4102- Chap 8 Environment.pdf

8/10/2019 4102- Chap 8 Environment.pdf

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MAAE 4012Strength & Fracture

Chapter 8

Professor R. BellDepartment of Mechanical & Aerospace Engineering

Carleton University

2013

Chapter 8 - EAC 1


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Environmentall Assisted Crackin

Department of Mechanical

& Aerospace Engineering

Under the action of a monotonic stress, corrosive environments can

Effect of Environment on Crack growth

levels that would not cause initiation or crack growth in inertenvironments

Occurs at a stress much less than the yield stress or at a stress

n ens y muc ess an e rac ure oug ness

Frequently no obvious indication of the effect of corrosion on sub-

critical crack growth Results in unex ected failure once the critical crack len th is

reached

Two Major Types of Environmentally Assisted Cracking

Stress Corrosion Cracking

Hydrogen Embritt lement

Chapter 8 - EAC 2


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Environmentally Assisted Cracking



No universal theory to explain these damage processes

Lack of theory is due to the complex nature of

Fracture Mechanics at the crack tip

The electrochemistry of the corrosion process

The physical metallurgy of the materials

Chapter 8 - EAC 3


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Examples of Stress Corrosion



Chapter 8 - EAC 4


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Chapter 8 - EAC 5


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Chapter 8 - EAC 6


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Material-Environment Corrosion Combinations



Material Susceptible Environments

Ordinary Steels NaOH solutions

,

Mixed acids (i.e. N2SO4HNO3)

HCN solutionAcidic H2S solutions

Seawater

Aluminum Alloys NaCl H2O2 solutions

NaCl solutions and seawater

Air and water

Stainless Steels Acidic chloride solutions (MgCl2, BaCl2)

NaCl-H2O2 solutions

Seawater

Copper Alloys Ammonia vapours & solutionsWater and water vapour

Not to be taken as complete

Chapter 8 - EAC 7


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Stress Corrosion Test Methods



Two main Categories of Corrosion Tests

Time to-Failure Tests

Crack growth rate tests

Tests based on Smooth specimens - crack initiation

Tests based on re-cracked FM s ecimens stress

corrosion crack growth

Chapter 1 - Introduction 8


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Stress Corrosion Test Methods



Ti-8Al-1Mo-1V alloy in 3.5% NaCl solution

Initial K plotted against time to failure

Threshold KISCC or KIEAC

No rigorous proof of an environmental

threshold, therefore the use of KIEAC or

treated with caution

< IEAC o a ure even a ter ong exposure

KIEAC < K < KIC Subcritical flaw growth with final fracture

K >K Immediate failure with initial loadin

Chapter 8 - EAC 10


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Cantilever Beam Test Specimen



Procedure to obtain K ISCCrequires multiple specimens

Chapter 8 - EAC 11


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Wedge-Opening-Loading Test Specimen



Basic principle of WOL specimen

Chapter 8 - EAC 12


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K ISCC - A Material Property



Canti lever beam specimen

Constant load

The KI value increases

as the crack lengthincreases

Increase in K leads

to fracture of specimen

Wedge Opening Loading specimen - WOL

The KI value decreases as the crack length increases Decreasing load

Constant displacement i.e. Constant crack opening displacement

Decrease in K leads to crack arrest at K

Chapter 8 - EAC 13


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K ISCC - A Material Property



Influence of specimen geometry and test duration

Elapsed Time (hrs) Apparent K ISCC(Ksi in)

100 170

1,000 115

,

Influence of Cut-off Time on KISCC measured

with Cantilever Bend Specimens

Influence of specimen geometry on time to

failure for 4340 steel Barsom & Rolfe p 288

Chapter 8 - EAC 14


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Crack Growth Rate Tests



Involves the measurement of the

crack growth rate per unit time da/dt as

a function of the instantaneous stress

intensity factor

3 Regions

Region I

rate of SCC growth is strongly

dependent on the magnitude of KI SIF below Kth cracks do not

propagate

Threshold SIF corresponds to

KISCC

Chapter 8 - EAC 15


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Crack Growth Rate Tests



Region II

SCC growth above KISCC

strength steels in gaseous H2 aswell as other environment- material

systems appear to be independent

Type B primary driving force

for crack growth is not mechanical

the crack tip such as chemical,

electrochemical, mass transport,

diffusion and absorption processes

Region III

rate of SCC growth increases

rapidly with KI as KI approaches

Chapter 8 - EAC 16

IC C


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Corrosion Fatigue



By definition Fatigue in a corrosive environment

Corrosion fatigue can occur in most materials

Corrosion fatigue is time dependent therefore

wave shape

Crack Initiation and crack growth both affected by

Different from SCCEffect of environment and frequency on

S-N curve of Mild Steel Schijve p45

Chapter 8 - EAC 17


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Corrosion Fatigue Crack Growth



Effect of load shape

Crack Growth rates below KISCC under

various load shapes Barsom &Rolfe, p 321

Chapter 8 - EAC 18


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Prevention of Corrosion Fatigue failures



Isolate the environment and material metallic coatings paint ceramic and rubber

liners clading

Alter the severity of the environment chemically remove aggressive constituents

increase pH decrease temp, flow rate and concentration

Alter the surface characteristics introduce surface compressive stresses (reduces crack

initiation and propagation)

Substitute a more corrosion resistant material

design the components to prevent the initiation and propagation of cracks to a critical size

