Phone Interview10122011
- 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