An overview of wireless structural health monitoring for civil structures

Wireless monitoring has emerged in recent years as a promising technology that could greatly impact the field of structural monitoring and infrastructure asset management. This paper is a summary of research efforts that have resulted in the design of numerous wireless sensing unit prototypes explicitly intended for implementation in civil structures. Wireless sensing units integrate wireless communications and mobile computing with sensors to deliver a relatively inexpensive sensor platform. A key design feature of wireless sensing units is the collocation of computational power and sensors; the tight integration of computing with a wireless sensing unit provides sensors with the opportunity to self-interrogate measurement data. In particular, there is strong interest in using wireless sensing units to build structural health monitoring systems that interrogate structural data for signs of damage. After the hardware and the software designs of wireless sensing units are completed, the Alamosa Canyon Bridge in New Mexico is utilized to validate their accuracy and reliability. To improve the ability of low-cost wireless sensing units to detect the onset of structural damage, the wireless sensing unit paradigm is extended to include the capability to command actuators and active sensors.     

Fracture mechanics in the characterisation of brick masonry structures

The analysis of the mechanical behaviour of masonry structures is based on the assumption that this material does not withstand tension. In this field many researchers are studying how to refine analytical and experimental procedures to obtain a more accurate correspondence between the real behaviour and the simulated one. However, this hypothesis cannot explain the masonry strength with respect to dynamic and seismic load conditions, which cause tensile stresses and dilating strains in masonry structures. The aim of this work is to give a complete set of experimental results in order to determine the mechanical properties of brick masonry. In particular, an extensive experimental programme was carried out to characterise the mechanical and structural behaviour of masonry composite in compression, along both the material directions, and in tension, along the horizontal mortar joint direction. Tensile properties in particular were estimated by means of four-point bending (FPB) tests owing to the lack of a standard tensile test and because bending tests are considered to be reliable indirect tensile ones and easy to carry out. The experimental results obtained using this testing procedure highlighted the behaviour of a bimodular material when subjected to tensile stresses, and these were then analysed by means of fracture mechanics theory to estimate the masonry toughness and stress intensity factor as further material characteristics.     

Physical and mechanical characterisation of 3D-printed porous titanium for biomedical applications

The elastic modulus of metallic orthopaedic implants is typically 6–12 times greater than cortical bone, causing stress shielding: over time, bone atrophies through decreased mechanical strain, which can lead to fracture at the implantation site. Introducing pores into an implant will lower the modulus significantly. Three dimensional printing (3DP) is capable of producing parts with dual porosity features: micropores by process (residual pores from binder burnout) and macropores by design via a computer aided design model. Titanium was chosen due to its excellent biocompatibility, superior corrosion resistance, durability, osteointegration capability, relatively low elastic modulus, and high strength to weight ratio. The mechanical and physical properties of 3DP titanium were studied and compared to the properties of bone. The mechanical and physical properties were tailored by varying the binder (polyvinyl alcohol) content and the sintering temperature of the titanium samples. The fabricated titanium samples had a porosity of 32.2–53.4 % and a compressive modulus of 0.86–2.48 GPa, within the range of cancellous bone modulus. Other physical and mechanical properties were investigated including fracture strength, density, fracture toughness, hardness and surface roughness. The correlation between the porous 3DP titanium-bulk modulus ratio and porosity was also quantified.     

Semi-quantitative mechanical characterisation of fibre composites in the sub-micron-range by SFM

The possibilities of using a Scanning Force Microscope (SFM) in tapping mode for a quantitative mechanical characterisation in the sub-micron-range are investigated. The test method and associated theory are presented and the technique is used to characterise glass fibre reinforced polyamide. Stiffness images are discussed with regard to the sample preparation method used and the mechanical properties of the interphase region between the fibres and the polyamide matrix.     

Investigation of the dynamic behaviour of civil engineering structures—new developments

EMPA Department Concrete Structures     

The slip casting and mechanical characterisation of magnesium-alumino-silicate glass-ceramics

An investigation of the slip preparation and casting of two magnesium-alumino-silicate glass powders has demonstrated the particular importance of the control of particle size distribution and slip viscosity in fabricating uniform glass-ceramics by powder methods. Measurement of strength and fracture toughness of fired material, coupled with particle size analysis and scanning electron microscopy has allowed interpretation of the quality of the material produced in terms of strength limiting flaws. Suggestions for the improvement of this type of material are made.     

