Adaptive, tolerant and efficient composite structures

New materials with superior properties are the basis to exceed existing technological barriers and to explore new fields of application. Especially composites as multiphase materials offer the possibility to influence their properties or to add even new functionalities by a proper choice and combination of the different phases. In this context it is of particular importance to understand the interactions between the different material phases. This includes for example the effect of nanoscale additives in resins as well as the effect of microscopic manufacturing defects, like pores, on the macroscopic material properties. A systematic material design is only possible if cause and effect on the different material scales is well understood. Nanotechnology gives the opportunity to manipulate the structure of materials on a level, allowing to realize properties and functionalities that can’t be achieved with conventional methods. Beside the improvement of mechanical, thermal, optical and electrical properties, the incorporation of new “smart materials” on a technical relevant scale is in the focus of our research.

Low profile actuators are a basic technology for smart structures. Bonded on surfaces or embedded in composite structures they work as actuators and sensors to control the structural behaviour. The simplest types are based on thin piezoceramic plates (typical thickness 200 μm) provided with surface electrodes to operate in the lateral d

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-mode. This type of actuator is able to generate strains of 500 μm/m. To achieve higher deformations it is necessary to use the d

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-effect. The difficulty is to generate the necessary in-plane electrical field. A common solution is the use of interdigitated electrodes consisting of two comb like electrodes with opposite polarity that are placed on the surface of the piezoceramic material. Known as Active Fiber Composites (AFC’s) or Macro Fiber Composites (MFC’s) these kinds of actuators can produce strains of 1,600 μm/m. The drawback of interdigitated surface electrodes is a very high driving voltage of up to 1,500 V. A promising concept to overcome this drawback is presented. It is based on the use of multilayer technology for low profile actuators. Within these actuators the electrodes are incorporated in the piezoelectric material during the sintering process as very thin layers with little impact on the actuator stiffness. This allows a significant reduction of the electrode distance and therefore also a reduction of the driving voltage. To utilize the multilayer technology for low profile actuators, standard multilayer stacks are diced into thin plates. In this configuration the electrodes are not only on the surface of the piezoelectric material but cover the whole cross section. In a second step these plates are embedded into a polymer to build a piezo-composite. Without the mechanical stabilization of the surrounding polymer the handling of the fragile multilayer plate would be extremely difficult or nearly impossible. Several prototypes have been build and achieved an active strain of 1,200 μm/m at a voltage of 200 V. Using other materials an active strain of 1,600 μm/m is possible.

Laminates of carbon fiber reinforced plastic (CFRP), which are manufactured by injection technology, are reinforced with boehmite particles. This doping strengthens the laminates, whose original properties are weaker than those of prepregs. Besides the shear strength, compression strength and the damage tolerance, the mode of action of the nanoparticles in resin and in CFRP is also analyzed. It thereby reveals that the hydroxyl groups and even more a taurine modification of the boehmites’ surface alter the elementary polymer morphology. Consequently a new flow and reaction comportment, lower glass transition temperatures and shrinkage, as well as a changed mechanical behavior occur. Due to a structural upgrading of the matrix (higher shear stiffness, reduced residual stress), a better fiber-matrix adhesion, and differing crack paths, the boehmite nanoparticles move the degradation barrier of the material to higher loadings, thus resulting in considerably upgraded new CFRP.

This chapter refers about the optimization of fire resistance of CFRP. For this optimization the most promising nano scaled additives are used and varied regarding the particle content, size and effect of flame retardancy. One major challenge is to optimize the particle dispersion and to determine the optimal particle concentrations in consideration of the effect of flame retardancy and the resulting material properties. Additionally a fire testing method has to be determined that resolves the potentially small differences in the used variations. Therefore standard fire and mechanical tests are used as well as a simple thermal material method, given with the thermo gravimetric analysis (TGA) including a single differential thermal analysis (SDTA) that also suits for a fast comparison of the materials fire properties. Hereby obtained results are compared and a related behaviour of the fire properties can be shown between standard fire tests and thermal material tests using the TGA-device.