Chapter 8 - EAC 19


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Cathodic Protection

Welded T Plate

Initiation Life

Chapter 8 - EAC 20


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Cathodic Protection

Welded T Plate

Total Life

Chapter 8 - EAC 21


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Cathodic Protection

Welded T Plate

Total Life

vs Air Data

vs design curve

Chapter 8 - EAC 22


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Corrosion Fatigue failures



1968, failure of pressure bulkhead

Loss of control of rudder and elevators

85 fatalities no survivors

Corrosion prevention by sealing paste

and drain hole

Corrosion cracks started at A and grew

along the edge of the bonded strip

At cruising altitude temp difference

across the bulkhead - condensation

small quantities of water is trapped at edgeof bonded strip local corrosion damage

and thus fatigue cracks

Inspection of this area difficult If bonded strip bonded outside bulkhead

Corrosion resistance of this Al alloy poor Bonded strip better than riveted strip

Sealer applied to prevent corrosion

If sealer applied to edge of bonded strip

corrosion fatigue avoided

Ref. Schi ve, 475

Chapter 8 - EAC 23


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Hydrogen Embrittlement

Department of Mechanical &

Aerospace Engineering

Hydrogen is present as atmospheric humidity

At high temperatures atmospheric molecular hydrogen can

This is capable of diffusing though metallic crystals Atomic hydrogen can also be present as a result of:

Electrolysis

Electroplating

Corrosion reactions

,zirconium, the dissolved hydrogen reacts to form brittle hydridecompounds

In zirconium this can lead to delayed hydride cracking

Chapter 6 - Fatigue 24


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In 1974 water leaked from the primary transport system into thegas annulus

The Zircaloy-2 tubes were removed and it was found that the

Chapter 8 - EAC 25

ea was n e area o e ro e o n


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In a Pickering reactor there are 139 pressure tubes, each about 6 m (240 in) long,

. . . .

They are made from a Zirconium alloy with 2.5% niobium Zr-Nb.

The pressure tubes are joined to end fittings which are made of stainless steel.

Chapter 8 - EAC 26


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-



The stainless steel end fitting was

revealed in the tube

When rolling the tubes into the end

the parallel part of the end fittingsto produce an over-extended joint

stresses

The cause of the cracking was

Cracking (DHC)

Chapter 8 - EAC 27


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The stainless steel end fitting was

in the area of high residual stress

Hydrogen, normally in the pressuretubes had migrated to the area of

hi h residual stress and a smallcrack initiated

Propagation was by fracture of thehydrides which are brittle when

cold Once initiated the crack proceed to

grow through the wall thickness byrepeated formation and fracture ofthe hydrides when the system was

When the system was hot thehydrogen was in solution and crack

growth did no proceed

Chapter 8 - EAC 28


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The cause of the rolling problem was

into the tapered section of the end fitting

The residual stress reached levels of 700MPa

MPa

Fuel channel safety was not significantly

effected because of the leak-before-breakcriterion

Cracks were about 15-20 mm (0.6-0.8in.) in surface length and were confined to

a very small region

The crack len ths were si nificantl less

than the critical crack length of about 3 in. The leak-before-break criterion was well

demonstrated

Chapter 8 - EAC 29

f h l


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J.M. Barsom and S.T. Rolfe, Fatigue & Fracture Control in Structures,Prentice Hall, 1987.

P.A. Ross-Ross, The Investigation into the Cracking of Pressure Tubes inPickering Units 3 and4, From Steam to Space, CSME 1996.

Standard Method for Plane-Strain Fracture Toughness of MetallicMaterials. ASTM Specification E-399-83


D t t f M h i l &


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N.E. Dowling, Mechanical Behavior of Metals, Prentice Hall, Newersey, .

A. Baumel and T. Seeger, Materials Data for Cyclic Loading,Elsevier, Amsterdam, 1990.

. ,Prismatic Bodies with Arbitrary Non Linear Stress-Strain Law,

ASME Journal of Applied Mechanics, vol. 28, 1961, pp. 544-550.

K. Molski and G. Glinka A Method of ElasticPlastic Stress andStrain Calculation at a Notch Root, Material Science andEngineering, vol. 50, 1981, pp. 93-100.

A. Moftakhar, A. Buczynski and G. Glinka, Calculation of Elasto-

Plastic Strains and Stresses in Notches under MultiaxialLoading, International Journal of Fracture, vol. 70, 1995, pp.357-373.




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References



J.M. Barsom and S.T. Rolfe, Fatigue & Fracture Control inStructures, Prentice Hall, 1987.

Bannantine, J.A., Comer, J.J. and Handrock, J.L. "Fundamental ofMetal Fatigue Analysis (Prentice Hall, 1990)

mar- aess, . a gue an oo - s ore ee s ruc ures .(Tapir Publishers, Trondheim, Norway, 1985)




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ReferencesDepartment of Mechanical &


J.M. Barsom and S.T. Rolfe,

Fatigue & Fracture Control in Structures , Prentice Hall, 1987.

P.A. Ross-Ross,

e nves ga on n o e rac ng o ressure u es n c er ngUnits 3 and4 , From Steam to Space, CSME 1996.

Standard Method for Plane-Strain Fracture Toughness of Metallic

Materials .ASTM S ecification E-399-83

Brock, D.

"Elementary Engineering Fracture Mechanics"

2nd Edition, Martinus Ni hoff Publishers, 1982

Brock, D.

"The Practr ical Use of Fracture Mechanics

Kluwer Academic Publishers 1988

Anderson, T.L Fracture Mechanics - Fundamentals and Applications

2 nd Edition CRC Press Boca Raton 1995


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