of free Mn, i.e., total Mn minus the
fraction tied up in the compound MnS. The free Mn is in solid solution where it
46
150 L
140 f-- • .. .••• • • , • •or·· • •
IJl • , •.::L • · ._e...
~.., '...<Ii 130 I-- • •:::> • • ••..... .- ••
120 f--
•
110 I I I .1 I I I I I
105 106 107
Cycles
(a) Effec t of Tensile Ultimate Strength
100 .. .... ... .....C 95 f- ... • •IVU"-IV 90 f-a.2:.~
85 --;:0IV0- 80 -
75I I I I I I I I I
105 106 107
Cycles
(b) Effect of Pearlite
1.0~0
j •0.9 l- • -
'E.... . -. .. •-.- -.. . • • • • -~ • •••
0 • •• • ...> 0.8 f- • • ••:;c:r • .W • •c
0.7 •0 f-.D
CIu
I I I i I I I I0.6
105 106 107
Cycles
(c) Effect of Cn rhan Equivalent
FIGURE 22. VARIATION OF LIFE WITH LEADING PREDICTORS
47
h~s a similar, but less effective, strengthening effect as Carbon. This is re
flected by the factor~. An AID analysis for incremental values of ~ was con
ducted from which a value of ~ = 0.1 was obtained as providing a 21 percent con
tribution to variances. Again, however, the statistical analysis was poorly
supported wi th the "shotgun" pattern of Figure 22 (~).
6.3.5. Correlation Analysis
The contrast between the statistical AID analysis and the weak evidence
of the graphical displays suggests that any numerical correlation is a coincidence
of "noise" in the data. At the same time, however, the complexity of a carbon or
other equivalent as suggested by other investigators(3) requires that other micro
structural details be included. In Reference (3), correlation functions were
derived between the TUS, TYS, and 20 ft-lb Charpy impact temperature for ferrite
pearlite steels. The functions are complex equations containing the percentage
of the various chemical constituents, volume fraction of pearlite, interlamellar
spacing and cementite plate thickness. At the present time, a positive conclusion
is not tendered. The analysis will be advanced as additional metallurgical
details are generated. However, the complex correlation functions as derived in
Reference (3) suggest that any correlation function may be very artificial.
Consequently, the generality of such functions is doubtful.
7. CATEGORIES FOR FURTHER RESEARCH
7.1 SELECTION OF CATEGORIES
The present test data provide the baseline information for the computa
tional failure model to be developed during the second phase of this research
program. However, for a complete failure model, more information is needed con
cerning crack growth under various circumstances. The effect of the following
parameters will have to be evaluated:
(a) Stress ratio, R
(b) Cycling frequency, F
(c) Temperature, T
(d) Specimen orientation.
In addition, the behavior of elliptical flaws and the behavior under mixed-mode
loading and variable-amplitude loading should be studied.
48
It is prohibitive to perform all this experimentation on all 66 rail
materials. Therefore, it is necessary to make a selection of a few materials to
be studied in more detail, under the assumption that the results obtained can be
generalized relative to the baseline behavior as observed in the present tests.
Although various possibilities exist to select the materials, the most obvious
criterion for selection is the rate of crack growth, because the differences in
crack-growth rates were so large.
Therefore, three categories were selected for further characterization,
consisting of materials with high (I), medium (II), and low (III) growth rates,
respectively. The basis for categorization was the crack propagation life from
a I-inch crack size to failure. This reflects the combined effects of n, C, and
K in a natural way. As a practical concern, the length of the sample avail-app
able for specimen manufacture was also a consideration.
The materials selected for Category I have crack growth lives (from
1 inch to failure) varying from 150 to 270 kilocycles. In Category II the lives
vary from 380-600 kilocycles, and in Category III the lives are 700 kilocycles
and higher. An appreciation of the crack growth behavior of the materials in
the three categories can be obtained from Figure 20. Specimen 3 in Figure 20
had a life of 211 kilocycles which is typical for Category I. The life of
Specimen 42 was 546 kilocycles, typical for Category II, and the life of Specimen
45 was 1,018 kilocycles, which is typical for Category III.
The samples selected for each category are listed in Table 11. Subse
quent testing will be done primarily on those materials. A more detailed metal
lographic and fractographic characterization of these materials will be required.
