Harald E.N. Bersee

Two-Degree-of-Freedom Active Vibration Control of a Prototyped “Smart” Rotor

This paper studies the load reduction potential of a prototyped “smart” rotor. This is, a rotor where the blades are equipped with a number of control devices that locally change the lift profile on the blade, combined with appropriate sensors and controllers. Experimental models, using dedicated system identification techniques, are developed of a scaled rotating two-bladed “smart” rotor of which each blade is equipped with trailing-edge flaps and strain sensors. A feedback controller based on H∞-loop shaping combined with a fixed-structure feedforward control are designed that minimizes the root bending moment in the flapping direction of the two blades. We evaluated the performance using a number of different realistic load scenarios. We show that with appropriate control techniques the variance of the load signals can be reduced up to 90%.

Process and performance evaluation of ultrasonic, induction and resistance welding of advanced thermoplastic composites:

The possibility of assembling through welding is one of the major features of thermoplastic composites and it positively contributes to their cost-effectiveness in manufacturing. This article presents a comparative evaluation of ultrasonic, induction and resistance welding of individual carbon fibre-reinforced polyphenylene sulphide (PPS) thermoplastic composite samples that comprises an analysis of the static and dynamic mechanical behaviour of the joints as well as of the main process variables. The induction welding process as used in this research benefitted from the conductive nature of the reinforcing fibres. Hence, no susceptor was placed at the welding interface. Resistance welding used a fine-woven stainless-steel mesh as the heating element and low welding pressures and times were applied to prevent current leakage. Triangular energy directors moulded on a separate tape of PPS resin were used to concentrate ultrasonic heat at the welding interface. The static single-lap shear strength of the joints was found similar for induction and ultrasonic welding. A 15% drop in the static mechanical properties of the resistance welded joints was attributed to incomplete welded overlaps following current leakage prevention. However, the fatigue performance relative to the static one was similar for the three sorts of joints. A comparative analysis of process variables such as welding time, required power and energy was also carried out.

Metal mesh heating element size effect in resistance welding of thermoplastic composites

The objective of this work is to determine the effects of metal mesh heating element size on resistance welding of thermoplastic composites. The materials to be resistance-welded consisted of carbon fiber/poly-ether-ketone-ketone (CF/PEKK), carbon fiber/poly-ether-imide (CF/PEI) and glass fiber/PEI (GF/PEI). Four different metal mesh sizes were used as heating elements. The samples were welded in a lap shear joint configuration and mechanically tested. Maximum Lap Shear Strengths of 52, 47 and 33 MPa were obtained for the CF/PEKK, CF/PEI and GF/PEI specimens, respectively. The ratio of the heating element’s fraction of open area and wire diameter was shown to be the most important parameter to be considered when selecting an appropriate heating element size.

Temperature Effect on Reinforced Thermoplastic Composite Properties for Primary Aircraft Structure Applications

[Abstract] Reinforced thermoplastic composites used for primary aircraft structures are subjected to thermal effects throughout theirs lives. That is why the understanding of the temperature impact on the mechanical properties of carbon fibre reinforced plastics is very important for the choice of the appropriate plastic for aircraft design. In this study we are mainly concerned with the behaviour of different carbon weave reinforced thermoplastic composites (PPS and PEEK) to find their temperature window without loss of properties by using mechanical tests. Microscopy analysis is also performed to see the impact of the temperature on the internal structure of the composite.

Determination of Cure Dependent Properties for Curing Simulation of Thick-Walled Composites

[Abstract] Because of higher specific strength and stiffness, low weight, and good resistance to corrosion, polymer based composite materials replace more often conventional metals in high performance aerospace, maritime, and automotive structures as well as in consumer goods. Although these structures are manufactured according to high-quality standards, these are often thin-walled. On the cont rary, thick-walled composite structures are hardly used in these industries to replace meta llic equivalents. One of the reasons is that thick-walled fiber reinforced structures (i.e., str uctures with a thickness larger than 10 mm) induce larger problems on the fabrication process c ompared to thin-walled fiber reinforced structures. One of the main problems to overcome is the heat generation during the exothermic reaction, which causes not only internal stress build-up but also other structural defects. Not only this problem but many others emphasize the necessity of improving the understanding of different phenomena occurring in the manufacturing process of thickwalled structures. Since the research field is quit e extensive, the focus will be on the thermochemical behavior of thermosetting resins. In this paper a generally applicable thermochemical model is set up and cure dependent material properties for a thermosetting resin called RTM 6 are determined in order to demonstrate the applicability of the thermochemical model to simulate the curing process and t o study the thermo-chemical behavior of RTM 6. In two numerical examples, the influence of the specific heat capacity on the reaction temperature and degree of conversion and the maximum achievable thickness when constrained by the degradation temperature will be discussed.

