PSI - Issue 77

Alireza Shadmani et al. / Procedia Structural Integrity 77 (2026) 221–228 Shadmani et al. / Structural Integrity Procedia 00 (2026) 000–000

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To mitigate the detrimental e ff ects of leading edge erosion, protective coatings are widely employed, with elas tomeric polymers, particularly polyurethane (PU), being the material of choice (Schramm et al. (2017)). These coat ings are applied to the leading edge to form a durable, resilient protection. The high elasticity and toughness of PU allow it to absorb and dissipate the kinetic energy from impinging liquid droplets, thereby protecting the underlying composite blade structure. The e ff ectiveness of a PU coating is determined by its ability to withstand millions of impact cycles without significant degradation. However, under the repetitive impact of raindroplets, these protective layers themselves are susceptible to fatigue and wear, eventually resulting in failure (Tempelis et al. (2025); Slot et al. (2021)). The erosion of a PU coating is a complex fatigue process that initiates at the microstructural level. Each raindroplet impact induces a transient stress wave within the material (Hoksbergen et al. (2023)), and while a single impact may cause no visible damage, the cumulative e ff ect of repeated impacts leads to the initiation and coalescence of micro cracks. This gradual accumulation of damage, rooted in the evolution of the polymer’s microstructure under cyclic loading, manifests as a progressive reduction in the material’s sti ff ness and strength, and the eventual exposure of the blade’s composite substrate (Zhang et al. (2024)). Understanding and predicting this transition from micro-scale damage to macro-scale failure is essential for designing more durable coatings and optimizing maintenance schedules. To accurately predict the service life and failure of these protective layers, a robust modeling framework that captures the physics of material degradation is required, especially given the recognized limitations of existing semi empirical models (Lopez et al. (2023)). The continuum damage mechanics (CDM) framework idealizes the develop ment of microscopic defects as a continuous field variable that represents the degradation of material sti ff ness (Pandey et al. (2023)). By integrating a CDM model within a finite element analysis framework, it becomes possible to sim ulate the initiation and evolution of damage under the complex, transient loading conditions induced by high-speed raindroplet impacts. Therefore, this paper presents a CDM-based numerical model to simulate the fatigue damage accumulation in PU coatings, providing insights into the material’s response to cyclic loading and o ff ering a method to predict the onset and progression of leading edge erosion. In order to simulate the erosion process of the PU coating, a finite element model was developed in ABAQUS / Explicit. The primary input of the model is the transient pressure profile resulting from high-speed rain droplet impacts. The pressure profile was obtained from a previously validated computational fluid dynamics (CFD) model (Ramachandran Nambiar et al. (2023)). The model simulates the impact of a single raindroplet with a diameter of 2 mm at a velocity of 110 m / s on a rigid wall. The impact generates a complex pressure distribution on the wall, which varies both spatially and temporally, as shown in Fig. 1(a)-(b). According to Fig. 1(a), this process can be classified into distinct stages based on the pressure profile. Initially, during the ”hammer stage” (t1), the pressure spikes to its maximum value at the impact center, an e ff ect known as water hammer. This is immediately followed by the ”excess stage” (t2-t6), where the peak pressure shifts to the expanding droplet edge and can briefly exceed the initial hammer value due to fluid compression. In the ”jetting initiation stage” (t7-t11), this compressed liquid begins to be ejected as lateral jets, which relieves the internal pressure. Finally, as the droplet continues to spread (t > t12), the wall pressure gradually decreases toward ambient conditions. The pressure distribution is non-dimensionalized by the hammer pressure. The evolution of shear stress on the wall, as demonstrated in Fig. 1(b), is a direct result of this pressure-driven flow and is the primary mechanism for erosion. The shear stress is non-dimensionalized by the dynamic pressure of impact. During the initial hammer and excess stages (t1-t6), shear stresses are negligible as there is no significant lateral outflow. However, the shear stress rises drastically during the jetting initiation stage (t7-t11), reaching its peak value due to the high-velocity ejection of the lateral jets. Although these laminar shear stresses are orders of magnitude lower than the impact pressure, they act as a powerful abrasive force in the repetitive impact scenario. This combination of the initial high-pressure shock stressing the material and the subsequent intense shear from jetting is the potent cause of damage, capable of stripping coatings and leading to erosive wear. 2. Materials and methods 2.1. Finite element model