Resistographic inspection of ancient timber structures for the evaluation of mechanical characteristics

Among non-destructive testing methods for the analysis of ancient timber structures, the Resistographic method is gaining great diffusion. This method is based on the registration of density variations through timber sections by means of an electronic drill and nowadays it is generally employed only to detect the presence of defects and damage. The present study is about the use of Resistographic method to evaluate mechanical characteristics of timber and, with this aim, an experimental program on wood-worm damaged timber has been carried on. Results show the usefulness of this non-destructive testing method as it allows the investigation through the interior part of timber and thus can give a quite reliable idea of local strength of wood in the examined section. That gives an useful parameter for the detection of recoverable elements in standing timber structures to be restored. In the present research, beech compressive strength has been evaluated from small clear wood specimens, while the evaluation of strength of dimensional lumber is outside the scope of the work.

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Résumé La méthode d'essai non-destructif dite Résistographique, est désormais la plus répandue pour l'analyse d'anciennes constructions en bois. Son application, qui se base sur l'enregistrement des variations de densité à l'intérieur d'une section en bois, à l'aide d'une perceuse électronique, n'est à présent utilisée que pour repérer d'éventuels défauts et endommagements. Cette étude concerne l'utilisation de la méthode Résistographique permettant d'évaluer les caractéristiques mécaniques du bois de construction. Dans ce but, on a expérimenté cette méthode sur des éléments de construction en bois de hêtre, en partie vermoulus. Les résultats démontrent l'utilité de cette méthode d'essai non-destructif qui permet d'examiner l'intérieur des bois, et d'en vérifier avec efficacité la résistance. Tout ceci nous permet d'individualiser tous les éléments récupérables dans les constructions en bois devant être restaurées. Dans cette recherche, on a derterminé la résistance du hêtre grace à des échantillons prismatiques, mais la résistance du bois des poutres en dimension d'usage réel, étant en dehors de l'objet de cette étude, n'a pas été mesurée.

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Editorial Note The University of Naples is a RILEM Titular Member.     

Experimental characterisation of the drying effect on uniaxial mechanical behaviour of mortar

The main objective of this present study was to evaluate, for a standard mortar, the drying effect on its mechanical behaviour. Numerous uniaxial compression tests were thus performed with loading-unloading cycles. They were carried out on different samples previously preserved under various conditions of conservation: preserved from desiccation, air drying and rapid drying at 60°C. The obtained results showed significant influences of these conditions on the material behaviour (increase in strength, decrease in Young's modulus and Poisson's ratio) and the necessity of taking into account the coupling effects between mechanical—poromechanical behaviours and drying.     

A practical engineering method for fuzzy reliability analysis of mechanical structures

The fuzzy sets theory in reliability analyses is studied. The structure stress is related to several other variables, such as structure sizes, material properties, external loads; in most cases, it is difficult to be expressed in a mathematical formula, and the related variables are not random variables, but fuzzy variables or other uncertain variables which have not only randomness but also fuzziness. In this paper, a novel approach is presented to use the finite element analysis as a “numerical experiment” tool, and to find directly, by fuzzy linear regression method, the statistical property of the structure stress. Based on the fuzzy stress–random strength interference model proposed in this paper, the fuzzy reliability of the mechanical structure can be evaluated. The compressor blade of a given turbocharger is then introduced as a realistic example to illustrate the approach.     

