PSI - Issue 57

Philipp Ulrich Haselbach et al. / Procedia Structural Integrity 57 (2024) 169–178 P. U. Haselbach and P. Berring / Structural Integrity Procedia 00 (2023) 000–000

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deformation. Moreover, the placement of the shear webs in conjunction with strong shell structures (spar caps) forms a box-beam like structure, which increases the flexural rigidity as well as it increase the buckling load resistance. Most wind turbine blade designs and manufacturing concepts are based on a method, where the subcomponents, like upwind and downwind aerodynamic shells as well as the shear webs, are manufactured separately in their moulds and subsequently adhesively bonded. In order to adhesively bond the subcomponents, structural adhesive bonding paste is applied to the specific regions of the individual components and then in an assembly step bonded together. In areas where too little structural adhesive has been placed or where, due to the curing process and the shrinking of the adhesive, residual stresses (tensile) rise, debonding occurs (Jørgensen, J. B. (2017)). During operation, wind turbine blades experience a variety of di ff erent loads, which also strain and stresses the adhesive joints, which typically are one of the first structural details in a blade to develop damage, which are defined as distributed adhesive cracks (Sørensen, B. F. et al. (2010), Jang, Y.-J et al. (2022). In consequence, manufacturing flaws or operational loads can cause areas with no bonding or debonding damages, among others, between the shear webs and spar caps. Depending on the load conditions and the length of the unbounded / defect area, this may a ff ect the structural sti ff ness of the blade significantly and can result in a lack of integrity or eventually blade failure. Therefore, it is an essential step towards the design of reliable prognostic and health management systems (PHM) not only to detect damaged and / or unbounded regions in the adhesive joints but also to evaluate the severeness of the defect reliably before the joint loses its capability to carry the designed load and failure occurs. Usually, non-destructive testing in form of ultrasonic evaluation and / or modal analyses are the two most common methods applied for identifying debonding damages of the blade structure. Automated ultrasonic scans are an im portant non-destructive evaluation methods for the quality and integrity of wind turbine blades in various lifetimes stages (Nielsen, S. A. (2015)). Nowadays, most wind turbine blades undergo an automated ultrasonic evaluation of the composite structure before leaving the factory. Moreover, ultrasonic scans are partly applied for in-service in spection. However, the quality assessment of rotor blades structures providing reproducible images and an operator independent documentation of the composite structure have been a challenge in the field of automated ultrasonic test ing and detection (Nielsen, S. A. (2015)). Artificial intelligence and machine learning processes are viewed helping to overcome existing shortcomings (FORCE Technology (2021)). Also, for the identification of debonding damages based on modal analyses, partly artificial neural networks using natural frequency relevant key features are used to enhance the detection of debondings in composite structures Movsessian, A. et al. (2021) Jang, Y.-J et al. (2021). Nevertheless, depending on the sensor amount and placement, the damages must have a significant size in order to identified by modal analyses Movsessian, A. et al. (2021). Thus, it is advantageous to ensure that joint designs are damage tolerant and damages develop in a stable manner, enabling detection before reaching a critical state (Sørensen, B. F. et al. (2010)). In order to evaluate when a specific debonding has reached a critical size, a detailed understanding of the crack initiation and propagation is important. A lot of research in the field of shear web and caps and as well as leading and trailing edge debonding of wind turbine rotor blades has been published. Eder, M. A. and Bitsche, R. D. (2015) demonstrated on specimens-level size the impact of the adhesive bond shape, crack detection and theoretical analyses for adhesive joints in the trailing edge using the Virtual Crack-Closure Technique (VCCT). Progressive failure analy ses on full-scale were demonstrated by Chen, X. et al. (2014), and with specific focus on the strain energy release rate (SEER) evaluation of the trailing edge for a wind turbine blade, which was subjected to a full set of design load cases applied in load directions of 15° angle by Haselbach, P. U. & Branner, K. (2015). Damage initiation and growth in the trailing edge region for a wind turbine blade subjected to ultimate loading was numerically modelled using progressive failure analyses and cohesive zone modelling (CZM) and finally compared to full-scale testing by Haselbach, P. U. et al. (2017). Also Jang, Y.-J et al. (2022) analysed recently the progressive failure for the NREL-5MW reference wind turbine blade simulating debonding damages in the spar cap–shear web and trailing edge based on the CZM method. Gulasik, H. and Coker, D. (2014) analysed numerically the debonding process of a T-joints of a wind turbine blade, where they focused particularly on the T-joint. Most of the existing literature refers either to small-scale analyses, where often material properties on coupon or small-scale length have been extracted and simulated, or where these knowledge has been applied numerically on subcomponent or full-scale level to model blade failure in generic wind turbine blade models. Other studies, focus on the experimental testing, where partly artificial failures are embedded as described in Al-Khudairi et al. (2017), without detailed numerical analyses on beforehand. At the Technical University of Denmark, at the Department of