This effort is already under way and some preliminary results are presented in
the following sections.
Some additional samples will be used for more detailed characterization
and testing. These samples will be selected on the basis of the AID analysis.
The criteria for selection will be discussed in Section 7.4. A test matrix and
experimental plan for the second phase of this program is presented in Section 7.5.
49
TABLE 11. THE TIIREE CATECO\{IES FOR PHASE 1I
Crack Growth LifeRail Sample From 1 Inch to Failure,
Category Number C Mn kilocycles
I 002 .74 .61 270
013 . 74 .89 216
014 .78 .74 269
016 .81 .93 150
023 .79 .92 155
025 .80 .91 153
030 .80 .90 197
II 006 ~72 .97 490
009 .61 1. 46 381
018 .75 .89 384
019 .74 .88 435
024 .81 .83 495
031 . 79 .76 596
032 .80 .94 404
III 001 .63 1. 48 736
007 .73 .93 796
020 .75 .83 1302
022 .78 .87 803
029 .72 .89 1256
035 .76 .80 1218
036 .75 .80 1269
50
7.2 MICROSTRUCTURAL ANALYSIS OF THREE CATEGORIES
7.2.1. Rail Samples Used
From the three categories of rails established on the basis of crack
growth rate, five rail. samples were chosen for more detailed microstructural
analyses. They were Samples 002 and 030 from Group I, Samples 006 and 024 from
Group II, and Sample 001 from Group III. The selection of the two samples from
Groups I and III was based primarily on major differences in their chemistry.
Sample 001 was selected because of the presence of internal fissures. Sample 004,
which was not categorized, was selected for further microstructural analysis,
since its microstructure consisted of a relatively high percentage (~ 15%) of
ferrite in a network morphology.
7.2.2. Grain-Size Measurements
Since standard metallographic preparation techniques do not reveal
prior austenite grain boundaries in pearlitic steels, an attempt was made to heat
treat the samples in such a way that the grain sizes could be measured. The heat
treatment employed was a partial isothermal transformation at approximately 1100 F,
designed to develop a structure consisting of a network of fine pearlite nodules
at austenite grain boundaries in a martensitic matrix. Partial isothermal trans
formation was successful using very small specimens, but the nucleation sites of
pearlite nodules were too random to discern a grain-boundary network. Attempts
to reveal prior austenite grains were made also using special etching reagents
on quenched and tempered specimens of rail samples. The reagents used were
(1) Vilella's reagent, an alcoholic solution of 1% picric - 5% hydrochloric acids,
(2) a saturated aqueous solution of picric acid containing 1 gram of sodium
triolecyl benzene sulfonate per 100 ml of solution, (3) a saturated aqueous solu
tion of picric acid containing 2 ml of Teepol (sodium alkyl sulfonate) per 100 ml
of solution, and (4) a solution of 1 gram of potassium metabisulfite and 2 drops
of Teepol in 100 ml of water. None of these etchants revealed prior austenite
grains satisfactorily for grain size measurements. Special etching techniques
were also used on quenched and tempered specimens of rail samples in attempts to
reveal the prior austenite grains, but these too were unsuccessful.
The prior austenite grains were revealed in Sample 004 by the ferrite
network present in its microstructure.· A similar network was present in the
other five samples at the rail surfaces where decarburization occurred during hot
51
rolling. The depth of decarburization was sufficient to produce a ferrite network
zone below the surface. The width of the zone generally encompassed several prior
;lustenitc grains. Therefore, grain-size determinations on the other five rails
were made in the decarburized surface zones.
Grain sizes were determined by the line i~tercept method. The number of
grains at 100X magnification intersected by a test line 10 cm long was obtained
three times on each specimen. The ASTM grain size, G, was calculated from
Hilliard's ~quation:
where L3
G = 10.00 - 6.64 log La
Total length of test linesTotal no. intersections x magnification
(12)
The results of prior austenite grain size measurements of the six rail
samples, and values computed from the grain-size measurements for average grain
diameters and average number of grains per unit volume also are given in Table 12.
