Structures should be designed in such a way that they do not fail during their expected / predicted safe-life

Transcription

1 Structures Any structure is built for a particular purpose Aircraft, Ship, Bus, Train Oil Platforms Bridgesand Buildings Towers for Wind energy, Electricaltransmission etc. Structures and Materials Structuresare built with variousmaterials Metallic Materials Aluminum alloys, Titanium alloys, Steels etc. Composites Metal matrix composites Ceramic matrix composites Polymer Matrix composites Non-Metals Wood Rubber Plastics etc. Mechanical loads on structural materials and components Structures and Materials Structures are subjected to various types of mechanical loads in service Tensile Compressive Shear Bending Fatigue In addition, structures may be subjected to High Temperature (aero-engines, thermal power plants etc.) Creep Oxidation etc. Corrosion (coastal environment) Wear / Abrasion etc. Structures should be designed in such a way that they do not fail during their expected / predicted safe-life 1

2 Failure of Structural Materials In spite of good design, structuresoften fail during service Failure represents an adverse situation wherein a component assemblyfails to perform its intended function satisfactorily Failureis the gap between expectation and performance Fatigue Failure Mechanisms Failure Modes Corrosion % failures Engg. Comp. Aircraft Comp Failure may be due to Excessive Elastic deformation Excessive Plastic deformation Fracture High temp. corrosion / Oxdn. SCC / Corrosion fatigue / HE Creep Wear/abrasion/erosion Others / unknown Need for Mechanical Properties Evaluation Testing and Evaluation Philosophy Building block approach To Determine Basic Design Data To Determine Residual Strength For Failure Analysis and Investigation To Validate Design To Characterize Newly Developed Materials etc. Full Scale level : One/two tests Component level : Few no. of tests Specimen level : large no. of tests 2

3 Servo-hydraulic Test Machine Servo-hydraulic Test Machine Parts Screw Driven Servo-hydraulic Electro-servo-hydraulic Specifications : Load capacity Power Pack Test Frame Controller Transducers Modes of operation Load Controlled Position/stroke controlled Strain Controlled Orientation : Anisotropy Considerations During Testing Forging, Rolling, Forming etc. introduce texture Tensile Testing Properties of Materials Evaluated L Elastic Modulus (E) Yield Strength / Proof Strength (YS) T L T LT TL Ultimate Tensile strength (UTS) Fracture Strength L Poisson s Ratio (ν) Ductility % Elongation % Reduction in C/S area 3

4 Tensile Testing Standard: ASTM E08M Tensile Testing Procedure Specimen Threaded / straight ends Miniature specimens Fix specimen in the grip Zero all transducers Set loading rate Fix extensometer Start / Acquire test data Load/strain/position Convert to stress/strain plot Evaluate all properties Tensile Testing Stress, MPa Stress, MPa Strain % Ductile and Brittle Failures Ductile Characterized by tearing of metals accompanied by gross plastic deformation Macroscopically they have a gray, fibrous appearance Brittle Characterized by rapid crack propagation without appreciable plastic deformation Macroscopically they have a bright, granular appearance Strain % 4

5 Some examples of fatigue failures 1. Failure of pins of an aero engine 1mm Circumferential deformation mark at 4.5 mm distance from the pinhead Transgranular fatigue crack Failed roller shaft of an automobile Half moon shaped region Striations 5

6 Fracture surface appearance Failure of LPTR blade of an aeroengine Fatigue failure Multiple crack initiation Failure of LPTR blade of an aeroengine Aloha aircraft accident: Hawaii, Apr. 28, 1988 More information available at 6

7 Fatigue General observations Surface Failure of materials under cyclic load It involves Crack initiation Crack propagation Final fracture Total fatigue life : Life to ( initiate + propagate) a crack Stage I Slip plane crack Stage II Striation mode fatigue Stage III Superimposed Static modes ; Cleavage, void etc Macroscopic characteristics of a typical fatigue failure Microscopic characteristics of a typical fatigue failure Beach marks Striations Beach marks (macro) Marks due to change in load amplitudes, delay, stop etc Striations (micro) Striations (micro) Marks on the fracture surface due to crack extension during every cycle 7

8 Mechanism of fatigue Some of the terms related to fatigue In a smooth specimen One complete cycle P (max) ; σ (max) ; K (max) to and fro slip in 45 0 slip plane, formation of PSB s Slip is not completely reversible Leads to intrusion and extrusion Intrusions act as a stress concentration Crack initiates due to high stress concentration Crack propagates under cyclic stress Unstable crack when it reaches fracture toughness of the material Load Time P ; σ; K P (min) ; σ (min) ; K (min) Defects in materials such as grain boundary, ppts., cavities, particles and engg. Shape such as bends, curves, holes etc act as stress concentrators Load ratio = R = P (min) / P (max) K = SIF range ; crack driving force Linear Elastic Fracture Mechanics (LEFM) Linear Elastic Fracture Mechanics (LEFM) Stress concentration stress stress notch distance crack distance σ app Stress Concentration Factor SCF = K t = σ loc / σ nom Stress Intensity Factor SIF = K = Y σ (πa) (MPa m) K t = (a/ρ) SIF provides a better representation of the crack tip stresses Fracture toughness = K IC = Y σ (πa) 8

9 Fracture Mechanics Answers to Queries What is the residual strength as a function of the crack size? K = σ π a Y σ = K Ic π a Y for a range of crack length Fracture Mechanics Answers to Queries What is the maximum permissible crack size under the service loading conditions? K = σ π a Y a max = 1 π KIc 2 Y σ service Need to know: K Ic for material Y-function for component σ Need to know: K Ic for material Y-function for component service stress, σ service a Representation of fatigue data S-N curve σ / 2 σ σ / 2 Endurance, fatigue limit σ / 2 = σ f (2 N f ) b No. of reversals to fail, 2N f 9