PSI - Issue 68

2

B. Li et al. / Structural Integrity Procedia 00 (2025) 000–000

Boyu Li et al. / Procedia Structural Integrity 68 (2025) 59–65

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*" water pressure g m capillary flow mass $ r x capillary flow length ̇ capillary flow speed ̈ H

gravitational acceleration depth of water layer

pressure of air remaining in capillary radius of circular capillary channel

capillary flow acceleration Δ m capillary flow mass increment per unit time contact angle between fluid and capillary wall viscosity of water water density surface tension of fluid 1. Introduction

Additively manufactured (AM) polymers are applied across various industries, though their use in the marine sector is still in the initial stage. The water-ageing effects of 3D-printed polymers remain inadequately studied, with few published works. Upadhyay et al. (2020) fabricated a polylactic acid (PLA) propeller through 3D printing and examined its degradation in water. It is understood that the degradation of polymers in aqueous environments is closely linked to the diffusion of water into the material. Recent studies revealed that voids generated during 3D printing significantly accelerate this diffusion process. Fichera and Carlsson (2016) investigated water diffusion in vinyl ester resin reinforced with carbon fibres by immersing three sizes of square composite panels in seawater for 60 days. These panels absorbed more water than pure-matrix specimens due to their higher void content. The data were fitted to a Fickian diffusion model to assess the capillary effect. The extent of water uptake in smaller panels varied, reflecting the randomness of void distribution. Numerical simulations using a capillary model resulted in faster saturation than experiments, likely due to assumptions about uninterrupted capillaries, which in reality may be obstructed by fiber matrix bonds. Further, Caggiano et al. (2018) conducted a mesoscale simulation of capillary-driven water absorption in cementitious materials, assuming evenly distributed porosity forming capillaries throughout the specimen. The average porosity was measured, allowing the diffusion rate due to the capillary effect to be applied in water-diffusion simulations. A prismatic specimen (40 × 40 × 160 mm³) was submerged in water to validate the model. The mesoscale approach closely aligned with the experimental results, indicating that microscale models can be scaled up to the macro level for representative simulation parameters. Tekinalp et al. (2014) observed that during the 3D printing process, gaps formed between printing beads due to the nature of material extrusion additive manufacturing (MEAM). These voids act as canals or capillaries, significantly enhancing water diffusion in polymer materials. The presence of such channels accelerates moisture diffusion, affecting material performance in marine environments. Therefore, it is crucial to investigate water movement within capillaries. Understanding the dynamics of moisture diffusion in capillaries can provide insights into the material's performance, durability, and overall behaviour in environments with significant water exposure. 2. Methodology Following our previous research (Li et al., 2024), a circular capillary channel with a diameter of 0.03 mm at the centre of a 0.1 mm × 1 mm × 100 mm specimen and aligned with its axis is modelled as shown in Fig. 1. The capillary tunnel - an inherent feature of MEAM - significantly accelerates the moisture diffusion in the polymer. Forces acting on the capillary flow are shown in Fig. 2; the constitutive equation of the model is: ( + ) ̈ = #$% + *% + !% + &% + '() (1)