PSI - Issue 68

Reza Afsharnia et al. / Procedia Structural Integrity 68 (2025) 1153–1158 Reza Afsharnia et al. / Structural Integrity Procedia 00 (2025) 000–000

1155

3

Considering the reinforcement fibers in the matrix as a defect, then the first term in Eq.2 ( N i ) could be neglected and the number of cycles to failure could be calculated using Model II, by just considering the crack growing regime. The number of cycles to failure while the crack is growing ( N a ), if the critical crack length is much bigger than the initial crack length ( a crit >> a i ), could be calculated using Eq.3 (Haibach 2006).

!

"

"

#

!

!

Eq. 3

"

! " # $ %' &( $" $#

%

!

"# "C

! $ %

!

= !

Where m is the exponent from Paris’s low (

).

3. Materials and methods For this study, two different types of short fiber reinforced polymers (SFRP) were used: polypropylene reinforced with 20 wt.% short glass fibers (PPGF20) and polypropylene reinforced with 40 wt.% short glass fibers (PPGF40). The materials were supplied in the form of injection-molded plates featuring a skin-core-skin structure, with dimensions of 120 x 80 x 2 mm. Specimens for both fatigue and fracture mechanics tests were extracted in two different orientations to capture the effects of fiber alignment. The specimens for fracture mechanics tests were prepared according to ASTM E647-11. Figure 2 illustrates the positioning of the specimens on the injection-molded plates.

(a)

(b)

Figure 2. Position of the (a) fatigue test specimens and (a) fracture mechanics tests on the plates in longitudinal (L) and transversal (T) direction

Both fatigue and fracture mechanics tests were conducted at room temperature with a stress ratio of R = 0.1. The testing frequency was set at 3 Hz for fatigue tests and 5 Hz for fracture mechanics tests. During the experiments, the temperature was continuously monitored to ensure that the temperature increase due to viscoelasticity remained below 5°C. Figures 3 (a) and (b) illustrate the testing setups for both the fatigue and fracture mechanics tests. Temperature measurements for the fatigue tests were performed using a thermocouple, while both temperature and crack growth measurements for the fracture mechanics tests were conducted with an IR camera.