Filtern
Dokumenttyp
- Beitrag zu einem Tagungsband (3)
- Vortrag (1)
Sprache
- Englisch (4) (entfernen)
Referierte Publikation
- nein (4)
Schlagworte
Organisationseinheit der BAM
Eingeladener Vortrag
- nein (1)
In situ thermography of crack growth in sandwich shell segments with manufactured imperfections
(2016)
A new test rig for the efficient material testing of curved GFRP composite specimens is developed. The presented intermediate scale test facility allows the fatigue testing of representative shell segments of rotor blades for the wind power industry. The non-destructive condition monitoring is performed by a combination of thermography and optical 3 D deformation analysis, automated in situ throughout the fatigue test. In many cases cracks in the shells of wind turbine rotor blades are detected long time before the calculated lifetime of 20 years. As a consequence of the harsh weather conditions damage progress occurs, that causes costly in-service repairs. Approximately 70% to 80% of defects in rotor blades are the consequence of imperfections through the manufacturing process or the rotor blade design. Production-related imperfections are supposed to be one possible reason for crack formation and crack growth. Therefore, a test rig for shell structures is constructed and launched into operation. In the test bench sandwich shells and full laminate shells of different sizes with artificially created defects (e.g. wrinkles, laminate change, etc.) are tested under cyclic tension and compression load. The characterization of the damage state and evolution during the fatigue test by simultaneous passive thermography and optical 3D deformation analysis is performed. The in- and out-off-plane deformations of sandwich shells under cyclic tension and compression load are determined by optical 3 D deformation analysis (with ARAMIS). For the non-destructive detection of developing material defects (e.g. cracks) in the shells due to cyclic loading thermography is used. The passive thermography is well suited for the characterization of the crack formation and crack growth of composites during fatigue tests. The cyclic strain energy introduces the damage and the damage progress can be recorded and documented in the material. By thermography the damage is recognized in the laminate long time, before the surface of the painted specimen is damaged. The experiments show a correlation between the hotspots, observed by passive thermography and the location of the final failure.
In the paper the design of the test bench and the results of the experiments will be presented.
Localized patches are a cost- and time-effective method for repairing fiber-reinforced polymer (FRP) sandwich wind turbine rotor blade shells. To increase the understanding of their effect on the fatigue of the blades, this study examines the effect of various layup methods of localized repair patches on the structural integrity of composite sandwich structures. Manufactured with the vacuum-assisted resin infusion (VARI) process, the shell test specimens are produced as a curved structure with glass fiber reinforced polymer (GFRP) sandwiching a polyvinyl chloride (PVC) foam core. Patch repairs are then introduced with varying layup techniques, and material properties are examined with cyclic fatigue tests. The transition region between patch and parent material is studied in greater detail with finite element method (FEM) simulations, with a focus on the effect of fiber orientation mismatch. Damage onset, crack development, and eventual failure are monitored with in-situ non-destructive testing methods to develop a robust understanding of the effects of repair concepts on material stiffness and strength.
A shell test bench was developed at BAM 5.3 which allows for static and fatigue testing of curved fiber-reinforced plastic (FRP) structures, during which in-situ the damage state can be non-destructively inspected by thermography and strain-field measurement techniques. Sandwich shell specimens with typical wind turbine blade manufacturing defects were designed and tested. The tested imperfections show a fairly significant reduction (up to 90%) of the shell test specimens‘ lifetime, depending on the type of imperfection. Using the in-situ NDT methods incorporated in the shell test bench, the location and cycle time of the initial defects and the damage evolution was investigated.
Common air-coupled transducers for non-destructive testing consist of a piezocomposite material and several matching layers. Better acoustical matching to air is achieved by transducers based on charged cellular polypropylene (PP). This material has about hundred times lower acoustic impedance than any piezocomposite, having about the same piezoelectric coefficient. The piezoelectric properties of cellular PP are caused by the polarization of air cells. Alternatively, a ferroelectret receiver can be understood as a capacitive microphone with internal polarization creating permanent internal voltage. The sensitivity of the receiver can be increased by applying additional bias voltage. We present an ultrasonic receiver based on cellular PP including a high-voltage module providing bias voltage up to 2 kV. The application of bias voltage increased the signal by 12 to 15 dB with only 1 dB increase of the noise.
This receiver was combined with a cellular PP transmitter in through transmission to inspect several test specimens consisting of glass-fiber-reinforced polymer face sheets and a porous closed-cell PVC core. These test specimens were inspected before and after load. Fatigue cracks in the porous PVC core and some fatigue damage in the face sheets were detected. These test specimens were originally developed to emulate a rotor blade segment of a wind power plant. Similar composite materials are used in lightweight aircrafts for the general aviation. The other inspected test specimen was a composite consisted of glass-fiber-reinforced polymer face sheets and a wooden core. The structure of the wooden core could be detected only with cellular PP transducers, while commercial air-coupled transducers lacked the necessary sensitivity. Measured on a 4-mm thick carbon-fiber-reinforced polymer plate, cellular PP transducers with additional bias voltage achieved a 32 dB higher signal-to-noise ratio than commercial air-coupled transducers.