A substructure test specimen for fatigue delamination crack growth initiating at ply drops
Topic(s) : Material and Structural Behavior - Simulation & Testing
Co-authors :
Xing-Yuan MIAO (DENMARK), Ashish Kumar BANGARU (DENMARK), Bent F. SØRENSEN (DENMARK)Abstract :
Thickness tapering is seen in many laminated composite structures e.g. helicopter rotor blades and wind turbine rotor blades, of which the geometrical configurations are tailored to satisfy specific design requirements such as aerodynamic efficiency and low weight. This is achieved by introducing ply drops at the locations where the geometries need to be changed. Ply drops refer to terminating or dropping off plies along the length of the laminates. When the structure is subjected to cyclic loading, there will be stress concentrations at the locations of the ply drops, due to the discontinuities in the material and geometry. Such stress concentrations can lead to initiation and growth of delamination cracks. The growth of the delamination cracks reduces the load-carrying capacity of the composite structures and eventually leads to catastrophic failures of the structures. From the perspective of structural damage tolerance, the predictability of the growth rate of delamination crack becomes critical.
In this work, we propose a substructure test specimen to explore the predictability of delamination fatigue crack growth driven by cyclic loadings. The design procedure to create a substructure test specimen and a corresponding DCB (double cantilever beam) fracture mechanics test specimen out of a full blade is presented. The focus of this work is on the design challenges of the specimens. An analytical ply drop model [1] and measured delamination fatigue crack growth rates [2] are used to determine the design parameters of the substructure test specimen, i.e., the ply thickness, the thickness of the underlying layers, and the load levels. The present substructure test specimen is designed for cyclic tension-tension loading.
Practical considerations related to the design are presented. This includes in-field investigation on a decommissioned blade to determine the cutting locations and dimensions in order to obtain consistent interfaces over a length of more than 2000 mm for substructure and DCB specimens. The locations and dimensions of blade segments to be cut out for manufacturing the specimens are carefully planned to avoid damages from the preceding commissioned time remaining in the segments as much as possible.
References
[1] Sørensen, B. F. (2020). Microscale testing and modelling for damage tolerant composite materials and structures. In IOP Conference Series: Materials Science and Engineering (Vol. 942, No. 1, p. 012004). IOP Publishing.
[2] Kenane, M., Benmedakhene, S., & Azari, Z. (2010). Fracture and fatigue study of unidirectional glass/epoxy laminate under different mode of loading. Fatigue & Fracture of Engineering Materials & Structures, 33(5), 284-293.
In this work, we propose a substructure test specimen to explore the predictability of delamination fatigue crack growth driven by cyclic loadings. The design procedure to create a substructure test specimen and a corresponding DCB (double cantilever beam) fracture mechanics test specimen out of a full blade is presented. The focus of this work is on the design challenges of the specimens. An analytical ply drop model [1] and measured delamination fatigue crack growth rates [2] are used to determine the design parameters of the substructure test specimen, i.e., the ply thickness, the thickness of the underlying layers, and the load levels. The present substructure test specimen is designed for cyclic tension-tension loading.
Practical considerations related to the design are presented. This includes in-field investigation on a decommissioned blade to determine the cutting locations and dimensions in order to obtain consistent interfaces over a length of more than 2000 mm for substructure and DCB specimens. The locations and dimensions of blade segments to be cut out for manufacturing the specimens are carefully planned to avoid damages from the preceding commissioned time remaining in the segments as much as possible.
References
[1] Sørensen, B. F. (2020). Microscale testing and modelling for damage tolerant composite materials and structures. In IOP Conference Series: Materials Science and Engineering (Vol. 942, No. 1, p. 012004). IOP Publishing.
[2] Kenane, M., Benmedakhene, S., & Azari, Z. (2010). Fracture and fatigue study of unidirectional glass/epoxy laminate under different mode of loading. Fatigue & Fracture of Engineering Materials & Structures, 33(5), 284-293.