U.S. Department of 

Transportation

Federal Railroad 

Administration

Fatigue Crack Growth and Fracture Behavior of 

Bainitic Rail Steels

Office of Railroad

Policy and Development

Washington, DC 20590

DOT/FRA/ORD-11/17 Final Report

September 2011
 

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Technical Report

4. TITLE AND SUBTITLE

Fatigue Crack Growth and Fracture Behavior of Bainitic Rail Steels

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DTFR53-02-G-00021

6. AUTHOR(S)

Heshmat A. Aglan

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Tuskegee University

218 Foster Hall

Tuskegee, AL 36088

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U.S. Department of Transportation 

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DOT/FRA/ORD-11/17

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Program Manager: Mahmood Fateh

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This document is available to the public through the FRA Web site at http://www.fra.dot.gov.

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13. ABSTRACT (Maximum 200 words)

The microstructure–mechanical properties relationships, fracture toughness, fatigue crack growth and fracture surface morphology 

of J6 bainitic, manganese, and pearlitic rail steels were studied. Microstructure–mechanical properties correlation of the three rail 

steels under consideration revealed that the bainitic rail steel has the highest mechanical properties as compared with austenitic 

manganese steel (AMS) and pearlitic rail steels. AMS shows very few signs of being work hardened or toughened, which usually 

increases the mechanical properties of the material. In pearlite, the microstructure has more detailed features that contribute to its 

mechanical properties. The bainitic steel has more alloying elements that when mixed, create so many dislocations that the 

dislocations cannot move against each other. This is where bainitic steel derives its strength. Fracture toughness analysis (KIc) of 

the three rail steels, namely bainitic, manganese and pearlitic was performed. A valid KIc was determined for the pearlitic and 

bainitic steels based on the American Society of Testing and Materials (ASTM) E399 criteria. It was found that the pearlitic steel 

had an average KIc of 41 MPa √m and the bainitic steel had an average value of 52 MPa √m. The manganese steel displayed 

elastic–plastic behavior, with a dominant plastic component. This invalidated the criteria of standard ASTM E399 and therefore a 

KIc could not be calculated. An approximate value for the energy release rate (680 kJ/m2 ), based on ASTM standard E1820-01,

was calculated instead for the manganese steel. 

14. SUBJECT TERMS

bainitic, pearlitic, manganese rail steels, microstructure-mechanical property relationship, 

hardness, fracture toughness, fatigue crack growth

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57

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ii

Acknowledgments

The author wishes to acknowledge the financial support for this research project provided by the 

Federal Railroad Administration (FRA). The author wishes to thank Mr. Mahmood Fateh, 

FRA’s Technical Representative, for his guidance during the course of this research, and Mr. 

Gunars Spons, FRA’s Resident Engineer at the Transportation Technology Center (TTC) 

(Pueblo, CO) for his valuable support, including TTC field visits and the steel samples for the 

research. The author is also grateful to Dr. David Y. Jeong of the U.S. Department of 

Transportation Volpe Center (Cambridge, MA) for his input and insightful comments on the 

draft version of this report.
iii

METRIC/ENGLISH CONVERSION FACTORS

ENGLISH TO METRIC METRIC TO ENGLISH

LENGTH (APPROXIMATE) LENGTH (APPROXIMATE)

1 inch (in) = 2.5 centimeters (cm) 1 millimeter (mm) = 0.04 inch (in)

1 foot (ft) = 30 centimeters (cm) 1 centimeter (cm) = 0.4 inch (in)

1 yard (yd) = 0.9 meter (m) 1 meter (m) = 3.3 feet (ft)

1 mile (mi) = 1.6 kilometers (km) 1 meter (m) = 1.1 yards (yd)

1 kilometer (km) = 0.6 mile (mi)

AREA (APPROXIMATE) AREA (APPROXIMATE)

1 square inch (sq in, in2) = 6.5 square centimeters (cm2) 1 square centimeter (cm2) = 0.16 square inch (sq in, in2) 

1 square foot (sq ft, ft2) = 0.09 square meter (m2) 1 square meter (m2) = 1.2 square yards (sq yd, yd2) 

1 square yard (sq yd, yd2) = 0.8 square meter (m2) 1 square kilometer (km2) = 0.4 square mile (sq mi, mi2) 

1 square mile (sq mi, mi2) = 2.6 square kilometers (km2) 10,000 square meters (m2) = 1 hectare (ha) = 2.5 acres

1 acre = 0.4 hectare (he) = 4,000 square meters (m2) 

MASS - WEIGHT (APPROXIMATE) MASS - WEIGHT (APPROXIMATE)

1 ounce (oz) = 28 grams (gm) 1 gram (gm) = 0.036 ounce (oz)

1 pound (lb) = 0.45 kilogram (kg) 1 kilogram (kg) = 2.2 pounds (lb)

1 short ton = 2,000 pounds 

(lb)

= 0.9 tonne (t) 1 tonne (t) =

=

1,000 kilograms (kg)

1.1 short tons

VOLUME (APPROXIMATE) VOLUME (APPROXIMATE)

1 teaspoon (tsp) = 5 milliliters (ml) 1 milliliter (ml) = 0.03 fluid ounce (fl oz)

1 tablespoon (tbsp) = 15 milliliters (ml) 1 liter (l) = 2.1 pints (pt)

1 fluid ounce (fl oz) = 30 milliliters (ml) 1 liter (l) = 1.06 quarts (qt)

1 cup (c) = 0.24 liter (l) 1 liter (l) = 0.26 gallon (gal)

1 pint (pt) = 0.47 liter (l)

1 quart (qt) = 0.96 liter (l)

1 gallon (gal) = 3.8 liters (l)

1 cubic foot (cu ft, ft3) = 0.03 cubic meter (m3) 1 cubic meter (m3) = 36 cubic feet (cu ft, ft3) 

1 cubic yard (cu yd, yd3) = 0.76 cubic meter (m3) 1 cubic meter (m3) = 1.3 cubic yards (cu yd, yd3)

TEMPERATURE (EXACT) TEMPERATURE (EXACT)

[(x-32)(5/9)] °F = y °C [(9/5) y + 32] °C = x °F

QUICK INCH - CENTIMETER LENGTH CONVERSION

0 1 2 3 4 5

Inches

Centimeters 0 1 2 3 4 5 6 987 1110 1312

QUICK FAHRENHEIT - CELSIUS TEMPERATURE CONVERSIO

 -40° -22° -4° 14° 32° 50° 68° 86° 104° 122° 140° 158° 176° 194° 212°

 

°F

 °C -40° -30° -20° -10° 0° 10° 20° 30° 40° 50° 60° 70° 80° 90° 100°

For more exact and or other conversion factors, see NIST Miscellaneous Publication 286, Units of Weights and 

Measures. Price $2.50 SD Catalog No. C13 10286 Updated 6/17/98
iv

Contents

Executive Summary ........................................................................................................................ 1

1 Introduction ............................................................................................................................ 3

1.1 Chemistry of Rail Materials ............................................................................................. 5

1.2 Processing, Microstructure, and Heat Treatment of Premium Rail Steels ...................... 5

1.3 Microstructure and Heat Treatment Influence on the Mechanical Properties of 

Premium Rail Steels .................................................................................................... 6

1.4 Fatigue and Fracture Failure Behavior of Rail Steels ...................................................... 7

1.5 Scope ................................................................................................................................ 8

2 Materials and Testing Methods .............................................................................................. 9

2.1 Materials Background ...................................................................................................... 9

2.2 Specimen Preparation ...................................................................................................... 9

2.3 Mechanical Testing ........................................................................................................ 11

2.4 Microstructure and Fracture Surface Morphology Examination ................................... 11

3 Microstructure and Mechanical Properties Relationships ................................................... 13

3.1 Introduction .................................................................................................................... 13

3.2 Results and Discussion .................................................................................................. 13

4 Fracture Toughness .............................................................................................................. 19

4.1 Introduction .................................................................................................................... 19

4.2. Theoretical Considerations ............................................................................................ 19

4.3 Fracture Toughness Results ........................................................................................... 20

5 Fatigue Crack Growth .......................................................................................................... 26

5.1 Introduction .................................................................................................................... 26

5.2 Results and Discussion .................................................................................................. 27

6 Hardness, Fracture Toughness and Fatigue Damage Relationship ...................................... 37

6.1 Introduction .................................................................................................................... 37

6.2 Results and Discussion .................................................................................................. 37

7 Concluding Remarks ............................................................................................................ 39

8 References ............................................................................................................................ 41

Abbreviations and Acronyms ....................................................................................................... 48
v 

Illustrations

Figure 1. Drawing of Railhead and Locations of Typical Slices for Tests .................................. 10

Figure 2. Specimen Geometry ..................................................................................................... 10

Figure 3. Compact Tension Specimens ....................................................................................... 11

Figure 4. Stress–Strain Diagram of Manganese, Bainitic, and Pearlitic Steel ............................. 14

Figure 5. Optical Global Microstructural Features, at 100× of (a) Pearlitic Rail Steel, (b) 

Manganese Rail Steel, and (c) Bainitic Rail steel ................................................................. 15

Figure 6. Optical Microstructure of the Pearlitic Rail Steel at 1,000× ........................................ 16

Figure 7. Optical Microstructure of the AMS Rail Steel at 1,000× ............................................. 17

Figure 8. Optical Microstructure of the Bainitic Rail Steel at 1,000× ......................................... 18

Figure 9. Load vs. Load–Point Displacement Diagram of Three Bainitic (1⁄2 T) Compact 

Tension Specimens ............................................................................................................... 20

Figure 10. Load vs. Load–Point Displacement Diagram of Three Manganese (1⁄2 T) 

Compact Tension Specimens ................................................................................................ 22

Figure 11. Load vs. Load–Point Displacement Diagram of Three Pearlitic (1⁄2 T) Compact 

Tension Specimens ............................................................................................................... 24

Figure 12. Fatigue Crack Length, a, vs. the Number of Cycles, N .............................................. 28

Figure 13. Fatigue Crack Growth Rate, da/dN vs. Crack Length, a ............................................ 28

Figure 14. Potential Energy, P, vs. Crack Length, a .................................................................... 29

