A fiber-reinforced composite wind turbine blade (WTB) is exposed to numerous impact threats during its service life causing damages that can be detrimental to its structural integrity. Currently, impact loads are not considered during blade design, so high safety factors are introduced, which result in a conservative design. However, as wind turbine blades become stiffer and lighter and health monitoring systems become more sophisticated, the design process is shifting toward damage-tolerant approaches. The design philosophy accepts damages to the structure, but it also requires that the damaged blade still meet structural and functional requirements. This design procedure requires a comprehensive understanding of different impact threats and their characteristics, which is currently unavailable in the public domain. This paper is a first attempt to review the impact loads on composite wind turbine blades. The aim of the current paper is to (a) identify different sources of impact threats on wind turbine blades during different stages of their service life, (b) describe their qualitative (causes and vulnerable regions) as well as quantitative characteristics (size, mass, and velocity of impactor), and to (c) provide modeling guidelines by comparing these impact threats using five different criteria - (i) relative deformability of projectile and wind turbine blade, (ii) impact velocity, (iii) kinetic energy of impact, (iv) repeatability of impacts and (v) nature of the impact. The review paper will be of special interest to researchers working on wind turbine blades and will serve as a baseline report for designing damage-tolerant blades. Recommendations are also provided for future research.

The presence of rain-induced leading edge erosion of wind turbine blades (WTBs) necessitates the development of erosion models. One of the essential parameters for erosion modeling is the relative impact velocity between rain droplets and the rotating blade. Based on this parameter, the erosion damage rate of a WTB is calculated to estimate the expected leading edge lifetime. The environmental conditions that govern this parameter have site-specific variations, and thus, rain and wind loading on a turbine differ for onshore and offshore locations. In addition, there are wave loads present in the offshore environment. The present paper tries to provide guidelines for erosion modeling and investigates whether there are differences in erosion of blades due to (1) varying rainfall conditions modeled using different droplet size distributions for onshore and offshore locations in combination with (2) winds of varying turbulence intensities and (3) wave-induced loads. Aero-hydro-servo-elastic simulations are carried out for an onshore wind turbine (WT) and a monopile-supported offshore WT. Furthermore, erosion variables such as the relative impact velocities and the associated erosion damage rate of a blade are analyzed for various blade azimuth angles. The study shows that the rainfall intensity and turbulence intensity minorly influence the impact velocity and pressure but have a substantial effect on the overall erosion damage rate. Additionally, a significantly higher erosion damage rate is found for blades exposed to offshore rainfall conditions than for blades under onshore rainfall conditions. Furthermore, no substantial influence on erosion is found because of wave-induced loads.

The rain-induced fatigue damage in the wind turbine blade coating has attracted increasing attention owing to significant repair and maintenance costs. The present paper develops an improved computational framework for analyzing the wind turbine blade coating fatigue induced by rain erosion. The paper first presents an extended stochastic rain field simulation model that considers different raindrop shapes (spherical, flat, and spindle), raindrop sizes, impact angles, and impact speeds. The influence of these raindrop characteristics on the impact stress of the blade coating is investigated by a smoothed particle hydrodynamics approach. To address the expensive computational time, a stress interpolation method is proposed to calculate the impact stress of all raindrops in a random rain event. Furthermore, coating fatigue analysis is performed by including the fatigue crack initiation in the incubation period and the fatigue crack propagation in the mass-loss-rate increasing period due to raindrop impact. Finally, the proposed computational framework is verified by comparing the estimated fatigue life with those obtained in literature. The results from the study show that by incorporating the statistics of rainfall data, the proposed framework could be used to calculate the expected fatigue life of the blade coating due to rain erosion.

The issue of leading edge erosion (LEE) of wind turbine blades (WTBs) is a complex problem that reduces the aerodynamic efficiency of blades, and affects the overall cost of energy. Several research efforts are being made at the moment to counter erosion of WTBs such as-testing of advanced coating materials together with development of high-fidelity computational models. However, the majority of these studies assume the coated surfaces as flat, while the surface curvature and the shape of the aerofoil at the blade's leading-edge exposed to such rain fields is neglected. The present study questions the assumption of a flat surface, in the context of LEE of WTBs, and provides guidelines for erosion modelling. The critical parameters associated with rain droplet impingement kinematics on leading edge are compared for blade impact with (a) flat surface assumptions together with (b) the effects of the blade's surface curvature. A parametric study is performed which includes WTBs of varying sizes and power ratings ranging from 750 KW to 10 MW, different positions along the blade length, and different rain droplet radii ranging from 0.1 mm to 5 mm for a land based wind turbine operating at rated wind speed. It is found in the study that droplet impingement kinematics are influenced by the surface curvature at the leading edge, the effect of which is significant for representing erosion at the blade tip for smaller blades, and for exposure to rainfall intensity with larger rain droplet size. A master curve describing the threshold level along the blade length is established for various WTBs and rainfall conditions, where flat surface approximation of the surface yields noticeable error and violates the impingement process. The results of the study are expected to aid the modeller in developing advanced numerical models for LEE for WTBs.