Materials Science &Technology

FATIGUE CRACK PROPAGATION FATIGUE CRACK PROPAGATION

BEHAVIOR OF BRAZED STEEL JOINTSBEHAVIOR OF BRAZED STEEL JOINTS

Dr. Dr. Tanya ATanya Aycan ycan BaşerBaşer

EMPAEMPA--Swiss Federal Laboratories for Materials Testing and ResearchSwiss Federal Laboratories for Materials Testing and Research

Laboratory for Joining Interface and Technology, Dübendorf, SwitzerlandLaboratory for Joining Interface and Technology, Dübendorf, Switzerland

CERNTR, 09/03/11

OutlineOutline

Introduction•Mechanical properties of materials

• What is fatigue?

• Why brazing?

• Problems occurred during brazing

• Application area of brazing

Materials and Methods• Base material and filler metals

• Brazing and heat treatment

• Experimental procedures

Results and discussion• Fatigue Crack Propagation (da/dN-ΔK) Curves

• Microstructural Investigations

• Fractographic Investigations

Brazing Quality• Effect of shield gas on mechanical properties

• Effect of sample geometry on mechanical properties

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IntroductionIntroduction

MALZEMELERİN MEKANİK ÖZELLİKLERİ

GEVREK-SUNEK DAVRANIS

()

()0 2 4 6 8 10 12 14 16 18 20

0

500

1000

1500

2000

Str

ess / M

Pa

Strain / %

Ela

sti

k b

olg

e

Plastik bolge

x Akma dayanimi

Cekme dayanimix kopma dayanimi

GERILIM-GERINIM DIYAGRAMI

Cekme testi

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IntroductionIntroduction

YORULMA NEDİR?

Malzemeyi zorlayan gerilmeler zaman icinde degisecek olursa malzeme cekme deneyinde elde

edilen kopma degerinin altinda bir gerilmede sunek de olsa plastik sekil degistirmeden kirilabilir. Bu olaya

yorulma denir.

Yukleme ve bosalmanin periyodik olarak cok sayida tekrari sonucunda malzemede yipranmalar

meydana gelir. Bunun nedeni yukun siddetinden cok onun periyodik olarak uzun bir sure uygulanmasidir.

İc mekanizmasi oldukca karisik olan bu olaya malzemenin yorulmasi denir.

Yorulma 3 asamada gerceklesir:

1-Catlak baslangici

2-Catlak ilerlemesi

3-Kirilma

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IntroductionIntroduction

Welding fusion takes place with melting of both the base metal and usually a filler metal. To

join metals by applying heat, sometimes with pressure and sometimes with an intermediate or

filler metal having a high melting point.

Brazing is a process for joining similar or dissimilar metals using a filler metal and heating

them the liquidus of the filler metals above 840°F (450°C), and below the solidus of the base

metals

Soldering has the same definition as brazing except for the fact that the filler metal used has

a liquidus below 840°F (450°C) and below the solidus of the base metals.

WHY BRAZING?

Brazing process is used because of the compressor

impeller geometry

Brazing

Defects such as incomplete gap filling, pores or

cracks may be formed during brazing and they

can act as stress concentration sites in the

brazing zone.

Under cyclic mechanical loading, fatigue cracks

can initiate and propagate from these defects,

leading to spontaneous failure.

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IntroductionIntroduction

/www.airbus.com/

/www.geae.com/

/www.nasa.org/

/www.manturbo.com/

Brazing is a quick and low-cost brazing method to produce strong joints and it is used in the aerospace and

other industries as well as for power generation, e.g. turbine parts or compressor impellers.

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Materials and MethodMaterials and Method

Base material and filler metals

The soft martensitic stainless steel X3CrNiMo13-4 was used as base material.

Chemical composition of X3CrNiMo13-4

As filler metal, foils of the binary alloy Au-18Ni with a thickness of 100 μm were applied (Melting

temperature is around 800 °C).

Brazing and heat treatment

Brazing was performed in an industrial shielding gas (93 vol.-% Ar, 7 vol.-% H2) furnace at a

temperature of 1020°C for 20 minutes.

