This document analyzes the fatigue behavior of two high-strength steels, HR590 and HR590DP. Low-cycle fatigue testing was conducted to characterize the mechanical properties of each steel and quantify their strain-life relationships. The precipitation strengthened HR590 showed more resistance to plastic deformation than the dual phase HR590DP. Future work is recommended to further study dislocation behavior, establish high-cycle fatigue properties, and test notched specimens and full components under dynamic loading.

1. Analysis of Fatigue Behavior, Fatigue Damage and Fatigue Fracture of Two High-Strength Steels
Charles G. Lester IV
Ph: 404-576-5921
Chuck.LesterIV@gmail.com
10/12/2011
Overview
Overview .................................................................................................................................... 1
Personal Background .................................................................................................................. 1
Advanced High-Strength Steels (AHSS) ..................................................................................... 2
Automotive Applications ............................................................................................................ 2
Steels Characterized in Fatigue Testing ....................................................................................... 3
Experimental Method.................................................................................................................. 5
Experimental Results .................................................................................................................. 6
Summary .................................................................................................................................. 10
Future Work ............................................................................................................................. 10
Personal Background
 B.S. Mechanical Engineering – Clarkson University
 B.S. Interdisciplinary Engineering and Management – Clarkson University
 M.S. Materials Science and Engineering – Georgia Institute of Technology
 Experience as a full-time employee managing a laboratory that tested construction
materials for code compliance and product development
 Experience as an intern running fatigue tests and analyzing fatigue data for a steel
manufacturer
 Experience running electric and hydraulic universal test frames
 Career objective is to combine knowledge from various degrees and perform research
focused on the mechanical behavior of materials
 Future goal is to broaden knowledge base to other materials used in structural
applications (e.g. FCC, HCP, composites)
1

2. Analysis of Fatigue Behavior, Fatigue Damage and Fatigue Fracture of Two High-Strength Steels
Advanced High-Strength Steels (AHSS)
 Used in automotive applications due to improved properties over conventional high-
strength steels
o Better formability to create complex shapes
o Better weldability
o Improved dent resistance
 Objective is to maintain strength with minimal losses in ductility (i.e. increase toughness)
by optimizing the microstructure
 Increase in toughness potentially provides superior fatigue resistance to conventional
high strength steels, however other factors need to be considered, such as
o The accommodation of strain within the microstructure
o Interfacial energy at grain boundaries and interfaces
o Dislocation motion and interactions
 By reducing the gauge thickness and improving the cross-section, reductions in overall
component weight can be realized. Weight reductions therefore require replacing
conventional high-strength steels with more ductile AHSS to maintain fatigue resistance.
Automotive Applications
 Grade is tailored to applications based on hardness, tensile strength, formability,
weldability and fatigue properties
o Tailored by precipitation hardening, grain refinement, work hardening, solid
solution hardening, bake hardening, etc.
 For example, the fatigue properties of an automotive wheel are more critical than the
fatigue properties of a door, therefore different microstructures should be considered for
each application
2

3. Analysis of Fatigue Behavior, Fatigue Damage and Fatigue Fracture of Two High-Strength Steels
Steels Characterized in Fatigue Testing
• HR590 • HR590DP, Dual Phase
– Continuous Cast, Hot-Rolled – Continuous Cast, Hot-Rolled
– 3.2mm thick sheet – 3.2mm thick sheet
– Precipitation strengthened ferrite – Martensite strengthened ferrite
matrix matrix
– Tensile Strength Grade: 590MPa – Tensile Strength Grade: 590MPa
– Average Yield Strength: 570MPa – Average Yield Strength: 420MPa
– Ultimate Strength: 650MPa – Ultimate Strength: 640MPa
– Uniform Elongation: 10.8% – Uniform Elongation: 11.3%
Table 1: Chemical composition of steels tested
C Mn Si Cr Nb V Ti Al P S N
HR 590 0.0855 1.36 0.12 0.043 0.042 0.005 0.034 0.017 0.012 0.006 0.0041
HR 590DP 0.0599 1.194 0.122 0.497 0.002 0.006 0.003 0.032 0.014 0.001 0.0056
700
600
Stress (MPa)
500
400
Regime of Low-Cycle Fatigue
300 Testing
HR590
200
HR590DP
100
0
0 0.05 0.1 0.15 0.2
Strain
Figure 1: Monotonic stress-strain behavior of steels tested
3