TABLE 12. PRIOR AUSTENITE GRAIN-SIZE MEASUREHENTS
Rail Group Calculated Average No.and/or ASTM Grain Diameter of of Grains
Sample No. Size No. Average Grain, mm per mm3
Group I002 4.3 0.081 1880030 4.7 0.071 2850
Group II006 3.5 0.107 820024 4.9 0.066 3500
Group III001 4.4 0.078 2100
004 3.2 0.12 600
7.2.3. Pearlite Interlamellar Spacing
True interlamellar spacing, So, is the perpendicular distance between
the planes of a single pair of contiguous lamellae. Because true spacing is diffi
cult to measure directly on metallographically prepared cross sections, the mean
random spacing, 0, of the pearlite lamellae observed in the six samples was meas
ured. The mean random spacing is defined as the reciprocal of NL, where NL is
52
the number of alternate lamellae intersected per unit length of random test lines.
True spacing was then calculated using So = ~ the validity of which has been
confirmed experimentally.
The mean random spacing of pearlite lamellae was measured on scanning
electron microscope (SEM) micrographs of the pearlite structures photographed at
5000X. No unresolved pearlite lamellae were observed at this magnification. Ex
amples of the pearlite, as revealed by the SEM micrographs, are shown in Figure 23.
Thirteen random fields on each specimen were photographed using the SEM.
Intercept measurements were made along six different test lines on each micro
graph. Each test line was 10 cm long. Thus, a total of 78 (6 x 13) test-line
measurements were made on each rail sample. A statistical analysis of the data
for each sample indicated the accuracy of the interlammelar spacings obtained to
be±lO to 14 percent.
The results of interlamellar spacing measurements are presented in
Table 13.
TABLE 13. PEARLITE INTERLAMELIAR SPACING
Rail Group Number ofTrue Spacing,and/or Intersections - 0 Accuracy,
Sample No. per mm, NL So' A ± percent
Group I002 1705 2932 10.2030 1385.5 3608 10.6
Group II006 1861. 5 2686 10.9024 1464.5 3414 13.8
Group III001 2025 2470 10.4
004 1202 4159 12.2
7.2.4. Other Microstructural Parameters
Determinations of the pearlite colony size and characterizations of
the nonmetallic inclusions in the six rail samples are planned but, as yet, have
not been made. Visual estimates of the volume fraction of free ferrite in the
samples are reported elsewhere. More precise determinations of volume fractions of
ferrite using established quantitative metallographic techniques also are planned.
53
5000X
(a) Sample 002L, Field 7
(b) Sample 006L, Field 13
FIGURE 23. TYPICAL SCANNINC ELECTRON HICROSCOPEVIEWS OF PEARLITE IN RAIL SAMPLES
54
7.3 FRACTOGRAPHY
A photomacrograph of the fatigue-test fracture surfaces of the six
rails is shown in Figure 24. The encircled areas on the fracture surfaces denote
the fracture surface locations along the direction of fatigue-crack propagation
where fractographic studies are being made. The locations were. selected from the
plots of crack lengths versus the number of load cycles and correspond to the
approximate midpoints of significant changes in the slopes (crack-growth rates)
of the curves. In addition to these locations, the following fracture surface
locations also are being examined: (1) the precrack fatigue origin, (2) the
approximate midpoint of the length of precrack propagation, (3) the approximate
beginning of constant cyclic load crack propagation, (4) a location approximately
midway between the point where the load frequency was lowered and the point of
unstable crack propagation, and (5) an area of unstable crack propagation. The
locations in terms of distance from the tip (origin of theprecrack) of the notch
on the test specimen are given in Table 14.
TABLE 14. LOCATIONS OF FRACTOGRAPHIC STUDIES
Sequence of Sample IdentificationLocation 004 002 030 006 001 024
Is t 0 0 0 0 0 02nd 0.18 0.17 0.17 0.17 0.18 0.18
3rd 0.31 0.33 0.30 0.26 0.31 0.27
4th 0.41 0.43 0.58 0.32 0.46 0.37
5 th 0.86 0.79 1.15 0.47 0.64 0.566th 1. 26 1.25 1.41 0.81 0.96 0.827th 1. 22 1.36 1. 20
8th 1.38
NOTE: Numbers shown represent distance from notch root ininches.
