PSI - Issue 77

Kumar C. Jois et al. / Procedia Structural Integrity 77 (2026) 405–412 Jois, et al./ Structural Integrity Procedia 00 (2026) 000–000

409

5

Table 2: Cohesive constitutive and damage properties from Höwer et al. [10]. E nn [MPa] E ss [MPa] E tt [MPa] t n0 [N/mm] t s0 [N/mm] t t 0 [N/mm] G IC [kJ/m2] G IIC [kJ/m2]

G IIIC [kJ/m2] 1.3

3500

7500

7500

15

55

55

1.4

1.3

3. Results 3.1. Void analysis

For the investigation of void formation, cylinders with a diameter of 350 mm were fabricated using various layup configurations, as shown in the Fig. 3. The cylinder diameter was chosen to replicate industrial-scale dimensions of a composite pressure vessel. Each cylinder was wound using four towpregs (resin pre-impregnated fibers), each subjected to a tension force of approximately 20 N.

Boundaries for void size (b)

(a)

a

32 o 32 o

C

b

5 cm

Fig. 3. (a) Wound cylinder and winding pattern; (b) Void size limitation based on band width.

Samples were extracted from different locations along the cylinder, and CT analysis was conducted to evaluate the void orientation distribution. The Fig. 4 (a) presents a representative result showing the angular distribution of voids for layers with winding angles of 30°, 45°, and 90°. The results indicate that the major axis of the voids aligns with the respective winding angle, suggesting that void formation is influenced by the fiber placement pattern. This alignment can be attributed to gaps between adjacent tows or successive winding passes within a layer. Such gaps may result from minor variations in the winding machine operation, inconsistencies in tow width, or a combined effect of both factors. Fig. 4 (b) presents the through-thickness void count histogram, overlaid with the laminate layup and approximate layer thicknesses. Light grey regions indicate helical layers, while dark grey denote hoop layers. Distinct spikes in void count are observed at the interfaces between helical and hoop layers, where large changes in winding angle occur. This can be attributed to mismatch in surface patterns between layers—similar winding angles allow better interlocking and fewer voids, whereas dissimilar angles create uneven interfaces that promote interlaminar void formation.