Phone Interview10122011

Analysis of Fatigue Behavior, Fatigue Damage and Fatigue Fracture of Two High-Strength Steels

1

Charles G. Lester IV

Ph: 404-576-5921

[email protected]

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)


2

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


3

Steels Characterized in Fatigue Testing

• HR590

– Continuous Cast, Hot-Rolled

– 3.2mm thick sheet

– Precipitation strengthened ferrite

matrix

– Tensile Strength Grade: 590MPa

– Average Yield Strength: 570MPa

– Ultimate Strength: 650MPa

– Uniform Elongation: 10.8%

• HR590DP, Dual Phase

– Continuous Cast, Hot-Rolled

– 3.2mm thick sheet

– Martensite strengthened ferrite

matrix

– Tensile Strength Grade: 590MPa

– Average Yield Strength: 420MPa

– Ultimate Strength: 640MPa

– 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

Figure 1: Monotonic stress-strain behavior of steels tested

0

100

200

300

400

500

600

700

0 0.05 0.1 0.15 0.2

Str

ess

(M

Pa

)

Strain

HR590

HR590DP

Regime of Low-Cycle Fatigue

Testing


4

(a)

(b)

Figure 2: Three-dimensional images of steels tested a) HR590 b) HR590DP

Microstructural attributes of HR590

o Grain Size: ~10µm

o Nearly all ferrite microstructure

o Pancaked grains

o Inclusions up to >20µm

o Centerline segregation consisting of

pearlite

Microstructural attributes of HR590DP

o Grain Size: ~10µm

o Ferrite/Bainite/Martensite microstructure

o Less pancaking of grains

o Inclusions up to <20µm

o Centerline segregation consisting of

martensite

Longitudinal

(Rolling Direction) Transverse

(Loading Direction)

Thickness


5

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

(a)

(b)

Figure 4: Etched microstructures using a) Nital + Sodium Meta-bisulfite b) Nital

Figure 3: Fatigue Test Apparatus


6

Experimental Results

Figure 5: Hysteresis loops for steels tested at 0Nf, 0.25Nf, 0.50Nf, 0.75Nf

(a)

(b)

Figure 6: Strain-Life curves for a) HR590 b) HR590DP

y = 0.5126x-0.622 R² = 0.9878

y = 0.0081x-0.113 R² = 0.9573

0.0001

0.0010

0.0100

0.1000

1.0000

100 1000 10000 100000 1000000

Lo

g S

tra

in A

mp

litu

de

Log Reversals to Failure(2Nf)

Plastic Strain

Elastic Strain

Total Strain - Experimental Data

Total Strain - Curve Fit

Power (Plastic Strain)

Power (Elastic Strain)

y = 0.2126x-0.457 R² = 0.9602

y = 0.0092x-0.14 R² = 0.9428

0.0001

0.0010

0.0100

0.1000

1.0000

100 1000 10000 100000 1000000

Lo

g S

tra

in A

mp

litu

de

Log Reversals to Failure(2Nf)

Plastic Strain Elastic Strain Total Strain - Experimental Data Total Strain - Curve Fit Power (Plastic Strain) Power (Elastic Strain)


7

200

250

300

350

400

450

500

550

600

650

700

0.001 0.010 0.100 1.000 10.000 100.000 1000.000

Av

g. A

lt.

Str

ess

(M

Pa

)

Log(Cumulative Strain)

2.0% Strain

1.1% Strain

0.5% Strain

0.2% Strain

200

250

300

350

400

450

500

550

600

650

700

0.001 0.010 0.100 1.000 10.000 100.000 1000.000

Av

g. A

lt.

Str

ess

(M

Pa

)

Log(Cumulative Strain)

2.0% Strain

1.1% Strain

0.5% Strain

0.2% Strain

Figure 8: Average alternating stress as a function of the total accumulated strain on the HR590DP specimen taken at four representative strain levels

Figure 7: Average alternating stress as a function of the total accumulated strain on the HR590 specimen taken at four representative strain levels


8

0

100

200

300

400

500

600

700

0 0.005 0.01 0.015 0.02 0.025

Str

ess

(M

Pa

)

Total Strain

Cyclic @ 0.5Nf

Monotonic @ 0.5Nf

Experimental Data

Figure 10: Cyclic and monotonic stress-strain data for HR590DP

Figure 9: Cyclic and monotonic stress-strain data for HR590

0

100

200

300

400

500

600

700

0 0.005 0.01 0.015 0.02 0.025

Str

ess

(M

Pa

)

Total Strain

Cyclic @ 0.5Nf

Monotonic @ 0.5Nf

Experimental Data


9

Table 3: Cyclic stress for strain resistance in MPa as calculated using the half-life data parameters

Figure 11: Optical images of fracture surfaces of tested steels

Figure 12: SEM images of fracture surfaces near the point of crack initiation

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


10

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.

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Phone Interview10122011