Transcript Diapositive 1

ASTM : American Society of Testing and Materials
1
Introduction:
Assuming a small plastic zone compared to the specimen dimensions,
a critical value of the mode-I SIF may be an appropriate fracture parameter .
KC
KIC : plane strain fracture toughness
plane stress
plane strain
KC : critical SIF, depends on thickness
KI > KC : crack propagation
K IC
Specimen Thickness
KIC : Lower limiting value of fracture toughness KC
Material constant for a specific temperature and loading speed
2
 ASTM E 399 first standardized test method for KIC :
- was originally published in 1970
- is intended for metallic materials
- has undergone a number of revisions over the years
- gives specimen size requirements to ensure measurements in the plateau region
 ASTM D 5045 -99 is used here for plastic materials:
Standard Test Methods for Plane-Strain Fracture Toughness and Strain
Energy Release Rate of Plastic Materials
Many similarities to E 399, with additional specifications important for plastics.
 KI based test method ensures that the specimen fractures under linear elastic conditions
confined plastic zone at the crack tip
3
1- Test specimens
Two specimen configurations for D 5045 -99 :
• Three Point Bend specimen (SENB):
• Compact tension configuration (CT):
2 W B  4
0.45  a W  0.55
4
For valid KIC measurements, the specimens are designed to ensure :
- a small plastic zone size / specimen thickness B.
- that plane strain conditions dominate around the crack tip.
K 
B  2.5  IC 
 Y 
2
K 
a  2.5  IC 
 Y 
2
K 
W  a  2.5  IC 
 Y 
2
Y yield stress of the material for the temperature and loading rate of the test
B must be sufficient to ensure plane strain.
W-a must be sufficient to avoid excessive plasticity in the ligament.
1
Plastic zone length c from Irwin Model (plane strain): c 
3
From ASTM 399
 KI 



 Y
2
Plane stress condition if c = B
Plane strain condition if c < B/25
25  K I 
Thus, B 


3   Y 
2
K 
2.5  I 
 Y 
2
5
2- Precrack:
• Remark:
The theory considers an ideal plane crack with zero notch radius r  0
Notch in reality
r
KC decreases with decreasing r until a limiting value rC  6.3 mm
Below rC , KC approximately constant:
The notch can simulate the theoretical crack
6
• Chevron starter notch (see p4-26):
The chevron forces crack initiation at its center.
B
W
Natural crack
A sharp crack of length af is propagated by
fatigue such that:
0.45  a W  0.55
where
a  a0  a f
af
a0
a
During fatigue the maximum SIF must not
exceed 60 % of KIC
The measured crack length a is the average of the crack lengths measured at:
- the center of the crack front: a1
- the end of the crack front on each surface: a2 and a3
a2
B
W
a1
a3
a   a1  a2  a3  3
W
Alternatively,
B
a2
a1
a3
Measurements of B and W to 0.1% accuracy
7
• Laboratory specifications for the specimen
W = 48 mm
a0 = 0.45 W
A
A
B
A-A
90°
8
Chevron
3- Experimental procedure
• Expression for the calculation of the SIF (CT- configuration):
KQ 
PQ
BW
12
a
f 
W 
0.2  a W  1
f shape function for the CT specimen (see A2.5 p8)
Accurate within ± 0.65% over the range
KQ conditional or trial KIC value.
• Fatigue crack propagation:
 half a day to complete
P(t)
P(t) (N)
120
3 Hz
Crack front
30
P(t)
Time
9
• Test procedure:
Servohydraulic testing system (Instron) used with constant displacement rate imposed.
(see section 8.3)
Loading rate and test temperature should be noted.
Both displacement and load recorded simultaneity during the test.
Several specimens tested (at least three) : reproducibility
10
4- Interpretation of test record and calculation of KIC
Pmax : maximum load sustained by the specimen
AB : initial compliance C = u / P
AB’ : compliance 5% greater than AB
Evaluation of PQ :
Pmax within lines (AB) and (AB’): PQ = Pmax
Calculus of KQ
Pmax outside lines (AB) and (AB’): PQ = intersection of (AB’) and the load curve
Pmax / PQ < 1.1
Calculus of KQ
Pmax / PQ > 1.1
Test invalid
11
Calculation of the trial value KQ with the relation :
KQ 
PQ
BW 1 2
a
f 
W 
If moreover,
 KQ 
B, a, W  a  2.5 


 Y 
2
then
KQ
is equal to K IC
When the test result fails to meet these requirements, use a 1.5 times larger
specimen. (see 9.1.5)
See Testing summary table 3 , p6.
http://lmafsrv1.epfl.ch/Fracture_course2006/KIC%20testing/
12
Typical KIC values:
13