CVE_3012_Session_04_Material_Properties_

CVE_3012_Session_04_Material_Properties_ - Session 4...

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Unformatted text preview: Session 4 Material Properties and Testing Part 3 – Failure and Safety; Testing CVE 3012, Fall 2011 Outline • Failure and Safety • Brittle Fracture - Lake Carling case study • Charpy V-Notch test • Laboratory Measuring Devices • Nonmechanical Properties Criteria for material selection 1. Economic Factors 2. Mechanical Properties 3. Nonmechanical Properties 4. Production and Construction 5. Aesthetic Characteristics 6. Material Variability 7. Laboratory Measuring Devices Fatigue S-N Curves The stress versus number of cycles to failure (S-N) curves for a tool steel and an aluminum alloy. S-N Curves Fatigue Strength Factors Reductions in the fatigue strength of cast steels subjected to various surface-finishing operations. Note that the reduction becomes greater as the surface roughness and the strength of the steel increase. Failure and Safety • Fatigue – repeated stresses below material strength – lower stresses require more repetitions – endurance limit for metals • Fracture – brittle materials at max stress – ductile materials due to excessive plastic deformations – Ductile materials brittle fracture at low temperature due to crack propagation • Buckling – unstable lateral deflection in compression members – in composites causes wrinkling of the fibers Mamlouk 1.2.9 Failure and Safety – Excessive deformation • serviceability • alter structural elements – General yielding • large scale yielding spread throughout in ductile materials Mamlouk 1.2.9 Factor Of Safety Applied for Design Depends on - Size of the structure - Cost of failure - Variability in material properties - Accuracy in identifying the loads - Local conditions - Allowable Stress design method σ failure - Single factor of safety σ allowable = FS - Strength (LRFD) design method - Load factors ΦR N ≥ RU γ i Qi - Resistance factors Mamlouk 1.2.9 Charpy V Notch Results • Plot the provided test results • Identify the ductile/brittle transition zone Excel Temp(F) 310 240 150 80 20 10 0 -10 -20 -40 Toughness (ft-lb) 80 79 78 57 33 26 18 10 7 5 Toughness (ft-lb) Charpy V Notch Test 100 80 60 40 20 0 -100 -50 0 50 100 150 Temperature (F) 200 250 300 350 Brittle Fracture Failure Modes Brittle vs. Ductile Lake Carling Photos Photo 1. The Lake Carling stopped in ice, 22 March 2002 (assisting tug, the Ryan Leet, on the port side) Lake Carling Photos Photo 2. Principal fracture Lake Carling Photos Photo 3. Frame 93 Photo 4. Frame 89 Photo 5. Frame 91 (aft) Photo 6. Frame 171½ starboard Brittle Fracture Ship Case Study • The Lake Carling bulk carrier • Constructed in Turkey in 1992 to DNV 1A1 and Polish Registry specifications. Strengthened for heavy bulk cargoes including alternate loading arrangements (holds 2 and 4 may be kept empty). • Constructed to DNV ice class 1C specifications. Grade A steel in the majority of the hull structure. Grade E steel in shear strake and strength deck. • March 2002, loaded iron ore in Quebec, Canada • According to the loading instrument, the greatest seagoing Still Water Bending Moments (SWBM) were located at frames 85 (hold 4) and 154 (hold 2), and were 90% and 86% of maximum http://www.safeship.ca/page7.php Brittle Fracture Ship Case Study • Next day, hold 4 opened for routine maintenance. Water streaming in on the port side through a 6m fracture. Air temperature -6° C; water temperature 0° C. • Lake Carling stopped in ice. Bracing work fitted to the inside of the fracture to reduce water ingress. • 22 March rescue tug arrived . Proceeded to the Bay of Gaspé. • Freezing spray was causing ice accretion on the forward third of the vessel, thus increasing the SWBM. Brittle Fracture Ship Case Study • Pre-existing cracks - One year earlier, the Lake Carling had extensive re-fit and dry-docking at Gdansk (Poland). No cracks documented in hold 4. During the post accident survey, five other cracks were observed at the bottoms of the side shell frames in this hold between frames 85 and 96. • Study of cargo operations since leaving dry-dock revealed an overstress event four months prior to the fracture. The overstress had occurred at the maximum SWBM locations between frames 86 and 91 and was at least 103% of the approved seagoing allowable limit. Most probable genesis of the cracks in hold 4 – culminating in the eventual brittle fracture at frame 91. Brittle Fracture Ship Case Study • Steel Tests – steel removed to make repairs was examined by the TSB. Charpy Vee Notch (CVN) energies were found to be relatively low. Fracture mechanics calculations showed that a 10 cm crack (similar to other pre-existing cracks in hold 4) would experience brittle fracture at approximately 11 ksi –well below the material’s ultimate tensile strength. • In 2003, a sister ship, the Ziemia Gornoslaska, called at the port of Montreal to repair small cracks discovered in the side shell. This steel was also tested by the TSB and CVN energies were found to be less than that found with the Lake Carling steel (TSB 2004). Laboratory Measuring Devices • Dial Gage • Linear Variable Differential Transducer (LVDT) • Strain Gage • Proving Ring • Load Cell • Extensometer Dial Gage • Smallest division = 0.001” • Max measurement called travel • Attach between two points to measure relative movement • Zero the gage by rotating the large scale • Read the 10-ths of an inch from the small scale • Read the 1000-ths of an inch from the large scale • Calibration – Micrometer – Gage blocks LVDT • Displacement transducer used to measure small movements or deformations (~ 0.001 inch) • Consists of – nonmagnetic shell with one