Figure 15. Energy Release Rate, J*, vs. the Fatigue Crack Length, a ......................................... 30

Figure 16. Crack Growth Rate, da/dN, vs. Energy Release Rate, J* ........................................... 31

Figure 17. Logarithmic plot of the Paris Law for the Bainitic, Manganese and Pearlitic Rail 

Steels ..................................................................................................................................... 32

Figure 18. An SEM Micrograph at 300× Taken from the Beginning of the Stable Crack 

Propagation Region of the Bainitic Rail Steel ...................................................................... 34

Figure 19. An SEM Micrograph at 300× Taken from the Beginning of the Stable Crack 

Propagation Region of the Manganese Rail Steel ................................................................. 34

Figure 20. An SEM Micrograph at 300× Taken from the Beginning of the Stable Crack 

Propagation Region of the Pearlitic Rail Steel, Showing a Ductile Fracture Mechanism 

Characterized by Ductile Tearing, Pulled-up Pearlite Lamella and Limited Microvoids 

Coalescence........................................................................................................................... 35

Figure 21. An SEM Micrograph at 1,000× Taken from the Middle of the Fast Crack 

Propagation Region for the Bainitic Rail Steel ..................................................................... 35

Figure 22. An SEM Micrograph at 1,000× Taken from the Middle of the Fast Crack 

Propagation Region for the Manganese Rail Steel Showing a Limited Number of 
vi 

Dimples, Oriented Tearing Flow Lines, and Ridges............................................................. 36

Figure 23. An SEM Micrograph at 1,000× Taken from the Middle of the Fast Crack 

Propagation Region for the Pearlitic Rail Steel, Showing Ridges, Pits, and Crevices 

along the Grain Boundaries Indicative of Quasi-Brittle Fatigue Failure .............................. 36
vii 

Tables

Table 1. Chemical Composition of the Bainitic, Manganese, and Pearlitic Rail Steel .................. 9

Table 2. Mechanical Properties of Pearlitic, AMS, and Bainitic Steel ........................................ 14

Table 3. Compact Tension Specimen Geometry and Test Results for Bainitic Steel.................. 21

Table 4. Compact Tension Specimen Geometry and Test Results for Manganese Steel ............ 23

Table 5. Compact Tension Specimen Geometry and Test Results for Pearlitic Steel ................. 25

Table 6. Average Values of Paris Law Constants........................................................................ 33

Table 7. Hardness Number for Leeb, Brinell, and Vickers Tests ................................................ 37
1

Executive Summary

The microstructure–mechanical properties relationships, fracture toughness, fatigue crack 

growth, and fracture surface morphology of J6 bainitic, manganese, and pearlitic rail steels were 

studied. Specimens were cut from the middle of each railhead along the longitudinal direction of 

the rail using electrical discharge machining. Fatigue crack propagation (FCP) tests were 

conducted under load control conditions using a servohydraulic Materials Testing System

(MTS). 

Microstructure–mechanical properties correlation of the three rail steels under consideration 

revealed that the bainitic rail steel has the highest mechanical properties as compared with

austenitic manganese steel (AMS) and pearlitic rail steels. Examination of the microstructural 

features reveals that bainitic steel has a more intricate structure than AMS and pearlitic steels. 

AMS shows very few signs of being “work hardened” or toughened, which usually increases the 

mechanical properties of the material. Mostly large grains are present in the microstructure of 

AMS. In pearlite, the microstructure has more detailed features that contribute to its mechanical 

properties. The bainitic steel has more alloying elements that, when mixed, create so many 

dislocations that the dislocations cannot move against one another. This is where bainitic steel

derives its strength. 

Fracture toughness analysis (KIc) of the three rail steels, namely bainitic, manganese, and 

pearlitic, was performed. Compact tension specimens (1⁄2 T) were machined and fatigue 

precracked according to American Society of Testing and Materials (ASTM) standard E399. 

Multiple identical specimens from each material were tested using an 810 MTS with a 

22-kilopound (kip) load cell. A valid KIc was determined for the pearlitic and bainitic steels, 

based on the ASTM E399 criteria. It was found that the pearlitic steel had an average KIc of 

41 Megapascal-square root meter (MPa √m) and the bainitic steel had an average value of 

52 MPa √m. The manganese steel displayed elastic–plastic behavior, with a dominant plastic 

component. This invalidated the criteria of standard ASTM E399, and therefore, a KIc could not 

be calculated. An approximate value for the energy release rate, based on ASTM standard 

E1820-01, was calculated instead for the manganese steel. An average value of 680 kJ/m2 was 

obtained for the manganese steel. 

Fatigue propagation tests, under load control, were performed on the three rail steels. Multiple 

identical specimens (at least three) were tested, and the average value of the FCP kinetics was 

taken. A simple form of the fatigue power law was used to rank the fatigue crack growth 

kinetics of the three materials. The results show that the bainitic steel has the highest fatigue 

damage tolerance as represented by the fatigue lifetime, FCP kinetics, and fatigue fracture 

surface morphological features. The fatigue damage tolerance of the manganese steel was 

superior to that of the pearlitic steel. 

Fracture surface analysis of the fatigue failed specimens revealed the various mechanisms by 

which the bainitic steel acquired its superior resistance to fatigue crack growth. The bainitic rail 

steel displayed more ductile fracture features such as tearing and extensive disoriented ridge 

formation during the stable crack propagation process than the manganese and pearlitic steels. 

These features were responsible for the crack deceleration and indicate a considerably high-
2

energy consuming process associated with the crack propagation of the bainitic steel. Ductile 

flow-like tearing in the direction of crack propagation with feather-like pattern is observed on the 

fatigue fracture surface of the manganese steel in the stable crack propagation region. Pulled-up 

pearlitic lamella, limited microcracks, and microvoid coalescence were found on the fracture 

surface of the pearlitic rail steel during the stable crack process. The unstable crack propagation 

region of the bainitic steel exhibits both large and small dimples indicative of high resistance to 

material separation. The manganese steel, however, shows a limited number of dimples 

compared with the bainitic steel. Evidence of oriented tearing flow lines and ridges are apparent 

on its fracture surface at this high magnification. In contrast, cleavage and intergranular 

separation are associated with the unstable crack region of the pearlitic steel.
3

1. Introduction

Two types of steels are generally used for railroad applications: pearlitic and AMS. Bainitic 

steel has recently been considered as a candidate material for railroad applications. Pearlitic 

steel accounts for most of the steel tonnage that is produced today [1]. AMS alloy steel that 

exhibits a distinct combination of properties such as high toughness, resistance to wear, and 

heavy impact loading is commonly used in railway frogs [2]. Use of bainitic steel is now being 

explored because of the unique properties that it offers. 

Pearlitic steel has a variety of alloying elements that include 0.72–0.8 percent carbon, 

0.60–1.25 percent manganese, 0.10–0.60 percent silicon, 0.25–0.50 percent chromium, 

0.25 percent nickel, with trace amounts of vanadium, molybdenum, sulfur, and phosphorus. 

Carbon is the main alloying element that has a variety of effects over a temperature range.

Manganese is used in steel to reduce oxides and to improve the ductility and is often used in 

place of nickel to reduce the cost of the steel. Nickel and manganese both exhibit similar 

behavior when added to steel. Nickel increases the strength, ductility, and toughness of the steel. 

Silicon and vanadium reduce oxidation and serve as hardening agents. Tungsten promotes age 

hardening, refines the grain size, and increases the corrosion resistance and heat resistance of 

steel [3, 4]. Pearlitic steels gain their strength from the grains of pearlite within the 

microstructure. The alternating arrangement of lamellae that consists of cementite and ferrite 

increases the mechanical properties of the pearlitic steel. The microstructure consists of bodycentered cubic (BCC) ferrite and BCC cementite. There are two main types of pearlitic steel: 

fine pearlite and course pearlite [5]. 

A typical AMS will contain 10–14 percent manganese, 0.95–1.4 percent carbon, and 

0.3–1.00 percent silicon. Obtaining fully austenitic steels requires careful balancing of chemical 

composition. The most visible microstructural features of this single-phase alloy are the 

austenite grain boundaries. Austenite is also a rather soft, low-strength phase; however, workhardening techniques produce substantial strengthening [6, 7]. The mechanical properties of 

AMS vary with both carbon and manganese contents. The grain size in austenitic manganese 

steel castings can vary widely and depends on the amount of superheat in the liquid metal during 

casting. The crystal structure of austenite is face-centered cubic (FCC) as compared with ferrite 

in bainitic and pearlitic, which has a BCC lattice. An FCC alloy has certain desirable 

characteristics; for example, it has low-temperature toughness and excellent weldability. 

Because of their FCC crystal structures, these alloys remain ductile irrespective of temperature or 

strain rate unless detrimental phase changes occur and are usually corrosion resistant [8]. 

Bainitic steel has a larger amount of alloying elements compared with pearlitic and AMS steels. 

Those elements are 0.23 percent carbon, 1.93 percent manganese, 1.96 percent silicon, 

1.84 percent chromium, with less than 0.15 percent of nickel, copper, columbium, aluminum, 

vanadium, tungsten, boron, sulfur, and phosphorus. Because most bainitic steels have low 

carbon content, it increases the weldability of the steel. Also bainitic steel has a higher strength 

level and good ductility without the use of heat treatment. Boron increases the hardening of the 

steel; however, when used with molybdenum, it increases the tensile strength in both AMS and 

bainitic steel. Bainitic steels have large amounts of lath-shaped aggregates of ferrite and 

cementite within the microstructure. There are two main types of bainitic steel: upper and lower 

bainite. Upper bainite is formed in the temperature range of 400–550°C, contains parallel lath-
4

shape constituents of ferrite, and has a feathery appearance when viewed with optical 

microscopy. Lower bainite is formed in the region of 250–400°C and appears to have a more 

acicular morphology than upper bainite. Although the laths are similar and have the same width 

of 0.5 micrometers, laths of ferrite in lower bainite are much broader and more disoriented than 

upper bainite. It has been found that lower bainite has higher strength and toughness than upper 

bainite. The main assumption for this is the nucleation of the ferrite laths at the austenite grain 

boundaries as seen in upper bainite, but there is nucleation occurring within the grains 

themselves, producing additional dislocations. Bainitic steels get their strength from the 

dislocations found in the structure of the ferrite [9–12].

