Progress Since the Last TIM (July 98) (On Modeling Corrosion Damage)

Transcription

1 Progress Since the Last TIM (July 98) (On Modeling Corrosion Damage) by V. Chandrasekaran (Chandru), Ph.D. David W. Hoeppner, P.E., Ph.D. International Incorporated 1146 S. Oak Hills Way Salt Lake City, UT

2 Outline of Presentation Experimental Study to Correlate Pit Depth to Fatigue Life of 2024T3 Aluminum Alloy Specimens Objective and hypothesis Experimental Details Test Specimens Prior Corrosion Method Quantification of Pit Depth Using Confocal Microscope Fatigue Test Results

3 Outline of Presentation Continued Analysis of Fracture Surfaces Macroscopic Analysis Microscopic Analysis (SEM) Fractographic Study Fatigue Life Prediction Based on Experimental Data Conclusions From Experimental Study Current and Future Efforts Concluding Remarks

4 Correlation of Pit Depth to Fatigue Life of 2024T3 Aluminum Alloy Specimens An Experimental Study

5 Objectives and Hypothesis The focus of our research effort is on modeling corrosion "damage" with pit depth as a controlling factor in determining the fatigue life of prior corroded 2024T3 aluminum alloy specimens. The greater the range of pit depth, the lower the fatigue life of 2024T3 aluminum alloy specimens. Using the experimentally generated data as well as fractographic observations, the fatigue life of prior pitted 2024T3 aluminum alloy specimens could be predicted using a fatigue life estimation relation.

6 Experimental Details Fatigue Test Specimen Configuration

7 Experimental Details (Continued) Prior Corrosion Technique + DC Power Carbon Rod (Cathode) 2024T3 Specimen (Anode) Solution (5% NaCl ppm of Cu Powder in 500 ml of distilled water)

8 Nf = C 1 ( 1.1 σ π ) n Fatigue Life Prediction 1 n 2 1 a 1 1 n n o a c Where C and n are empirical parameters. σmax = 138 MPa (20 ksi); σmin = 13.8 MPa (2 R = 0.1 Specimen ID ao ac (Average Pit Depth) Crack Depth (Fractographic study) 1 (3566 µm) 50.5 µm 1.4 mm 2 (60110 µm) 85 µm 0.5 mm 3 (50230 µm ) 140 µm 0.26 mm 4 (5590 µm ) 72.5 µm 1.4 mm

9 KC135 Program FCG 2024T3 Corroded Doubler Specimens C = 1.60E13 m/cycle; n = 6.6 (Estimated from da/dn vs. K)

10 Experimental Fatigue Life data vs. Predicted Fatigue Life data Fatigue Life (Cycles) 1.E+08 1.E+06 1.E+04 1.E+02 1.E E E E04 Average Pit Depth (m) Fatigue life from experiments Predicted fatigue life

11 Fatigue Life (Cycles) Predicted Fatigue Life Cycles at Stresses 138, 207, 276, and 345 MPa 1.00E E E E E E E E04 Average Pit Depth (m)

12 Nf = C 1 ( 1.1 σ π ) n Fatigue Life Prediction 1 n 2 1 a 1 1 n n o a c Where C and n are empirical parameters. σmax = 138 MPa; σmin = 13.8 Mpa C = 1.60E13 m/cycle; n = 6.6 (Estimated from da/dn vs. K) Specimen ID ao ac (Maximum Pit Depth) Crack Depth (Fractographic study) 1 (3566 µm) 66 µm 1.4 mm 2 (60110 µm) 110 µm 0.5 mm 3 (50230 µm ) 230 µm 0.26 mm 4 (5590 µm ) 90 µm 1.4 mm

13 Experimental Fatigue Life data vs.predicted Fatigue Life data Fatigue Life (Cycles) 1.E+08 1.E+06 1.E+04 1.E+02 1.E E E E04 Max. Pit Depth (m) Fatigue life from experiments Predicted fatigue life

14 Fatigue Life (Cycles) Predicted Fatigue Life Cycles at Stresses 138, 207, 276, and 345 MPa 1.00E E E E E E E04 3.0E04 Max. Pit Depth (m)

15 Fatigue Life Prediction of Prior Pitted 2024T3 Aluminum Alloy at Different Stress Levels with Pit Depths Ranging From µm ao ac (Pit Depth) Crack Depth (Fractographic study) µm 1.4 mm µm 0.5 mm µm 0.26 mm Using, KC135 Program FCG 2024T3 Corroded Doubler Specimens C = 1.60E13 m/cycle; n = 6.6 (Estimated from da/dn vs. K)

16 Predicted Fatigue Life vs. Pit Depth at Stresses 138, 207, 276, and 345 MPa Fatigue Life Cycles 1.E+09 1.E+06 1.E+03 1.E+00 0.E+00 2.E04 3.E04 Pit Depth (m)

17 Conclusions From this experimental study the following conclusions can be made: The stress concentration effect of corrosion damage was so severe that fatigue cracks that nucleated from corrosion damage were responsible for the final fracture of 2024T3 aluminum alloy specimens. Pit size in terms of pit depth was found to be a controlling factor in relation to fatigue life of 2024T3 aluminum alloy specimens when tested in laboratory conditions. The greater the range of pit depth, the lower the fatigue life of 2024T3 aluminum alloy specimens.