A comparative evaluation between flat and traditional energy directors for ultrasonic welding of CF/PPS thermoplastic composites

Energy directors, responsible for local heat generation in ultrasonic welding, are neat resin protrusions traditionally moulded on the surfaces to be welded. This study evaluates an alternative energy directing solution for ultrasonic welding of thermoplastic composites based on the usage of a loose flat layer of neat resin at the welding interface, referred to as ‘flat energy director’. Analysis of dissipated power, displacement of the sonotrode, welding energy and time as well as weld strength compared to more traditional energy directing solutions showed that flat energy directors, which significantly simplify ultrasonic welding of thermoplastic composites, do not have any substantial negative impact in the welding process or the quality of the welded joints.

A high-rate shape memory alloy actuator for aerodynamic load control on wind turbines

This article discusses the development of a high-rate shape memory alloy–driven actuator. The concept of the actuator was developed to act as aerodynamic load control surface on wind turbines. It was designed as a plate or beam-like structure with prestrained shape memory alloy wires embedded off its neutral axis. Moreover, the shape memory alloy material was embedded in channels through which air was forced to actively cool the wires when the recovery load was to be released. Wires were implemented on both sides of the neutral axis to deflect the beam in both directions. Thermal analysis of the cooling channels showed that they increased the cooling rate up to 10-fold in comparison to the same set-up without forced convection. Subsequently, a fuzzy logic controller was designed to control the thermo-mechanical system. The inputs were the error between the deflection and the set point, the value of the set point and the time derivative of the set point. The output consisted of two signals to the valves that controlled the flow through the channels and a signal heating signal that was split into both sets of wires, depending on its sign. The controller was tested on an antagonistic set-up, through which a similar thermo-mechanical behaviour as with the actuator was obtained, but eliminating the beam dynamics. The results were satisfactory; an actuation bandwidth of 1 Hz was attained. Subsequently, the controller was tested on the actuator. With increasing actuation frequency, until 0.6 Hz, a relatively small error between the set point and the actual deflection was observed. Above that frequency, the error increased, but the sinusoidal response was lost. This is believed to be due to snap-through behaviour around the neutral position of the actuator. This was substantiated by the apparent inability of the actuator to track the set point around the neutral position in tracking a composite sinusoidal set point.

Thermal Aspects in Resistance Welding of Thermoplastic Composites

This paper presents a comprehensive experimental study on the thermal aspects in resistance welding of thermoplastic composites. A special test set-up was developed to perform the experiments. Glass fiber reinforced polyetherimide was the material used for manufacturing the welding specimens. Stainless steel mesh was used for production of heating elements. The temperature distribution was monitored using type-K thermocouples connected to a data acquisition system. The main objective of the study was investigating a possible solution for the edge effect. Temperature profiles over the weld length and over the weld width were monitored. The focus was on the transient temperature profiles at the edges of the weld. The temperature distribution through the weld thickness was also monitored. The influence of factors like insulation and power level was investigated. Finally, conclusions are drawn and options for improving the temperature distribution and modification of the models are being discussed.Copyright

A study on the contraction and cooling times of actively cooled shape memory alloy wires

Shape memory alloys (SMAs) are thermally activated and show a strong thermomechanical coupling (i.e. the relationship between strain, stress and temperature). SMA wires are commonly used to develop SMA-based actuators. The most important characteristics of SMA wires is their capability to exert linear forces with strokes up to 8% of their original length. This make SMAs an attractive smart material for actuation purposes. However, the main weakness of SMAs is their limitation on actuation bandwidth. This limitation comes from the rate at which the SMA wires are able to heat up and cool down, releasing heat energy to the surroundings. This paper focuses on the effects that different heating and cooling rates have on the SMA wire’s working rate that can be attained. An SMA wire has been experimentally tested under different conditions of heating, cooling and applied load to find the influence of these parameters on the contraction and cooling times of an SMA wire and, therefore, on its attainable frequency. In addition, a model for SMAs has been implemented in a finite element analysis software and the experiments have also been simulated, which has been used to corroborate the conclusions drawn from the experiments.

Flexural properties of glass fiber/thermoplastic composites reinforced with metal mesh layers

In this paper, the flexural properties of glass fiber reinforced polymer composites with metal mesh layers were investigated. Two thermoplastic polymers, polyetherimide (PEI) and polyphenylene sulfide (PPS) were used as the matrix. Stainless steel was chosen as the mesh material due to its excellent strength and stiffness properties. The position of the metal mesh within the laminate stack was varied and the effect on the flexural properties and damage mechanisms was compared to that of the plain laminate. Placement of the mesh in the middle of the laminate led to the greatest improvement in flexural properties for the PEI laminates with an increase in specific flexural stress and strain of over 11% in both cases. However, the same case was not found for the PPS laminates and this was attributed to the poor adhesion between the PPS and the metal mesh.