Nanocomposites based on silica nanoparticles and high performance epoxy resins are investigated for their suitability as a new type of matrix for fiber-reinforced polymers (FRP) using injection technologies (LCM). The key focus is on the determination of the processing parameters at varying silica nanoparticle content. The homogeneous distribution of the nanoscaled silica in the epoxy matrix is proven by photon cross correlation spectroscopy (PCCS) and scanning electron microscopy (SEM) analysis. Depending on the silica content of the composite, its stiffness, strength and toughness can be increased significantly compared with the neat resin. The mechanical performance is discussed by failure mechanisms based on the analysis of the fracture surface morphology. Moreover, resin shrinkage and the thermal expansion are significantly reduced both important for lowering internal stress in FRP. The injectability of the nanocomposite for the purpose of lamination using the LCM technology is nearly unaffected. Epoxy-silica nanocomposites are now proven to be a new high performance polymer matrix for FRP structures manufactured by the low cost LCM techniques.

The outstanding electrical and mechanical properties of single carbon nanotubes (CNT) are the motivation for an intensive research in various fields of application. The actuation effect constitutes the foundation for any application as a multifunctional material and within the field of adaptronics. The effect is in the majority of cases investigated by a CNT configuration of stochastically aligned CNT, so-called bucky-paper, in an electrolytic environment. The chapter presents an analytical model for a detailed understanding and investigation of the actuation process. The complete description and parameterization of the model is documented. Initial results from experiments with aligned CNT structures and the application of solid electrolytes are presented.

The success of active vibration control in adaptronics decisively depends on the performance of the actuators used as multifunctional structural elements. DLR’s concept of impedance matched actuators proposes novel actuators in honeycomb design (Fig.

8.1

). The partitions of their hexagonal cells are made of piezoceramic materials. The honeycomb geometry guarantees lightweight properties with nearly perfect load distributions in the plane perpendicular to the cell tubes. Furthermore there is a dynamic flexibility in this plane even when the static stiffness is very high. New fabrication methods are required to realize honeycomb actuators with very small cells, low wall thicknesses and perfect rounded corners. In order to fulfill the lightweight conditions self-organizing effects were used to guarantee optimal 3D geometries. In this context, a local thermal treatment of ceramic structures by employing lasers initiates self-organizing mechanisms via material flow. Experimental comparisons between piezoelectric honeycomb actuators and conventional monolithic actuators demonstrate a reduction of electrical power by a factor of 500. Honeycomb actuators seem to be the most promising candidate for technical applications that require low energy consumption.

Fig. 8.1

Piezoceramic honeycomb actuator

Current industrial demands for fiber composite primary structures in the area of aeronautics require innovative, experimentally validated simulation methods and tools, to support a cost and weight efficient design and to reduce their time-to-market utilizing virtual (simulation based) testing. Reliable application of numerical analysis in upfront design challenges not only verification (“solve the equation right”) aspects but also the validation of the numerical methods (“solve the right equations”) with trustworthy experimental investigations. This chapter provides an overview on different aspects of the validation process, starting with a concise insight in the terminology in modeling and computational simulation, followed by a description of the selected approach to validate structures at different levels of detail, the comparison of numerical results with experimentally extracted data and finally a brief outlook with respect to transferability prospects and limitations. These aspects will be reflected exemplarily utilizing numerical and experimental investigations on buckling and postbuckling of stiffened CFRP panels.

Two facts are the main drivers for a steady rise of models simulating fiber composites: an increasing demand on optimal utilization of the material and a drastic improvement in computational power. Though this process is still in full swing a review is considered reasonable since it facilitates guidelines for future research. Topics which comprise major development lines are micromechanics, laminate theories, design and optimization, damage and failure, and manufacturing, though a strict separation is not in all cases useful. In reviewing these topics it will turn out that their model status is of rather different maturity. Areas are identified where there are plenty of models available which are really not needed, whereas other problems cannot be modeled adequately as yet.

In this chapter, a numerical multiscale modeling approach is presented and discussed. It bases on the FE² approach in which a simultaneous finite element computation of the mechanical response at two different length scales is carried out at each macroscopic integration point. The approach is suitable to obtain the global load response of composite structures without omitting the effect of physical phenomena at the local scale, as for example process-induced defects like voids or fiber waviness.

Non Crimp Fabric (NCF) provides a low-cost potential and competitive advantages for thick composite structures. In this chapter, a method will be presented to determine the interlaminar failure under combined through-thickness load conditions. Additionally, the in-plane failure behaviour of NCF composite is discussed and analysed. A new test setup, based on the idea of Arcan, determines the material properties. Test results of combined through-thickness loading are presented by in the form of a shear-compression failure curve. The tests are reproducible and reliable. The failure envelope is finally used to verify known failure criteria.

