PSI - Issue 64

Omid Hassanshahi et al. / Procedia Structural Integrity 64 (2024) 81–88 Hassanshahi et al. / Durability of GFRP Composites Produced by Pultrusion under Thermal Environments

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3.1. Effects on tensile strength The tensile strength retention of the GFRP_UP and GFRP_VE composites is illustrated in Fig. 2a (GFRP_UP) and Fig. 2b (GFRP_VE) for the longitudinal direction (0°), and in Fig. 3a (GFRP_UP) and Fig. 3b (GFRP_VE) for the transverse direction. In general, Fig. 2 demonstrates that both composites exhibited minimal variation in longitudinal tensile strength when subjected to constant temperatures and thermal cycles, with tensile strength retention values generally above 90% for both types of matrices.

Table 1. Mechanical properties of references (unaged) series for both UP and VE matrices.

GFRP_UP

GFRP_VE

Mean value [MPa]

CoV [%]

Mean value [MPa]

CoV [%]

Test type

Tensile (0°) strength Tensile (90°) strength Compressive strength

426 54.8 546 519 72.6 21.9

1.7 4.4 8.2 6.1 5.5 3.3

456 62.8 545 539 84.6 42.1

1.7 4.0 9.6 3.4 3.9 3.2

Flexural strength

In-plane shear strength Interlaminar shear strength

0

1

2

3

4

5

6

7

0

1

2

3

4

5

6

7

120%

120%

(b)

GFRP_VE

(a)

GFRP_UP

110%

110%

99.86% (4.37%)

100%

99.32% (6.45%)

100%

98.09% (2.62%)

97.31% (3.72%)

96.32% (1.15%)

93.59% (4.78%)

93.77% (8.09%)

92.68% (4.46%)

92.66% (3.37%)

91.68% (5.96%)

90.61% (3.73%)

90%

90%

87.77% (3.07%)

Tensile strength rentention

Tensile strength rentention

80%

80%

T20

T60

T-15

T40

T20

T60

T-15

TC100 TC200

T40

TC100 TC200

Ageing Environment

Ageing Environment

Fig. 2. Tensile strength retention in the longitudinal direction (0°): (a) GFRP_UP; (b) GFRP_VE.

With respect to constant temperature exposure, the maximum longitudinal tensile strength reduction was observed at 60 °C for both matrices, with a strength retention of ~92%. After exposure to thermal cycles, slightly higher reductions in tensile strength retention were observed, with values of 87.8% and 90.6%, respectively, for the GFRP_UP and GFRP_VE composites after 200 thermal cycles. It should be noted, for the studied GFRP composites (pultruded laminates with fibres mostly aligned in the pultruded direction), the longitudinal tensile strength is mostly governed by fibre characteristics rather than by the matrix properties (although the resin also plays an important in the tensile behaviour). Therefore, given the range of temperatures used in this experimental program (well below the T g ) and the reported literature, e.g., CEN/TS 19101:2022, minor reductions were expected following exposure to constant temperature. However, the potential degradation of the interfacial fibre/matrix bond caused by differences between the corresponding coefficients of thermal expansion (CTE) may justify the higher degradation in the case of the TC series. In contrast to the longitudinal direction, when the applied tensile load is transverse to the pultrusion direction, the tensile strength retention of GFRP composites is primarily governed by the matrix properties rather than by the fibre characteristics (given the fibre architecture of the pultruded laminates used in the tests) and this may justify the obtained results. In fact, when compared to the longitudinal direction, transverse tensile strength retention results have shown rather different trends, with retention values raging between 74% and 103%, as depicted in Fig. 3.