ce 671 lect 13 bw
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Fracture Toughness Tests
CE671 – Lecture 13
Fracture MechanicsWhat is Fracture Mechanics?– Mathematical analysis of solids with
notches, cracks, or defects
Applied Stress Intensity Factor (SIF) ‘K’
Parameter used to assess or predict fracture due to applied stressCharacterizes applied stress at notches or cracks‘K’ has unusual units – ksi-√in or MPa-√m
Function of:– Size of crack or flaw– Nominal stress away from crack
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‘K’ as a Material PropertyKc or Kd material fracture toughness– Measure of material resistance to fracture
Kc - static or slow loadingKd - dynamic loading
Similar to σyield, E, or any other material property Characterized in terms of applied K at onset of fractureKI, KII, KIII refer to mode of loading
Material Tests to Obtain K
Plane strain fracture toughness– Used to obtain Klc or Kld
– Specimens sized to approximate plane strain
Thick specimens often requiredSize required is dependent on Klc
– Expensive tests to conduct
Requirements on Thickness
in 0.004 CTOD inksi 100 K
valueslfor typica 5" B b, thus
CTOD 1500 ksi 50 σfor i.e. σK 2.5 B b,
K 399 E ASTM
y
2
y
IC
IC
==
≥
≥=⎟⎟⎠
⎞⎜⎜⎝
⎛≥
All dimensions, a, B, and W are dependent on KIC, which is what is to be measured??
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Why is Thickness Important?
Must be thick enough so linear elastic behavior occurs so that effects of plastic zone ahead of crack can be neglectedNeed to ensure there is a triaxial tensile stress present so shear stress is very low compared to maximum normal stress– Ensures Mode I
Estimated from Charpy Tests (CVN)– Correlation gives dynamic fracture
toughness KId
Must convert to KIc for slower load ratesDrop the subscript “I” for plain strain except when K is from a valid fracture toughness test
Material Tests to Obtain K Cont’d
Compact Tension Test
See ASTM E399
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Three-point Bend Test
2.1W (Min) 2.1W (Min)
W
W/2 See - ASTM E-399
Charpy Testing
Not a direct measure of fracture toughnessExcellent for material screeningExcellent for quality controlLow cost and easy
Will likely remain favored method
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Since not a Direct Measure, we Need Correlation between ‘K’ and CVN
KId= [5(CVN)E]1/2
– KId = Dynamic fracture toughness (psi-in1/2)– CVN = Charpy energy (ft-lbs)– E = Modulus of elasticity (psi)
Valid for “lower shelf” and lower portion of transition regionNote that this yields Dynamic fracture toughness KId
Dynamic Fracture Toughness
Measure of material’s resistance to brittle fracture subjected to dynamic loading [(105 ksi√in)/sec] (Running Fracture)Bridge load rates are much slower– 0.2 to 1.0 sec
Since fracture toughness is affected by load rate, must account for slower loading rates in bridges– Interested in KIc – static to slow load rates
Impact Loading Rate Shift
Experimentally shown that slower load rates tend to shift toughness curves to left – i.e., increases toughness
Magnitude of the shift is function of yield strength of materialAccount for lower loading rate by applying “temperature shift”
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Impact Loading Rate Shift
Energy level at which temp. shift is measured
Impact
Slow Bend
Temperature shift in transition temp. range
Ft-lb shift in upper-shelf level
Test Temperature
Abs
orbe
d En
ergy
in C
VN T
est
Impact Loading Rate Shift
Calculate temperature shift as: – Tshift = 0.75 (215 - 1.5 σys)– 36 ksi < σys < 140 ksi
For common steels:– 36 ksi » 120 deg F– 50 ksi » 105 deg F
Barsom & Rolfe includes equations which account for strain rate– Tshift = (150 - σys)(ε')0.17
Example
Given:– Set of CVN data– Steel with σys = 40 ksi
Develop:– KId curve– KIc curve
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Charpy Testing
Charpy Testing
Example
Test Temperature (deg. F)-120 -100 -80 -60 -40 -20 0 20 40 60 80 100 120
CV
N E
nerg
y (f
t-lb)
0
20
40
60
80
100
120
140Test DataAASHTO Non-fracture CriticalAASHTO Fracture Critical
Step 1 – Develop Charpy curve from testing
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ExampleStep 2 – Apply relationship - KId= [5(CVN)E]1/2
to convert from CVN to KId
Temperature (deg. F)-240 -210-180 -150-120 -90 -60 -30 0 30 60 90 120
KIc (
ksi -
in1/
2 )
0
20
40
60
80
100
120
140
160KId Dynamic Load RateKIc Bridge Load Rate
ExampleStep 3 – Adjust to intermediate load rate
Apply temperature shift - Tshift = 0.75 (215-1.5 σys)
Temperature (deg. F)-240 -210-180 -150-120 -90 -60 -30 0 30 60 90 120
KIc (
ksi -
in1/
2 )
0
20
40
60
80
100
120
140
160KId Dynamic Load RateKIc Bridge Load Rate
Tshift = 116 F
Effect of Temperature on Fracture Toughness
Fracture toughness decreases with decreasing temperatureBridges are subjected to large temperature variationsMinimum service temperature is critical– Related to geographic location
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How does Specification address effect of temperature on fracture toughness?
Requires minimum CVN energy at various service temperaturesThree temperature “zones” defined as per AASHTOIndiana is Zone 2
Temperature Zone Designations
3-31 F to -60 F
2-1 F to -30 F
10 F and above
Temperature Zone
Min Service Temperature
Minimum Charpy requirements for bridges and buildings
Miminum Service Temperature
Material: -18oC -34oC -51oC
Joules@oC Joules@oC Joules@oC
Steel: non-fracture critical members*,** 20@21 20@4 20@-12
Steel: fracture critical members*,** 34@21 34@4 34@-12
Weld metal for non-fracture critical* 27@-18 27@-18 27@-29
Weld metal for fracture critical* 34@-29oC for all service temperatures
AISC: Jumbo sections and plates thicker than 50 mm** 27@21oC for all service temperatures
SAC: weld metal for seismic applications 27@16oC below service temperature
not to exceed -18oC
* These requirements are for welded steel with minimum specified yield strength up to 350 MPa up to 38 mm thick. Fracture critical members are defined as those which if fractured would result in collapse of the bridge.
** The requirements pertain only to members subjected to tension or tension due to bending.
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Charpy Specifications
To assure ductile/brittle transition temperature is below minimum service temperatureTemperature shift (38 C or 70 F) allowed because Charpy test has much higher strain rate than service loads
Questions ?
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