Advance in Fatigue Life Prediction Techniques for Pre- corroded Aluminium Alloy

Transcription

1 January 2012 Vol. 7 No V O A doi /j. issn Advance in Fatigue Life Prediction Techniques for Pre- corroded Aluminium Alloy CHEN Bo 1 LIU Jian-zhong 1 WANG Hao-wei 2 MA Shao-jun 1 1. Beijing Institute of Aeronautical Materials Beijing China 2. China Special Vehicle Research Institute Hubei Jingmen China Abstract Corrosion especially pit corrosion is a key problem facing aluminium aircraft structures. Prediction of the fatigue life of pre-corroded aluminum alloy is critical to the safety of aged aircrafts. In this paper the fatigue life prediction techniques for pre-corroded aluminum alloy were discussed and summarized. And main analysis models for fatigue life prediction based on fracture mechanics and their critical techniques were introduced in detail. Key words pre-corrosion pit corrosion fatigue life prediction fracture mechanics Goswami % 8%

2 1 51 Pit nucleation Pit growth Short crack growth Long crack growth t t Transition t Transition t Fracture pn pg sc tc for pit to from short short crack to long growth crack t Fig Seven stages of pitting corrosion fatigue life Gruenberg 4 4 4a 4b 2024-T3 3 3 Leek Howard 3 4c 4a FASTRAN 2 2 4b 4c 5 Cycles to Failure ksi LT(e) L(e) 26 ksi LT(s) LT(e) Experiment Prediction 32 ksi LT(e) Load Direction Fracture Surface Randomly Distributed Pits Single Semi elliptical Surface Crack Develops at a Pit Some 鄄 where on the Corroded Face K t Effect Negligible (K t<5) Crack 0 2 Hours of Corrosion Exposure Corroded Face Pit Fig. 2 Predicted fatigue life and experimental results Sankaran 2 Initial Crack Dimension= Average or Maximum Pit Size AFGROW Corroded Face Length DuQuesnay 5 Sankaran 3 Fig Medved 6 Propagating Crack Ciack 7475 Walde 7 Depth Schematic illustration of the assumption used in the modeling of fatigue crack growth and life prediction 2

3 a Growth of only the largest nucleating pit Sankaran h b Growth of all nucleating pits without connection between each other Maximum Stress /MPa Fatigue Life /cycles Average Fatigue Life LT,138MPa, 6 h Fig. 5 Legeng: Measured life(specimens exposed for 96 h in prohesion spray) Predicted life (average pit size of specimens exposed for 96 h in prohesion spray) c Growth of all nucleating pits Predicted life (maximum pit size of specimens 4 exposed for 96 h in prohesion spray) Measured life (non-corroded specimens) Fig. 4 Three prediction models for LT30 5 LT,138MPa, 24 h LT 180 MPa 24 h Experimental Method 1:Single Largest Nucleating Pit Method 2:All Nucleating Pits and Nonnuc leating Pits Method 3:All Nucleating Pits 6 h 24 h 6 h 24 h LT,220MPa, LT,220MPa, 6 h 24 h Average experimental life and predicted life Gruenberg Gruenberg 6 96 h Fig. 6 Predicted and measured fatigue life of 7075-T6 pre-corroded for 96 h DuQuesnay mm 1. 5 mm 2. 5 mm 7 Lankford 8 Newman 9

4 Crack Length of 1.27 mm Cumulative Distribution of Numer of Cycles to Reach a SFH,PFH Experiment AFGROW(2c=2.0 mm) AFGROW(2c=1.5 mm) AFGROW(2c=2.5 mm) Empirical Trend Pit Depth, a /mm Cumulative Density Function F /N cycles Weibull CDF(AFGROW Data) AFGROW TTCL 1.27mm-c Experimental TTCL 1.27mm-c 7 Fig. 7 (a) (b) (c) Fig. 8 AFGROW Comparison of AFGROW life prediction data and experimental data based on average surface width of corrosion pits 8 Observation Flaw area=a Transformation Semi-circular model Flaw area=a Equivalent processing method for the fatigue nucleation sites EIFSD K Dolley 11 a /t 0 EIFSD EIFSD Medved 6 EIFSD AF- AFGROW NASGROW FAS- GROW TRAN AFGROW EIFSD AFGROW mm TTCI 9 NASGROW NASA EIFSD 6 FASTRAN da /dn - K eff K max / K min 9 Fig mm TTCI Cumulative distribution of number of cycles to achieve the crack length of mm based on AFGROW EIFSD EIFSD 2. 2 SIF SIF Newman - Raju 12 Newman - Raju 13 Newman - Raju 14 SIF -

