Report

Metallurgical Aspects of Fatigue Failure of Steel Dr. Ahmed Sharif Associate Professor Department of Materials and Metallurgical Engineering Bangladesh University of Engineering and Technology (BUET) Dhaka-1000, Bangladesh 1 Materials Tetrahedron Processing Performance Microstructure Dr. Ahmed Sharif, MME, BUET Properties 2 Microstructural Constituents of Steel Body Centred Cubic Ferrite Face Centred Cubic Austenite Orthorhombic Cementite Dr. Ahmed Sharif, MME, BUET 3 Fe-Fe3C Equilibrium Diagram Austenite Ferrite Cementite Pearlite Part of the iron –carbon thermal equilibrium diagram Dr. Ahmed Sharif, MME, BUET 4 Microstructural Constituent of Steel -Continued Pearlite Bainite Body centred Tetragonal Dr. Ahmed Sharif, MME, BUET Martensite 5 Microstructure and Property Relationship of Plain Carbon Steels Dr. Ahmed Sharif, MME, BUET 6 Failure Tensile failure mode Brittle Failure Failure in Torsion Failure in Compression Failure in Bending Fatigue Failure Dr. Ahmed Sharif, MME, BUET 7 Materials Failure Material failure corresponding to deformation and fracture Dr. Ahmed Sharif, MME, BUET 8 Fatigue Failure On March 27, 1980 the floating drill platform "Alexander Kielland" suffered a catastrophic failure Part of the I-5 bridge in Washington collapsed on May 24, 2013, sending cars and people into the water. Dr. Ahmed Sharif, MME, BUET 9 Fatigue Fatigue is the name given to failure in response to alternating loads (as opposed to monotonic straining). Static loading Cyclic loading Until applied stress intensity factor K applied can be well below Kc (K) reaches critical stress intensity (3 MPa m for example). Over factor (Kc) (30 MPa m for time, the crack grows. example) the crack will not grow. The design may be safe considering static loads, but any cyclic loads must also be considered. Dr. Ahmed Sharif, MME, BUET 10 Fatigue: General Characteristics The three different stages of fatigue 1. Crack initiation 2. Crack growth 3. Final rupture Cyclic slip Crack nucleation Initiation period Dr. Ahmed Sharif, MME, BUET Micro crack growth Macro crack growth Final failure Crack growth period 11 Fatigue Tests -Test Specimens Dr. Ahmed Sharif, MME, BUET 12 Fatigue Tests -Testing Arrangements Rotating-bending Axial loading Constant deflection amplitude cantilever bending Dr. Ahmed Sharif, MME, BUET Rotating cantilever bending Combined in-phase torsion and bending Three point flexural 13 Standard Practices Designation ASTM E466 Title Conducting Force Controlled Constant Amplitude Axial Fatigue Tests of Metallic Materials. ASTM E467 Verification of Constant Amplitude Dynamic Forces in an Axial Fatigue Testing System. ASTM E468 Presentation of Constant Amplitude Fatigue Test Results for Metallic Materials. ASTM E606 Strain-Controlled Fatigue Testing. ASTM E647 Measurement of Fatigue Crack Growth Rates. ASTM E739 Statistical Analysis of Linear or Linearized Stress-Life (S-N) and StrainLife (-N) Fatigue Data. ASTM E1012 Verification of Specimen Alignment Under Tensile Loading ASTM E1049 Cycle Counting in Fatigue Analysis. ASTM E1823 Standard Terminology Relating to Fatigue and Fracture Testing. Dr. Ahmed Sharif, MME, BUET 14 Fatigue Testing, S-N curve High Cycle Fatigue Low Cycle Fatigue Fatigue limit S-N curve is concerned chiefly with fatigue failure N > 104 cycles high cycle fatigue (HCF). N < 104 cycles low cycle fatigue (LCF). Dr. Ahmed Sharif, MME, BUET 15 Metallurgical Control on Stress-life Curves The fatigue limit has historically been a prime consideration for long-life fatigue design. Fatigue limit has an enormous range depending on: Surface finish Microstructural constituents Strength Ductility Inclusion Heat treatment Casting porosities and Residual stresses. Dr. Ahmed Sharif, MME, BUET 16 Metallurgical Control: Surface Finish Effects Effect of decarburization Dr. Ahmed Sharif, MME, BUET 17 Metallurgical Control: Microstructural Constituent (0.78% C, 0.27% Mn, 0.22% Si, 0.016% S, and 0.011% P) Effect of martensite content on fatigue limit Effect of microstructure on fatigue behavior of carbon steel Dr. Ahmed Sharif, MME, BUET 18 Metallurgical Control: Strength sa smean 3 > smean 2 > smean 1 s s s mean 1 AlSl 4340 alloy steel mean 2 mean 3 log Nf Fatigue limit is about