55
\Jl
0'
,
Gro
upI
Gro
upII
Gro
upII
I
En
circ
led
are
as
den
ote
loca
tio
ns
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fra
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ph
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ud
ies.
1.66
X
Rail
Sam
ple
(Cate
go
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00
4(1
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at
Fail
ure
55
(ksi~)
002
(1)
51
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30
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54
50
FATI
GU
ET
EST
FRA
CTU
RE
SUR
FAC
ES
001
(III)
7002
4(II)
47
Some general fracture surface characteristics are apparent in the frac
ture surfaces shown in Figure 24. Significant observations made at magnifications
up to 100X using optical microscopy are described in Table 15.
TABLE 15. GENERAL FRACTURE-SURFACE CHARACTERISTICS
RailSampleNumber Low-Magnification Observations
004 • The length of the fatigue crack zone was ~30 mm.
• A cleavage facet was located very near the tip (precrack origin area) of the notch.
• Some scattered cleavage facets were located throughout thefatigue-crack zone.
• The fatigue-crack zone terminated abruptly and was followed byuns~able cleavage fracture.
• Final rupture, about 2 - 3 mm in length, was ductile.
002 • The length of the fatigue-crack zone was ~28 mm.
• A cleavage facet was located a little below, and on one sideof, the notch tip.
• Some scattered cleavage facets were located throughout the fatigue-crack zone to a crack length of ~20 mm. Several cleavage facets were located from 20 mm to the end of the fatiguecrack zone.
• The fatigue-crack zone terminated fairly abruptly and was followed by unstable cleavage fracture.
• Final rupture, about I - 2 mm in length, was ductile.
030 • The length of the fatigue-crack zone was ~30 mm.
• Cleavage fracture was predominant at the tip of the notch.
• Some scattered cleavage facets were located throughout thefatigue-crack zone to a crack length of ~15 mm. At approximately 18, 23, 25, and 27 mm of crack length, there appearedto be arrest zones containing increasing amounts of cleavagefracture in each successive zone.
• The fatigue-crack zone terminated fairly abruptly and was followed by unstable cleavage fracture.
• Final rupture, about 2 rom in length, was ductile.
57
RailSampleNumber
TABLE 15, (Continued)
Low-Magnification Observations
006 • The length of the fatigue-crack zone was ~25 rom.
• Several cleavage facets were located a short distance from thenotch tip.
• Some scattered cleavage facets were located throughout the fatigue-crack zone to a crack length of ~12 rom. Beyond 12 rom,the amount of cleavage fracture increased rapidly to more than50 percent at the termination.of the fatigue-crack zone. From17 to 25 rom of crack length there was some tendency for cleavage to concentrate in apparent arrest zones.
• The fatigue-crack zone seemed to terminate by a gradual transition from fatigue to cleavage fracture over the last 13 romof fatigue-crack length and was followed by unstable cleavagefracture.
• Final rupture, about 0.5,rom or less in length, was ductile.
001 • The length of the fatigue-crack zone was ~21 rom.
• Some cleavage facets were located in the area of the notch tip.However, fracture-surface features were partially obliteratedby corrosion.
• Some scattered cleavage facets were located throughout the fatigue-crack zone to a crack length of ~10 rom. The amount ofcleavage increased be tween 10 and 21 rom of crack length.Cleavage tended to be concentrated in ~3 arrest zones between15 and 19 rom of crack length.
• The fatigue-crack zone terminated in a rapid transition fromfatigue to cleavage over the last 6 rom of fatigue-crack length.
• Final rupture, less than 0.5 rom in length, was ductile.
024 • The length of the fatigue-crack zone was '~25 rom.
• Very little cleavage was located in or near the notch tip.
• Some scattered cleavage facets were located throughout the fatigue-crack zone to a crack length of ~13 rom. Beyond 13 rom,cleavage occurred in increasing amounts.
• The fatigue-crack zone terminated in a rapid transition fromfatigue to cleavage over the last 7 - 8 rom of crack length.
• Final rupture, ~1.5 rom in length, was ductile.
58
Fractographic studies of the six rails using electron microscopy are
incomplete. Initial scanning electron microscopic (SEM) examinations resulted in
some confusion with respect to the interpretation of detailed fracture features.