After brazing, the specimens were tempered at 520 °C for 5.5 h in nitrogen atmosphere.

Element C Si Mn P S Cr Mo N Ni

Min. 12.00 0.30 0.02 3.50

Max. 0.05 0.70 1.50 0.04 0.01 14.00 0.70 4.50

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Materials and MethodMaterials and Method

Experimental procedures

Fatigue crack propagation tests were performed on a resonant testing machine.

Fractured specimens were investigated by SEM.

Geometry of the DCB specimen

(90 x 60 x 8 mm)Set-up of the fatigue crack propagation test

ASTM E647

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nKCdN

da

1 10 1001E-11

1E-10

1E-9

1E-8

1E-7

1E-6

1E-5

1E-4

K [MPa m1/2

]

da/d

N [m

/cyc

le]

R = 0.1 R = 0.3 R = 0.5 R = 0.7

R C n

0.1 1.309E-22 11.17

0.3 4.071E-23 12.17

0.5 7.234E-22 12.64

0.7 8.489E-21 12.81

Calculated C and n parameters at different R values

The Paris equation:

ResultsResults

• Fatigue Crack Propagation (da/dN-ΔK) Curves

5.02

5.01225.138

h

a

h

a

Bh

FK I

da: difference of crack length

dN: difference of number of cycles

C,n: experimentally measured material constants

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ResultsResults

R. H. Dauskardt, Acta Metall Mater. Vol 41 9 (1993) 2765.

1 101E-11

1E-10

1E-9

1E-8

1E-7

1E-6

1E-5

K [MPa m1/2

]

da/d

N [m

/cycle

]

R = 0.1 R = 0.3 R = 0.5 R = 0.7

Material n

Metals 3-4

Ceramics -50

The Paris Exponent

Brazed components 11-13

?

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SEMSEM

Taramali elektron mikroskobu

Çalışma Prensibi

Taramalı Elektron Mikroskobu üç temel kısımdan oluşmaktadır:

Optik Kolonda elektron demetinin kaynağı olan elektron tabancası, elektronları

numuneye doğru hızlandırmak için yüksek gerilimin uygulandığı anot plakası, ince

elektron demeti elde etmek için yoğunlaştırıcı mercekler, demeti numune üzerinde

odaklamak için objektif merceği, bu merceğe bağlı çeşitli çapta apatürler ve

elektron demetinin numune yüzeyini taraması için tarama bobinleri yer almaktadır.

Numune hucresine numune yerlestirilir.

Görüntü sisteminde elektron demeti ile numune girişimi sonucunda oluşan

çeşitli elektron ve ışımaları toplayan dedektörler, bunların sinyal çoğaltıcıları ve

numune yüzeyinde elektron demetini görüntü ekranıyla senkronize tarayan

manyetik bobinler bulunmaktadır.

Nasil goruntu elde edilir?

Taramalı Elektron Mikroskobunda (SEM) görüntü, yüksek gerilim ile

hızlandırılmış elektronların numune üzerine odaklanması, bu elektron

demetinin numune yüzeyinde taratılması sırasında elektron ve numune

atomları arasında oluşan çeşitli girişimler sonucunda meydana gelen

etkilerin uygun algılayıclarda toplanması ve sinyal güçlendiricilerinden

geçirildikten sonra bir katot ışınları tüpünün ekranına aktarılmasıyla elde

edilir.

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ResultsResults

•Microstructural Investigations

SEM-cross section

20 µm

Au-rich

Solid solution

Ni-rich

Solid solution

100 µm

X3CrNiMo13-4

steel

Au-18Ni

braze Diffusion zone

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ResultsResults

• Microstructural Investigations

SEM-cross section

20 µm20 µm

5 µm

Crack tip

20 µm

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ResultsResults

•Microstructural Investigations

SEM-cross section

200 µm

100 µm 20 µm

200 µm

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ResultsResults

• Microstructural Investigations

SEM-cross section

20 µm

Crack tipCrack tip

32 32 µmµm

95 95 µmµm20 µm

Pre-damaged zones well

ahead of the crack tip

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20 µm 50 µm

20 µm

ResultsResults

200 µm

• Fractographic Investigations

SEM-cross section

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ResultsResults

• Fractographic Investigations

200 µm

The stepped nature of the fracture pattern is clearly evident

200 µm

porespores

3 mm

stereo SEM

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Fractographic InvestigationsBrittle fracture ductile fracture

1018 steel

transcrystalline steel

BMG

Steel wire

Al alloy

Cu alloy

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ResultsResults

• Fractographic Investigations

Brittle or ductile?

