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Lecture_20 - NE 125: Lecture 20 NE Failure Analysis and...

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Unformatted text preview: NE 125: Lecture 20 NE Failure Analysis and Prevention II Instructor: William K. O’Keefe, P.Eng. wkokeefe@engmail.uwaterloo.ca “Office Hours”: Mondays 1:30 to 2:20 RCH 106 Teaching Assistant (TA): Hua (Leanne) Wei h6wei@engmail.uwaterloo.ca Today’s Lecture Reading: Shackelford 6th Ed, Chapter 8 Next week’s quiz: 3 questions from Chapter 13, Chapter 8 and Chapter 6, sections 6.4 to 6.6 Assignment # 6 due Monday March 3, 2008 NE 125: Introduction to Materials Science and Engineering Introduction Review: Failure Modes in Metals Ductile Fracture Brittle Fracture Creep Fatigue Static Fatigue (Ceramics) Thermal Fatigue Corrosion Fatigue Stress Corrosion Cracking Hydrogen Embrittlement and Hydrogen Attack Liquid Metal Embrittlement Cavitation Pitting Corrosion Fretting Corrosion (Wear Failure) Crevice Corrosion Galvanic Corrosion Graphitization (Carbon steels) Ductile to Brittle Transition Intergranular Corrosion Complex Failure NE 125: Introduction to Materials Science and Engineering Introduction Review: Failure Modes and Effects Analysis (FMEA) “Engineering design has as its first and foremost objective, the obviation of failure” - Henry Petroski, Engineering To Engineer is Human, Random House Inc. (1992) To FMEA A systematic methodology for the identification of failure modes and the management of risk in the engineering of processes, systems, subsystems, mechanical designs etc. mechanical Eg. For a process FMEA 1. 1. 1. 1. 1. 1. For each step in the process, potential failure modes are identified. For The probability of failure is estimated (p) The ability to detect the failure mode is quantified (d) The consequence of failure is identified and assigned a severity rating (s) A risk priority number (RPN) for the failure mode is calculated RPN =p*d*s NE 125: Introduction to Materials Science and Engineering Introduction Factors that Affect Fatigue Life Average Stress Average Increasing the mean stress in a load cycle will reduce fatigue life Increasing Surface Effects Cracks initiate at the surface of the material due to localized plastic deformation resulting in Cracks discontinuities (intrusions and extrusions) discontinuities Polishing and Surface Treatments Polishing and surface treatments can minimize surface defects and enhance fatigue Polishing resistance by increasing surface hardness and inducing compressive stresses to counteract tensile forces tensile Mechanical Design Avoid geometry which gives rise to stress raisers NE 125: Introduction to Materials Science and Engineering Introduction Review: Nitriding of Steel Case Hardening The fatigue resistance and surface hardness of a steel can be enhanced by carburizing or The nitriding. Corrosion resistance can be remarkably improved by nitriding. nitriding. Nitriding: Nitriding: Nitrogen diffuses into steel at elevated temperature (500 to 550 °C). Growth of the compound layer Nitrogen Is controlled by controlling diffusion rate Gamma prime phase (hard) Growth is controlled and usually removed If outer surface all gamma prime, it is If known as the white layer known Epsilon phase Fe4N Fe3N Some solid solution strengthening Due to atomic N diffusion into interstitial Due sites sites Epsilon and Gamma prime phases are both observed at the surface in roughly equal proportions. The proportion of the epsilon phase can be increased by increasing temperature and adjusting gas flow. NE 125: Introduction to Materials Science and Engineering Introduction Review: Engineering to Minimize Fatigue Anticipating failure Case Hardening The fatigue resistance and surface hardness of steel alloys may be enhanced by nitriding or The carburizing. carburizing. The metal is exposed to nitrogen or carbonaceous atmosphere at elevated temperature. The C or N diffuses into metal. The outer layer of metal becomes very hard and resistant to crack formation and propagation formation Shot Peening Residual compressive stresses are introduced to external surface by local plastic deformation Residual from having small particles (0.1 to 1mm) projected at very high velocity into the surface from Polishing and Surface Treatments Polishing and surface treatments can minimize surface defects and enhance fatigue