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1 ASTM : American Society of Testing and Materials.

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1 1 ASTM : American Society of Testing and Materials

2 2 Assuming a small plastic zone compared to the specimen dimensions, a critical value of the mode-I SIF may be an appropriate fracture parameter. K IC : plane strain fracture toughness plane strain K C : critical SIF, depends on thickness plane stress K I > K C : crack propagation Specimen Thickness KCKC K IC : Lower limiting value of fracture toughness K C Material constant for a specific temperature and loading speed Introduction:

3 3 Standard Test Methods for Plane-Strain Fracture Toughness and Strain Energy Release Rate of Plastic Materials  K I based test method ensures that the specimen fractures under linear elastic conditions confined plastic zone at the crack tip Many similarities to E 399, with additional specifications important for plastics.  ASTM E 399 first standardized test method for K IC : - was originally published in 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 is used here for plastic materials:

4 4 Two specimen configurations for D : Three Point Bend specimen (SENB): Compact tension configuration (CT): 1- Test specimens

5 5 For valid K IC measurements, the specimens are designed to ensure : B must be sufficient to ensure plane strain. yield stress of the material for the temperature and loading rate of the test W-a must be sufficient to avoid excessive plasticity in the ligament. - a small plastic zone size / specimen thickness B. - that plane strain conditions dominate around the crack tip. Plastic zone length c from Irwin Model (plane strain): Plane stress condition if c = B From ASTM 399 Plane strain condition if c < B/25 Thus,

6 6 2- Precrack: K C decreases with decreasing  until a limiting value  C  6.3  m The theory considers an ideal plane crack with zero notch radius  Notch in reality Below  C, K C approximately constant: The notch can simulate the theoretical crack Remark:

7 7 Chevron starter notch (see p4-26): The chevron forces crack initiation at its center. A sharp crack of length a f is propagated by fatigue such that: where Natural crack a0a0 W a The measured crack length a is the average of the crack lengths measured at: - the center of the crack front: a 1 - the end of the crack front on each surface: a 2 and a 3 During fatigue the maximum SIF must not exceed 60 % of K IC B a1a1 a2a2 a3a3 B W Measurements of B and W to 0.1% accuracy Alternatively, a2a2 a3a3 B W a1a1

8 8 Laboratory specifications for the specimen W = 48 mm a 0 = 0.45 W A A A-A B Chevron 90°

9 9 3- Experimental procedure Expression for the calculation of the SIF (CT- configuration): f shape function for the CT specimen (see A2.5 p8) Accurate within ± 0.65% over the range K Q conditional or trial K IC value. Fatigue crack propagation: P(t) Time P(t) (N)  half a day to complete 3 Hz Crack front

10 10 Servohydraulic testing system (Instron) used with constant displacement rate imposed. Several specimens tested (at least three) : reproducibility Both displacement and load recorded simultaneity during the test. Loading rate and test temperature should be noted. Test procedure: (see section 8.3)

11 11 4- Interpretation of test record and calculation of K IC P max : maximum load sustained by the specimen AB : initial compliance C = u / P AB’ : compliance 5% greater than AB Evaluation of P Q : P max within lines (AB) and (AB’): P Q = P max P max outside lines (AB) and (AB’): P Q = intersection of (AB’) and the load curve Calculus of K Q P max / P Q < 1.1 Calculus of K Q P max / P Q > 1.1 Test invalid

12 12 Calculation of the trial value K Q with the relation : If moreover, then is equal to 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.

13 13 Typical K IC values:


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