R. Christen

A sandwich beam with electrostatically tunable bending stiffness

The tuning of the bending stiffness of structural elements is of interest for, among other things, the suppression of vibrations related to resonance phenomena. For a given cross-sectional area and geometry, the variation of the elastic properties of the material composing the structure provides a viable approach to this task. Only very limited options are available for such changes in material properties. The use of NiTi shape memory alloys has been proposed for this purpose. A new, energetically less expensive method for the modification of the bending stiffness of sandwich beams is presented. The proposed method makes use of electrostatic forces to modify the transfer of shear stresses at the interface between the faces and the core of the sandwich. Changes in bending stiffness of up to 18 times could be obtained for a prototype beam. A simple model for describing the behavior of the beam is presented.

Electrostatically tunable bending stiffness in a GFRP?CFRP composite beam

The suppression of vibrations in structures is commonly considered a useful measure for the extension of their lifetime, when high amplitude vibrations are observed. In the experiments presented in this work, the modification of the stiffness of a beam as a means to suppress vibrations due to resonance is proposed as an alternative to the introduction of discrete damping devices. The stiffness of a beam is modified by applying an electric field between the main element of the structure and additional stiffening elements applied to its surface, thus coupling the latter to the former by transfer of shear stresses. The effect of electrostatic tuning of the bending stiffness (and consequently of its eigenfrequencies) of a large size GFRP–CFRP beam is shown by the shift of the resonance peak for the first bending mode to higher frequencies. The discrete character of the stiffness increase in multi-layer beams (n≥3) is postulated.

Mechanical performance of cold-curing epoxy adhesives after different mixing and curing procedures

Abstract This paper presents strength, stiffness, and porosity characteristics of commercially available cold-curing epoxy adhesives for structural engineering applications in the field of externally bonded and/or near-surface mounted composite strip reinforcements. Depending on specific requirements, accelerated curing of the adhesive under high temperatures might be necessary. Experimental investigations aimed at assessing the possible differences in strength and stiffness between samples cured at elevated temperatures for a defined time span and the ones cured at room temperature. It could be demonstrated that for the same specimen age, nominal tensile strength and stiffness are lower after an initial accelerated curing process at elevated temperatures. Furthermore, it could be shown that the specimens after an accelerated curing at elevated temperatures exhibited an increased porosity. The development of a numerical code for image analysis allowed a detailed inspection of several fracture surfaces and subsequently to assess the level of decrease in available cross-section due to an increased overall porosity. Cross-section area losses in the range of 10–15% compared to the reference specimens could be deduced. The subsequent derivation of the actual tensile strength exhibits smaller differences between the room and high temperature exposed specimens while curing. Regardless of the short-term material strength, the observed porosity might be subject of important durability issues on a long-term and needs further investigation.

Frictional behaviour of polymer films under mechanical and electrostatic loads

Different polymer foils, namely polyimide, FEP, PFA and PVDF were tested on a setup designed to measure the static coefficient of friction between them. The setup was designed according to the requirements of a damping device based on electrostatically tunable friction. The foils were tested under different mechanically applied forces and showed reproducible results for the static coefficient of friction. With the same setup the measurements were performed under an electric field as the source of the normal force. Up to a certain electric field the values were in good agreement. Beyond this field discrepancies were found.

Electrostatic tuning of the bending stiffness of simple slender multilayer composite structures

Vibration control and suppression in structures plays a central role in the extension of their service life and improvement of their reliability. While in many cases the solution of this problem implies the introduction of external damping devices, it is also conceivable to adaptively modify their vibratory properties, so that the occurrence of severe vibrations due to resonance phenomena can be curbed at its origin. The modification of the shear stress transfer at the interface between the core and the faces of a sandwich beam has been shown to have a remarkable effect on the bending stiffness of the structure. Such modification can be obtained by applying a normal stress between the core and the un-bonded, electrically insulated faces of the sandwich by means of a strong electrical field. An intermediate behavior between fully bonded and un-bonded layers in terms of inter-laminar shear stress can be achieved by temporary electrostatic bonding of the components. The outlined approach to the reduction of transversal vibrations in thin multi-layer beams is promising and can in principle be applied to multi-layer plates.

A simple approach to the localization of flaws in large diameter steel cables

Experimental work performed on several full-scale stay-cable models as well as on RAMA IX Bridge in Bangkok has confirmed that the application of magnetic flux leakage (MFL) methods is a viable approach to the non-destructive evaluation of large diameter steel cables. Such method allows for a high sensitivity and high-resolution detection of fractured wires in stay cable systems. So far, the information obtained from the recorded data (intensity of the MFL on the surface of a cable) was limited to the accurate position of detected flaws along the axis of the cable and a qualitative indication of the position of the flaws within the cross-section. The ability to accurately determine the position of flawed wires within the cross-section of a cable is especially useful in the case of multi-strand systems, in which individual strands can be replaced if damaged. Such information can be obtained by computation with finite element models or sophisticated dipole approximations. An alternative to such computing intensive approach, based on a simple mathematical model of the MFL function is proposed in this work. The function is used for a non-linear fit of the measured data. The method has been tested successfully on simulated and measured data.

