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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1. Introduction Glass Fibre Reinforced Polymer (GFRP) composites have emerged as an alternative material for structural engineering applications due to their advantageous properties, including lightweight, high specific strength and modulus, corrosion resistance, and minimal maintenance requirements during service life. Likewise, pultruded GFRP profiles have emerged as an alternative to steel and stainless steel, particularly in harsh environments (Bazli et al. 2016; Karbhari et al . 2003). Despite their widespread use and proven performance, concerns remain regarding the long-term durability of pultruded elements, particularly for profiles with relatively thick walls (flanges and webs). Several factors, including environmental exposure and mechanical loading, can influence the service life of composite structures, leading to degradation at both the material and structural levels (Nguyen et al. 2024; Karbhari et al . 2007). Thermal effects are among the primary concerns affecting the durability of GFRP composites. Thermal ageing, characterized by exposure to particularly elevated or low (e.g. sub-zero) temperatures over time, and/or thermal cycling, involving repetitive temperature fluctuations, can significantly impact the mechanical properties of GFRP materials (CEN/TS 19101:2022). The lack of comprehensive data on the behaviour of relatively thick pultruded GFRP laminates under varying thermal conditions makes it difficult to predict their long-term durability. To address this knowledge gap, an extensive experimental program has been conducted in the scope of Durable FRP R&D Project to assess the durability of thick pultruded GFRP laminates produced using unsaturated polyester (GFRP_UP) and vinyl ester (GFRP_VE) resin matrices. The test programme involved laboratory and in-situ field tests, with specimens exposed to different environmental agents, both unloaded and subjected to different stress states. This paper presents preliminary results of the Durable-FRP project in terms of the effects of exposure to thermal ageing environments, including constant temperatures of -15 °C to 60 °C for 12 months, and thermal cycles (100 and 200) between -15 °C and 60 °C. The degradation of mechanical properties, namely in tension (in both longitudinal and transverse directions), compression, flexure, in-plane shear, and interlaminar shear, was evaluated following exposure to those environments. The pultruded GFRP laminates employed in the experimental program were made with unsaturated polyester (GFRP_UP) and vinyl ester (GFRP_VE) resin matrices. The fibre architecture consists of seven layers: four layers of chopped strand mat and in-between layers of rovings oriented in the pultrusion direction. The laminates were produced with nominal width and thickness of 250 mm and 8 mm, respectively. Calcination tests were performed in accordance with ISO 1172:2023, indicating fibre contents (in mass) of 59% and 63% for GFRP_UP and GFRP_VE laminates, respectively. For practical reasons, plates of 900 mm (length) × 250 mm × 8 mm were used for accelerated laboratory ageing. 2.2. Thermal ageing environments Two types of thermal ageing environments were simulated in laboratory conditions: (i) a first group of specimens was subjected to constant temperatures of -15 °C, 20 °C, 40 °C, and 60 °C, for a period of 12 months; (ii) a second group of specimens underwent thermal cycling between -15 °C and 60 °C, for 100 and 200 cycles (additional tests will be carried out for 300 cycles). The following thermal cycling protocol was adopted: each cycle had a duration of 12 hours, comprising 5 hours at -15 °C and 5 hours at 60 °C (relative humidity of 50%), with 1 hour of heating or cooling (rates of 1.25 °C/min), respectively. 2.3. Test methods Several test methods were used to assess the mechanical properties of the GFRP laminates before and after ageing, namely, tensile, compressive, flexural, in-plane shear, and interlaminar shear tests, as depicted in Fig. 1. Furthermore, 2. Materials and methods 2.1. Materials description