5 Sankaran 2 50 μm 16 K FASTRAN DKEFF da /dn - K eff da /dn - K 10 Walde 7 1.E-04 1.E-05 R=0.1 da/dn /(m/cycle) 1.E-06 1.E-07 1.E-08 1.E-09 1.E-10 Aloca Long CrackData Newman's Short and Long Crack Data Back-Calculated 1.E ΔK applied /MPa m 1/2 Fig Back-calculated short crack and long crack growth data C - T C - T C - T C - T 22 C T C - T 16 C - T T

6 1 55 Sankaran K t = K t = 2 11 Gruenberg 10 3 Rokhlin Shi 1 National Research Council Committee on Aging of U. S. Air 24 Force Aircraft. Aging of U. S. air force aircraft final report MSD Publication NMAB M. Washington DC National A- 25 cademy Press Maximum Stress /MPa Fatigue Life /cycles 10 6 Legend: h h 384 h 96 h No Exposure Mil-Hdbk-5 Data,K t=1(r=0.1) Mil-Hdbk-5 Data, K t=2(r=0.1) Medved J J Breton A M Irving P E. Corrosion pit size distributions Fig. 11 Fatigue property of pre-corroded 2 mm thick 7075-T6 compared to design data and fatigue lives a study of the EIFS technique for fatigue design in the presence of corrosion J. International Journal of Fatigue Lankford J. The growth of small fatigue cracks in 7075-T6 alumi- num J Structures Sankaran K K Perez R Jata K V. Effects of pitting corrosion on the fatigue behavior of aluminum alloy 7075-T6 modeling and experimental studies J. Materials Science and Engineering Goswami T K Hoeppner D W. Pitting corrosion fatigue of structural materials J. Structural integrityin aging aircraft 1995 AD Gruenberg K M Craig B A Hillberry B M et al. Predicting fatigue life of pre-corroded 2024-T3 aluminum J. International Journal of Fatigue DuQuesnay D L Underhill P R Britt H J. Fatigue crack growth from corrosion damage in 7075-T6511 aluminium alloy under aircraft loading J. International Journal of Fatigue Walde K Brockenbrough J R Craig B A et al. Multiple fatigue crack growth in pre-corroded 2024-T3 aluminum J. International Journal of Fatigue Fatigue and Fracture of Engineering Materials and 9 Newman J C Edwards P R. Short-crack growth behavior in an a- luminum alloy an AGARD cooperative test programme R. AGARD Report Gruenberg K M Craig B A Hillberry B M et al. Predicting fatigue life of pre-corroded 2024-T3 aluminum from breaking load 1 tests J. International Journal of Fatigue 627. fatigue life J and Structures Dolley E J Lee B Wei R P. The effect of pitting corrosion on. Fatigue and Fracture of Engineering Materials

7 Newman J C Reuter W G Aveline J C. Stress and fracture analyses of semi-elliptical surface cracks C. St. Louis MO Walde K. Corrosion-nucleated fatigue crack growth D. Purdue University Newman J C Edwards P R. Short crack growth behaviour in an aluminum alloy - An AGARD cooperative test programme M. AGARDR Advisory Group for Aerospace Research and Development Wu X R Newman J C Zhao W et al. Small crack growth and fatigue life predictions for high-strength aluminium alloys Part I Experimental and fracture mechanics analysis J. Fatigue and Fracture of Engineering Materials and Structures Piascik R S. Willard S A. The Growth of small corrosion fatigue cracks in alloy 2024 J. Fatigue and Fracture of Engineering Materials and Structures Piascik R S. The Growth of small corrosion fatigue cracks in alloy 7050 J. International Journal of Fatigue C - T J M J J Rokhlin S I Kim J Y Nagy H et al. Effect of pitting corrosion on fatigue crack initiation and fatigue life J. Engineering Fracture Mechanics T3 24 Shi P Mahadevan S. Damage tolerance approach for probabilistic pitting corrosion fatigue life prediction J. Engineering Frac- J ture Mechanics Shi P Mahadevan S. Corrosion fatigue and multiple site damage reliability analysis J. International Journal of Fatigue 櫬櫬櫬櫬櫬櫬櫬櫬櫬櫬櫬櫬櫬櫬櫬櫬櫬櫬櫬櫬櫬櫬櫬櫬櫬櫬櫬櫬櫬櫬櫬櫬櫬櫬櫬櫬櫬櫬櫬櫬櫬櫬櫬櫬 檿檿檿檿檿檿檿檿 檿檿檿檿檿檿檿檿檿檿檿檿檿檿檿檿檿檿檿檿檿檿檿檿檿檿檿檿檿檿檿檿檿檿檿檿檿檿檿檿檿檿檿檿檿檿檿檿檿檿檿檿檿檿檿檿檿檿檿檿檿檿檿檿檿檿檿檿