S-N curve Low Cycle Fatigue, LCF. Use of S-N curve for Constant Amplitude (CA) fatigue. High Cycle Fatigue, HCF

Transcrição

1 N f, 1 n 1 / N f, 1 n 2 / N f, 2 n 3 / N f, 3 n 4 / N f, 4 Representation of fatigue data S-N curve Low Cycle Fatigue, LCF High Cycle Fatigue, HCF Designing for fatigue Use of S-N curve for Constant Amplitude (CA) fatigue R = -1 σ 1 3 No. of reversals to fail, 2N f Completely reversed loading No. of cycles to fail, N f Designing for fatigue Use of S-N curve for Constant Amplitude (CA) fatigue Miner s rule of damage accumulation Damage is accumulated for each load cycle in a spectrum load sequence When damage fraction is one ; specimen fails R =.1 σ S 3 S 1 Tension-Tension Fatigue S S 4 S 2 S 1 S 1 n 1 S 2 n 2 S 3 n 3 S 4 n 4 No. of cycles to fail, N f No. of cycles to fail, N f D = Σ n i / N f, i 1

2 Designing for fatigue Use of S-N curve for spectrum load fatigue Stress, MPa Total no. of preversals = ε-n curve (Coffin-Mansion Law) ε / 2 ε / 2 = ε f (2 N f ) c No. of reversals to fail, 2N f Limitations of S-N and ε-n N curves No. of cycles to fail, N f They are based on total life only Do not distinguish between crack initiation and propagation Representation of fatigue data Fatigue Crack Growth Rate Curve High K regime Designing components against fatigue Fail safe approach Component is given a specified life based on fatigue data and expected loads Component is REPLACED at the end of life irrespective of whether it is still good or not da/dn Near-threshold regime K th K Paris regime ; da/dn = C ( K) m Safe-Life approach Component is given a specified life based on fatigue data and expected loads Component is INSPECTED at the end of its life It is lifed once again, if found suitable Process of inspection continued at frequent intervals of time and life is extended Retired from service when economically or technically not viable to continue using this component any more 2

3 Service loads are random in nature ; They are generally termed as, Spectrum load Variable amplitude (VA) load Random Load etc. What is Damage Tolerance? Inspection intervals Stress / Load stress time Time Crack length, a (mm) K max = f (a/w) σ max (πa) = K IC No. of cycles / blocks Fatigue crack growth prediction Method of Determining FCGR curve for the material Material FCGR General Procedure - Cycle counting - FEA / Experiment/ Handbooks K = f(a/w) σ (π a) da/dn = C [ ( K)] m Take CT or M(T) or Bend specimen Induce a fatigue pre-crack in it Apply a constant amplitude cyclic load Monitor crack length as a function of cycles Obtain a-n data Convert to da/dn Vs K K data da/dn = C [ ( K)] m Use crack closure data to get da/dn Vs K eff plot Fit the equation, get constants 3

4 FCGR specimens FCGR Test Set-up Single edge-notch, bend: SE(B) E399, E813, E1737, E129 Compact tension: C(T) E399, E647, E813, E1737, E129 Middle(-crack) tension: M(T) E647 Disk-shaped compact tension: DC(T) E399, E1737 Arc-shaped tension: A(T) E399 Cord-supported, arc-shaped bend: A(B) E399 FCGR testing Crack length monitoring by electrical method a da dn I II III N m K = Y σ (πa) C K th K d a d N = C K m 4

5 Fracture surface of fatigue tested specimen FEA to get K function P P Compliance Technique V V V a EB (V/P) = f(a/w) P Concept of Fatigue Crack Closure A phenomenon where in crack faces come in contact with each other before reaching the minimum load in the fatigue cycle Concept of Crack Closure Surface roughness induced K (app) K eff oxide induced crack closure da/dn = C [ ( K)] m da/dn = C [ ( K eff )] m

6 Plasticity induced crack closure Experimental determination of crack closure Compliance measurements Load COD, Strain etc. An example of FCGR Prediction Under spectrum load sequence Full Scale Fatigue Testing of an Aircraft 2 Total no. of preversals = Stress, MPa Crack Length (mm) Expt-test-1 Expt-test-2 Predicted Number of blocks 6

7 Aircraft Structure Fatigue loads on the airframe structural components Airframe experiences fatigue loads Main parts of an airframe structure Wings Fuselage Vertical Fin Horizontal stabilizer Engine ( on wings/fuselage) Landing Gears Lift loads (wings) Cabin pressurization (fuselage) Taxiing loads (Entire airframe) Passenger / Stealth loads (fuselage / wings) Fuel and its consumption with flight (fuselage / wings) Gust loads (fuselage) Maneuver loads (fin, wings) Landing impact loads etc. (landing gears) Ageing Aircraft.. A serious concern An aircraft is subjected to various types of cyclic loads ( eg., Lift, gust, maneuver load etc.) There are innumerous stress concentration locations on the airframe ( eg., Rivet hole, cut-out etc. ) Many of these locations are potential sites for a fatigue crack to initiate and grow Designing against fatigue should ensure the safe operation of airframe within the specified technical life Fatigue studies are important in airframe materials and components All airframes have a specified Total Technical Life (TTL) Due mainly to economic considerations, it is necessary to use them for some more time beyond their specified TTL Fatigue is a major concern in such aged airframe The fatigue life of an ageing airframe could be extended through Full Scale Fatigue Testing (FSFT) Aircraft is loaded under spectrum load sequence Airframe is frequently checked through NDT Possible period of extended life is decided 7

8 Full Scale Fatigue Testing Major Milestones Analysis of flight data records to derive a representative flight load sequence Selection of test airframe from the fleet Derivation of test load sequence CFD analysis to determine air pressure distribution on the frame Design and fabrication of loading rig Fatigue Loading of the airframe to specified number of flight hours Frequent NDT checks and small repairs, if any Frequent Analysis of Strain data Final decision for life extension using scatter factor Video : FSFT 8