Bainitic rails appear to be a very promising new development in rail track technology that first 

appeared in the mid-1990s. Only a limited amount of research has so far been conducted on 

bainitic steels. Comparison between pearlitic, manganese, and bainitic rail steels [13–16] 

revealed superior flaking resistance, good wear resistance, and excellent weldability of bainitic 

rail steels. A detailed comparison of the wear characteristics of bainitic and pearlitic steel has 

been conducted, of which the results showed that the bainitic steels are potential candidates for 

wear-resistant rail application [17]. Su and Clayton [14] gave a review of experimental research 

on wheel-rail contact, pointing out that the use of bainitic rail steel remained a fertile area for 

research, and started their studies on rolling contact fatigue of pearlitic and bainitic steels. They 

found that rolling contact fatigue cracks are associated with plastic deformation and embryo 

cracks can be produced even under dry running conditions. 

Studies on the failure behavior of rails have demonstrated that FCP and fracture is one of the 

major causes of derailment and other severe accidents [18]. The limitations of fracture 

toughness and fatigue strength criteria are obvious because of the diversified composition of rails 

and the complicated live and residual stress conditions. Efforts to study the fatigue behavior of 

rails have been made [18, 19]. For example, Orringer et al. [20–22] studied the FCP of detail 

fracture in rails. More recently, Glowacki and Kuziak [23] have investigated the effect of 

coupled thermal-mechanical processes on the evolution of microstructure in rails. Headhardening behavior and rolling contact fatigue have also been studied [24, 25]. Fracture 

toughness and fatigue strength have been applied as criteria for characterization of the fatigue 

damage resistance of railway rails [26]. Another factor must be addressed that data on the 

fatigue properties of rails is considerably limited, which makes it difficult to further understand 

the fatigue behavior and to assess the fracture resistance of railway track alloys under cyclic 

loading conditions. Thus, it is necessary to acquire more fatigue data to study the related fracture 

and failure mechanisms and to propose new criterion of damage tolerance for rail steel materials. 

This is especially true for new materials, such as bainitic steel, if they are to be accepted in 

widespread application. So far, studies on the FCP behavior of bainitic rail steels have not been 

reported in the open literature.

To understand the relationships between the chemistry, manufacturing, and postprocessing and 

the mechanical properties of modern rails, a comprehensive review of such aspects is given. To 

obtain better track stability, longer rail life, reduced maintenance costs, reduced rail failure, and 

minimum interruptions to traffic, scrupulous materials selection, manufacturing procedures 

optimization and innovative track design have been proposed for a long time [27–32]. Among 

these various aspects, materials selection is the most basic one to achieve the above-mentioned 

goals. The following review will provide some information about the rail steel chemistry.
5

1.1 Chemistry of Rail Materials

Various metallurgical materials have been used for rail manufacturing. Cast metals and copper 

alloys are among those materials that have been used for a long time [33, 34]. However, copper 

alloys are not used as commonly as steel. Steel has been a universal material of choice for the 

manufacturing of rails ever since the beginning of the railway industry, primarily because of its 

cost-effectiveness. Although different manufacturing procedures such as quenching, hot rolling,

etc., can change the mechanical properties of rails [35], the chemical composition of the steels 

remains the most essential factor that alters of the properties of rails [36, 37]. Medium carbon 

steel has been used for most of the sections of ordinary rails including upper table, lower table, 

web, foot, and head [38–41]. Most of the rails are rolled from open-hearth basic steel and 

contain from 0.50 to 0.60 percent by weight of carbon, from 0.90 to 1.20 percent manganese, 

from 0.10 to 0.30 percent silicon, and not more than 0.06 percent of sulfur and phosphorus [38]. 

Each element fulfills a different function in the steels. Iron is the basic element, whereas carbon 

serves as the most significant strengthening element. Silicon is a deoxidizing element. It 

increases the fluidity of the molten metal and condenses the alloy, thus producing solid ingots 

with fine grains. Phosphorus and sulfur harden the steel but decrease the ductility at much 

higher rates than any other element in the steels. Thus, the combination of the last two elements 

cause a brittle effect, and their presence is not desired in most cases. 

Alloy steels are generally used for heavy-duty rails in places where high-stress concentration 

exists such as points, crossings, and sharp curves [42]. They are also used to make some special 

parts for rail stabilization, including, bearing-plates, fish-plates, spikes, fang-bolts, hook-bolts, 

Stewart’s keys, chairs, open coiled keys, etc. Most of the commonly used alloy rail steels can be 

divided into three classes. These are medium manganese steel, high manganese steel, and 

chromium steel. Typically, medium manganese steel contains 1.10–1.40 weight percent (wt%)

manganese and 0.45–0.55 wt% carbon. The content of silicon may be higher than 0.05 wt%, 

whereas the content of phosphorus and sulfur is generally lower than 0.05 wt%. High 

manganese steel contains higher manganese and higher carbon than the medium manganese 

steel. The composition of high manganese steel is manganese from 10.50 to 15.00 percent and 

carbon from 0.95 to 1.35 percent. Manganese hardens the steel, and provides more resistance to 

wear. However, manganese is expensive to use. In addition, the higher the content of 

manganese, the more difficult it is to manufacture the rails, because these factors imposed some 

limitations in the wide use of high manganese steel. Chromium steel has a typical composition 

of carbon from 0.42 to 0.53 percent, chromium from 0.75 to 1.00 percent, manganese from 

0.65 to 0.85 percent. Chromium steels are even more expensive than manganese steels.

Chromium steels are only used in rail joints or places where intensive traffic exists. The high 

cost of chromium steels may be partially offset with the reduction in maintenance and the 

increase in service lifetime. 

1.2 Processing, Microstructure, and Heat Treatment of Premium Rail Steels

The microstructure of the premium rail steel is mainly pearlitic [43]. This provides the rail steel 

with excellent wear resistance. The premium steel can be made by open hearth, basic oxygen, or 

electric furnace, followed by a continuous casting process into hot topped ingots. To remove any 

undesired segregation, rolling has to be used in the manufacturing processes. The as-rolled rails 

will be fully heat treated or on-line hardened to achieve a head-hardening effect. Such 

procedures can further increase the wear resistance of the railhead made of the premium rail 
6

steel. Following casting and hot rolling, one of the most important procedures in manufacturing 

premium rails is the head-hardening heat treatment. Such a treatment process can change the 

properties of the rail steel significantly. The head-hardening heat treatment is an on-line airquenching process. It was found that it increases the hardness and yield strength of the rail steel 

significantly. It can also reduce strain softening in later use and thus can improve the 

deformation and wear resistance of the rail under service loading conditions. It was reported that 

heat treatment can increase the wear life of rails by at least 1,000 times [44, 45]. Beynon et al.

[46] studied three pearlitic rail steels to investigate the head-hardening effect on the wear 

behavior of the steels. Comparative studies on several eutectoid steels were made. It was 

concluded that the head-hardened grade eutectoid steel had the highest resistance to rolling 

contact fatigue. The naturally hard eutectoid steel was the next best, whereas the lowest strength 

steel gave the worst wear performance. These researchers also studied the crack initiation 

behavior of these eutectoid steels. It was found that loading level is important in the formation 

of different shapes of cracks. Crack networks were formed in the specimens tested at nominal 

contact stresses of 1,200 and 1,500 MPa, whereas isolated cracks were produced in the 

specimens tested at 1,800 MPa.

1.3 Microstructure and Heat Treatment Influence on the Mechanical Properties 

of Premium Rail Steels

Although the contact fatigue resistance of rail steels can be increased significantly through headhardening heat treatment such a heat treatment process can also exert adverse effects on 

tension/tension fatigue properties of the rail steels as reported by Hellier and Merati [47]. In 

their work, fatigue tests were conducted on single-edge–notched rectangular bar specimens 

machined from the head of a hardened heavy haul railway rail, manufactured using an ingot 

route. An increasing load technique was used for threshold determination with a small positive 

stress ratio R. The tests resulted in a fatigue threshold of 11.4 MPa √m for R = 0.11, which is 

lower than the established fatigue threshold for standard carbon rail steel of approximately 

14 MPa √m. In shear deformation dominated fatigue, the heat treatment caused an increase in 

the fatigue damage tolerance [48]. Rail rolling contact fatigue studies have been investigated,

based on the performance of naturally hard and head-hardened rails in track by Muster et al. [25]. 

Rolling contact fatigue is reduced substantially by the use of harder rails. Fatigue defects 

occurred in the form of head splitting on the high rails of the curves; the phenomenon was much 

more pronounced for the non-heat-treated steel (naturally hard). Spalling defects also occurred 

in the case of the naturally hard steel. The naturally hard steel exceeds its ductility limit earlier 

because of pronounced cold deformation so that plastic shear deformations and ultimately 

cracking and spalling occur. On the contrary, the head-hardened rails prove far more resistant to 

contact fatigue damage because of their higher yield point. Cold deformation is much lower, so 

crack initiation becomes more difficult. In view of fatigue failure controlled by wear, the service 

life of head-hardened rails can therefore be expected to be much higher than that of the naturally 

hard rails.

Head-hardening heat treatment can also change the fracture toughness of the rail steels. 

Subsequently, the fracture and failure behavior of the rails can be altered. The relationship 

between fracture toughness and the failure behavior of rail steels is a very important aspect in the 

evaluation of the performance of rail steels. Singh et al. [49] presented the results on fracture 

toughness of standard carbon rail steel, wear-resistant rail steel, and two high-strength rail steels. 
7

It has been found that, in contrast, higher wear resistance is usually achieved at a higher hardness 

of the rail steel; however, higher hardness can produce lower fracture toughness and higher 

fatigue crack growth rate. The quality of rails was evaluated by both the basic tensile properties 

and fracture toughness [50]. This was to establish the correlations between mechanical 

properties and fracture resistance as well as to calculate the critical flaw sizes inside rails. 

