A Preliminary study to correlate pit depth to short

D.W. Hoeppner, V. Chandrasekaran and A.M. Taylor FASIDE

May 5, 1999

A Preliminary study to correlate pit depth to "short" fatigue crack growth rates of 2024-T3 aluminum alloy specimens

by

David W. Hoeppner, P.E., Ph.D.V. Chandrasekaran (Chandru), Ph.D.Amy M. Taylor, M.S.

FASIDE International Incorporated1146 S. Oak Hills WaySalt Lake City, UT 84108-2026


May 5, 1999

Outline of Presentation• Introduction

• Objective and hypothesis

• Experimental Details-- Test Specimens and Fatigue Test Setup-- Prior Corrosion Method-- Quantification of Pit Depth Using Confocal

Microscope

• Fatigue Experimental Results (‘a’ vs. N and da/dN vs. ∆∆∆∆K)


May 5, 1999

• AFGROW analysis of test data

• Fractographic Study

• Conclusions From Experimental Study

• Recommendations for Future Work

Outline of Presentation - Continued


May 5, 1999

Introduction

Aircraft Location of Failure Cause Incide nt Seve rity Place Year From

Bell Helicopter Fuselage, longeronFatigue, corrosion and pitting present Serious AR 1997 NTSB

DC-6 Engine, master connecting rod Corrosion pitting Fatal AK 1996 NTSBPiper PA-23 Engine, cyclinder Corrosion pitting Fatal AL 1996 NTSBBoeing 75 Rudder Control Corrosion pitting Substantial damage to plane WI 1996 NTSBEmbraer 120 Propeller Blade Corrosion pitting Fatal and serious, loss of plane GA 1995 NTSBGulfstream GA-681 Hydraulic Line Corrosion pitting Loss of plane, no injuries AZ 1994 NTSBL-1011 Engine, compressor assembly disk Corrosion pitting Loss of plane, no injuries AK 1994 NTSBEmbraer 120 Propeller Blade Corrosion pitting Damage to plane, no injuries Canad1994 NTSBEmbraer 120 Propeller Blade Corrosion pitting Damage to plane, no injuries Brazil 1994 NTSB

Mooney Mooney 20 Engine, interiorCorrosion pitting, improper approach Minor injuries TX 1993 NTSB

C-130 Bulkhead 'Porkchop' Fitting Fatigue, corrosion pitting Pressurization leaks 1995 LMAS

C-141 FS998 Main FrameCorrosion pitting, stress corrosion cracking Found crack during inspection 1991 LMAS

Incidents and Accidents Caused by Corrosion Pitting Since 1983*

* Prepared for LMAS on CFSD program


May 5, 1999

Introduction (Continued)

Digitized confocal microscope image showing numerous pits around a rivet hole of aspecimen extracted from a JSTAR fuselage panel (Range of pit depth 5-20 µm)


May 5, 1999

Objectives and Hypotheses

To characterize corrosion damage quantitatively in relationto "short" fatigue crack growth rates of 2024-T3 aluminumalloy specimens including a specimen made from adismantled JSTAR fuselage panel.

The shorter the range of pit depth, the longer the fatiguecycles to form the first detectable crack length.

The crack growth rates for the prior-corroded specimensand the JSTAR specimen will be greater than theuncorroded specimen.

The greater the range of pit depth, the lower the fatigue lifeof 2024-T3 aluminum alloy specimens.


May 5, 1999

Experimental Details

Fatigue Test Specimen Configuration (All dimensions in mm)

25.40

R = 12.70 +0.07 -0.00Ø9.53+0.07

-0.00

TWO HOLES

76.20

50.80

18.42+0.07-0.00

1.52+0.07-0.00

C

C

Ø5.00+0.10-0.10


May 5, 1999

Experimental Details (Continued) - PriorCorrosion Technique

+

-

DC Power

Carbon Rod(Cathode)

2024-T3 Specimen (Anode) Solution (5% NaCl + in 500 mlof distilled water)


May 5, 1999

SEM micrograph showing artificiallyproduced pits on 2024-T3 aluminum

alloy specimen tested in this study(X80) (5-10 µµµµm).

Photograph showing surface corrosiondamage on the JSTAR specimen

tested in this study (2-6 µµµµm).