primary and two secondary electric coils – magnetic core connected to a push rod • An electric voltage is input to the LVDT and an output voltage is obtained • As the core moves into one of the secondary coils the result is that the sum of their outputs creates an output of certain magnitude • Requires periodic calibration Strain Gage • Very thin wire arranged in a series of loops • The wire is cemented to a backing which is glued to the material where strain is to be measured • Strain is change in length with respect to the original length, called gage length • As the material elongates, the wire increases in length and decreases in diameter, causing the electrical resistance of the gage to increase • The change in resistance of the wire associated with strain of the material is measured • A Wheatstone bridge used to measure change in resistance Proving Ring • Used to measure forces • Consists of a steel ring with a dial gage attached • When force is applied, the ring deforms and the attached dial gage registers a reading • Through calibration charts the dial gage reading is related to the force value • Periodic calibration required Load Cell • Force measuring device used for axial loading and bending tests • An electric voltage is input to the load cell and an output voltage is obtained • If the relation between the force and the output is known, the force can be easily determined by measuring the output voltage • Periodic calibration required Extensometer • Available in different sizes • Two cantilever beams separated by a fixed distance at each end – Fixed end measures deformations. – Sample end is free to move apart as the test piece to which they are attached stretches • Each arm of the cantilever has a strain gauge attached to its upper and lower surfaces • Appropriate deflection range to accommodate the expected strain Nonmechanical Properties • Density and Unit Weight • Thermal Expansion • Surface Characteristics Mamlouk 1.3 Density and Unit Weight • Density • Mass per unit volume of material • Units kg/m3 or lb/ft3 = • Variety: • total (wet, moist) density • solid density (particles) • Unit Weight • Weight per unit volume of material • Equals density times acceleration of gravity • Units kN/m3 or lb-force/ft3 • Variations: • saturated • moist • dry ρ m V W γ= V γ = ρg Mamlouk 1.3.1 Volumes for Determining Density • • • • Loose Compacted Total particle Volume inaccessible to water • Volume of solids Specific Gravity • Ratio of the mass of a substance to the mass of an equal volume of water at a specified temperature • Density of water 1 Mg/m3 or 62.4 lb/ft3 • Gs is dimensionless • Types - bulk-dry - bulk-SSD - apparent ρ Gs = ρw Specific strength (tensile strength/density) Specific Strength and Specific Stiffness Specific stiffness (elastic modulus/density) Specific Strength versus Temperature Thermal Expansion • Thermal deformations • At high temperature – expansion • At low temperature – contraction α L= ΔL ΔT = ε L ΔT • Linear coefficient of thermal expansion • Units - μ ε /Cº • Volumetric coefficient of thermal expansion • For isotropic materials • Applications Mamlouk 1.3.2 α V = 3α L αV = ΔV ΔT V Thermal Expansion Example • Given: Unconstrained steel rod length 200mm, diameter 20 mm at 15°C. The rod is heated uniformly to 115 °C. Coefficient of thermal expansion = 12.5x10-6 m/m/°C. • Find: Length after heating. δL = (α L )(δT )( L 0 ) δL = (12.5 x10 −6 m / m / °C )(115°C − 15°C )(200mm) δL = .25mm L = 200.25mm • Is there any stress in the rod? Why? Mamlouk 1.26 Thermal Expansion Example (cont) • If the rod is fixed on both ends (fully constrained) what stress is induced? δL = (α L )(δT )( L 0 ) σ = Eε δ L (α L )(δ L)(L0 ) ε= = L0 L0 ε = (12.5 x10−6 m / m / °C )(115°C − 15°C ) ε = 12.5 x10−4 m / m σ = (207GPa)(12.5 x10−4 m / m) σ = .2588GPa( 145,037 psi / GPa) = 37,540 psi Mamlouk 1.26 Physical Properties of Selected Materials at Room Temperature Metal Aluminum Aluminum alloys Copper Copper alloys Iron Steels Lead Magnesium Magnesium alloys Nickel Nickel alloys Titanium Titanium alloys Tungsten Zinc Nonmetallic Ceramics Glasses Graphite Plastics Wood Density 3 (kg/m ) Melting Point (°C) Specific heat (J/kg K) 2700 2630–2820 8970 7470–8940 7860 6920–9130 11,350 1745 1770–1780 8910 7750–8850 4510 4430–4700 19,290 7140 660 476–654 1082 885–1260 1537 1371–1532 327 650 610–621 1453 1110 –1454 1668 1549–1649 3410 419 900 880–920 385 377–435 460 448–502 130 1025 1046 440 381–544 519 502–544 138 385 Thermal conductivity (W/m K) 222 121–239 393 29–234 74 15–52 35 154 75–138 92 12–63 17 8–12 166 113 2300–5500 2400–2700 1900–2200 900–2000 400–700 — 580–1540 — 110–330 — 750–950 500–850 840 1000–2000 2400–2800 10–17 0.6–1.7 5–10 0.1–0.4 0.1–0.4 Surface Characteristics • Deterioration – Corrosion (metals and ceramics) • Deterioration by a reaction with environmental chemicals • Types: chemical or electrochemical – Degradation (asphalt and concrete) • Asphalt – aging • Concrete – carbonation, salt formation, sulfate attack Mamlouk 1.3.3 Surface Characteristics • Abrasion and Wear Resistance – Pavements – skid resistance • Surface Texture – Smooth on concrete aggregate – Rough on asphalt aggregate Mamlouk 1.3.3 Continued leakage in the expansion joint leading to concrete degradation at the beam seat level and the appearance of cracks. Production and Construction • Production – Availability – Labor • Intensity • Skill • Construction – Available equipment – Tradition (trained work force) – Regionally available materials Mamlouk 1.4 Summary • Failure and Safety • Brittle Fracture - Lake Carling case study • Charpy V-Notch test • Laboratory Measuring Devices • Nonmechanical Properties ...
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