During air quenching of the steels, phase transformation from austenite to pearlite occurs [51–

54]. Because of the difference in cooling speed, the crystallization and grain growth kinetics of 

pearlite in different locations inside the railhead vary. The steel can be strengthened by solid 

solution, precipitation, increasing dislocation density, and pearlite volume fraction and grainrefining [55]. The microstructural change increases the hardness and tensile strength as often 

found in high carbon steel [56]. Microstructural effects, especially the grain size effect, have 

long been the focus in various metallic materials [57–59]. The microstructural effect on the 

tensile behavior of a ferrite-pearlite C-Mn steel has been studied by Hussain and De Los Rios 

[60]. This steel has a composition that is very similar to the rail steel. The ferrite phase in the 

direction of maximum shear stress has been identified as the preferable site for crack nucleation 

under tensile stress. Similar results have been reported by Nomura [61]. The ferrite precipitated 

along the austenite grain boundary is regarded as small cracks in medium carbon ferrite pearlite 

steels with fine prior austenite grain size. Thus, increase in carbon or alloy element contents 

such as manganese and vanadium contents raises the strength and fracture toughness through 

increasing the pearlite volume fraction and refining the ferrite grain size. Rosenberg and Kovove 

[62] investigated the influence of grain size on brittle fracture of medium carbon steel. The 

results indicated that resistance against brittle fracture evaluated by standard Charpy V-notch test 

in comparison with dynamic fracture toughness may give contradictory information about 

influence of microstructure.

1.4 Fatigue and Fracture Failure Behavior of Rail Steels

Various defects are normally the origins of fatigue failure. Efforts have been made to detect the 

formation of defects in rails by metallurgical and nondestructive methodologies [63–72]. 

Intensive studies on the wear and protection of rails have been performed in recent years [73, 

74]. Wolfe, Spiegelberg, and Evangelisti [75] have used exploratory metallurgical evaluation of 

worn rails. Rail wear limits, based on rail strength investigations, have been estimated by Jeong, 

Tang, and Orringer [76]. Surface modification has been proposed to mitigate the wear and to 

prevent the formation of surface cracks on the rails [77, 78]. Laser modification [76] and 

diamond-like surface coating [77] have been applied to reduce the chance of failure.

Studies on the failure behavior of rails have demonstrated that FCP and fracture are two of the 

major reasons of derailment and other severe accidents [78]. Efforts to study the fatigue 

behavior of rails have been made [18, 78]. For example, Orringer [19–21, 67] studied the life of 

detail fracture in rails. More recently, Glowacki, and Kuziak [23] have investigated the effect of 

coupled thermal-mechanical processes on the evolution of microstructure in rails. Headhardening behavior and rolling contact fatigue have been studied also [23, 79]. 

Fracture-toughness and fatigue-strength have been applied as criteria for characterization of the 

fatigue damage resistance of railway rails [25]. The limitations of fracture-toughness and 

fatigue-strength criteria are obvious because of the diversified composition of rails and the 

complicated life and residual stresses conditions. Another factor must be addressed in that data 

on the fatigue properties of rails is considerably limited, which makes it difficult to further 
8

understand the fatigue behavior and to assess the fracture resistance of railway steel alloys under 

cyclic loading conditions. Thus, it is necessary to acquire more fatigue data, to study the related 

fracture and failure mechanism, and to propose new damage tolerance criteria for rail steels. 

1.5 Scope

In this report, the microstructure–properties relationships, fatigue crack growth, and fracture 

surface morphology of bainitic rail steel, manganese rail steel, and conventional premium 

pearlitic rail steel are studied. Microdamage formation and evolution during crack movement, 

particularly on the fracture surface will be evaluated. The mechanical properties will be 

correlated with the microstructure of each material. The fatigue crack growth of the materials 

will be assessed and correlated with the fracture surface morphological features of the three rail 

steels. A simple form of the Paris equation will be used to rank the three materials. 
9

2. Materials and Testing Methods

2.1 Materials Background

The materials used in the present study were a J6 bainitic rail steel and a pearlitic rail steel that 

was provided by the Transportation Technology Center, Inc. (Pueblo, CO), and a manganese rail 

steel that was provided by Nortrak of Birmingham, AL. No reported details on the 

manufacturing of the J6 bainitic rail are available, but it is believed that no heat treatment was 

done. The chemical compositions of the three steels are given in Table 1.

Table 1. Chemical Composition of the Bainitic, Manganese, and Pearlitic Rail Steel

Element C S P Si Cr Ni Mn Cu Mo Ti Al V W B

% by weight

Bainitic 

Steel 0.23 0.008 0.012 1.96 1.84 0.14 1.93 0.13 0.43 0.034 0.045 0.007 0.01 0.0047

Pearlite 

Steel

0.72–

0.78 0.037 0.035

0.1–

0.6

0.25

–0.5 0.25 0.6–12.5 - 0.1 - - 0.03–0.05 - -

Manganese 

Steel 0.9–1.05 - 0.07

1 

max - - 11.5–14 - - - - - - -

2.2 Specimen Preparation

Specimens were prepared from their specific railhead by using electrical discharge machining 

(EDM). For fatigue tests, the railhead was sliced into thin layers for the preparation of test 

specimens; the schematic representation can be seen in Figure 1. In the present study, the middle 

layer at a depth of approximately 20 millimeters (mm) from the top of the head was chosen for 

the fatigue tests.

The specimens have nominal dimensions of 152.3 mm in length, 100 mm in gauge length, 

25.4 mm in width, and 2 mm in thickness. At the center of one free edge of the specimens, a 60°

notch of 6.2 mm depth was introduced by using EDM. The notch depth to sample width ratio 

(a/W) was 0.24. The sample geometries for the unnotched and notched specimens are shown in 

Figure 2. For fracture toughness, 1⁄2 T were cut from railheads along the direction of the rail by 

using EDM. The geometry of the compact tension specimens is shown in Figure 3 along with its 

nominal dimensions.
10

Figure 1. Drawing of Railhead and Locations of Typical Slices for Tests

Figure 2. Specimen Geometry

Tension Fatigue
11

Figure 3. Compact Tension Specimens

2.3 Mechanical Testing

2.3.1 Static Tensile Tests

Static tensile experiments on unnotched specimens were performed using an 810 MTS equipped 

with a 100 kilonewton (kN) load cell. This was carried out under displacement control condition 

at a crosshead speed of 0.02 mm per second (s). The specimens were gripped between two 

hydraulic wedge grips typed 647.10A-01. The gage length was 100 mm. The static test results,

based on the unnotched specimens, were used to calculate the ultimate strength of the rail steels.

2.3.2 Fracture Toughness

1⁄2 T were machined and fatigue precracked according ASTM standard E399. Multiple identical 

specimens (at least three) from each material were tested using an 810 MTS with a 22-kilopound

load cell. The rate of loading was 20 kN per minute (min).

2.3.3 Fatigue Tests

FCP tests were conducted at room temperature of approximately 25°C under load control 

conditions using a sinusoidal waveform and a frequency of 1 hertz. The maximum stress was 

200 MPa for the bainitic and pearlitic steels and 97 MPa for manganese steel, and the ratio of 

minimum stress to maximum stress was 0.1. The crack length at various intervals at a specific 

number of cycles was recorded during the tests. A video camera with a zoom lens was used to 

view the crack tip region, to measure the crack length, and to capture the damage associated with 

the crack growth. A total of three samples were tested from each material, and data from each 

sample was used for the FCP analysis and the fatigue damage species examination.

2.4 Microstructure and Fracture Surface Morphology Examination

The microstructures of the bainitic, manganese and the pearlitic rail steels were studied using an 

optical microscope. Various microstructural features of each rail steel were identified. The 

W = 22.86 mm

ao = 8.89 mm

ap = pre-crack length

t = 11.43 mm

B = 9.14 mm

L2 = 27.43 mm

L1 = 13 97 mm

 

W

a

L1

ap ao

t

B

L2
12

fracture surfaces for all specimens were examined using a Hitachi S-2150 Scanning Electron 

Microscope operated at acceleration voltage of 20 kilovolts. Typical micrographs revealing the 

fracture surface morphology were taken using a PCI-Image Management System.
13

3. Microstructure and Mechanical Properties Relationships

3.1 Introduction

The materials used in the present work were bainitic, manganese, and pearlitic rail steels. The 

chemical composition of all three rail steels is given in Table 1. Small samples were sectioned 

from the regular rectangular specimens and mounted on an epoxy mold of 1.25 inches in 

diameter. The samples were polished using both a BETA/VECTOR and the MINIMET 1000 

Polisher/Grinder according to ASTM A128. The samples were then etched in 2-percent Nital 

solution for 60 s. The optical microstructures were taken with an OLYMPUS GX51 Inverted 

System Metallurgical Microscope with a Pixelink C-Mount digital camera and PAX-IT version 

5.4a Windows-based imaging system. Rectangular specimens with a length of 150 mm, a width 

of 25.4 mm, and a thickness of 2 mm were sectioned from the middle of each rail were used for 

the mechanical properties evaluation. 

Air quenching is normally done on the top layer of pearlitic steel rails [22]. Static tensile tests of 

the specimens were performed using a MTS 810 servohydraulic machine equipped with a 

100-kilonewton load cell and controlled by the MTS TestStar II hardware and software. 

Displacement control conditions with a crosshead speed of 0.02 mm/s were used to carry out the 

static tests. The gage length for the specimens was 100 mm. Force and displacement data was 

acquired in real-time mode using the PC. 

3.2 Results and Discussion

The stress–strain relationships of the three rail steels are shown in Figure 4, and a summary of 

their mechanical properties is given in Table 2. Bainitic steel has the highest yield strength, 

ultimate strength, and strain to failure as compared with both the pearlitic and AMS steels. 

Optical global microstructural features, at 100×, for the three materials are shown in Figure 5. 

The microstructure of the bainitic steel is seen to be very intricate. The high mechanical 

properties found in the bainitic steel correspond to the low amount of carbon and the increased 

amount of other alloying elements in the chemical composition. An increase in the amount of 

alloying elements in the pearlitic rail steel as compared with the manganese steel has made a 

contribution to the increased mechanical properties, compared with the AMS steel. Other 

processing methods such as heat treatment can play a part in the strength of the pearlitic steel. 