4. Analysis of Fatigue Behavior, Fatigue Damage and Fatigue Fracture of Two High-Strength Steels
Thickness
(a) (b)
Longitudinal
Transverse
(Rolling Direction)
(Loading Direction)
Figure 2: Three-dimensional images of steels tested a) HR590 b) HR590DP
 Microstructural attributes of HR590  Microstructural attributes of HR590DP
o Grain Size: ~10µm o Grain Size: ~10µm
o Nearly all ferrite microstructure o Ferrite/Bainite/Martensite microstructure
o Pancaked grains o Less pancaking of grains
o Inclusions up to >20µm o Inclusions up to <20µm
o Centerline segregation consisting of o Centerline segregation consisting of
pearlite martensite
4

5. Analysis of Fatigue Behavior, Fatigue Damage and Fatigue Fracture of Two High-Strength Steels
Experimental Method
 Experiments involved mechanical testing, fractography, metallography
Table 2: Test parameters for fatigue testing
Control Mode Axial Strain
Strain Rate 0.005/second
Strain Amplitudes 0.0200, 0.0170, 0.0140, 0.0110, 0.0080, 0.0050, 0.0035,
0.0029, 0.0023, 0.0020
R-Ratio 1.0 (Fully Reversed)
Waveform Triangular
Failure Criteria 50% of Estimated Max. Load
 Due to imperfect crystal structure localized plastic
deformation can be unavoidable during extreme
loading conditions, however in situ observations can
be difficult to see
 By performing tests in strain control, stable hysteresis
loops are formed with constant deformation
 Data is statistical, therefore a test plan is required that
addresses outliers and deviations
 Test plan is designed to address the curvature of a
strain-life curve that has a plastic and elastic
component (i.e. bi-lineal relationship)
 Fractography was performed to determine crack
initiation
 Specimens were acid etched to reveal microstructure
Figure 3: Fatigue Test Apparatus
(a) (b)
Figure 4: Etched microstructures using a) Nital + Sodium Meta-bisulfite b) Nital
5

6. Analysis of Fatigue Behavior, Fatigue Damage and Fatigue Fracture of Two High-Strength Steels
Experimental Results
Figure 5: Hysteresis loops for steels tested at 0Nf, 0.25Nf, 0.50Nf, 0.75Nf
1.0000 1.0000
Plastic Strain Plastic Strain
Elastic Strain Elastic Strain
Total Strain - Experimental Data Total Strain - Experimental Data
Total Strain - Curve Fit Total Strain - Curve Fit
0.1000 Power (Plastic Strain) 0.1000 Power (Plastic Strain)
Log Strain Amplitude
Log Strain Amplitude
Power (Elastic Strain) Power (Elastic Strain)
0.0100 0.0100
y = 0.2126x-0.457
R² = 0.9602
0.0010 y = 0.5126x-0.622 0.0010
R² = 0.9878
y = 0.0081x-0.113 y = 0.0092x-0.14
R² = 0.9573 R² = 0.9428
0.0001 0.0001
100 1000 10000 100000 1000000 100 1000 10000 100000 1000000
Log Reversals to Failure(2Nf) Log Reversals to Failure(2Nf)
(a) (b)
Figure 6: Strain-Life curves for a) HR590 b) HR590DP
6