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ResultsResults

• Fractographic Investigations

20 µm

Plastic deformation features

containing ductile dimples

50 µm

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DiscussionDiscussion

Damage and Fracture Behaviour of Brazed

Joints Under Cyclic Loading

• After crack initiation, high stresses can lead

to the formation of cavities well ahead of the

crack tip.

• New cavities develop and grow every loading

cycle.

• The fatigue crack then propagates along

these predamaged zones and coalescence

and final failure occurs.

X3CrNiMo13-4

X3CrNiMo13-4

notchAu-18Ni

High Paris exponent, n, was explained by the triaxial stress state in the filler metal, which is a

result of the different elastic-plastic material properties of the filler metal and the base material.

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Brazing qualityBrazing quality

Defects such as incomplete gap filling, pores or cracks may be formed

during brazing and they can act as stress concentration sites in the brazing

zone.

Under cyclic mechanical loading, fatigue cracks can initiate and propagate

from these defects, leading to spontaneous failure. Therefore, defect

assesment of brazed components should be considered.

200 µm

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Brazed steel plates of different batches

brazed steel

platesshielding gas

Batch A 93 vol.-% Ar, 7 vol.-% H2

Batch B, C 93 vol.-% Ar, 7 vol.-% H2

Batch D, E 100 vol.-% H2

Batch F, G 100 vol.-% H2

Brazing qualityBrazing quality

The addition of hydrogen to the argon allows removing the oxide film on the

stainless steel surface, which is essential for filler metal wetting. BUT....

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100 100 µmµm

(93 vol.-% Ar, 7 vol.-% H2)

100 100 µmµm

(100 vol.-% H2)

Brazing qualityBrazing quality

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Materials and MethodMaterials and Method

Base material and filler metals

The soft martensitic stainless steel X3CrNiMo13-4 was used as base material. As filler metal, foils of the

binary alloy Au-18Ni with a thickness of 100 μm were applied.

Brazing and heat treatment

Brazing was performed in an industrial shielding gas (93 vol.-% Ar, 7 vol.-% H2 and 100 vol.-% H2 ) furnace at

a temperature of 1020°C for 20 minutes. After brazing, the specimens were tempered at 520 °C for 5.5 h in

nitrogen atmosphere.

Experimental procedures

Fatigue tests were performed on a standard electro-mechanical and servohydrolic testing machine.

Fractured specimens were investigated by SEM.

Geometry of the t-joint specimen

(t=16mm, W=8 mm) standart electro-mechanical

testing machine

Servohydrolic

testing machine

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Brazing qualityBrazing quality

! In general, fracture occured on the base material instead

of the brazing zone

The addition of hydrogen to the argon

allows removing the oxide film on the

stainless steel surface

Joint strength was improved under 100% H2

atmosphere

crack

Brazing zone

σnom = 700 MPa

N = 8304

σnom = 650 MPa

N = 11787

σnom = 700 MPa

N = 8642

(100 vol.-% H2)

Specimen

geometryshielding gas Rm [MPa]

Base material - 975±25

T-joint93 vol.-% Ar, 7 vol.-% H2 882±15.7

100 vol.-% H2 1120±4.8

Standart round 100 vol.-% H2 1084±3.6

brazing

standart round specimen

(Ø1=5 mm, Ø2=4 mm)

CERNTR, 09/03/11

S-N curves

102

103

104

105

200

300

400

500

600

700

800

900

1000

1100

1200 T-joint-defect free-93 vol.-% Ar, 7 vol.-% H

2

T-joint-defect free-100 vol.-% H2

round shape 100 vol.-% H2

Nf

n

om

, m

ax (

MP

a)

• The only difference in between these specimens is different shielding gases.