In this chapter, efficient methodologies to evaluate impact resistance and damage tolerance of composite structures are introduced. Internal non-visible or barely visible impact damage (NVID, BVID) can provoke a significant strength and stability reduction in monolithic composite structures as well as in composite sandwich structures. Therefore, methodologies have been developed to reliably simulate the dynamic response and to predict the impact damage size that develops during low-velocity impact (LVI) events. Additionally, methods for the prediction of the compression-after-impact (CAI) strength are presented. Special attention is given to the impact assessment methodologies, which have been implemented in the DLR in-house tool CODAC. Simulation results of CODAC are presented and compared to experimental results.

This section deals with the experimental and numerical stability analysis of thin walled composite structures. Two different types of composite structures are considered. An unstiffened cylinder which is very sensitive to imperfections and used for basic research, and a stringer stiffened panel that is more related to aircraft fuselage structures. Both types of structures are loaded in axial compression. A set of different measurement systems has been applied before and during the tests in order to gather as much information as possible about the structure and its imperfections, as well as about the behaviour under load. Advanced optical measurement systems, for instance, have been utilized to monitor the failure in the skin-stringer interface of the stiffened panels. Exemplary test results are presented and compared to numerical simulations.

The relation between design and manufacture is of particular importance within the composite structure development process. Therefore, a continuous composite process chain beyond state-of-the-art is described in this section. Such an all-embracing process chain realizes a concurrent engineering, where iteration loops are enabled and, thus, product improvements and higher process efficiency are achieved. Concurrent engineering comprises the various interdisciplinary working phases and provides the necessary connectivity. In contrast to the traditional one-way relation from design to manufacture, the improved process chain also deals with the feedback from manufacture to design, based on numerical simulations. This is demonstrated by the example of composite parts made by Tailored Fiber Placement (TFP), including effects of the feedback on load bearing capacity.

In general, tests can be divided into four categories: parameter estimation (e.g. material strength), phenomenological investigation, validation and qualification. According to this classification tests are carried out on a structural or component level and on a coupon level. For structural testing a Buckling Test Facility, a Variable Component Test Facility and a thermo-mechanical test field are described. Furthermore, information is given on specimen level tests with devices for standard test machines: Stringer Pull-off Device and 3D-Biax Device.

The aggregation of functionalities offers additional benefits to the customers such as reduced weight, reduced life cycle costs and an increased range of applications. For a compliant aggregation of functionalities according to given requirements clear instructions on how to conduct lightweight design are essential, but often not available today. High performance lightweight structures are made from carbon fiber reinforced plastics increasingly. Due to the specific composite manufacturing process four different levels of function-integration are conceivable. The pre-fabrics or components of the composite can include smart materials with enhanced functionalities. The structure design can better exploit the composite potentials of anisotropic material properties. Passive components integrated into the structure provide additional functionalities as for example de-icing and lighting protection. In adaptive systems active elements significantly improves the ability of the structure to adapt changing environmental conditions. The development of the potentials resulting from the compliant aggregation of functionalities is presented in this chapter.

Deployable structures are necessary to realize large but weight-efficient space systems. DLR provides a deployable mast that can be used either at once to realize e.g. long dipole antennas of some ten meters or to setup structures that use this mast as basic building block structure. This section shall, therefore, enable a basic insight on the concept and of the resulting challenges. Moreover, a deployment test series under weightlessness is presented and evaluated to show possible concepts of deployment control and demonstrate the potentials.

Composite technologies have been proven to be advantageous in allowing for the development of aircraft and spacecraft structures which feature highly integral design concepts. However, structural joining using conventional mechanical fastening techniques still remains an indispensable issue within the design of advanced composite structures. Crucial challenges facing structural joining are the inherent complexity of the stress state at the bolt locations on the one hand and the multifaceted fracture mechanics of composite material on the other. The aerospace industry’s increasing requirement for weight reductions and a more efficient use of composites demands not only an accurate understanding of this material’s mechanics and its damage behavior at composite joints but the development of advanced joining techniques as well in order to be able to fully exploit the outstanding capabilities of composite material. The use of a local hybridization with metal represents a suitable and technologically feasible means to increase the mechanical efficiency of highly loaded composite bolted joints which allows for a significant improvement of the overall structural efficiency of real composite structures. This chapter presents the hybrid reinforcement concept and addresses some fundamental topics of this technology’s mechanical behavior.

In comparison to other transport systems, launch vehicles are characterized by relatively light but extremely valuable payloads. The launcher’s upper stage structures, e.g. payload adapter and fairing, offer the highest weight saving potential. An effective weight reduction can only be achieved by the combined utilization of high performance materials and adapted construction methods. To improve the structures damage tolerance a new hybrid lay-up has been developed, which combines the properties of both, steel and carbon fiber reinforced plastics (CFRP). This chapter presents a preliminary design of a payload adapter as a framework, which is based on the high performance material properties of unidirectional CFRP-steel-laminates, offering a considerable weight saving potential.

