fatigue studies at uob

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University of Bristol Department of Aerospace Engineering FATIGUE LIFE PREDICTION FOR COMPOSITE MATERIALS - A NEW APPROACH: 

University of Bristol Department of Aerospace Engineering FATIGUE LIFE PREDICTION FOR COMPOSITE MATERIALS - A NEW APPROACH Mostefa Bourchak, Dr Ian Farrow, Dr Ian Bond

Current Fatigue Assessment For Composites: 

Current Fatigue Assessment For Composites There is no generally applicable and reliable fatigue life prediction method available for fatigue of CFRP under spectrum loading. Instead use: Conservative static design @ low stress/strain levels for static notch/impact allowables Conservative test validation hot/wet cycling damage tolerance demonstration

Classical Approach: 

Classical Approach Three stages: Load Cycle History Idealisation Aircraft load history monitoring and idealisation Counting methods Spectrum representation exceedence curves Standard load spectra Summary of load history idealisation Data Manipulation Cyclic data manipulation & empirical constant life diagrams Damage Accumulation Cumulative damage rule (as before) OR Property degradation e.g. RESIDUAL STIFFNESS, RESIDUAL STRENGTH “wear-out”, PROBABILISTIC...

Classical Approach: 

Unconservative Classical Approach Typical Results Classical fatigue analysis prediction for carbon-fibre composites. A 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 1.E+07 1.E+08 1.E+09 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 1.E+07 1.E+08 1.E+09 Prediction Test CEP-50/50-4BT YH CEP-50/50-4BC YH CEF-0/91/9-FAT IB CEF-0/91/9-FAT IB CEF-0/91/9-2BT lm SD CEF-0/91/9-2BT hm SD CEF-0/91/9-FAT SD CEF-0/91/9-FAC SD CEF-0/91/9-FAT FV CEF-0/91/9-SFT FV CEF-0/91/9-EFT FV CEF-0/91/9-FAC FV CEF-0/91/9-SFC FV CEF-0/91/9-EFC FV CEF-0/91/9-2BT lm FV CEF-0/91/9-2BT hm FV 32 34 36 CEP-25/50/25-SGC YD CEF-25/50/25-FA PB CEF-25/50/25-FAC SD CEF-25/50/25-FAC FV CEF-25/50/25-SFC FV CEF-25/50/25-EFC FV CEP-25/50/25-TWC P CEP-25/50/25-TWC LOM P CEP-25/50/25-TWC 2LOM P CEP-25/20/25-TWC 4LOM P CEP-25/50/25-TWC 4LOM P CEP-25/50/25-TWC 4H2LOM P CEF-25/50/25-FAC IB CEF-25/50/25-SFC IB CEF-25/50/25-EFC IB Conservative

New Approach: 

At present; no general consistent design approach for modelling cumulative damage & predicting residual strength/fatigue life long term behaviour is currently unpredictable (probably!) not possible to derive cumulative damage law [based on C/A data] which will be superior to Palmgren/Miner developments MUST include effects of interactions between stresses at different levels New Approach

New Approach: 

Load Cycle History Idealisation plus DAMAGE EVENT DEFINITION Data Manipulation Cyclic data manipulation & empirical constant life diagrams Damage Accumulation Cumulative damage rule (as before) OR Property degradation e.g. RESIDUAL STIFFNESS, RESIDUAL STRENGTH “wear-out”, PROBABILISTIC... New Approach

Damage Event Definition: 

Damage Event Definition Mix Events E.g. Significant increasing +ve or decreasing -ve events

Slide8: 

Damage Event Definition Account for mix - methodology based on 2-block base data e.g. by R ratio, severity, sm & sa Investigation of significant damage events by thermal transient monitoring

Slide9: 

‘Mix’ Event

Slide10: 

Effect of Cycle Mixing

Slide11: 

Effect of Cycle Mixing

Slide12: 

Test S-N data Constant life fitting +++ + + + S N ** ** * ** + ++ +++ ++ R m a +  Data Manipulation + Mix S-N data

Damage Accumulation: 

Damage Accumulation e.g. D = {A(ni / Ni) + B(ni/Ni)C} N = total of applied cycles when D = {A(ni / Ni) + B(ni/Ni)C} = Df dD = f(m, a, D) dN N =  dD  f(m, a, D) f

Fatigue Life Prediction: 

Fatigue Life Prediction

Current UoB Activities: 

“Fatigue Life Prediction For Fibre Reinforced Plastics Under Complex Loading” EPSRC/AWHL/DAP CASE Studentship £60k over 3 yrs 1. Damage event characterisation ‘Mix’ counting algorithm (FALSTAFF,TWIST, HELIX etc.) Investigate damage event transient wear-in states by thermal/p.d./acoustic monitoring Flexural testing of “damage feature” specimens, modified lathe (>10Hz) Laminate specimens with ply drop, fibre wrinkle, tow break 2. Prediction methodology verification Two-block testing to complement AWHL/DAP programmes Assess prediction method (FALSTAFF,TWIST, HELIX etc.) Computer controlled servo-hydraulic test machine (100kN, <5Hz) Standard in-plane specimens (e.g. CRAG type) with ply drop, fibre wrinkle, tow break Current UoB Activities

Summary: 

Summary (Currently!) No reliable, generally applicable method of predicting fatigue response for any situation/application. Fatigue = primary design issue More efficient designs@higher stress/strain levels  Need for reliable fatigue life prediction UoB new approach: Damage event definition within a load-time history  Experimental evidence of ‘mix’ effect on fatigue life  Significant improvement in predictive accuracy

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