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Fracture Toughness Testing
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Organization for Expts. 5 and 7 (weeks of 4/2 and 4/16)
Experiment 5• Lab in 127 Norris• Can visit lab in advance
Monday 4/2 and Monday 4/16 from 9am-12pm
• Submit logbook preparation to TA in advance as normal
• Two teams working together to test three samples. Lab is over when samples tested and logbooks complete (may not take full lab period).
• Your regular TA will be there.• Each team submits their
completed logbook to their TA before leaving lab
Experiment 7• Lab in Stability Tunnel• Can visit in advance Friday
3/30 and Friday 4/13 from 8am to 12pm and 1pm to 5pm.
• Submit logbook preparation to TA in advance as normal
• First team* arrives at start of lab period. Has 75 mins to complete test. Second team arrives 90 minutes after start of lab, has 75 mins for test.
• Regular TA not there. Submit logbook to wind tunnel operator before leaving lab
* First team is the team with the lowest number on the experiment schedule
Don’t forget laptops
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Organization for 5th Instrumentation Lab Period (week of 4/9)
• Lab in Randolph 25 (next to the open-jet tunnel)• Apply digital data acquisition and processing, including
your own LabView programs, to the dynamic beam structure of Experiment 6
• Works just like a regular experiment– Read manual (chapter 5 of instrumentation lab)– Meet with your team in advance– Visit the lab– Do a logbook preparation– Logbook submitted and end of lab is graded
• Note your logbook preparation should be submitted to Dustin Grissom ([email protected]) not your regular TA.
Don’t forget laptops
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Fracture
• Breaking of structural components into two or more parts
• Brittle fracture - low energy absorption• Ductile fracture - large energy absorption
DEFLECTION
LOAD× ×
××
BRITTLE
DUCTILE
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Tanker SS Schenectady(24 hours after launch)
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Aloha Airlines
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Aloha Airlines
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Why does fracture occur?
• Load increases to a point where cracks grow catastrophically.
• There are always cracks– They form as part of the manufacturing– They develop over time as a result of fatigue
• The strength of materials in the presence of cracks is therefore critical in defining when they will fail.
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F-111 Crack
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The Ideal CrackThe Linear Elastic Fracture Mechanics (LEFM) Approach
x
σ (uniform background stress)
Infinite flat plate
Plastic zone (radius r0)
y
Local stressσx, σy
Distance from crack x
2a
xK
yx πσσ
2, =
Elastic solution
aK πσ=where
Plastic zoneMaterial yields
Even though the crack is embedded in a plastic zone, it is the elastic solution, in particular K that determines the stress field through which the crack would advance
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Stress Intensity Factor K
• Scale of the elastic stress field generated by the crack
• Units of – Pa√m (usually MPa√m)– p.s.i.√in (usually k.s.i.√in)
• The stress intensity when the crack advances catastrophically is a measure of the strength of the material in the presence of a crack.
• This is called the Fracture Toughness Kc
The Linear Elastic Fracture Mechanics (LEFM) Approach
Local stressσx, σy
Distance from crack x
xK
yx πσσ
2, =
Elastic solution
aK πσ=where
Plastic zoneMaterial yields
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Effect of Thickness
x
σ (uniform background stress)
Infinite flat plate
Large plastic zone radius r0
y2a
Thin Plate
σz=0
σy
t ≤ r0
(Relieved by Poisson
contraction) Fracture by tearing on 45o
plane
Deformation in z results in:• stresses in only x and y - Plane Stress• relief of some of the stress around crack• a large fracture toughness, that depends
on thickness• ductile fracture takes place by tearing on
45o plane
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Effect of Thickness
x
σ (uniform background stress)
Infinite flat plate
Small plastic zone radius r0
y2a
Thick Plate
σz≠0 but εz=0
σy
t>>r0
(Poisson contraction
prevented by surrounding
material)
Fracture by cleavage
Lack of deformation in z results in:• strain in only x and y - Plane Strain• no relief of stress around crack• a lower fracture toughness, that is
independent of thickness• brittle fracture on horizontal plane
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Effect of ThicknessFracture toughness Kc
Thickness t
KIc
Plane Strain Fracture toughness
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KIc for different materials
* heat treated for higher strength
14 2070 52100
60 1515 *4340
99 860 4340 Steel
55 1035 *Ti-6Al-4V
115 910 Ti-6Al-4V Titanium
24 495 7075-T651
26 455 2024-T851 Aluminium
MPa√mMPa
toughness yield material
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Experiment 5• Objective
– To measure this material property, the plain strain fracture toughness, for Aluminum
• Approach– Break samples of different thickness using fracture
toughness testing procedures specified in ASTM Standard No. E399
• Organization– Two teams of students have 3 samples between them
• Location– ESM Materials Lab, Norris 127
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Samples• Measure
displacement vs. load until sample breaks.
• Identify load at fracture PQ
• Use this to determine fracture toughness
‘Machine’ crack
Real crack
Load P
Displacement
v
Pre-crack generated by fatiguing the sample
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Load and DisplacementExtensometer (senses crack opening displacement)
Testing machine(applies displacement, senses force)
Jaws fit in notches in side of sample
Clevis grips hold sample in machine
Computer with LabView (reads and records load and displacement)
Controller and A/D
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Broken SamplesPre-crackPre-crack
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Load at Fracture?• Ductile or brittle?
• Elastic or plastic?
• Note that deciding when fracture occurs involves some choices.
0.0
0.2
0.4
0.6
0.8
1.0
0.00 0.01 0.02 0.03 0.04 0.05
0.0
1.0
2.0
3.0
0.00 0.01 0.02 0.03 0.04 0.05
Displacement
Load
Load
‘Pop in’
Loadcell range 10 kipsExtensometer range 0.075“
Manufacturer gives:Loadcell accuracy 10lbsExtensometer accuracy 0.000075”
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Fracture Toughness?
Load P• Real sample is finite and
applied stress is not uniform• However, if plastic zone is
small enough then a region exists where stress field behaves as though sample is infinite.
• The stress intensity in this region is a function of the applied load and the sample geometry. This function is known in the form of a curve fit, derived from finite element analysis.
Plastic zone
Region where stress field influenced by sample edges
Region where stress behaves as though sample infinite (region of K dominance)
tw
ai
⎟⎠⎞
⎜⎝⎛=
wa
fwt
PK i
aK πσ=In ∞ case
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Ex. 5 Summary• Pre-defined goal and procedures• Two teams collaborate to break 3 samples• Measure/photo samples before and after
fracture• Analyze the load vs displacement curves and
the sample dimensions are analyzed to yield– The fracture toughness– The plastic zone size– Whether the sample is in plane stress or plane strain
• When you have completed all analysis, and have a plot of fracture toughness vs. sample thickness you are done.