Similar difficulties were encountered during replication transmission electron
microscopic (RTEM) examinations. However, it is anticipated that continued exam
inations by both techniques will bring clarification.
Observations of the fracture surface at the tip of the notch (the fatigue
precrack origin) made during SEM examinations of the six rails are shown in Figures
25 through 30. Cleavage facets, indicated by the letter "c" in the figures, are
apparent in some cases. Fatigue striations do not seem to be discernible at the
lower magnifications. The features which appear to be bubbles at the top and to
the right in most of the micrographs are globules of molten metal on the electri
cal discharge machined surface of each test specimen. The globules are most
evident in Figure 30.
Two SEM views of an area of the fracture surface located 0.17 inch from
the notch tip of Sample 002 are shown in Figure 31. The views are considered to
be typical of the appearance 6f the fracture surface areas of most of the samples
when using the SEM. Note the fibrous striated brittle appearance of the crack
surface. The lines in Figure 31 appear to be fatigue striations but they are
actually pearlite lamellae on the fracture surface. Note the similarity between
the pearlite interlamellar spac~ng shown in Figure 23(a) at 5000X magnification
and the spacing of the .lines in Figure 31 at 5000X magnification.
Some random RTEM views of fracture surfaces are shown in Figures 32
through 35. The RTEM micrograph in Figure 32 has an appearance similar to the
SEM micrograph in Figure 31; however, the magnifications differ by a factor of
4. Some striations observed in Sample 004 which appear to be clearly fatigue
striations are shown in Figure 33(a). These striations may be located in ferrite,
since Sample 004 contained a high percentage of ferrite in the microstructure. On
the other hand, similar striations in Figure 33(b) were observed on the fracture
surface of Sample 030 which contained essentially no ferrite.
Occasionally, cross-hatched lines were observed as shown in Figure 34.
Since the replicas were shadowed in a direction toward the crack origin, the lines
in Figure 34 most nearly perpendicular to the direction of shadowing are likely to
be fatigue striations. (These are the striations running approximately up and
down in Figure 34.) The other lines, those that are parallel to the direction of
shadowing, are likely to be pearlite lamellae •.
The RTEM view presented in Figure 35 shows primarily cleavage fracturing.
No evidence of ductile overload cracking has been observed in any of the fatigue
fracture zones.
59
lOOX
soox
FIGURE 25. FRACTURE SURFACE OF SAMPLE 004 ATTHE NOTCH TIP
"c" denotes cleavage fracture.Tip of notch is at upper right.
60
100X
500X
FIGURE 26. FRACTURE SURFACE OF SAMPLE 002 ATTHE NOTCH TIP
"e" denotes cleavage fracture.Tip of notch is at upper right.
61
lOOX
500X
FIGURE 27. FRACTURE SURFACE OF SAMPLE 030 ATTHE NOTCH TI P
"c" denotes cleavage fracture.Tip of notch is at upper right.
62
lOOX
500X
FIGURE 28. FRACTURE SURFACE OF SAMPLE 006 ATTHE NOTCH TIP
"C" deno tes cleavage frac ture.Tip of notch is at upper right.
63
lOOX
.,.
~~;~~~~~ .. "
500X
FIGURE 29. FRACTURE SURFACE OF SAMPLE 001 ATDiE NOTCH TI P
Tip of notch is at upper right.
64
lOOX
500X
FIGURE 30. FRACTURE SURFACE OF SAMPLE 024. ATTHE NOTCH TI P
Tip of notch is at upper right.
65
1000X
5000X
FIGURE 31. FRACTURE SURFACE OF SAMPLE 002 0.17 INCHFROM THE NOTCH TIP, ~K~ 17 ksi-in.~
Compare lines at SOOOX with Figure 23(a).