A straightforward approach to predict spring-in deformations of angled composite parts is presented. Therefore, a proposal by Radford is extended in order to calculate the spring-in contribution due to chemical shrinkage. For this, the volumetric shrinkage of neat thermoset resin, which is in the range of 2–7%, is transferred to equivalent strains on ply level assuming no shrinkage in fiber direction. As the fiber volume fraction (FVF) affects mechanical and chemical properties significantly, the spring-in angle is affected as well. Therefore, the numerical investigation accounts for the spring-in angle and its thermal and chemical contributions depending on the FVF. Classical laminate theory (CLT) is utilized to homogenize layup expansion and shrinkage properties. For validation purposes, model predictions are compared with measurement results gained from one manufactured test specimen. Good agreement between analytical and experimental results is found. Furthermore, the chemical contribution of the total spring-in angle ∆φ turned out to be significantly larger than the thermal contribution.

Increasing environmental, economical, and social issues force future car concepts to strive for maximum efficiency. As metal designs have reached a very high level of maturity, further potentials are seen especially with extremely lightweight (carbon) fiber reinforced composites (CFRP) exhibiting high strength and stiffness, which are advantageously integrated into multi-material designs. This section focuses on improvements in weight reduction, safety and modularisation strategies for future cars. A novel Rib and Space-Frame concept incorporating carbon fiber composites is presented. Herein, an essential component replacing the former B-pillar is the B-rib, which uses a novel mechanical principle to meet the side impact crash requirements. Furthermore, using the Rib and Space-Frame concept as an example, the entire process chain of fiber composites—from materials to design, sizing, prototype manufacture, and testing—is being presented also opening up perspectives for future mass production strategies.

Today’s bonded composite repairs rely heavily on manual grinding. The human influence on scarf tolerances and consequently on assembly and structural performance can be reduced by automating the process of scarf manufacturing. A process based on contact free surface scanning, surface reconstruction, automated repair design and automated milling of the repair scarf is presented. A machine and software design for validation purposes is described. Several repair specific design considerations relevant for the construction of a mobile scarfing machine are discussed. The redesign of a standard 3-axis milling machine to a mobile automated scarfing unit is presented and the architecture of the associated software framework is described. An outlook to future validation steps is given.

Technical properties of composite materials highly depend on their distribution of fibre and matrix components. To combine these two components in the best possible configuration a variety of technologies has been developed. However a remaining challenge is that all semi finished products have decisive tolerances that may interact during production and lower the level of structural performance. To avoid extended safety margins, a variety of strategies has been developed to maximize reproducibility in the crucial production steps by detecting and compensating dominating tolerance issues in the process chain.

Preforming is required for complex shaped profiles manufactured in liquid composite moulding (LCM) processes. The stacking is made of dry fiber fabrics, which are infiltrated in a later process step. The complete stacking is called preform. The fixing of the textile layers can be realised through stitching or binder technology. The main disadvantage is the immense rate of manual work within the preform process. In consequence, the manufacturing is costly in terms of time and high effort for quality control. Automated preforming can reduce the costs by increasing the output and production rate while minimising waste. Preform profiles with variable outlines and non-extrudable sections are of particular interest for the aviation and automotive industry. The DLR Institute of Composite Structures and Adaptive Systems has developed an innovative device to overcome the previous limitations and to fulfil the industrial demands.

This paper reports on different parameters that influence the closed mould resin transfer moulding (CM-RTM) process for fiber reinforced plastics. A sensitivity study of selective parameters is performed. This includes material parameters (i.e. viscosity, permeability), process parameters (i.e. temperature) and geometrical parameters (i.e. position of preform in the tool). Furthermore, fiber type and targeted fiber volume content (FVC) are considered to validate the full range of fiber reinforced plastics. As an example for the sensitivity study, the aeronautical carbon fabric G0926 and epoxy resin system RTM6 (both manufactured by Hexcel) are analyzed for targeted fiber volume contents in a range of ~60%. The infiltration of a rectangular panel was simulated with the flow simulation software RTM-Worx by Polyworx. It was found that the infiltration of a simple geometry can differ by app. factor 3 in terms of duration, when only considering the tolerances of material and process parameters (“upper tolerance limit” vs. “lower tolerance limit” scenario). For different types of composite materials observed in this study, it can even go up to Factor 1,000. To achieve a reliable RTM process, these aspects—material types and range of tolerance—have to be considered.

