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I-DEAS® Durability Training A course presented by Advanced Test and Analysis (ATA) San Diego, CA

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2 WELCOME The ATA Industrial Group at SDRC Course Instructor: Tom Deiters Advanced Test and Analysis Engineering

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3 COURSE OBJECTIVES demonstrate methods for performing fatigue analysis of structures share fatigue-related expertise of the ACE industrial group teach the fundamental science of fatigue analysis show how I-DEAS Durability task can be used to extract fatigue information from finite element models

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4 Course Overview Day 1 Morning –Introduction –Review of Concepts –Workshop: Linear Damage Rule –Working With Functions –Workshop: Working With Functions Day 1 Afternoon –SN Curve And Material Properties –Workshop: Create SN Curves –Damage Direction Uniaxial/Biaxial –Workshop: Uniaxial/Biaxial Effect On Damage Day 2 Morning –Define A Static Event/Evaluate Event –Workshop: Use Trailer Hitch FE Model; Create Static Event and Evaluate Responses –Basic Approach To Damage Calculation using Durability –Workshop: Continue trailer hitch example and find highest damage location Day 2 Afternoon –Define A Dynamic Event –How To Use Notch Factors –Workshop: Understand effect of notch factor using trailer hitch event –Modeling and Fatigue Analysis of Welds –Workshop: Perform fatigue analysis for a welded detail –Creating Duty Cycles and Other Fatigue Tools –Workshop: Create a duty cycle and predict whether location will endure the duty cycle. –Customer Applications/Questions

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5 This Is The Aerospace Center of Excellence (ACE) We conducted the modal test to flight qualify the C-17 but we also consult in fatigue applications... …also dynamic simulations of Pegasus (a re-useable launch vehicle), and many others We specialize in test and analysis vibration consulting

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6 Most Of Our Fatigue Work Comes From The Entertainment Industry First major “simulator ride” Commercial six DOF motion platform Amusement park environment proved very “harsh” SDRC redesigned the simulator cabin –Extensive field testing to understand loads –Integrated design, analysis, test, manufacture I-DEAS used exclusively –Final drawing package in 18 weeks New cabins installed in 1994

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7 How Do Specimens Fail In Fatigue? Fatigue is a failure brought about by repeated loading Ultimate Strength rates a material’s ability to resist failure due to a single applied load [1] [2] Crack initiation here Crack propagation Overload failure DuctileBrittle

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8 Course Focus Is On Crack Initiation ASTM Definition of Fatigue The process of progressive localized permanent structural change occurring in a material subjected to conditions which produce fluctuating stresses and strains at some point or points and which may culminate in cracks or complete fracture after a sufficient number of fluctuations. - “Standard Definitions of Terms Relating to Fatigue Testing and Statistical Analysis of Data,” ASTM Designation E206-72. progressive - fatigue occurs over a period of time localized - fatigue operates on local areas cracks - cracks form and grow under specific combinations of stress and cycle history fracture - ultimate failure due to a crack which has grown to a point at which the remaining material can no longer tolerate the stresses or strains [1]

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9 SN Curve Relates Crack Initiation To Stress (S) Amplitude and Cycles (N) Fatigue Or Endurance Limit Stresses below the fatigue limit are assumed to do no fatigue damage SN Curve indicates stress level vs. number of cycles required to initiate a crack [1]

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10 Most fatigue data is gathered from uni- axially stressed specimens Many SN curves provide fatigue properties for materials under ideal conditions (smooth, polished specimens, uni-axially loaded) Real world conditions such as mean stress, stress concentrations, corrosion, multi-axial loading, must be taken into account [1]

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11 Many Empirically Based Formulations Of SN Curves Stress Life Strain Life Smith Watson Topper (SWT) ASME Boiler Code BWI (Welds) User Defined [1]