Fracture mechanical characterisation of mixed-mode toughness of thermoplast/glass interfaces

According to studies conducted by, e.g. Liechti and Chai [J. Appl. Mech. 58 (1991) 680], Yuuki et al. [Eng. Fract. Mech. 47 (3) (1994) 367] and Ikeda and Miyazaki [Eng. Fract. Mech. 59 (6) (1998) 725], a significant increase of interfacial toughness is observed, whenever the magnitude of the bond tangential shear load of the asymptotic elastic mixed-mode state is increased in either direction. Between these extremes the interfacial toughness curve exhibits a pronounced minimum, which, according to Hutchinson and Suo [Mater. Sci. Eng. A 107 (1989) 135] is believed to represent the so-called intrinsic adhesion, i.e. the failure toughness under pure local mode I loading. Within linear elasticity, the biaxial, singular near-tip solution for an open interface crack may be employed for characterising the local stress state as long as non-linearities such as, e.g. crack-wall contact and plastic flow are contained within a zone small enough compared to the extension of the singular opening-dominated fields. Then, the critical stress state is given in terms of bimaterial stress intensity factors K_1,c, K_2,c and the fracture toughness under mixed-mode loading may be expressed in terms of the critical energy release rate as a function of the mode-mixity ψ=tan⁻¹K_2,c/K_1,c. The stress intensities have to be extracted from a stress analysis of the specimen under the critical load, which in the present work is performed by means of an FE-model of the loaded sample.     

Principal component thermography for flaw contrast enhancement and flaw depth characterisation in composite structures

This study reports on the application of principal component thermography to the nondestructive inspection of composite structures. The technique is based on a singular value decomposition of the measured response to pulsed thermal excitation as applied in the course of a normal flash thermographic inspection. Trials on synthetically generated data show evidence of excellent noise-rejection qualities which manifest high levels of flaw contrast relative to that present in the unprocessed data. A simple analytical expression is then derived that relates a characteristic time furnished by the decomposition to the flaw depth, providing a basis for flaw depth estimation. This framework is shown to provide flaw-depth estimates with good levels of precision and robustness. Finally, application to experimental data is considered. An AS4/3501 composite laminate sample containing an assortment of teflon inserts is subject to active thermographic inspection and the response data analysed using the proposed approach. The results conclusively demonstrate the practical efficacy of the approach, and confirm the attractive properties outlined in relation to the numerical tests.     

An engineering method for reliability analyses of mechanical structures for long fatigue lives

Reliability analyses of mechanical structures designed for long fatigue lives require: some information on the probability distribution of material fatigue strength at long fatigue lives. In order to address this need, three-parameter P-S-N curves are adopted to represent the results of fatigue tests based on the conventional method. The parameters of the P-S-N curves are estimated by using the least squares fitting method and maximizing the correlation coefficient. The three-parameter P-S-N curves obtained are then used to estimate the fatigue strengths for different survival probabilities at an arbitrarily long fatigue life. These fatigue strengths are used to define the probability distribution of fatigue strength at the long fatigue life when a normal. distribution is used to approximate the probability distribution of fatigue strength. The method is illustrated by analyzing the results of fatigue tests of steel #45 (Chinese steel) notched specimens subject to axial loads with a stress concentration factor k_t = 2·0. Then, the method is applied to the fatigue reliability analysis of the runner of a hydraulic turbine. It is shown here that the reliability at long lifetimes can be easily estimated by the proposed engineering method and the conventional method may give a non-conservative design at long fatigue lives due to the assumption of bi-linear P-S-N curves.     

Combined numerical–experimental model for the identification of mechanical properties of laminated structures

A combined numerical–experimental method for the identification of six elastic material modulus of generally thick composite plates is proposed in this paper. This technique can be used in composite plates made of different materials and with general stacking sequences. It makes use of experimental plate response data, corresponding numerical predictions and optimisation techniques. The plate response is a set of natural frequencies of flexural vibration. The numerical model is based on the finite element method using a higher-order displacement field. The model is applied to the identification of the elastic modulus of the plate specimen through optimisation techniques, using analytical sensitivities. The validity, efficiency and potentiality of the proposed technique is discussed through test cases.     

Development of fatigue cracks in complex mechanical structures

The paper summarizes some theoretical and experimental activities concerned with reliability problems of mechanical structures within advanced Czechoslovakian scientific and research institutions. The aim of the paper is directed to the system concept of the fatigue damage mechanism as acting on real mechanical structures in real operational conditions or in those being simulated in laboratories. Such a concept—which has long been emphasized by Prof. A. M. Freudenthal—is based herein on the knowledge gained by studying large-scale structure failures namely in aeronautics.