High precision measurement of surface cracks using an optical system

In this contribution an advanced technique to measure the width of surface cracks in concrete is presented. The goal is to provide a system for laboratory applications with a high resolution in order to be able to estimate the stabilization of surface cracks in the range of 0–250 µm during laboratory loading tests. Using digital imaging and suitable image processing algorithms, an automated system for the determination of crack widths was developed. A 4 megapixel monochrome digital camera was used in combination with a special telecentric lens, in order to achieve a resolution of 1 µm per pixel. The main challenge in this work was to develop a robust image processing algorithm, which determines the surface crack width under changing environmental conditions, especially in terms of ambient light.

A simple model for the prediction of the discrete stiffness states of a homogeneous electrostatically tunable multi-layer beam

The adaptive modification of the mechanical properties of structures has been described as a key to a number of new or enhanced technologies, ranging from prosthetics to aerospace applications. Previous work reported the electrostatic tuning of the bending stiffness of simple sandwich structures by modifying the shear stress transfer parameters at the interface between faces and the compliant core of the sandwich. For this purpose, the choice of a sandwich structure presented considerable experimental advantages, such as the ability to obtain a large increase in stiffness by activating just two interfaces between the faces and the core of the beam. The hypothesis the development of structures with tunable bending stiffness is based on, is that by applying a normal stress at the interface between two layers of a multi-layer structure it is possible to transfer shear stresses from one layer to the other by means of adhesion or friction forces. The normal stresses needed to generate adhesion or friction can be generated by an electrostatic field across a dielectric layer interposed between the layers of a structure. The shear stress in the cross section of the structure (e.g. a beam) subjected to bending forces is transferred in full, if sufficiently large normal stresses and an adequate friction coefficient at the interface are given. Considering beams with a homogeneous cross-section, in which all layers are made of the same material and have the same width, eliminates the need to consider parameters such as the shear modulus of the material and the shear stiffness of the core, thus making the modelling work easier and the results more readily understood. The goal of the present work is to describe a numerical model of a homogeneous multi-layer beam. The model is validated against analytical solutions for the extreme cases of interaction at the interface (no friction and a high level of friction allowing for full shear stress transfer). The obtained model is used to better understand the processes taking place at the interfaces between layers, demonstrate the existence of discrete stiffness states and to find guidance for the selection of suitable dielectric layers for the generation of the electrostatic normal stresses needed for the shear stress transfer at the interface.

Filtering of NDT signals obtained from wrapped steel cables

The main cables of suspension bridges are often wrapped with a steel wire, in order to compact the cable and hold it in shape. If a non-destructive evaluation by means of magnetic methods is performed on such a cable, disturbances due to the wrapping can be expected in the measured signal. In the presented work, these disturbances shall be quantified and compared to the flaw signals. Different approaches for the separation of the disturbance and the flaw signal are discussed. Additionally, the possibility to detect wire breaks and corrosion within an unwrapped steel cross-section could be shown in laboratory measurements. The influence of the wrapping was investigated using finite element (FE) simulations and experimental laboratory measurements. A parameter study was performed in order to obtain data in which the components from a flaw and the wrapping can be separated. The parameters varied in this study were chosen depending on the prospect of success and the cost of the realization. Using these data sets different filtering methods, such as wavelet analysis, were implemented. A final comparison of the different methods suggests the most efficient way to assess the condition of such cable systems using magneto-inductive testing. Finally, it can be concluded that the use of FE simulation is a very useful tool for the development of new data analysis methods, even if a real set-up and data from measurements exist.

Electrostatic tuning of the bending stiffness of a large scale GFRP-CFRP beam

The suppression of vibrations of a structure is commonly considered a necessary measure for the extension of its lifetime, when high amplitude vibrations are observed. As an alternative to the introduction of discrete damping devices, the modification of the stiffness of a beam is proposed as a means to suppress vibrations due to resonance, thank to the ability to reject mechanical energy input at specific frequencies. Previous work has outlined the principle and the potential advantages of such an approach based on the behavior of a small scale system. In order to confirm the feasibility of the approach on macro-scale systems, such as a light weight pedestrian bridge, experiments for the tuning of a 2.5 m long glass fiber reinforced polymer I-beam were performed. The results of the experiments show that it is possible to modify the bending stiffness of structural elements that can be used for real life engineering applications. Measurements show that it is possible to shift the resonance peak of a beam while maintaining a reasonably good q-factor in the transfer function, thus indicating that the change in behavior happens in connection with an increased stiffness rather than with the introduction of substantial damping. Based on the presented feasibility study, the development of an adaptive bridge deck will be considered.