• Brazing quality was improved under 100 vol.-% H2

• Fracture occurred on the base material in specimens which were brazed under 100 vol.-% H2

Nu=20 000 cycles

Brazing qualityBrazing quality

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stereo

ResultsResults

Fractographic Investigations- Comparison

SEM

(100 vol.-% H2)

3 mm

σnom = 650 MPa

N = 11787

200 µm

The step fractured pattern

could not be observed

200 µm

(93 vol.-% Ar, 7 vol.-% H2)

3 mm

σnom = 400 MPa

N = 5630

The step fractured pattern

Stronger bonding was

obtained and better

interface reaction

occured under 100% H2

atmosphere

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TEŞEKKÜRLER

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AdditionalAdditional--II

Rp0.2

[MPa]

Rm

[MPa]

A5

[%]

τe

[MPa]

τmax

[MPa]

KIc

[MPa

m0.5]

X3CrNiMo-13-4 920 ± 5 975 ± 25 17.5 ± 2.5 620 ± 5 660 ± 10 ~270

X3CrNiMo13-4 -AuNi18 923 ± 7 976 ± 15 6 ± 0.5 245 ± 10 539 ± 7 49 ± 1.5

Element C Si Mn P S Cr Mo N Ni

Min. 12.00 0.30 0.02 3.50

Max. 0.05 0.70 1.50 0.04 0.01 14.00 0.70 4.50

Table 1. Chemical composition of X3CrNiMo13-4.

Table 2. Mechanical properties of X3CrNiMo 13-4 and X3CrNiMo13-4 – Au-18Ni

braze joints.

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5.02

5.01225.138

h

a

h

a

Bh

FK I

The stress intensity for mode I loading, KI, as a function of the specimen geometry and the applied

load can be calculated according to;

where F is the applied force, B and h the specimen geometry and a the total crack length measured

from the load initiation point.

AdditionalAdditional--IIII

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0 75 150 225 3000

10

20

30

40

batch 1 batch 2 batch 3

N (103)

a [m

m]

0

10

20

30

40

batch 1 batch 2 batch 3

0

10

20

30

40

batch 1 batch 2 batch 3

ΔF1, ΔF2, ΔF3=constantΔF1 = 6.8 kN

ΔK1 = 23 MPa m1/2

ΔF2= 5.7 kN

ΔK2= 19 MPa m1/2

ΔF3= 4.6 kN

ΔK3= 16 MPa m1/2

ResultsResults

• Fatigue Crack Growth Curves

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0 50 100 150 200 250 3000

5

10

15

20

25

30

35

F1=6.8 kN

F2=5.7 kN

F3=4.6 kN

a (

mm

)

N [103]

ResultsResults

The fatigue crack growth rate of brazed components is extremely sensitive to

the load range.

• Average of Fatigue Crack Growth Curves (batch 1, 2 and 3)

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M. C. Tolle, M. E. Kassner, Acta Metall. Mater. Vol 43 (1995) 287.

Nucleation based theory for ductile fracture

under high triaxial stress

(a) A few nanometer-sized cavities nucleate with

small plastic strain.

(b) Additional nucleation occurs with a small

additional macroscopic strain.

(c) At a critical stage, cavities get sufficiently

close, and there is a coalescence between the

small nanometer-sized cavities, and larger

cavities form.

(d) Additional nucleation occurs in regions near

the cavities and further interlinkage occurs.

(e) The interlinkage of larger cavities through

continued nearby nucleation leads to final

failure.

Nucleation based theory for ductile fracture under high triaxial stress

Mechanism of ductile fracture in pure silver under high-triaxial stress states under static loading

AdditionalAdditional--IIIIII

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AdditionalAdditional--IVIV

Damage and Fracture Behaviour of Brazed

Joints Under Cyclic Loading

ΔK1

ΔK2

Δa

BULK MATERIALS

BRAZED JOINTSAu-18Ni

ΔK1

ΔK2

ΔK1 < ΔK2

Δa`

Δa` >> Δa

a1

ΔK1 < ΔK2

X3CrNiMo13-4

X3CrNiMo13-4

a1

CERNTR, 09/03/11