The microstructure of the AMS is an indication of a soft material, with no work hardening 

having taken place. If manganese has not been work hardened, the material will have low 

mechanical properties. Also, AMS has a relatively low amount of alloying elements in its 

chemical composition. The manganese steel has only four to six alloying elements. The 

mechanical properties of AMS is in the same range as those found in the as cast properties. 
14

Figure 4. Stress–Strain Diagram of Manganese, Bainitic, and Pearlitic Steel

Table 2. Mechanical Properties of Pearlitic, AMS, and Bainitic Steel

Material

Yield 

Strength

(Offset=0.2%)

(MPa) 

Ultimate 

Strength

(MPa) 

Failure 

Strain

(%) 

Pearlitic 700 1100 11

AMS 400 575 8

Bainitic 1100 1550 12.5

0

200

400

600

800

1000

1200

1400

1600

0 2 4 6 8 10 12 14

Strain (%)

Stress (MPa)

manganese

Bainitic

Pearlitic
15

Figure 5. Optical Global Microstructural Features, at 100× of (a) Pearlitic Rail Steel, (b) 

Manganese Rail Steel, and (c) Bainitic Rail steel

(a) Pearlitic Rail Steel

(b) Manganese Rail Steel

(c) Bainitic Rail Steel
16

pearlite

Lamella 

structure of 

ferrite and 

cementite

3.2.1 Microstructure of Pearlitic Rail Steel

The microstructure of pearlitic rail steel at 1,000× is shown in Figure 6. This steel has a lower 

amount of alloying elements than bainite but more alloying elements than the manganese. Each 

pearlitic grain has its own orientation. The lamellae of ferrite and cementite, with pearlite 

spectral, are seen throughout the grains. The lamellae are orientated in the same direction within 

each grain. At this higher magnification, the spacing of lamellae is clear. The mechanical 

properties of the pearlitic steel are determined by the interlayer spacing of the lamellae of the 

ferrite and cementite. The formation of ferrite and cementite is clearly shown at grain 

boundaries, with some parent austenite remaining in the grains. 

Figure 6. Optical Microstructure of the Pearlitic Rail Steel at 1,000× 

3.2.2 Microstructure of Manganese Rail Steel

Typical microstructural features of AMS are shown in Figure 7 at 1,000× magnification. The 

grain boundaries seen in the AMS are very clear and well defined. Austenite is formed at higher 

temperatures than pearlite and bainite; therefore, more dislocations are present in AMS. Pitting 

and other inclusions can form dislocation loops in the microstructure. Pitting, which is often 

seen in the microstructure of AMS, is seen in Figure 7 on the left side. The dislocation loops 

formed can cause a decrease in the mechanical properties. Twinning, a phenomenon that also 

provides strength in AMS, is seen in the larger grain of this micrograph. This is an as-cast 

manganese sample that has not been heat treated or work hardened. The microstructure is 

representative of a single-phase alloy of an austenite matrix with cementite and a few patches of 

pearlite present. The patches of pearlite often provide additional strength for the material. 

Inhomogeneity of the grains that exists in the microstructure has resulted in the segregation of 

the alloying elements and carbide precipitation. Since manganese is a carbide-forming element, 

a sufficient amount is present to reduce the mechanical properties in the material.
17

Figure 7. Optical Microstructure of the AMS Rail Steel at 1,000× 

3.2.3 Microstructure of Bainitic Rail Steel

The microstructure of bainitic rail steel at 1,000× is shown in Figure 8. The grain boundaries are 

thin and clear in the bainitic sample. Primary laths formed at the grain boundary and secondary 

laths were formed from the primary laths within the grains. The cementite, which also forms 

parallel groups, did not fully precipitate in the ferrite laths. The laths are of parallel orientation 

within the grains, depending on the formation of the ferrite. This microstructure resembles a 

mixture of upper and lower bainite. In Figure 8, the bright parts with some dark spectral within 

are ferrite and upper bainite. The dark portion as indicated by the arrows is lower bainite. The 

intricate structure within the bainitic steel is caused by the low amount of carbon, the large 

amount of alloying elements, and the wide range of cooling rates used to process the bainitic 

steel.

Twinning 

inside the 

grain

Parent austenite grain

pitting
18

Figure 8. Optical Microstructure of the Bainitic Rail Steel at 1,000× 

Thin grain 

boundary lines 

Ferrite and 

upper 

bainite

Lower 

bainite
19

4. Fracture Toughness

4.1 Introduction

Compact tension specimens were subjected to fatigue precracking to introduce a sharp notch for 

fracture testing. An intended precrack of approximately 2.5 mm was introduced into each 

specimen to have a target a/W of 0.5. For the purposes of calculation, the actual crack length 

was measured from the fracture surface following static failure. Observations made during the 

precracking are as follows:

• All of the pearlitic steel samples precracked in the normal fashion with relatively straight 

precracks. 

• In the bainitic steel, the first attempt at precracking resulted in a crack growing from an 

angle out of the notch tip. This was attributed to residual stresses at the machined notch 

tip. The samples were then stress relieved at 850°F for 30 min. The remainder of the 

samples was precracked in a normal fashion with relatively straight precracks.

• Initially, the manganese samples could not be precracked. This was due to suspected 

residual stresses in the notch tip as well as blunting, because of the difficulties in 

machining. Manganese alloys are known for work hardening, and machining would have 

caused this at the crack tip. Therefore, the manganese steel samples were stress relieved 

at 850°F for 30 min. After this treatment, precracking was achieved. However, 

controlling the crack growth proved to be very difficult, and a consistent total crack 

length was not achieved among the manganese samples. 

4.2. Theoretical Considerations

The general expression of stress intensity factor for a compact tension specimen is:

(1)

where P is the load, B is the thickness, W is the distance from the center of the loading holes to 

the edge of the specimen, a is the total crack length (initial plus precrack), and f(a/W) is a 

geometrical correction factor. The geometrical correction factor, f(a/W), in eq. 1 can be 

expressed as:

(2)

( ) 1 W

a f

B W

P K =

3 / 2

2 3 4

(1 ( ))

(2 ( ))[0.886 4.64( ) 13.32( ) 14.72( ) 5.6( ) ]

( )

W

a

W

a

W

a

W

a

W

a

W

a

W

a f

−

+ + − + −

=
20

4.3 Fracture Toughness Results

4.3.1 Bainitic Steel

The bainitic steel demonstrated elastic behavior and cleavage fracture; therefore, ASTM 

Standard E 399 was used for the analysis. The load versus load–point displacement behavior of 

three specimens is shown in Figure 9. The data for all three samples tested along with their 

calculated values of KIc, based on eq. 1 is presented in Table 3. The average value of KIc was 

found to be 52 MPa √m. 

Figure 9. Load vs. Load–Point Displacement Diagram of Three Bainitic (1⁄2 T) Compact 

Tension Specimens

0

1

2

3

4

5

6

7

0 0.25 0.5 0.75 1 1.25 1.5

Displacement (mm)

Load (kN)

Linear (Bainitic 3)

Linear (Bainitic 2)

Linear (Bainitic 1)

B3

B2

B1
21

Table 3. Compact Tension Specimen Geometry and Test Results for Bainitic Steel

Sample W

(mm)

ao

(mm)

ap

(mm)

a

(mm)

(a/W) f(a/W) Pm

(kN)

Pq

(kN)

B

(mm)

Kq

(MPa m 1/2)

KIc

MPa m 1/2

#1 22.4 8.89 3.0 11.89 0.531 10.66 6.6 6.6 9.14 51.43 51.43

#2 22.4 8.89 3.5 12.39 0.553 11.48 6.3 6.3 9.14 52.87 52.87

#3 22.6 8.89 3.4 12.29 0.544 11.13 6.4 6.4 9.14 51.44 51.44

Average value of KIc . 52

W = the distance from the center of the loading holes to the edge of the specimen

ao = initial machined notch length

ap = fatigue precrack

a = total crack length

Pm = maximum load

Pq = load at which Kq is calculated

B = thickness

Kq = stress intensity factor calculated from eq. 1

KIc = mode I fracture toughness

To validate the calculated value of K as a true KIc fracture toughness, the following conditions 

must be met. 

a and B ≥ 2.5 (KQ/σy)

2 (3)

Pmax/PQ < 1.1 (4)

For the bainitic steel, the value of 2.5 (KQ/σy)

2 = 2.5 (52/1,100)2 × 1,000 = 2.24 mm. This value 

is less than B, which is 9.14 mm and less than a, which is approximately 12 mm; therefore, the 

condition of eq. 3 is met. In addition, Pmax equals PQ so the condition of eq. 4 is met. Therefore,

these fracture tests for the bainitic steel yield a valid value of KIc according to ASTM E399. 

4.3.2 Manganese Steel

The manganese steel demonstrated elastic–plastic behavior with stable ductile crack propagation. 

The residual strength versus load–point displacement for three specimens is shown in Figure 10. 

One of the reasons for the difference in the behavior is that these samples all had different 

precrack lengths, especially sample 2. As discussed earlier, fatigue precracking of manganese 

steel posed some difficulties in controlling the final target crack length. It can also be seen in 

Figure 10 that the clearly observed linearity in both the pearlitic and bainitic steels is not 

displayed here. 
22

Figure 10. Load vs. Load–Point Displacement Diagram of Three Manganese (1⁄2 T) 

Compact Tension Specimens

Because of the nonlinearity, the value of Pmax/PQ in all the manganese specimens is much higher 

than 1.1. This does not validate one of the requirements for a valid KIc test according to ASTM 

standard E399. Nevertheless, a K value was calculated, based on the maximum load sustained 

by each specimen. These values are given in Table 4 in a similar fashion as given for the 

pearlitic and bainitic steels. The average K value was used to check the condition, a and B ≥ 2.5 

(KQ/σy)

2

. For the manganese steel, the value of 2.5 (KQ/σy)

2 = 2.5(109/400)2 is 185 mm. This 

also clearly invalidates any calculations based on E399. The full data for the samples tested 

along with their calculated values is presented in Table 4. 