Experimental Details (Continued)


May 5, 1999

Schematic illustration of the recording system and test setup

Experimental Details (Continued)


May 5, 1999

Fatigue Test Parameters and Test Results

Specimen ID Material Test Parameter Range ofPit Depth(µm)

FatigueCycles atFracture

Plain oruncorroded

2024-T3 σmax = 20 ksi (Netstress); R = 0.1, f =10 Hz

Notapplicable

291,200

Prior-corroded#1or prior-pitted #1


5-10 122,100



5-10 106,400



5-15 29,000



5-15 92,000

JSTAR 2024-T3 σmax = 20 ksi (Netstress); R = 0.1, f =10 Hz

2-6 188,700


May 5, 1999

Specimen ID Range ofPit Depth(µm)

Length offirstdetectablecrack (mm)

Fatiguecycles tofirstdetectablecrack

Final cracklengthmeasured(mm)**

Fatigue cyclesto fracture

Plain oruncorroded

Notapplicable

0.201 70,200 2.849 291,200


5-10 0.157 20,000 2.653 122,100


5-10 0.149 20,100 2.679 106,400


5-15 0.189 5000* 2.398 29,000


5-15 0.154 20,000 2.417 92,000

JSTAR 2-6 0.216 35,000 2.507 188,700*Tested at σmax = 30 ksi (Net stress). All other tests were conducted at σmax = 20 ksi (Netstress). R = 0.1, f = 10 Hz, Room temperature (laboratory air).** That is when unstable fracture occurred.

Cycles to the first detectable through crack for uncorroded andprior-corroded 2024-T3 aluminum alloy specimens


May 5, 1999

Stress intensity factor (KI) values for one crack (through) from the hole for the specimen configuration used in this study

0

200

400

600

800

1000

1200

0.00 0.50 1.00 1.50 2.00 2.50 3.00

Crack Length 'a' in mm

K in

MPa

sqr

t. m

m

The stress intensity values for the specimen geometry used in this study

Providedby

APES Inc.


May 5, 1999

'a' vs. N data ("short" and long regime) for 2024-T3 aluminum alloy specimens (Plain as well as prior-pitted specimens)

0

0.5

1

1.5

2

2.5

3

0.0E+00 5.0E+04 1.0E+05 1.5E+05 2.0E+05 2.5E+05 3.0E+05 3.5E+05

Fatigue Cycles (N)

Crac

k Le

ngth

(mea

sure

d in

mm

)

Material: 2024-T3 aluminum alloyMax. Net stress = 20 ksiR = 0.1Frequency = 10 Hz.

1. Plain specimen (20 ksi)2. JSTAR Fuselage panel specimen (20 ksi) (2-6 micron pit depth)3. Prior-pitted specimen (20 ksi) (5-10 micron pit depth)4. Prior-pitted specimen (20 ksi) (5-15 micron pit depth)5. Prior-pitted specimen (30 ksi) (5-15 micron pit depth)

4 32 1

5

Cycles to first detectable through crack


May 5, 1999

Fatigue Crack Growth Rate Data for 2024-T3 Aluminum Alloy Specimens

1.00E-10

1.00E-09

1.00E-08

1.00E-07

1.00E-06

1.00E-05

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

Delta K (MPa sqrt. m)

da/d

N (m

/cyc

le)

Series1Series2Series3Series4Series5

Material: 2024-T3 aluminum alloyMax. Net stress = 20 ksiR = 0.1Frequency = 10 Hz.Room temperature (Lab. air)

Series 1:Plain specimen (20 ksi)

Series 2 & 3:Prior-pitted specimen (20 ksi) (5-10 micron pit depth)

Series 4:Prior-pitted specimen (20 ksi) (5-15 micron pit depth)

Series 5:JSTAR Fuselage panel specimen (20 ksi) (2-6 micron pit depth)


May 5, 1999

AFGROW analysis of experimental results

Comparison of AFGROW results with fatigueexperimental results

Specimen ID Range ofPit Depth(µm)

Length of the firstdetectable crack(mm)/Initial cracklength 'C'

Fatigue cycles tofracture(AFGROW result)

Fatigue cyclesto fracture(Test result)

Plain oruncorroded

Notapplicable

0.201 183,700 291,200


5-10 0.157 147,000 122,100


5-10 0.149 149,000 106,400


5-15 0.154 148,500 92,000

JSTAR 2-6 0.216 144,100 188,700


May 5, 1999

SEM Analysis of Fracture Surfaces

Crack origin at the corner of the holeand fracture features of uncorrodedspecimen (X200)

Crack origin at corrosion pit on prior-corroded specimen #2 (X200)


May 5, 1999

SEM Analysis of Fracture Surfaces (Continued)

A crack between corrosion pits on prior-corroded specimen #2 (X1.0k)


May 5, 1999

SEM Analysis of Fracture Surfaces (Continued)

Crack origin inside rivet hole on JSTAR specimen (X150)


May 5, 1999

From this experimental study the following conclusions can bemade:

! The shorter the range of pit depth, the longer thefatigue cycles to form the first detectable crack length.

