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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the thermomechanical properties of unaged GFRP laminates were also assessed by means of dynamic mechanical analysis (DMA). DMA tests on aged specimens are also planned. The DMA tests were conducted using a dynamic mechanical analyzer, model Q800 DMA (TA Instruments), following the recommendations of ISO 6721:2019 (parts 1 and 5). Specimens with 60 mm × 10 mm × 4 mm were heated from 25 °C to 200 °C at a rate of 2 °C/min, while subjected to a sinusoidal load in a dual cantilever-type clamp system, at a constant frequency of 1 Hz and a strain amplitude of 15 μm. The specimens were prepared in a CNC milling machine by reducing the laminate thickness from 8 mm to 4 mm on one of their surfaces. The glass transition temperature, T g , was determined from the onset of the decay of the storage modulus ( E ’) curve, considering the average of three replicates. All mechanical tests were carried out in an MTS Exceed E45.105 universal testing machine with a load capacity of 100 kN (see Fig. 1). Tensile tests were performed in the longitudinal direction (0°, pultrusion direction) and in the transverse direction (90°), following EN ISO 527-4:1997, in specimens with 300 mm × 25 mm × 8 mm, loaded at a rate of 2 mm/min. The compressive tests (in 0° direction) were performed according to ISO 14126:1999, in specimens with 153 mm × 25 mm × 8 mm, loaded at a rate of 1 mm/min. The flexural tests were performed according to EN ISO 14125:1998 in specimens with geometry of 240 mm × 15 mm × 8 mm, loaded at a rate of 5 mm/min. The in-plane shear tests were carried out according to ASTM D5379/D5379M:2012, using specimens with 76 mm × 19 mm × 8 mm, comprising a double V-notch at mid-span, loaded at a rate of 2 mm/min. Finally, the interlaminar shear tests were conducted as per ISO 14130:1997, in specimens with 80 mm × 40 mm × 8 mm, loaded at a rate of 1 mm/min. Six replicate specimens were used per test type.

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(b)

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Fig. 1. Mechanical tests: (a) tensile, (b) compressive, (c) in-plane shear, (d) flexural, (e) interlaminar shear.

3. Results and discussion The DMA tests provided values of the glass transition temperature, T g , of 80.6 °C (coefficient of variation, CoV=5.1%) and 101.0 °C (CoV=0.8%) for the laminates produced with unsaturated polyester (UP) and vinyl ester (VE) resin matrices, respectively. These values are significantly (more than 20 °C) higher than the maximum temperature applied (60 °C) in the ageing environments. Table 1 presents the results of the initial mechanical properties in tension, compression, flexure, in-plane shear, and interlaminar shear, conducted on both unaged GFRP_UP and GFRP_VE composites, in terms of mean values and corresponding CoV. These results are then compared in the following sections with those obtained for the aged series under different thermal conditions in terms of the retention value of the analysed property, i.e. the ratio between the property analysed for aged and unaged conditions. This means that when the retention is less than 100% there is a reduction in the property of the aged material with respect to the unaged material, whereas when the retention is greater than 100% there is an increase in the property of the aged material.