0

2

4

6

8

10

12

14

16

0.0 1.0 2.0 3.0 4.0 5.0

LPD (mm)

Load (kN)

M1, a/W =.57

M2, a/W =.76

M4, a/W =.56
23

Table 4. Compact Tension Specimen Geometry and Test Results for Manganese Steel

Sample W

(mm) 

ao

(mm) 

ap

(mm) 

a

(mm) 

(a/W) f(a/W) Pm

(kN) 

Pq

(kN) 

B

(mm) 

KI

(MPa m 1/2) 

A

(kN-mm) 

JI

(kJ/m2) 

#1 22.3 8.89 3.85 12.7 0.571 12.24 10.7 9.14 96 28.0 712

#2 22.4 8.89 8.2 17.1 0.763 31.43 4.8 9.14 110 17.5 766

#3 22.5 8.89 Load was removed after crack jumped, sample 

was not fractured for future observations. 9.14

#4 22.4 8.89 3.7 12.6 0.562 11.85 13.8 9.14 120 23.0 572

Average value 109 683

W = the distance from the center of the loading holes to the edge of the specimen

ao = initial machined notch length

ap = fatigue precrack

a = total crack length

Pm = maximum load

Pq = load at which Kq is calculated

B = thickness

Kq = stress intensity factor calculated from eq. 1

KI = mode I fracture toughness 

Energy Release Rate, aWB )(

A J I − = η 

W

aW )(522.0 2 − η += for compact tension specimens 

A = area under the load vs. displacement curve

To understand the fracture behavior of the manganese steel the fracture toughness JI was 

calculated based ASTM E1820-01. This is based on the area under the load–displacement 

curves in Figure 10. It should be mentioned that the fracture toughness JI is given as the sum of 

the elastic and plastic portions.

I EL PL += JJJ (5) 

From the behavior of the manganese steel, Figure 10, it is reasonable to assume that JEL is very 

small in comparison with JPL, thus J ~ JPL. 
24

)(

2

aWB

JJ I PL − == η (6) 

The values of JI for the three manganese samples were calculated and are given in Table 4, as a 

rough approximation due to the peculiar behavior of this manganese steel. On the basis of this, 

an average value of JI of about 680 kJ/m2

 was obtained. This value is similar to that reported for 

an A285 carbon steel [23, 80].

4.3.3 Pearlitic Steel

The pearlitic steel demonstrated elastic behavior and cleavage fracture; therefore, ASTM 

Standard E 399 was used for the analysis. The load versus load–point displacement curves can 

be seen in Figure 11. The data for all three samples along with their calculated values of KIc,

based on eq. 1, is presented in Table 5. The average KIc value was found to be 41 MPa √m. 

Figure 11. Load vs. Load–Point Displacement Diagram of Three Pearlitic (1⁄2 T) Compact 

Tension Specimens

0

1

2

3

4

5

6

7

0 0.25 0.5 0.75 1 1.25 1.5

LPD (mm)

Load (kN)

Linear (P3)

Linear (P2)

Linear (P1)

P3

P2

P1
25

Table 5. Compact Tension Specimen Geometry and Test Results for Pearlitic Steel

Sample W (mm)

ao

(mm)

ap

(mm)

a

(mm)

(a/W) f(a/W) Pm

kN

Pq

kN

B

(mm)

Kq

MPa m 1/2

KIc

MPa m 1/2

#1 22.4 8.89 3.2 12.09 0.540 10.98 5.7 5.7 9.14 - 45.75

#2 22.3 8.89 3.3 12.19 0.547 11.25 4.7 4.7 9.14 - 38.74

#3 22.4 8.89 3.7 12.19 0.562 11.85 4.3 4.3 9.14 - 37.25 

Average value of KIc . 41

W = the distance from the center of the loading holes to the edge of the specimen

ao = initial machined notch length

ap = fatigue precrack

a = total crack length

Pm = maximum load

Pq = load at which Kq is calculated

B = thickness

Kq = stress intensity factor calculated from eq. 1

KIc = mode I fracture toughness 

To validate the calculated value of K as a true KIc fracture toughness the following conditions 

must be met. 

a and B ≥ 2.5 (KQ/σy)

2 (7)

Pmax/PQ < 1.1 (8) 

For the pearlitic steel, the value of 2.5 (KQ/σy)

2

 = 2.5 (41/700)2 × 1,000 equals 8.6 mm. This 

value is less than B, which is 9.14 mm and less than a, which is approximately 12 mm; therefore, 

the condition of eq. 7 is met. In addition, Pmax equals PQ, so the condition of eq. 8 is met. 

Therefore, these fracture tests for the pearlitic steel yield a valid value of KIc according to ASTM 

E399. 
26

5. Fatigue Crack Growth

5.1 Introduction

In existing fatigue models, stress dominated crack growth behavior has been considered to 

describe the FCP kinetics of metallic materials under cyclic loading. Classical FCP laws express 

cyclic crack extension in terms of some function of the applied load or stress field in the region 

near the crack tip. Parameters such as the stress intensity factor, the stress intensity factor range, 

and the energy release rate are commonly used as correlative tools. The equation proposed by 

Paris and Erdogan [81] has gained the widest acceptance. In this law, the crack growth rate is 

related to the stress intensity factor range, which can be expressed as:

m KA

dN

da ∆= )( .......................................................(9) 

where a is the crack length, N is the corresponding number of cycles, A and m are parameters 

that depend on the material properties, and ΔK is the stress intensity factor range. The Paris law 

is valid for describing the commonly observed linearity of the FCP rate within the stable crack 

propagation region. The remaining problem of how to predict FCP rates at both low and high 

values of the stress intensity factor has led to the modification of this fatigue model [81–86].

Jeong, Tang, and Orringer [85, 87] have proposed damage-tolerant strategies for rail steels, 

based on fracture toughness analysis and FCP kinetics. In their approach, stress intensity factor 

consideration was applied first to determine the critical defect sizes of detail fracture in rails. 

Then, a modified Paris equation was used for calculating the time for a detail fracture to grow 

from a given size to the critical size at which rail failure occurs. The stress intensity factor was 

calculated using the following formula [87].

aaaMTaaaM

a

b

I MTaK s R σσ T G B πσ

π

)].().(),()().[().(. 2 ),( = 1 ++ ............. (10) 

where the defect size, a, is expressed in terms of the semimajor axis length of an elliptical flaw, 

Ms stands for the elliptical shape factor of the defect, M1 accounts for the boundary condition of 

the defect, MG reflects the local stress gradients, σR is the residual stress, σT is the thermal stress, 

and σB is the stress from the bending of the rail. The growth of the crack was assumed to obey 

the following modified Paris law as proposed by Walker [86]:

n

m

R

K A

dN

da

)1(

)(

−

∆ = ...................................................... (11) 

where R is the ratio of the minimum to maximum stress, n is an empirical constant. Under a 

simplified loading spectrum, assuming the four wheel loads produce five stress cycles (five pairs 

of minimum and maximum stresses), the safe-growth time, N, for a defect to propagate from an 

initial defect length to the critical length can be calculated from the following [87]:

m

a

a

i

i n

i m aaF

da

TaR

Ta A

N

c

o ]).([

*

)],(1[

)],([ .

1

5

1

)(

∫ )(

∑= −

∆ = σ

..................................... (12) 
27

where ao is the initial crack length, ac is the critical crack length, and F(a) is a geometrical 

function that depends on the defect configuration. It can be expressed as: 

)().(. 2 )( 1 aM

a

b MaF S

π = ............................................. (13) 

The determination of the stress intensity factor range in the above approach is considerably 

complex, and the calculation of the integral is also very difficult. Furthermore, the defect size 

and shape are highly irregular in actual cases. Considering the inaccuracy of field measurement 

of the defect size, the calculated safe-growth lifetime may vary by several orders of magnitude. 

The risk of rail failure becomes very difficult to be predicted. 

5.2 Results and Discussion

In the current fatigue studies, the maximum fatigue stress used was 200 MPa for the bainitic and 

pearlitic materials. The rationale was based on approximately 40 percent of the yield strength of 

the pearlitic steel. The value of 40 percent represents a design safety factor of 2.5. A maximum 

stress of 97 MPa was used for the fatigue testing of manganese steel. This was calculated based 

on 20 percent of the maximum stress of the manganese steel.

5.2.1 Fatigue Crack Growth Rates

The average crack length, a, versus the number of cycles, N, for the bainitic, manganese, and 

pearlitic rail steel is shown in Figure 12. It can be seen from Figure 12 that the total fatigue 

lifetime of the bainitic steel is much higher than that of the pearlitic steel. This is based on the 

average of three macroscopically identical specimens from each material. The total average 

fatigue lifetime for the bainitic steel is approximately 78,000 cycles, while that for pearlitic is 

approximately 11,000. The fatigue lifetime of manganese steel is approximately 79,000, which 

is slightly higher than that of the bainitic steel. This difference can be explained by a much 

smaller maximum load applied to the manganese steel, which is approximately 50 percent less 

than the bainitic and pearlitic. It is also shown that both initiation lifetime and propagation 

lifetime for the bainitic steel is higher than that for the pearlitic. Although direct comparison 

between the lifetime of manganese steel and both bainitic and pearlitic cannot be made here, 

the data will be compared later on a normalized plot of FCP kinetics and energy release rate.

The slopes of the curves in Figure 12 are taken as the average crack growth rate at each crack 

length. The relationships between the crack growth rate, da/dN, and crack length, a, for all 

three rail steels are shown in Figure 13. The curves shown in this figure demonstrate crack 

growth kinetics of a sigmoidal feature, which can be divided into three stages. The first stage 

is the crack initiation or the threshold. In the second stage, a stable crack is observed. The 

curve approached asymptotic values in the third stage (critical stage). In all three stages, the 

crack growth rate varied. It is obvious that the bainitic rail steel displays a slower crack 

growth rate at any crack length than the other two rail steels. At stage I the crack growth rate 

of manganese steel was almost the same as pearlitic steel and much higher than bainitic steel. 

