PSI - Issue 77

Shadmani et al. / Structural Integrity Procedia 00 (2026) 000–000

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

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(a)

(b)

Fig. 5: (a) Damage distribution at 110 m / s impact velocity and (b) damage slice at y = 50mm

To calculate the damage evolution, the maximum principal stress and stress triaxiality were extracted from the coating. Using the stress data obtained from the finite element model, the damage evolution in the PU coating was calculated using the CDM framework described in the previous section. The damage evolution was simulated for repeated droplet impact, with the damage accumulation tracked over 10,000 impacts. The damage distribution within the PU coating, as shown in Fig. 5(a), reveals a pronounced damage zone centered around the impact location, with damage values approaching unity at the surface, indicating imminent failure. The damage propagates radially outward, forming a ring-like pattern that reflects the stress wave propagation observed in the stress analysis. To further analyze the damage distribution, a cross-sectional slice of the damage field at y = 50 mm is presented in Fig. 5(b). The cross section view provides the most critical insight into the failure mechanism. It unequivocally shows that the point of damage initiation, where the damage variable D first reaches its critical value of 1.0, is not on the surface but at a subsurface location, approximately z = − 0 . 1 mm . The maximum principal compressive stress, which is a primary driver for the damage model, was shown to have its peak magnitude at a similar subsurface depth. This correlation strongly suggests that the initial damage is driven by the high compressive stresses induced by the droplet impact. The repeated application of the localized stress field, with its peak compressive and shear values occurring below the surface, leads to the progressive accumulation of damage at that specific location. A crack would therefore be expected to nucleate at this depth and subsequently propagate outwards and upwards towards the surface, ultimately leading to the pitting and erosion observed in such a phenomena. In this study, a continuum damage mechanics (CDM) model was developed to simulate the fatigue damage in polyurethane (PU) coatings of leading edge erosion of wind turbine blades under repeated high-speed raindroplet impact. This work elucidated the link between the transient stress dynamics of a single impact and the long-term accumulation of material damage. The most significant finding of this work is the unequivocal prediction that material failure initiates not on the surface, but at a subsurface location. The continuum damage model, driven by the repeated application of the transient stress field, showed that the damage variable D consistently reaches its critical value first at a depth of approximately − 0 . 1 mm . This directly correlates with the location of the maximum principal compressive stress identified in the tran sient analysis. The direct correlation between the subsurface stress concentration and the point of incipient failure strongly supports the hypothesis that for this material and impact condition, erosion is governed by a subsurface initiated fatigue mechanism. A crack is expected to nucleate at this weakened subsurface point, which would then propagate towards the surface with subsequent impacts, ultimately leading to the pitting and material loss character istic of leading edge erosion. 4. Conclusions