Polishing resistance resistance Mechanical Design Avoid geometry which gives rise to stress raisers Eg. Add rounded fillet to mitigate Eg. stress amplification in rotating shaft shaft NE 125: Introduction to Materials Science and Engineering Introduction Review: Engineering to Enhance Fracture Toughness (Ceramics) Anticipating failure Transformation Toughening Tetragonal ZrO2 polymorphs are added to the Tetragonal matrix consisting primarily of cubic ZrO2 matrix crystals crystals Intensification of stress at crack tip causes Intensification tetragonal ZrO2 to transform to monoclinic tetragonal form form The increase in specific volume associated The with the polymorphic change results in compression which inhibits crack propagation compression (a) Transformation Toughening (b) Microcracks intentionally introduced to the specimen by processing can inhibit the propagation of cracks Illustration from Shackelford “Introduction to Materials Science for Engineers”, 6th Ed. NE 125: Introduction to Materials Science and Engineering Introduction Failure Analysis and Prevention Microscopic Analysis of Fracture Surfaces Fracture surfaces exhibit features characteristic of the mode of failure. Consequently, microscopic examination of fracture surfaces may reveal the failure mode of the specimen. SEM is typically used Eg. Ductile fracture gives a fracture surface with characteristic dimpled texture Eg. Fatigue Failure The fracture surface created from fatigue failure will exhibit characteristic striations observable by electron microscopy (SEM or TEM) in a circular or semi-circular pattern about crack origin. The striations correspond to propagation of crack during each cycle of stress application Macroscopic concentric ridges lead away from the origin of the crack called “Clamshell” or “Beachmark” ridges may be seen with the naked eye. Regions of slow crack propagation is relatively smooth and lustrous whereas the region of fast fracture has a dull and fibrous texture with characteristic ridges radiating from crack origin NE 125: Introduction to Materials Science and Engineering Introduction Failure Analysis and Prevention Nondestructive Testing Nondestructive X-Radiography Voids and cracks on the order of millimeter size may be detected by variation in lightness / darkness in a shadowgraph Beer-Lambert Law I = I 0exp{ − µx} µ = molar absorption coefficient (extinction coefficient) I0 I Voids will result in relatively light regions in shadowgraph and high density inclusions will lead to dark spots x NE 125: Introduction to Materials Science and Engineering Introduction Failure Analysis and Prevention Nondestructive Testing Nondestructive Ultrasonic Testing Acoustic waves refract when entering a new medium (speed of sound changes). Some sound waves are reflected. The intensity of the reflected waves are dependent on the acoustic impedance of the materials. Z=ρ vs Ultrasonic Transducer I r ( Z 2− Z 1 ) R= = I i ( Z 2+ Z 1 ) 2 NE 125: Introduction to Materials Science and Engineering Introduction Sample Problem 2: Shackelford 8.33 In doing an X-radiography of steel, assume the film can detect a variation in In radiation intensity represented by ∆ I/I0 = 0.001. What thickness variation could be detected using this system for the inspection of a 12.5 mm thick plate of steel using a 100 keV beam? using NE 125: Introduction to Materials Science and Engineering Introduction Mid Term Exam Results Raw Scores Raw Scores NE 125 Mid Term 2008 35 30 25 20 15 10 5 0 < 20 20 to 30 to 40 to 50 to 60 to 70 to 80 to 90 to 29.5 39.5 49.5 59.5 69.5 79.5 89.5 100 NE 125: Introduction to Materials Science and Engineering Introduction Mid Term Exam Results Final Marks Final Scores NE 125 Mid Term 2008 35 30 25 20 15 10 5 0 < 20 20 to 30 to 40 to 50 to 60 to 70 to 80 to 90 to 29.5 39.5 49.5 59.5 69.5 79.5 89.5 100 NE 125: Introduction to Materials Science and Engineering Introduction Sample Problem: Mid Term Exam 2008 5. (8 marks) Atomic Structure and Bonding (Chapter 2) For an NaCl ion pair, the attractive (EA) and repulsive (ER) energies in electron volts (eV) depend on the interatomic separation distance (r) in nanometers (nm) according to the following relations: 1.436 E A= − r 7.32 x10−6 E R= r8 Cl- pair. Ionic radii: Calculate the bonding energy in eV for this Na+ Cl- = 0.181 nm; Na+ = 0.098 nm ...
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