As the crack length is increased, as can be seen in stage II of Figure 13, the crack growth rate 

of manganese is decreased, and by the end of stage II and in stage III, the crack growth rate of 

manganese is close to that of bainitic steel and is much lower than that of pearlitic steel.
28

Figure 12. Fatigue Crack Length, a, vs. the Number of Cycles, N 

Figure 13. Fatigue Crack Growth Rate, da/dN vs. Crack Length, a 

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

0 10000 20000 30000 40000 50000 60000 70000 80000

Number of Cycles, N (cycles)

Crack Length, a (mm)

Manganese

Bainite

Pearlite

1.00E-08

1.00E-07

1.00E-06

1.00E-05

1.00E-04

0 2 4 6 8 10 12 14 16 18 20 22

Crack Length, a (mm)

Crack Growth Rate, da/dN (m/cycle)

Manganese

Bainite

Pearlite
29

5.2.2 The Potential Energy and the Energy Release Rate

The potential energy, P, was calculated from the hysteresis loops recorded at intervals of 

number of cycles. It is the area above the unloading curve at each crack length. On this basis, 

the relationship between the potential energy and the fatigue crack length, a, was established. 

Figure 14 shows the calculated results of the potential energy of the bainitic, manganese, and 

the pearlitic steel specimens. It can be seen from Figure 14 that the potential energy, at the 

critical crack length of the bainitic steel, is higher than that of the manganese and pearlitic 

steel. It is 1.35 J for the bainitic steel, 0.95 J for manganese steel, and 0.8 J for the pearlitic 

steel. Also, from the graph, it can be seen that there is a significant difference between the 

pearlitic steel when compared with the manganese and the bainitic steel. 

Figure 14. Potential Energy, P, vs. Crack Length, a 

The relationship between the potential energy and the fatigue crack length was used to 

determine the energy release rate J* based on the following equation:

a

P

B

J

∂

∂ = 1 * ................................................... (14) 

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

1.3

1.4

0 5 10 15 20

Crack Length, a (mm)

Potential Energry, P (Joules)

Manganese

Bainite

Pearlite
30

0

5

10

15

20

25

30

35

40

45

50

0 2 4 6 8 10 12 14 16 18 20 22

Crack Length, a (mm)

Energy Release Rate, J* (kJ/m2)

Pearlitic Bainitic Manganese

where P is the potential energy, a is the crack length, and B is the specimen thickness. 

Figure 15 illustrates the average energy release rate, J*, as a function of the crack length, a, for 

the bainitic, manganese, and pearlitic rail steels. The value of J* increases with increasing 

crack length, a. The critical value of J* for the bainitic steel is approximately 40 kJ/m2, 

whereas that of manganese is approximately 36 kJ/m2 and for the pearlitic steel is 

approximately 13 kJ/m2. The ratio between the critical crack length and specimen width ( W

ac ) 

is higher for bainitic and manganese steels than that for pearlitic steel. The value of 

W

ac is 

approximately 0.8 for both bainitic and manganese steels and 0.63 and pearlitic steel.

Figure 15. Energy Release Rate, J*, vs. the Fatigue Crack Length, a

5.2.3 FCP Kinetics

The crack growth rate versus the energy release rate J* for bainitic, manganese, and pearlitic rail 

steel are shown in Figure 16. The crack deceleration in the case of the bainitic and manganese 

rail steel started after a value of J* of approximately 10 kJ/m2. This is indicative of material 

damage ahead of the crack tip. It can also be seen from Figure 16 that the first stage of FCP 

kinetics is well developed in the bainitic and manganese steel, while that for the pearlitic is less 

pronounced. Under the same J*, the crack growth rate of pearlitic steel is higher than that of 

bainitic and lower than manganese steel, which means that bainitic rail steel has highest 

resistance to FCP in stage I. 
31

Figure 16. Crack Growth Rate, da/dN, vs. Energy Release Rate, J*

The second stage of FCP kinetics is also different in all three materials. The bainitic and 

manganese steels exhibit approximately the same rate of change in crack growth rate. These 

steel samples have a higher rate of change in crack growth rate than that of pearlitic steel as 

indicated by the lower slope of the curves in Figure 16. The pearlitic steel, does not exhibit such 

a change in crack growth rate and a clear transition from the first to the second stage of crack 

propagation kinetics is not shown. It is the second stage of FCP that determines the resistance to 

FCP of the material. During this stage, for corresponding values of J*, the manganese steel has a 

significantly higher crack growth rate than that of bainitic steel. Also, the bainitic and 

manganese steels dissipate about the same amount of energy, which is more energy than that of 

the pearlitic steel. This takes more energy from the fracture process and thus reduces the crack 

growth rate. Fracture surface analysis of fatigue-failed specimens should reveal the mechanisms 

by which the bainitic steel acquires its superior resistance to fatigue crack growth. 

In the third stage (unstable crack propagation), the pearlitic steel displays an approximate 

asymptotic behavior. This indicates an avalanche like fracture after the second stage. The 

bainitic and manganese steel, however, shows a more gradual transition, which indicates a 

graceful failure-like behavior.

1.00E-08

1.00E-07

1.00E-06

1.00E-05

1.00E-04

0 10 20 30 40

Energy Release Rate, J* (kJ/m2)

Crack Growth Rate, da/dN (m/cycle)

Bainitic

Pearlitic

Manganese
32

To calculate the Paris law constants, A and m as given in eq. 9, it is necessary to first substitute 

J* for K and take the log of both sides. This results in eq. 15. 

log log A mlog(J*) dN

da  = + 

 



 (15)

The data in Figure 16 for the bainitic and pearlitic rail steels is plotted in Figure 17 in the form of 

eq. 15 to obtain the values of A and m. The average values of A and m for the two materials are 

given in Table 6. As can be seen, m for the bainitic steel is slightly lower than the manganese 

steel, but it is approximately one-fifth of the value for the pearlitic steel. The value of A for the 

bainitic steel is almost the same for manganese steel, and both steels are over 3,000 times greater 

than that of the pearlitic steel. However, the influence of the value of m in the above equation is 

much greater than the influence of the constant A because of its role as an exponent rather than 

as a multiplicative factor. The lower value of m reflects the considerably lower crack growth 

rates at the same value of J* for the bainitic steel and manganese, even with the much higher 

value of A. 

Figure 17. Logarithmic Plot of the Paris Law for the Bainitic, Manganese, and Pearlitic 

Rail Steels

y = 1.1304x - 8.0031

y = 5.4423x - 11.517

y = 1.816x - 7.7525

-9

-8

-7

-6

-5

-4

-3

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

log(J*)

log(da/dN) 

Manganese

Pearlite

Bainite
33

Table 6. Average Values of Paris Law Constants

Material A m

Bainitic rail steel 1.0 × 10–8 1.13

Pearlitic rail steel 3.04 × 10–12 5.44

Manganese 1.68 × 10–8 1.82

5.2.4 Fatigue Fracture Surface Morphology

Fracture surface morphology examination was performed on typical fatigue-failed specimens of 

each material to identify the fatigue damage species. The fracture surface can mainly be divided 

into two regions according to the morphological features. Region I is the stable crack 

propagation region, which contains stages I and II in the FCP kinetics. Region II represents the 

unstable crack propagation process, which represents stage III of the FCP kinetics. It should be 

mentioned that differentiating stages I from II is difficult since both are stable events and stage I 

is normally associated with a very short crack. 

Micrographs (300×) taken from the stable crack propagation region, ahead of the notch tip, are 

shown in Figure 18 to Figure 23 for the bainitic, manganese, and pearlitic rail steels,

respectively. Ductile tearing and extensive disoriented ridge formation are associated with the 

bainitic steel (Figure 18). Ductile flow-like tearing in the direction of crack propagation with 

feather-like pattern is observed on the fatigue fracture surface of the manganese steel

(Figure 19). Pulled-up pearlite lamella, limited microcracks, and microvoid coalescence can be 

found in the pearlitic steel (Figure 20). In general, these features reflect the crack deceleration 

and indicate a considerably high-energy–consuming process associated with the crack 

propagation. It appears to be the case that the bainitic rail steel displays a greater number of 

randomly oriented ductile fracture features in the first region than the manganese or pearlitic 

steels. As it was stated earlier, the creation of these fracture species consumes more energy and 

therefore reduces the crack growth rate in the stable crack propagation region of the bainitic 

steel. 

The unstable crack propagation region was also examined using SEM. At 1,000×, the bainitic 

steel (Figure 21) exhibits both large and small dimples, indicative of high resistance to material 

separation. The manganese steel, however, shows a limited number of dimples compared with 

the bainitic steel. Evidence of oriented tearing flow lines and ridges are apparent on its fracture 

surface at this high magnification (Figure 22). In contrast, cleavage and intergranular separation 

are associated with the fast crack region in the pearlitic steel (Figure 23). It is obvious from the 

fracture surface morphological features displayed in these micrographs that the bainitic rail steel 

offers more resistance to material separation than the manganese or pearlitic rail steels. 

Manganese displays more ductile features than the pearlitic steel, indicative of its superior 

fatigue separation performance to pearlitic.
34

Figure 18. An SEM Micrograph at 300× Taken from the Beginning of the Stable Crack 

Propagation Region of the Bainitic Rail Steel (Ductile fracture mechanisms characterized by 

extensive tearing and ridge formation are seen.)

Figure 19. An SEM Micrograph at 300× Taken from the Beginning of the Stable Crack 

Propagation Region of the Manganese Rail Steel (Ductile flow-like tearing in the direction of crack 

propagation. This produces an extended feather looking pattern seen at 300×.)

Crack propagation direction
35

Figure 20. An SEM Micrograph at 300× Taken from the Beginning of the Stable Crack 

Propagation Region of the Pearlitic Rail Steel, Showing a Ductile Fracture Mechanism 

Characterized by Ductile Tearing, Pulled-Up Pearlite Lamella and Limited Microvoids 

Coalescence

Figure 21. An SEM Micrograph at 1,000× Taken from the Middle of the Fast Crack 

Propagation Region for the Bainitic Rail Steel (Clear ductile fracture dimples and dense tearing 

ridges are seen.)