!The number of fatigue cycles to first detectable crackfor the uncorroded specimen was much higher whencompared to the prior-corroded as well as the JSTARspecimen.

!The final crack length of the plain specimen wasconsiderably longer when compared to that of theprior-corroded and the JSTAR specimens.

Conclusions


May 5, 1999

! The crack growth rates for the prior-corrodedspecimens and the JSTAR specimen were greater thanthe uncorroded specimen. In addition, as expected, thegreater the range of pit depth, the lower the fatigue lifeof the specimens.

! At low ∆∆∆∆K values (for example @ 0.1 MPa√√√√m and0.25 MPa√√√√m), the crack growth rates of the prior-corroded specimens in the "short" region wereconsiderably faster than the uncorroded specimen.

Conclusions (Continued)


May 5, 1999

! In general, the analysis of the experimentally generated datausing the AFGROW program resulted in the over-prediction of thefatigue life of prior-corroded 2024-T3 aluminum alloy specimens.However, the fatigue life estimation by the AFGROW for theuncorroded and the JSTAR specimens was conservative.

! On the uncorroded specimens the origins of cracks were at thecorner of the rivet hole where the stress concentration was highest,due to no other damage on the surface. The prior corrodedspecimens cracked at the rivet hole and also at the points of largecorrosion pits.



May 5, 1999


! The most important result was the crack origin ofJSTAR specimen, which was inside the rivet hole.This shows the extent of the damage inside the rivetholes that may go undetected but that may result infailure.

! From the experimentally generated results, it isthereby concluded that prior-corrosion indeed affectsthe fatigue crack growth rates of 2024-T3 aluminumalloy specimens in the "short" crack region as well asit alters the cracking mechanism(s) as observed fromfractographic analysis.


May 5, 1999

The Challenge

Crack(s) form at one or more holes


May 5, 1999

Short crack propagation occurs in uncorroded material.Question: Is crack propagation rate of short cracks faster/slower/no different than long cracks?

The Challenge


May 5, 1999

Short crack propagation occurs in corroded material.Question: Is short crack propagation different (faster) in corroded material than uncorroded material?

The Challenge


May 5, 1999

Potential Significance of Results

• The faster crack growth rates in the “short” crack regime on theprecorroded specimens has a potential significant effect on lifeprediction.

• Results using uncorroded data could lead to unconstructive lifeestimates or inspection interval estimates.

• As well, MSD accumulation could occur much more rapidly thanenvisioned due to prior corrosion effects on fcp in the “short” crackdomain.

• The role of corrosion in potentially modifying SSI/PSE selection ismore significant from these results.


May 5, 1999

Recommendations for Further Work

! Statistically planned fatigue experiments should beconducted on three different aluminum alloys in theplain (uncorroded) and prior-corroded or prior-pittedcondition in laboratory air.

! Statistically planned fatigue experiments should beconducted on three aluminum alloys in 3.5 % NaClsolution and other environment at three differentfrequencies (e.g. 1 Hz, 5 Hz, 10 Hz). Due to the lengthof time required for these experiments three fatiguemachines should be dedicated to this purpose.


May 5, 1999

!Attempts must be made to characterize the pitsgeometrically and in the second case to establish therate of growth of pits under cyclic loading.

! Enough additional experiments must beconducted to stop several tests to section pits forcharacterization at various stages of their growth.

! As well, observations of the transition of pits tocracks must be made and the principal factors thatinfluence this transition must be identified.

Recommendations for Further Work (Continued)


May 5, 1999

! Additional measurements of "short" crackgrowth rates must be made especially for shortercracks to continue an evaluation of the effect ofprior-corrosion and concomitant corrosion andfatigue on the growth rates of "short" cracks.

! Finally, in further work a simple computerprogram should be developed to allowtransference of the corrosion pitting fatigue modelcalculations to other engineers as required.

Recommendations for Further Work (Continued)


May 5, 1999

Concluding RemarksEnergy And Damage Mechanics Approaches Are Being

Pursued to Model Corrosion “Damage”.

Transition of Pits to “Small” Cracks and TheirCrack Propagation Rates Will Be Studied.

Thank You!

Any Questions email

[email protected]

[email protected]

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A Preliminary study to correlate pit depth to short