The most time consuming step in manufacturing of high performance composites usually is the manually driven preforming step. Individual layers are draped in their formed position and fixed to each other by activating a binder. The innovative technology based on inductive heating presented in this chapter speeds up this process. As many parameters affect the inductive heating rate, a simple analytical model based on empirical data will be introduced. With this model, the resulting temperature can be predicted for given process parameters. Conversely, the best fitting process parameters can be found given a required heating temperature (depends on the binder).

This chapter presents a novel manufacturing technology for the production of integrated structures made of carbon fiber composite materials. This manufacturing process is able to considerably reduce the production time of large assemblies for primary structural applications. It is based on a combination of the prepreg and the resin infusion technology, both of which are already established in industrial aerospace production. Experimental studies have been carried out to demonstrate the utility of this process. These investigations focused on the characterization of the generated contact zone through the use of two matrix systems as well as its mechanical properties.

The most critical step within the process chain is the curing of the matrix because here the different semi-finished products are transformed into a completely new compound material. In case of thermoset matrix systems all geometrical characteristics of the produced composite component are frozen as well. Tolerances in the chemical composition of the matrix or simple aging aspects can influence the crosslinking reaction while typical weight tolerances of the fibre product may affect the fibre content, the laminate thickness or even the global geometrical shape of the composite component through the so called “Spring-In” phenomenon. Interactively controlling the curing step is a promising approach to solve most of the above mentioned problems.

The vision behind so-called autonomous composite structures is the creation of a new class of composite lightweight structures with significantly enhanced capabilities with respect to traditional design. This includes health monitoring features to increase the maintainability and to enlarge the achievable design layouts as well as the incorporation of noise reduction and vibration control capabilities directly into the structure. The overall quantity to be minimized is the weight per surface ratio, which can only be put below a certain application dependent threshold value by active methods. Therefore three different key technologies have to be merged into one autonomous system: energy harvesting, smart structures and fiber composites. This section gives an overview about the requirements for current and future research to make this vision real and presents examples which demonstrate that some key aspects of autonomous composite structures are already realizable with “state of the art” techniques.

To make use of low-drag future generation wings with high aspect ratio and low sweep for natural laminar flow, new high lift devices have to be developed [ACARE (Addendum to the Strategic Research Agenda, 2008), Horstmann (TELFONA, Contribution to Laminar Wing Development for Future Transport Aircraft, 2006)]. At the wing leading edge a smart e.g. morphing high lift device is being developed which provides a high-quality surface without gaps and steps. Due to the low maturity of morphing skins (Thill et al. (The Aeronautical Journal, 112:117–138)) the challenge of high strains has to be solved by an adequate design and sizing process. The presented design process comprises the requirements of a smart leading edge device, the structural pre-design and sizing of a full-scale leading edge section for wind tunnel tests.

Individual Blade Control (IBC) for helicopter rotors promises to be a method to increase flight performance and to reduce vibration and noise. Quite a few concepts to realize IBC Systems have been proposed so far. Some of them have already been tested in wind tunnels or on real helicopters. A drawback of all systems that include discrete mechanical components like hinges, levers or gears is their vulnerability in a helicopter environment with high centrifugal loads and high vibration levels. That’s why the idea of using smart materials that are directly embedded in the rotor blade structure is very attractive for this application. Operating as solid state actuators they can generate a twist deformation of the rotor blade without any friction and wear. In the common DLR-ONERA project “Active Twist Blade” (ATB), DLR designed and build a 1:2.5 mach scaled BO105 model rotor blade incorporating state of the art Macro Fiber Composite (MFC) Actuators. The design of the blade was optimized using a finite element code as well as rotor dynamic simulations to predict the benefits with respect to vibrations, noise and performance. Based on these tools a blade was designed that meets all mass and stiffness constraints. The blade has been intensively tested within some bench- and centrifugal tests. The mechanical properties of the blade obtained within the bench tests showed a good correlation between measured and calculated values. The centrifugal test comprised a measurement of the active twist performance at the nominal rotation speed of 1,043 RPM at different excitation frequencies from 2 up to 6/rev. It was proven, that also under centrifugal loads the predicted twist amplitudes can be achieved.