Crack propagation direction
36

Figure 22. An SEM Micrograph at 1,000× Taken from the Middle of the Fast Crack 

Propagation Region for the Manganese Rail Steel Showing a Limited Number of Dimples, 

Oriented Tearing Flow Lines, and Ridges 

Figure 23. An SEM Micrograph at 1,000× Taken from the Middle of the Fast Crack 

Propagation Region for the Pearlitic Rail Steel, Showing Ridges, Pits, and Crevices along 

the Grain Boundaries Indicative of Quasi-Brittle Fatigue Failure

Crack propagation direction
37

a

b

V

V HL 1000 =

6. Hardness, Fracture Toughness and Fatigue Damage 

Relationship

6.1 Introduction

All three samples (pearlitic, bainitic, and manganese) were tested for their hardness. A total of 

five tests were done on each sample and the average of the tests was recorded. The hardness 

tester used was the Time TH150 Hardness Tester, which was developed by Dr. Leeb. This type 

of tester is known as a scleroscope, it measures the height of rebound of an indenter dropped 

onto a material. In details, the indenter is dropped on the material being tested and the impact 

and rebound velocity is measured when the indenter is 1 mm above the surface. The Leeb 

number is then calculated by taking the ratio of the rebound velocity to the impact velocity. 

(16)

where Vb and Va are the rebound and impact velocity, respectively. The same instrument is 

capable and was used to measure the Brinell and the Vickers number.

6.2 Results and Discussion

The results obtained from the test are shown in the Table 7. For the Leeb Hardness test (HLC), it 

can be seen that the bainitic rail steel has the highest value of 611, followed by the pearlitic rail 

steel with a value of 549, and then the manganese with value of 357. Both the Brinell hardness 

(HB) test and Vickers hardness (HV) test shows a similar trend in the hardness of the materials. 

It can also be seen that the Brinell and Vickers test have hardness values that are very similar.

Table 7. Hardness Number for Leeb, Brinell, and Vickers Tests

 Test Method

Materials (Leeb Test) HLC (Brinell Test) HB Vickers Test (HV)

Bainitic Rail Steel 611 290 299

Pearlitic Rail Steel 549 199 201

Manganese 357 93 90

6.2.1 Hardness Comparison to Stress–Strain Relationship

The hardness can be compared with the stress–strain curves of the specific material to gain a 

better understanding of the properties of the materials. The stress–strain diagram of the bainitic, 

pearlitic, and manganese rail steel is shown in Figure 4, with the corresponding yield strength, 

ultimate strength, and failure strain percent in Table 2. As shown in the stress–strain diagram

(Figure 4), the bainitic rail steel has the highest yield strength, ultimate strength, and failure 

strain, followed by the pearlitic then the manganese. This trend is similar to that of the hardness 

test, which shows that the hardness of the material plays an important role in the yield and 
38

ultimate strength of a material. The bainitic rail steel has the smallest grain size and the highest 

hardness number, and manganese rail steel has the largest grain size and the smallest hardness 

number. The pearlitic rail steel, in contrast, had both a grain size and a hardness number that fell 

between the bainitic and manganese rail steel. This shows that as the grain size decreases the 

hardness increases, which is also known as the Hall–Petch effect.

6.2.2 Hardness Comparison to Fracture Toughness

When the hardness number to the fracture toughness of the rail steels is compared, it can be seen 

that there is a similar trend between bainitic rail steel and pearlitic rail steel. The hardness 

number is higher for bainitic rail steel than for the pearlitic rail steel; these results are shown in 

Table 7. The same trend is seen for the fracture toughness of bainitic and pearlitic rail steel that 

has an elastic behavior. When the ASTM standard E399 is used, the fracture toughness of 

bainitic rail steel was found to be 52 MPa √m and that of pearlitic rail steel was 41 MPa √m; 

these data can be seen in Table 7 and Table 5, respectively. As for the manganese steel, a severe 

nonlinearity does not validate one of the requirements for a valid KIc test according to ASTM 

standard E399; therefore, ASTM standard E1820-01 was used. On this basis of E1820-01, a 

value of the energy release rate J was calculated. The manganese rail steel under consideration 

displays a dominant plastic deformation, although it is lower than that of the bainitic and pearlitic 

rail steels. Thus, there is no correlation between hardness and fracture toughness when the 

material displays considerable plastic deformation.

6.2.3 Hardness Comparison to Fatigue Crack Growth

There is no clear relationship between the hardness and the fatigue crack growth of the different 

rail steels. However, from Table 7, it is seen that the bainitic steel has the highest hardness, and 

from Table 6, it has the smallest Paris Law constant m, and its Paris Law constant A is between 

pearlitic and manganese. Although manganese has the lowest hardness number, its Paris Law 

constant m is between that of bainitic and pearlitic rail steel, and it has the highest Paris Law 

constant A. As for the pearlitic steel, its hardness is between that of bainitic and manganese 

steel, but it has the largest Paris Law constant m and the smallest Paris Law constant A. The 

influence of the Paris Law constant m has a much greater influence than the constant A because 

of its role as an exponent rather than as a multiplicative factor. On the basis of the current study, 

it appears that there is no direct correlation between hardness and the fatigue crack growth 

behavior of rail steels.
39

7. Concluding Remarks

The bainitic rail steel has the highest mechanical properties as compared to AMS and pearlitic 

rail steels. Examination of the microstructural features reveals that bainitic steel has a more 

intricate structure than AMS and pearlitic. AMS shows very few signs of being work hardened 

or toughened, which usually increases the mechanical properties of the material. As the number 

of alloying elements increase, the microstructure of the steel becomes more complex, resulting in 

the increase of mechanical properties and fracture toughness.

To further investigate the fracture toughness, fracture mechanics tests were conducted on three 

steels, namely pearlitic, bainitic and manganese using a (1⁄2 T) compact tension specimens. At 

least three specimens from each material were tested. It is concluded that:

• The pearlitic steel obeyed the linear elastic fracture mechanics theory with an average 

KIc of 41 MPa √m, based on ASTM standard E399.

• The bainitic steel obeyed the linear elastic fracture mechanics theory with an average 

KIc of 52 MPa √m, based on ASTM standard E399.

• The manganese steel behaved in an elastic plastic manner with dominant plastic 

deformation. ASTM standard E399 could not be invoked because of such behavior. 

Therefore, ASTM standard E1820-01 was used to determine an approximate value of 

the energy release rate JI for the manganese steel. This average value was found to be 

680 kJ/m2

. Difficulties were encountered in both machining and fatigue precracking 

the manganese compact tension specimens. This has caused residual stresses that 

have affected the procedures for producing a consistent initial fatigue precrack. 

The fatigue behavior was investigated by performing FCP analysis of J6 bainitic, manganese,

and pearlitic rail steels. It was found that the bainitic rail steel has the highest initiation and 

propagation lifetimes, followed by the manganese steel then the pearlitic steel. The average total 

fatigue lifetime of the bainitic steel specimens is approximately seven times that for the pearlitic, 

tested under the same conditions. The critical value of the energy release rate J*, obtained from 

the hysteresis loops during fatigue testing, was found to be approximately 40, 36, and 

approximately 13 kJ/m2 for the bainitic, manganese, and pearlitic rail steels, respectively. This 

occurred at a critical crack length to specimen width ratio of 0.80 for bainitic and manganese 

steels and 0.63 for pearlitic steel. It was found that the crack growth rate for the bainitic steel is 

lower than that of the pearlitic and manganese steel over the entire range of the energy release 

rate. Stage I for manganese steel showed a higher crack growth rate than pearlitic steel. In 

stages II and III the gradient of the crack growth rate decreased significantly for the manganese 

steel and was much lower than that of the pearlitic steel. The bainitic and manganese steel 

exhibits a higher rate of decelerating the crack growth rate in the second stage, as indicated by 

the lower slope of the FCP curve in comparison with the pearlitic steel. The fatigue crack 

growth of the rail steel was expressed in the form of a power law, m (da / dN) = A(J*) , where A 

and m are empirical constants. It was found that the value of m for the bainitic and manganese 

steel was approximately the same and they were about one fifth of the value for the pearlitic 

steel, reflecting the considerably lower crack growth rate at the same value of J* for the bainitic 

and manganese steel. This indicates the superior resistance of bainitic and manganese rail steel 

to FCP (i.e., higher fatigue damage tolerance).
40

According to the morphological features, the fatigue fracture surface can mainly be divided into 

two regions. Region I is the stable crack propagation region, which contains stages I and II in 

the FCP kinetics. Region II represents the unstable crack propagation, which is stage III of FCP 

kinetics. The bainitic rail steel displays more ductile fracture features in the first region than the 

pearlitic steel. Ductile tearing and extensive ridge formation are associated with the bainitic steel 

in this region. These features reflect the crack deceleration and indicate a considerably high 

energy consuming process associated with the crack propagation of the bainitic steel. Pulled-up 

pearlite lamella, limited microcracks and microvoid coalescence were found on the fracture 

surface of the pearlitic rail steel in the first region. The unstable crack propagation region of the 

bainitic steel exhibits both large and small dimples indicative of high resistance to material 

separation. In contrast, cleavage and intergranular separation are associated with the fast crack 

region in the pearlitic rail steel. 

In regard to the relationship between hardness, fracture toughness and fatigue crack growth 

behavior, it has been concluded that:

• The bainitic rail steel has the highest hardness number followed by the pearlitic then 

the manganese rail steel, this is the same trend for the yield strength and ultimate 

strength.

• The smaller the grain size the higher the hardness number.

• For bainitic and pearlitic rail steel that possess elastic behavior, the higher the fracture 

toughness of the material the higher the hardness. But for manganese rail steel that 

has a nonlinear elastic–plastic behavior, the fracture toughness cannot be correlated to 

the hardness. This is due to the dominant plastic deformation properties of the 

manganese steel.

• There is no clear relationship between the hardness of the rail steels and fatigue crack 

growth behavior.
41

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Abbreviations and Acronyms

1⁄2 T compact tension specimen

a/W notch depth to sample width ratio

AMS austenitic manganese steel

ASTM American Society of Testing and Materials

BCC body-centered cubic

BLE Brotherhood of Locomotive Engineers

EDM electrical discharge machining

FCC face-centered cubic

FCP fatigue crack propagation

HB Brinell hardness

HLC Leeb Hardness test

HV Vickers hardness

KIc fracture toughness analysis

kN kilonewton

min minute(s)

mm milliliter(s)

MPa √m megapascal-sqaure root meter

s second(s)

SEM scanning electron microscopy

wt% weight percent