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.

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.

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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 nearsurface 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 10e15% 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.

Mortar and concrete exhibit low tensile strengths. Hence, cracks develop easily due to shrinkage and extemal actions. They can be prevented by applying prestress, thus obtaining crack-free products. Such products exhibit a high bending and tensile strength, are leak proof and of high dmability. Presh•ess can be realized using extemal or internal wires or cables. In thin walled products, however, this is not feasible. For this purpose, short fibers of shape memory alloy (SMA) wires were embedded in m01iar. The wires had been shaped by inelastic elongation into loop-and star-shaped fibers. After hardening of the m01iar, the specin1ens were heated up in order to activate the tensile stress in the fibers, thereby causing a presh•ess of the surrounding m01iar. The effect was monitored by length measurements both on specimens with and without fibers. Compression sh•esses in the cement m01iar were estimated by multiplying the difference in strain between fiber-reinforced and reference prisms by the Yollilg's modulus. Thus, compression of some 7 MPa was reached in the experiments. For practical applications, alloys with suitable temperature domains of austenitic and maiiensitic trai1sfonnation, most likely Fe-based, and efficient methods for the production of such fiber mo1iars are to be developed.