66
FIGURE 32. FRACTURE SURFACE OF SAMPLE 0240.56 INCH FROM THE NOTCH TIP,tlK R:: 22 KSI-IN.\
(b) Sample 030. 1.15 inchesFrom N~tch Tip, tlKR:: 43ksi-in.~
SURFACE STRIATIONS
Sample 004. 0.86 inchFrom No ten Ti p, tlK R::29 ksi-in.~
FIGURE 33. . EXAMPLES OF FRACTURE
9500X
(a)
67
FIGURE 34. CROSS-HATCHED LINE PATTERN SAMPLE 024, 1.21 INCHES FROMNOTCH TIP, 6K R:: 45 KSI-IN.J:i
2800X
FIGURE 35. CLEAVAGE FRACTURE SAMPLE 024, 1.21 INCHESFROM NOTCH. TIP, 6K R::
45 KSI-IN.~
68
6K) in favorably
(controlled by K ).max
threshold experiments
The fractographic results obtained ~o far are in agreement with those
reported in rdJrences I' arid 2.' the 'two referenced publications indicate that
the topography of the examined fatigue fractures is as com~lex with:an irregular
()('currence of striations, transgranular pearlite cracking, and some cleavage.
The observation of a gradual increase in the amount of cleavage fracture
is in agreement with other reports.(4,5) During ;~ase II Df the program, quanti
tative estimates will be made of the amount of cleavage encounter'ed during the
fatigue crack growth in various rail steels.
The scattered cleavage facets observed close to the notch tip in various
specimens will also be a point of further examination. A two-component mechanism
for crack extension at very low growth rates was proposed in reference 6. This
mechanism accounts for planar fracture damage (controlled by
oriented grains, followed by failure of the unbroken grains
It is expected that ,the tests at different R-ratios and the
may shed some further light on this matter.
7.4. PROJECTED EXPERIMENTS FOR PHASE II
The objective of Phase II is to obtain the more detailed information
on fatigue-crack propagation necessary for the development of the failure model.
As pointed out in the foregoing sections, this information will be generated for
a limited number of rail samples. For this purpose, three groups of samples were
selected with low, medium, and high crack propagation rates. It was attempted to
compose each group of rail samples with nearly the same carbon and manganese
content (Table 11).
In addition to these three groups, other samples were to be selected
for further testing on the basis of the data analysis. However, no clear-cut
correlations with other properties as might appear from between fatigue-crack
growth rates and metallurgical variables emerged. Therefore, the, selection of the
additional samples were somewhat arbitrary. The weak correlations found with
carbon and manganese content, carbon equivalent, and fraction of pearlite were
used as a starting point for the selection.
The 10 samples chosen are listed in Table 16. Reasons for selection are
indicated, and it is also shown in which growth rate category each sample would
belong. Two additional experiments will be performedo~ each ,sample in order to
obtain further information for the AID analysis. In addition, detailed metall
ography and fractography will be performed on 20 samples used in Phase II. This
work involved the determination of pearlite lamella size, pearlite colony size,
69
TABLE 16. SAMPLES SELECTED FOR ADDITIONAL TESTING
RailSampleNumber Category c: Mil Reason for Selection
004 I-II .61 .62 85% pearlite, high sulfur
010 I .63 .74 90% pearlite, low sulfur
014 I .78 .74 low sulfur
026 I .78 .94 low sulfur
027 III .78 .87 low ratio, TYS/TUS
037 II .72 .93 low sulfur
038 III .57 1.48 93% pearlite, low C, high Mn
040 I-II .58 .64 99% pearlite, low C, low Mn
045 III .65 .65 85/0 pearlite, low sulfur
058 III .83 .84 heat treated
TABLE 17. EXPERHlENTS IN PHASE III
Number ofSpecimen Tests per
Types Category
CT 2
CT, SEN 8
CT 11
Orientations TL, SL
R = -1. 0, 0.5
-40, +140 F
R = 0, 0.5
Frequency 2, 20 Hz
R = 0, 75 F
I-II, I-III
R = -1. 0, 0, 0.5
Orientation
Test Type Parameters
;
Variable Amplitude
Temperature
Mixed Mode
Threshold
SF 2
Bend 8
CT, SEN 2
CT, SEN -lQTotal 43
Total for 3 Categories 129
Check tests on io addi'tional samples listed in table
Surface Flaw
Stress Ratio
R = 0, Orientation LT and TL ~
Total Number of Tests 149
70
prior austenite grain size, inclusion content and fraction of various fracture
mechanisms. This will permit an exercise of complex correlation functions as
presented in reference 3.