half the ultimate tensile strength. Heat treatment or alloying addition that increases the strength (or hardness) of a steel can be expected to increase its fatigue limit Dr. Ahmed Sharif, MME, BUET 19 Metallurgical Control: Ductility Effect of hardness level on plot of total strain versus fatigue life Hardness Ductility Fatigue strength Ductility is generally important to fatigue life only under low-cycle fatigue conditions. e.g. short with variable amplitude of loading during earthquake. Dr. Ahmed Sharif, MME, BUET 20 Metallurgical Control: Inclusions Effect of nonmetallic inclusion size on fatigue of AISI-SAE 4340H steels Fatigue limits of SAE 4340 steel prepared by vacuum melting and electric melting Process Longitudinal fatigue limit MPa ksi Electric furnace melted 800 116 Vacuum melted 960 139 Transverse fatigue Ratio of transverse Hardness, limit to longitudinal HRC MPa ksi 545 79 0.68 27 825 120 0.86 29 Metallurgical Control: Heat Treatment • Increasing hardness tends to raise the endurance limit for high cycle fatigue. This is largely a function of the resistance to fatigue crack formation (Stage I in a plot of da/dN). Mobile solutes that pin dislocations fatigue limit, e.g. carbon in steel Dr. Ahmed Sharif, MME, BUET 22 Metallurgical Control: Casting Porosity Affects Gravity cast versus squeeze cast versus wrought Al-7010 • Casting tends to result in porosity. Pores are effective sites for nucleation of fatigue cracks. Castings thus tend to have lower fatigue resistance (as measured by S-N curves) than wrought materials. Dr. Ahmed Sharif, MME, BUET 23 Metallurgical Control: Residual Stresses The effect of quenching medium (quench severity) on the magnitude of the residual stress and its variation along the cross-sectional area Compressive stress fatigue strength . Dr. Ahmed Sharif, MME, BUET increases 24 Comparison of Fatigue Testing Techniques Dr. Ahmed Sharif, MME, BUET 25 Fatigue Life Improvement Techniques • Surface rolling - Compressive stress is introduced in between the rollers during sheet rolling. • Shot peening Sheet rolling - Projecting fine steel or cast-iron shot against the surface at high velocity. • Polishing - Reducing surface scratches • Thermal stress - Quenching or surface treatments introduce volume change giving compressive stress Dr. Ahmed Sharif, MME, BUET Shot peening 26 Design for fatigue Several distinct philosophies concerning for design for fatigue 1) Infinite-life design: Keeping the stress at some fraction of the fatigue limit of the material. 2) Safe-life design: Based on the assumption that the material has flaws and has finite life. Safety factor is used to compensate for environmental effects, varieties in material production/ manufacturing. 3) Fail-safe design: The fatigue cracks will be detected and repaired before it actually causes failure. 4) Damage tolerant design: Use fracture mechanics to determine whether the existing crack will grow large enough to cause failure. Dr. Ahmed Sharif, MME, BUET 27 Case Study-1 Low‐cycle fatigue model by ‘rain ﬂow cycle counting’ approach 10‐storey steel building located in San Fernando Valley, California Nastar, Navid, et al. "Effects of low‐cycle fatigue on a 10‐storey steel building." The Structural Design of Tall and Special Buildings 19.1‐2 (2010): 95-113. . Case Study-2 Fatigue life analysis of a reinforced concrete railway bridge Considering the stress level s= 79.8 MPa Calculated crack growth curve for current axle loads of 247KN. Fatigue life variation as a function of number of trains. Frangopol, Dan, et al. Proceedings Bridge Maintenance, Safety, Management, Resilience and Sustainability. Vol. 1. No. EPFL-CONF-180270. CRC Press/Balkema, 2012. References • Mechanical Behavior of Materials (2000), T. H. Courtney, McGraw-Hill, Boston. • Fatigue and Fracture (1996), ASM Handbook, ASM International, Ohio. • Fatigue Resistance of Steels (1990), B. Boardman, ASM International, Metals Handbook, 10th Ed. • Deformation and Mechanics of Engineering Materials (1976), R. W. Hetzberg, Wiley, New York. • Metal Fatigue in Engineering (2001), R. I. Stephens, Wiley, 2nd Ed. New York. • Designing Against Fatigue (1962), R. E. Heywood, Chapman & Hall, London. Dr. Ahmed Sharif, MME, BUET 30