Vibrations are problems encountered at almost every technical application when there are moving parts or fluids included. Upon the need for lightweight structures, especially in aerospace applications or electric mobility, conventional damping concepts are insufficient because of their extra-weight and low performance at low frequencies. Measures for active noise and vibration reduction have the potential to solve the drawbacks of passive systems. However, a limitation of these concepts arises from the need for complex system models for deriving the controllers. This leads to another possibility to reduce vibrations consisting of piezoelectric transducers attached to the structure and connected to hybrid electric networks. Within this paper, the basic principles of shunt damping, two hybrid electric networks circuits for shunted damping and the experimental validation of a damped system will be shown. Finally—as a challenging example—a circular saw blade is equipped with piezoelectric transducers and negative capacitance hybrid electric networks to reduce the vibration and noise amplitude excited by the cutting process.

The turbulent boundary layer (TBL) is one of the dominant external noise sources in high subsonic aircrafts. Especially in modern aircrafts where common materials for fuselages are currently substituted by carbon-fiber-reinforced-plastics (CFRP), it is essential to avoid a decrease of passenger comfort as a result of an inferior transmission loss of the new materials. To increase the transmission loss of CFRP panels they are equipped with active noise reduction systems. In this paper the results of an experimental study in the aeroacoustic wind tunnel of the German Aerospace Center (DLR) are presented. An active panel excited by a TBL is tested at flow speeds up to Mach 0.16. The CFRP panel (500 × 800 × 2.7 mm

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) is equipped with five piezo-ceramic patch actuators and ten accelerometers. Active structural acoustic control (ASAC) and active vibration control (AVC) are used to reduce the broadband TBL noise transmission in the bandwidth from 1 to 500 Hz. Feedforward (FF) and feedback (FB) control algorithms are applied in the experiments and show high performance even in presence of plant uncertainties. To improve control results the generalized plant framework of robust control is utilized for global feedback control. Finally, an overview of the achieved results is given.

The oil pan of large diesel engine trucks is a significant contributor of external noise radiation. Especially at lower frequencies below 500 Hz, this undesired broadband noise cannot be treated effectively by passive measures due to weight and size restrictions. Augmenting such systems with an Active Structural Acoustic Control (ASAC) system is a promising way to effectively damp the sound radiation at critical frequency ranges. Such a system was to be realized within the European Union (EU) project “Intelligent materials for Active Noise Reduction” (InMAR) for the oil pan of a Volvo MD13 truck engine. Piezoceramic patch actuators have been used in a laboratory test stand to alter the vibrations in a broadband noise reduction manner. This chapter discusses the actuator placement strategy and how to obtain an estimation of the broadband sound power minimization capability. Finally the chosen actuator layout was validated by experimental observations of a serial production oil pan.

The poor sound insulation of windows especially at low frequencies constitutes a severe problem, both in transportation and in the building sector. Due to additional constraints on vehicles or aircrafts regarding energy efficiency and lightweight construction, the demand of light-weight-compliant noise-reduction solutions is amplified in the transportation industry. Simultaneously, in order to satisfy the customer demands on visual comfort and modern design, the relative size of glazed surfaces increases in all sectors. The experimental study presented below considers the feasibility of actively controlled windows for noise reduction in passenger compartments by using the example of an automobile windshield. The active windshield consists of the passive windshield, augmented with piezoceramic actuators and sensors. The main focus of the subsequent work was the development and evaluation of feedforward and feedback control strategies with regard to interior noise reduction. The structural excitation of the windshield was realized by an electrodynamic exciter (shaker) applied at the roof brace between the A-pillars. By this choice it was possible to emulate the structural excitation of the windshield through the car body, induced by coasting and motor-force harmonics. The laboratory setup does not permit the consideration of hydrodynamic and acoustic loads, which might be important as well. However, the experimental results indicate the high noise reduction potential of active structural acoustic control of structure-borne sound that radiates into a cavity.

Structural Health Monitoring (SHM) based on ultrasonic guided waves, so-called Lamb waves, is a promising method for in-service inspection of composite structures without time consuming scanning like conventional ultrasonic techniques. Lamb waves are able to propagate over large distances and can be easily excited and received by a network of piezoelectric actuators and sensors. In principle different kinds of structural defects can be detected and located by analyzing the sensor signals. The chapter describes recent research activities at DLR on Structural Health Monitoring. The research are focused on the visualisation of Lamb wave propagation fields based on air-coupled ultrasonic technique, the simulation of virtual sensors, mode selective actuators as well as manufacturing of actuator and sensor networks. Additionally, the chapter present the development of a SHM system for impact detection in a helicopter tailboom (Eurocopter—EC 135).