PSI - Issue 25

Jamaloddin Jamali et al. / Procedia Structural Integrity 25 (2020) 180–185 Author name / Structural Integrity Procedia 00 (2019) 000–000

184

5

Where P is the maximum or fracture load. w and t are the width and the thickness and a is the crack length. The angle α shows the loading angle in the above equations. The effective fracture toughness and overall toughness can be calculated from the mode I and mode II values. In this work the testing was carried out on two cases first was pure mode I (α=0°) and the second specimen was under mixed mode I/II loading where the force was applied at angle of 15 degree (α=15°).

Table 1. Mechanical Properties of Woven Carbon Fiber Epoxy Max. stress(MPa) Fracture strain

E (GPa)

469.73

0.060

9.1

Table 2. Fracture Properties of Woven Carbon Fiber Epoxy Angle of loading → Mechanical properties ↓ Maximum force (N)

0° 15°

K � ������ √m� �� ������ √m� ��� ������ √m� K K

9363.7

8334.5

34.86

30.08

0

4.212

34.86

30.37

The moving cross head displacement rate for these tests were 5 mm/min and the displacement for each point of the crack front and notch was measured using the method explained in part VI (displacement tracking). The maximum force at which crack opens up was measured by a 50kN load cell. The maximum forces that open the crack, the effective fracture toughness as well as the values of critical energy strain release rate for the composite are provided in Table 2. For the calculation of fracture toughness using equations 1 and 2, the crack length was 35 mm, the width of the

specimen was 75mm and the specimen thickness was 3.1 mm were used. The effective fracture toughness was calculated using below relation: ��� � � � � � � � �

(3)

In both cases of loading it was observed that the carbon fiber epoxy specimen starts fracture at notch tip. For pure mode I, where α = 0°, the crack propagated along the same direction of the notch of the specimen. For the mixed mode loading, where α = 15°, the crack propagated with an angle of 7 degree with regard to the direction of the notch. Fig. 4a shows the specimen under pure mode I with the crack propagation along α = 0° and Fig. 4b shows the specimen under mixed mode I/II loading which has a crack that extended with an angle of almost 7 degree with regard to the direction of the notch. The drop in the value of effective fracture toughness matches with previous studies [7].

7. Conclusion The mixed mode fracture behavior of woven carbon fiber epoxy was studied in this work. The testing was performed using a compact tension test which was 3D designed already. The value of maximum force resulting in the crack