The test matrix for Phase II is ~resented in Table 17. The top part
shows the detail testing to be performed on the three categories. The parameters
for investigation are indicated. All this information will be used in the devel
opment of the failure model. It requires 129 crack growth tests.
The bottom part of Table 17 shows the experiments to be performed on
the 10 additional samples listed in Table 16. Hence, a total of 149 experiments
will be performed in Phase II. All experimental data will be used for a further
evaluation with the AID program.
8. REFERENCES
1. Evans, P. R. V., Owen, N. B., and Hopkins, B. E., "Fatigue Crack Growth andSudden Fast Fracture in a Rail Steel", J. of the Iron and Steel Inst., June1970, pp 560-567.
2. Evans, P. R. V., Owen, N. B., and McCartney, L. N., "Mean Stress Effects onFatigue Crack Growth and Failure in a Rail Steel", Eng. Fracture Mechanics,~, 1974, pp 183-193.
3. Gladman, T., McIvor, I. D., and Pickering, F. B., "Some Aspects of the Structure-Property Relationships in High-Carbon Ferrite-Pearlite Steels", J. ofthe Iron and Steel Inst., Dec. 1972, pp 916-930.
4. Beevers, C. J., et al., "Some Considerations of the Influence of SubcriticalChange Growth During Fatigue Crack Propagation in Steel", Metal Science,i,l (1975), pp 119-126.
5. Cooke, R. J., and Beevers, C. J., "Low Fatigue Crack Propagation in PearliticSteels", Mat. Science Engineering, 11 (1974), pp 201-210.
6. Robinson, G. L., and Beevers, C. J., "The Effects of Load Ratio, InterstitialContent and Grain Rise on Low-Stress Fatigue-Crack Propagation ina -Titanium",Metal Science, l,i (1973), pp 153-159.
71/72
APPENDIX A
BASELINE CRACK-GROWTH DATA
The following tabulations present the crack length measurements and
associated cycle count for the 66 material samples received for evaluation in
this program. A total of 67 data sets are presented with a reproducibility
demons tration provided in duplicate tes ting of Specimen Nos. 027 and 027A. Speci
men No. 029A replaced Specimen No. 029 for which unanticipated crack growth to
failure occurred during an untended cycling period.
These crack growth data sets are presented sequentially in ascending
order of sample number. The t"irst measurement point represents the precrack length
on the specimen surface after crack initiation and generation out of the chevron
. notch. The final crack length represents the last crack length that could be rna,,"
itored by -visual following with a traveling microscope.
Note: Specimen 27 was cycled at 2 kips, Specimen 46 at 5 kips, Specime~
58 at 4.5 kips. All other specimens were cycled at 2.5 kips. R = 0 for all tests.
A-I
-... -_ .......•...-.- .._- ..•..•....S~E.C l"'~N vh11
CYCLECOUNT,
N, I< C
CRACI<L.Er~r,TH,
A, INCH
SPI:.CIMi:.N "i:l3-------_.-.-_.-..--_._-----.--_._.
evCLl::CUUNT,
N, KC
CRAC"Lt::~Jr" TI'i,A, INCH
--------_.-.-.-.-..-----~---_._-_.
C:YCL.~
l,;OUNT,~, ~c::
CI'ACKL.ENr.TH,A, INCH
.910
.Q471.~21
1 .111 b 01.1ft151.1J51.11:161.2381.31/)91.341171.3Y41. 4 761.5321.~Y2
1.6221.64181.7131. 7451.7t191. 841 3~.137
2.102
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CRACKLt:'NGTH,A, IN[;H
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Cjo(AL;1(
LENGTH,A; INCH
CYCLECOUNT,
N, KC
SPE.r:IHI:.N Qlk,14-.-.----.-....-.. -------_._....-.- -_.-.-----.._---~
.9.;31.~1l9
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A-24
APPENDIX B
REPORT OF INVENTIONS
After a diligent review of the work performed to generate the
aforementioned information, it is believed that no patentable innovation,
or invention was made.
However, this report does contain data on static strength and
fatigue-crack-propagation properties of rail steels presently in use in
the United States - data which is not widely available. Therefore, it is
considered that the data base generated here, although still limited, is
a unique compilation of importance for the improvement of safety and
performance of railroads in the USA.
220 copies
B-I/B-2
·,
