PSI - Issue 72

P.M.M. da Silva et al. / Procedia Structural Integrity 72 (2025) 69–76

74

3.3. Tensile fracture tests The DCB tests followed the ASTM D5528 standard in a mechanical test machine at 8 mm/min. The P -  curves were extracted from the software and the a values were obtained by photographs each 5 s with a Canon ® EOS 650D camera. Three approaches were used to obtain G IC . The compliance calibration method (CCM) and corrected beam theory (CBT) require measurement of a during the test, while the compliance-based beam method (CBBM) is based on an equivalent crack length ( a eq ) and only required P and  (Campilho et al. 2014). As shown in Fig. 6 (a), the random orientation of the fibers in the SMCs leads to some deviations and oscillations in the measured P during the tests.

1

50

40

0.8

30

0.6

0.4 G I [N/mm]

P [N]

20

10

0.2

0

0

0

10

20

30

40

50

60

30 40 50 60 70 80 90 100 110 120

a or a eq [mm] CBBM CCM CBT

δ [mm] P2 P4 P6

a)

b)

Fig. 6. P - δ curves for SMC 20% (a) and R -curves by each method in a DCB specimen with SMC 20% (b). The average values of the stabilization zone of the R curve gives G IC . Fig. 6 (b) highlights the R -curve by each method of a specimen in SMC 20%. The CBBM curve is offset to the right due to using a eq , which accounts for the fracture process zone (FPZ) (Monteiro et al. 2015). Due to polynomial fitting difficulties, the CCM curve is truncated. Table 5 summarizes G IC by the different methods. CBBM is most reliable by not requiring measuring a and including FPZ effects (Constante et al. 2015). The highest differences to this method were 10.8% (CCM) and 5.9% (CBT) in the SMC 20% data. Early failures were observed for SMC 15%, which prevented measuring a .

Table 5. G IC values obtainedby each method through DCB.

Material

CCM [N/mm]

CBT [N/mm]

CBBM [N/mm]

SMC 15% SMC 20% SMC 30% SMC 45%

-

-

0.1455 0.4721 0.4955 0.7912

0.5231 0.4962 0.8073

0.5149 0.4912 0.8012

Fig. 7 shows G IC via CBBM versus FVF. Increasing FVF leads to higher G IC . Between the limit FVF, the G IC improvement was 443.7%. This behavior may be considered excessive since the reinforcement fibers are parallel to the plane of crack propagation. However, the presence of fiber bridging in the SMC increases the energy to propagate a crack, thereby improving the fracture toughness of the composite (Sørensen and Jacobsen 1998).

1.0

0.8

0.6

0.4 G IC [N/mm]

0.2

0.0

0

10

20

30

40

50

FVF [%]

Fig. 7. FVF influence on G IC (CBBM).