Fracture and Crack
Propagation in Weldments
Prepared by: DSc Dževad Hadžihafizović (DEng)
Sarajevo 2023
Fracture and Crack Propagation in Weldments
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Introduction
• Content
 Specific aspects of weldments
 Determination of fracture toughness
 Determination of the crack driving force
 Shallow crack propagation and fatigue
strength
 Shallow crack propagation and fatigue
strength
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Fracture mechanics of weldments: Specific aspects
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Potential welding defects of manual arc (stick) welding of typical mild steels from the
point of view of a failure analyst; according to Gagg (2005).
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ISO 5817: Arc welded joints in steel -Guidance on
quality levels for imperfections
• 26 different types of weld imperfections
• Can be assigned to distinct groups from the perspective of mechanical integrity
(a)Cracks and crack-like imperfections have to be avoided or –if they occur –are
immediately subject to fracture mechanics analysis
(b) Material imperfections which act as crack initiation sites of paramount
importance for fatigue strength and fatigue life analyses of paramount importance
for fatigue strength and fatigue life analyses
(c) Geometric discontinuities increase the local stresses, affect crack initiation,
propagation and final failure
(d) Imperfections which probably are of no effect on fracture or fatigue life
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Fracture mechanics of weldments: Specific aspects
• Inhomogeneous
• microstructure
The influence of welder
technique on the risk of slag
inclusions when welding with a
basic MMA (7018) electrode
Inhomogeneous
microstructure
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Material inhomogeneity
Figure according to T oyoda, 1998
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Fracture mechanics of weldments: Specific aspects
Crack along the coarse grain
structure in the HAZ
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Strength mismatch
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Strength mismatch
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Welding residual stresses
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Welding residual stresses
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Welding residual stresses
Residual stress profiles
1. Individual determination
2. Compendia (upper bound curves to literature data) to
literature data)
3. Membrane stress (as-welded: max. value: yield strength)
4. Post weld treatment: σ+σ≥σ
Membrane stress
(yield strength at annealing temperature + correction
for ratio of E modules at room & annealing temperatures
5. Mechanical post weld treatment
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Welding residual stresses
Types of misalignment:
(a)Axial misalignment between flat plates
(b)Angular misalignment between flat plates
(b)Angular misalignment between flat plates
(c)Angular misalignment in a fillet welded joint
Consequence:
Notch effect/local bending stress
 Strong effect of fatigue life and shallow crack
propagation
 Effect on long crack fatigue propagation and
(sometimes)on failure load
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Fracture toughness determination
Modifications compared to testing of non-welded
material
 Specimen geometries most appropriate for
weldments, e.g., shallow cracked bend specimens
 Weldment specific aspects of specimen
preparation such as the introduction of the notch,
minimization of residual stresses and
misalignment
 Generation of a straight crack front
 Validity criteria
 Required number of test specimens
 Strength mismatch effects for testing in the net
section yielding range
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Fracture toughness determination: Scheme
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Fracture toughness determination
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Fracture toughness determination
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Fracture toughness determination
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Fracture toughness determination
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Fracture toughness determination
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Fracture toughness determination
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Specific features because of strength mismatch
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Fracture toughness determination
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Definition of weld width H for other than prismatic welds
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Effect of strength mismatch on constraint and toughness
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Effect of strength mismatch on toughness and crack path deviation
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Stress-strain curves
Micro tensile tests
e.g., Kocak et al., 1998
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Specific features because of residual stresses
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Crack driving force and fracture assessment
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Crack driving force: R6 type assessment
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Mismatch corrected limit load FYM
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Mismatch corrected limit load FYM
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Primary and secondary stresses
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Crack driving force due to primary
and secondary stresses
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Interaction factor V
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Determination of V
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Fracture analyses including residual stresses
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Fracture analyses including residual stresses
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Fracture analyses including residual stresses
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Fracture analyses including residual stresses
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Initial defects in engineering alloys
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Weld discontinuities and defects
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Example: Weldment quality grades: VOLVO
Group Weld Quality Standard 181-0004, 2008
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Contributions to fatigue life
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Specific of mechanically short cracks
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Literature
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