R. Loendersloot

Vibration Based Structural Health Monitoring and the Modal Strain Energy Damage Index Algorithm Applied to a Composite T-Beam

A Finite Element based numerical model for a vibration based damage identification method for a thin-walled slender composite structure is discussed in this chapter. The linear dynamic response of an intact and a locally delaminated 16-layer unidirectional carbon fibre PEKK reinforced T-beam is analysed. The capabilities of the modal strain energy damage index algorithm to detect and localize a delamination is assessed. Both bending and torsion modes of the structure are used in the algorithm. Both an experimental set-up and a numerical model are discussed. Measurements are performed on an intact and an artificially delaminated structure, using a laser-vibro measuring system to determine the response to a force excitation. A commercially available Finite Element package is employed for the numerical model. The aim of the numerical model is to perform a parametric study. The study is preceded by an experimental verification of the numerical model. Subsequently, it is used to analyse the effect of the size and location of a delamination, as well as the number of data points employed, on the damage index.

Investigation of PVC physical ageing in field test specimens using ultrasonic and dielectric measurements

Physical ageing in PVC is studied using two techniques: a) non-linear ultrasonic measurements based on the non-collinear wave interaction theory and b) dielectric measurements. The ultrasonic measurement results are compared with dielectric measurement results. The comparison shows that the used non-linear ultrasonic measurement technique is more sensitive to the effects of physical ageing than the dielectric measurements. The influence of temperature on the ultrasonic measurements is studied experimentally. Finally, the ultrasonic measurements, linear and initial non-linear, are used for C-scanning of a natural crack in an old PVC water supply pipe. The linear ultrasonic measurements are performed in a pulse-echo mode. The experimental results show that the natural crack, which consists of a through-thickness crack and microcrack zones, is detected using both measurement methods. All measurements are performed in field PVC test specimens.

Smart Actuation for Helicopter Rotorblades

Successful rotorcrafts were only achieved when the differences between hovering flight conditions and a stable forward flight were understood. During hovering, the air speed on all helicopter blades is linearly distributed along each blade and is the same for each. However, during forward flight, the forward motion of the helicopter in the air creates an unbalance. The airspeed is increased for the blade passing in the advancing side of the helicopter, while it is reduced in the retreating side. Moreover, when each blade enters the retreating side of the helicopter, a reverse flow occurs around the profile where the blade speed is lower than the forward speed of the helicopter. The balance of a rotorcraft is solved by a cyclic pitch control, but trade-offs are made on the blade design to cope with the great variety of aerodynamic conditions. A smart blade that would adapt its characteristics to this large set of conditions would improve rotorcrafts energy efficiency while providing vibration and noise control.nSmart rotor blades systems are studied to adapt the aerodynamic characteristics of the blade during its revolution and to improve the overall performances. An increase in the lift over drag ratio on the retreating side has been studied to design a blade with better aerodynamic efficiency and better stall performances in the low-speed region. The maximum speed of a rotorcraft is limited by the angle of attack that the profile can sustain on the retreating side before stall. Therefore, increasing the maximum angle of attack that a profile geometry can sustain increases the rotorcraft flight envelope. Flow asymmetry and aerodynamic interaction between successive blades are also investigated to actively reduce vibrations and limit noise.nThese improvements can be achieved by deploying flaps, by using flow control devices or by morphing the full shape of the profile at a specific places during the blade revolution. Each of the listed methods has advantages and disadvantages as well as various degrees of feasibility and integrability inside helicopter blades. They all modify the aerodynamic characteristics of the profile. Their leverage on the various aerodynamic effects depends on the control strategy chosen for actuation. Harmonic actuation is therefore studied for active noise and vibration control whereas stepped deployment is foreseen to modify the stall behaviour of the retreating side of the helicopter.nHelicopter blades are subjected to various force constraints such as the loads from the complex airflow and the centrifugal forces. Furthermore, any active system embedded inside a rotor blade needs to comply with the environmental constraints to which a helicopter will be subjected xa0during its life-span. Other concerns, like the power consumption and the data transfer for blade control, play an important role as well. Finally, such a system must have a life-time exceeding the life-time of a rotor blade and meet the same criteria in toughness, reliability and ease of maintenance.nSmart system is an interplay of aerodynamics, rotor-mechanics, material science and control, thus a multidisciplinary approach is essential. A large part of the work consists in building processes to integrate these domains for investigating, designing and testing smart components.nPiezoelectric actuators are a promising technology to bring adaptability to rotor blades. They can be used directly on the structure to actively modify its geometry, stiffness and aerodynamic behaviour or be integrated to mechanisms for the deployment of flaps. Their large specific work, toughness, reliability and small form factor make them suitable components for integration within a rotor blade. The main disadvantage of piezoelectric actuators is the small displacement and strain available. Amplification mechanisms must be optimised to produce sufficient displacement in morphing applications.nSmart actuation systems placed inside rotor blades have the potential to improve the efficiency and the performances of tomorrows helicopters. Piezoelectric materials can address many of the challenges of integrating smart components inside helicopter blades. The key aspect remains the collaboration between various domains, skills and expertise to successfully implement these new technologies.