Fracture Mechanics
Session 13
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Material Testing (K1c,J1c & CTOD)
&
Crack detection by NDT
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Materials
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Why metals tested ?
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Ensure quality & properties
Detect the defects/cracks
Factor of Safety under worst condition
Prevent failure in use
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Material Quality for Design against
fracture
The material must have sufficient fracture
toughness, Energy release and displacement
properties (K1c, J1c & CTOD) so a noticeable crack
can be detected before failure
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Why NDTs are important in fracture
failures?
• Very hard to build a structure that is defect
free i.e. completely without cracks
– Cracks already in material (inclusions or voids).
– Cracks caused by shrinkage in castings and welding.
– Cracks caused by machining.
– Cracks caused by cyclic loading (fatigue).
– Cracks caused by corrosion.
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Motivation…
To what extent ?
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Roll of Toughness
• Fracture toughness, K1c, is the key material property and
Toughness is the resistance of a material to the propagation
of crack.
• A material with low fracture toughness, if it contains a
crack, may fail before it yields. Less energy absorbed prior
to fracture.
• A tough (high K1c) material will yield, work harden even
when cracked – the crack makes no significant difference
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UTM
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Specimen Specifications
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Specimen Specifications
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Notches
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Plastic Zone Size Measurement Techniques
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Plastic Zone Size Measurement Techniques
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Digital Image Correlation
Deformed Grain Measurements
Transmission Electron Microscopy
Micro hardness Measurements
Foil Strain Gauge Technique
Photo-elastic coatings
XRD
Moire- patterns
Surface replicas
Etching
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Non-destructive testing (NDT)
(crack detection)
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Liquid Penetrant inspection
Ultrasonic Testing
Radiographic Imaging
Magnetic Particle Inspection
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Example- CTOD
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Test Data
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Example
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ENERGY RELEASE RATE
Where
C = Compliance
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CTOD & J
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Example
Tensile panel 15mm thick 55 mm wide of 4340 steel with an edge
crack tested for KR measurement. Given material properties are: σY
= 1240 MPa and Kic=117.5MPa ݉. Following fracture loads and
the corresponding instantaneous crack sizes were recorded. Draw
the variation of KR with crack extension over the span of crack
growth. The specimen is under plane stress condition.