7. Analysis of Fatigue Behavior, Fatigue Damage and Fatigue Fracture of Two High-Strength Steels
700
2.0% Strain
650
1.1% Strain
600
Avg. Alt. Stress (MPa)
0.5% Strain
550
0.2% Strain
500
450
400
350
300
250
200
0.001 0.010 0.100 1.000 10.000 100.000 1000.000
Log(Cumulative Strain)
Figure 7: Average alternating stress as a function of the total accumulated strain
on the HR590 specimen taken at four representative strain levels
700
2.0% Strain
650
1.1% Strain
600
Avg. Alt. Stress (MPa)
0.5% Strain
550
0.2% Strain
500
450
400
350
300
250
200
0.001 0.010 0.100 1.000 10.000 100.000 1000.000
Log(Cumulative Strain)
Figure 8: Average alternating stress as a function of the total accumulated strain
on the HR590DP specimen taken at four representative strain levels
7

8. Analysis of Fatigue Behavior, Fatigue Damage and Fatigue Fracture of Two High-Strength Steels
700
600
500
Stress (MPa)
400
Cyclic @ 0.5Nf
300
Monotonic @ 0.5Nf
200
Experimental Data
100
0
0 0.005 0.01 0.015 0.02 0.025
Total Strain
Figure 9: Cyclic and monotonic stress-strain data for HR590
700
600
500
Stress (MPa)
400
Cyclic @ 0.5Nf
300
Monotonic @ 0.5Nf
200
Experimental Data
100
0
0 0.005 0.01 0.015 0.02 0.025
Total Strain
Figure 10: Cyclic and monotonic stress-strain data for HR590DP
8

9. Analysis of Fatigue Behavior, Fatigue Damage and Fatigue Fracture of Two High-Strength Steels
Table 3: Cyclic stress for strain resistance in MPa as calculated using the half-life data
parameters
Steel Life Level in Reversals
500 1000 5000 10000 50000 100000
HR590 622 593 532 508 456 435
HR590DP 553 520 452 426 370 349
Figure 11: Optical images of fracture surfaces of tested steels
Figure 12: SEM images of fracture surfaces near the point of crack initiation
9

10. Analysis of Fatigue Behavior, Fatigue Damage and Fatigue Fracture of Two High-Strength Steels
Summary
 Low–cycle fatigue testing was performed to characterize the mechanical properties of
two steel microstructures that utilize different strengthening mechanisms to achieve the
same tensile grade
 Fatigue data was quantitatively analyzed and microstructural attributes were qualitatively
analyzed
 Using parameters from experimental data, a relationship for the magnitude of the
resistance to a given amount of strain was developed and showed that the precipitation
strengthened ferrite microstructure (HR590) showed more resistance to the onset of
plastic deformation than the dual phase microstructure (HR590DP)
 Fatigue behavior is often complicated and cannot be completely described by uniaxial
low-cycle fatigue testing
 Other elements that affect fatigue life are
o Changes in loading direction or combinations of loading directions
o Material sensitivity to geometric discontinuities
o Different distributions of stress (e.g. bending)
o Deformation within the high-cycle regime (e.g. bulk elastic)
Future Work
 The motion and interaction of dislocations are of great importance when studying fatigue,
therefore a more quantitative approach to characterizing the microstructure can be
established to understand this phenomenon. This approach often involves the use of
electron microscopy to see dislocation substructures.
 Although plastic deformation can occur in areas where the microstructure is non-
homogeneous, elastic deformation is of importance when establishing fatigue criteria and
therefore high-cycle fatigue testing should also be considered. For steel this may be used
to establish a fatigue limit, however for other materials this may be required to establish
service life.
 Components often have geometric discontinuities, or notches, that negatively impact
fatigue life. For monotonic loading, notches are compensated for by a stress
concentration factor based on geometry alone. Similar factors need to be established for
fatigue, as the fatigue behavior is dependent on both the notch geometry and the
sensitivity of the microstructure. Therefore, notch fatigue tests should be performed at
stress amplitudes that elastically deform the material, but cause plastic deformation at the
notch root.
 After the fatigue behavior of the material is clearly established, scale component tests in
which dynamic loads are cyclically applied should be run to evaluate the true service life
and establish criteria for combination loading.
10