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Application of fracture mechanics to welds with crack origin at the weld toe: a review Part 1: Consequences of inhomogeneous microstructure for materials testing and failure assessment

  • U ZerbstEmail author
Research Paper
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Abstract

This two-part paper provides an overview on the state-of-the-art in the application of engineering fracture mechanics to weldments. This, of course, cannot be exhaustive but is limited to butt and fillet welds with crack initiation at weld toes. In the present first part, the authors briefly focus on the susceptibility of welds to cracks and other defects. Following this, they discuss in more detail the consequences of material inhomogeneity across the weld for fracture mechanics. Inhomogeneity causes scatter in fracture toughness and strength mismatch effects both of which have to be considered in fracture toughness testing, crack driving force determination, and fracture assessment of welded components. Part 2 of the paper series will add a discussion on welding residual stresses and questions of applying fracture mechanics to residual as well as total lifetime estimation of welds under cyclic loading.

Keywords

Fracture toughness Crack driving force Material inhomogeneity Strength mismatch 

Nomenclature

a

Crack length (crack depth for surface cracks)

B

Specimen thickness (fracture mechanics specimen)

C

Half crack length at surface (semi-elliptical crack)

CTOD

Crack tip opening displacement

da/dN

Fatigue crack propagation rate

E

Modulus of elasticity (Young’s modulus)

f(Lr)

Plasticity correction function (monotonic loading)

F(x)

Cumulative probability

FY

Yield or limit loads

H

Width or half width of the weld strip (strength mismatch consideration)

HV

Hardness according to Vickers

J

J-Integral

Jmat

Fracture resistance, monotonic loading (general term), Eq. (2)

J0.2;BL

Resistance against stable crack initiation (monotonic loading)

J0.2

Resistance against stable crack initiation (alternative definition)

K

Stress intensity factor (K-factor)

\( {K}_c^J \)

Monotonic fracture resistance (formally derived from J-integral)

K0

Scale parameter in 3-parameter Weibull distribution

Kmat

Fracture resistance, monotonic loading (general term), Eq. (2)

Kmax

Maximum K-factor in the loading cycle, cyclic loading

Kmin

Shift parameter in 3-parameter Weibull distribution

Kmin

Minimum K-factor in the loading cycle, cyclic loading

Kr

Ordinate of the FAD diagram (= K/Kmat)

Lr

Ligament yielding parameter (monotonic loading)

Lrmax

Maximum Lr (plastic collapse limit)

m

Shape parameter in 3-parameter Weibull distribution

M

Strength mismatch ratio (commonly σYW/σYB)

N

Number of loading cycles

N

Number of specimens in a statistical test set

Nc

Number of loading cycles up to fracture

P

Failure probability

R

Loading ratio (= σmin/σmax or Kmin/Kmax)

ReL

Lower yield strength (materials showing a Lüders’ plateau)

Tp

Peak temperature during welding

U

Energy dissipated in monotonic fracture mechanics test

W

Specimen width or half width (fracture mechanics specimen)

Δa

Crack extension

δ5

Definition of the CTOD

εa

Strain amplitude (=½ Δε)

ΔK

K-Factor range (Kmax − Kmin), cyclic loading

ν

Poisson’s ratio

σa

Stress amplitude (= ½ Δσ)

σapp

Applied stress

σmax

Maximum stress in the loading cycle, cyclic loading

σmin

Minimum stress in the loading cycle, cyclic loading

σref

Reference stress (reference stress approach, FAD)

ηp

Geometry function in monotonic J-integral testing

σ

Stress

σ0

Reference yield stress

σY

Yield strength, general (either ReL or Rp0.2)

σm

Hydrostatic stress

\( {\sigma}_Y^{\prime } \)

(Stabilized) Cyclic yield strength

σYB

Yield strength of base metal

σYW

Yield strength of weld metal

Abbreviations

ASTM

American Society for Testing and Materials

bcc

Body-centered cubic (lattice)

BM

Base metal

BS

The British Standards Institution

c

Critical

CG

Coarse grain (HAZ)

FAD

Failure assessment diagram

fcc

Face-centered cubic (lattice)

FG

Fine grain (HAZ)

h

Stress triaxiality

HAZ

Heat-affected zone

IIW

International Institute of Welding

ISO

International Organization for Standardization

M(T)

Middle crack tension (fracture mechanics specimen)

NASGRO

Computer program for fatigue crack propagation, provided by NASA

OM

Strength overmatching (σYW > σYB)

R-curve

Crack resistance curve

TTT

Temperature-time-transformation (diagram)

UM

Strength undermatching (σYW < σYB)

WM

Weld metal

Notes

References

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Copyright information

© International Institute of Welding 2019

Authors and Affiliations

  1. 1.Bundesanstalt für Materialforschung und -prüfung (BAM)BerlinGermany

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