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12 Low Cycle/High Cycle Fatigue Imply Loading Amplitude High Cycle Low stress ampl. Stress/strain relationship is linear Use Stress Life or SWT Low Cycle High stress ampl. Stress/strain in plastic region Use Ramberg-Osgood to relate stress and strain Use Strain Life/SWT/ASME Boiler

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13 Strain is a better predictor of fatigue damage for high amplitudes (low cycle) For low amplitudes either stress or strain can be used to predict damage. For best correlation to test results, convert linear FEA strain history to stress history using Ramberg-Osgood. Then use that stress history to count cycles and calculate damage, life, etc., as opposed to using linear FEA stress history. Stress and Strain are converted back and forth using: Linear Ramberg-Osgood (preferred even for linear stress-strain) Power Hardening At high amplitudes, small changes in stress produce large changes in strain

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14 Typically Stress Ranges Are Counted. SN Curves Are Stress Ampl Vs. Cycles It is easy to convert from cycles to reversals and amplitude to range; just multiply by two Make sure you check SN data carefully to be sure you know what is being reported

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15 There are many methods for counting ranges…Rainflow is most common Range Pair - counts all reversals. Racetrack - only counts reversals outside a tolerance band. Rainflow - counts cycles. (Follows stress/strain hysteresis loops in Strain Life.)

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16 Palmgren-Miner Rule: Damage Accumulates Linearly Palmgren used it for ball bearing life Miner used it for aircraft Damage for one cycle = 1/Life Life is the number of cycles to crack initiation for a given stress amplitude (from SN curve) Damage for n such cycles = n/Life Total Damage = n/Life n + p/Life p +... where n is number of cycles with Amp n, Mean n, and p is number of cycles with Amp p, Mean p Total Life = 1/Total Damage (if <1, cracks initiate)

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17 Example Using Palmgren-Miner Rule

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18 A Duty Cycle Comprises Many Service Histories Example of an Aircraft landing gear: 1,000 take offs per year 1,000 landings per year 1,000 loading/unloading plane per year 2,000 hours of random and sinusoidal vibration from flight 200 hours of random vibration from rough weather [2] Each event may have its own service history and will be repeated N times during the parts life A duty cycle sums the damage from each service history using the Palmgren-Miner (linear damage) rule

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19 Stress/Strain Can Originate From Test or Simulation Analytical Experimental Cycle Count Damage Calculation FE Model Test Article Static Solution Test Measurement Post Process Stress/Strain Data Dynamic Solution Durability Event Strain gages Single gage - uniaxial Rosette - three gages; principal stress

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20 When are Dynamic Models Necessary? Use static model when frequency content of excitation is more than an order of magnitude below first natural frequency of structure. When excitation frequency is close to a natural frequency of the structure, dynamic amplification of the response is possible. A dynamic (modal) model should be used.

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21 Von Mises Is Combined Stress Result, Useful As A Yield Failure Criterion When calculating fatigue damage, maximum principal stress should be used. Not Von Mises [2] Ductile Brittle

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22 Notch factors account for local plastic yielding not predicted by model Fuch’s Metal Fatigue In Engineering has a good chapter (6) on Notch Factors You could use a stress concentration factor appropriate for the notch geometry to scale up the predicted stress history and then use this to predict the life. Unfortunately this will give a pessimistic life estimate. Fatigue notch factors account for the beneficial effect of the steep stress gradients near the actual notch. [1]

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23 Many good references on fatigue... 1. Fuchs, H.O., Stephens, R.I., Metal Fatigue in Engineering, John Wiley & Sons, Inc., New York, 1980. 2. Dowling, Norman E., Mechanical Behavior Of Materials, Prentice Hall, Upper Saddle River, NJ, 1999. 3. I-DEAS Dynamics and Durability User’s Guides For MS7 http://www.sdrc.com/nav/support-training/tech_info/ click tech tips browse (enter your site ID # and “sdrc” as the password) pick Simulation and then I-DEAS Dyn. And Dur. User’s Guides for Master Series 7 Download pdf document These references were used extensively in the preparation of this course

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