18 Conclusions (Continued) The fractographic study clearly revealed that numerous secondary cracks could result from corrosion pits that would significantly affect the fatigue life of 2024T3 aluminum alloy specimens. The fractographic study also revealed that cracks that nucleated from pits of less depth could propagate deeper into the specimen when compared to those of greater depths (example specimen #1 and 4). However, with pits of larger depth the same material was found to fracture in less number of fatigue cycles and cracks that nucleated from those pits were found to propagate only a small depth into the specimen subsurface (example specimen #3).

19 Conclusions (Continued) Furthermore, the SEM analysis of fracture surfaces clearly showed that fatigue cracks that formed from the uncorroded area of the specimens had a negligible effect on the fatigue life of the prior corroded 2024T3 aluminum alloy specimens. In other words, cracks that nucleated from the uncorroded area of the specimens remained as surface cracks and did not propagate further into the specimen subsurface. This observation was further supported by the fractographic analysis as characteristic fatigue features were not seen on the fracture surfaces corresponding to the uncorroded area of the specimens.

20 Conclusions (Continued) Fatigue life prediction with quantified average pit depth of prior corroded 2024T3 specimens as well as using the measured crack depth from fractographic analysis revealed a fair correlation with the experimental fatigue life data.

21 Current and Future Efforts

22 Energy Concept for Quantification of Corrosion Griffith s Energy Criterion P σ f = 2 E γ πa s (1) where, σ f is fracture stress 2a γ s is surface energy a is half crack size P

23 Irwin s Elastic Energy Release Rate Concept Irwin utilized Griffith's energy criterion to define the energy release rate, G, as the rate of change in potential energy with crack area for a linear elastic material. σ = EG πa (2) where, σ G is is the length fracturestress elastic increment a is half crack size energy G = per unit crack du da At instability, the critical strain energy release rate is given by Gc = P 2 max 2B dc da Where C is compliance /P (KIc 2 )/E = Gc

24 Energy Concept for Quantification of Corrosion Continued From beam theory, for a double cantilever beam specimen, /2 = Pa 3 /3EI, where I = Bh 3 /12, is displacement with a known crack size 'a'. The elastic compliance C = /P = 2a 3 /3EI Substituting C into a well known relation for G = (P 2 /2B)(dC/da), G c = P 2 a 2 /BEI (3) where, G c is critical energy release rate P is load at fracture a is half crack size B is specimen thickness E is modulus of elasticity I = Bh 3 /12.

25 Schematic showing the experimental procedure based on the compliance concept.

26 Schematic illustrating the concept of compliance

27 Energy Concept for Quantification of Corrosion Continued D1 D2 D1 < D2 < D3 LOAD D3 D = Corrosion damage SLOPE α STIFFNESS COMPLIANCE α 1/STIFFNESS EXTENSION Schematic showing the proposed compliance concept to model corrosion damage

28 Energy Concept for Quantification of Corrosion Continued As mentioned before, Irwin utilized Griffith's energy criterion to define the energy release rate, G, as the rate of change in potential energy with crack area for a linear elastic material. However, to model corrosion damage using Irwin's energy release rate, the definition of G could be modified as the rate of change in potential energy with corrosion area for a linear elastic material. In this case a corrosion damage parameter such as the pit depth (P d ) can be incorporated in calculating the energy release rate of the specimen rather than the crack size 'a'. Therefore, replacing the half crack size term 'a' in the relation (3) with pit depth (P d ) yields,

29 Energy Concept for Quantification of Corrosion Continued G cp = P f 2 (P d ) 2 /BEI (4) where, G cp is critical energy release rate with pit depth as a controlling factor. P f is load at fracture P d is pit depth B is specimen thickness E is modulus of elasticity I = Bh 3 /12. Using (4) G cp can be calculated. As well, using (K Icp2 )/E = G cp, the stress intensity factor with certain pit depth (P d ) can be found.

30 Concluding Remarks Energy And Damage Mechanics Approaches Are Being Pursued to Model Corrosion Damage. Transition of Pits to Small Cracks and Their Crack Propagation Rates Will Be Studied. Thank You! Any Questions or