W. P. De Wilde

Damage identification in reinforced concrete structures by dynamic stiffness determination

Service loads, environmental and accidental actions may cause damage to constructions. Regular inspection and condition assessment of engineering structures are necessary so that the early detection of any defect can be made and the structures remaining safety and reliability can be determined. When the structural damage is small or it is in the interior of the system, its detection cannot be carried out visually. A useful more elaborate non-destructive evaluation tool is vibration monitoring. It relies on the fact that the occurrence of damage or loss of integrity in a structural system leads to changes in the dynamic properties of the structure. In this paper, different techniques will be presented and compared to derive from experimentally determined modal characteristics of a reinforced concrete beam its dynamic bending stiffness. The degradation of stiffness, due to the cracking of the reinforced concrete, gives information on the position and severity of the damage that has occurred.

Identification of the damping properties of orthotropic composite materials using a mixed numerical experimental method

A mixed numerical experimental approach is the basis of a new method for the identification of the material damping properties of fibre reinforced polymers, which provides an answer to many problems encountered in experimental damping characterization. Experimental modal parameters, measured on a plate specimen, are compared with corresponding results from a numerical calculation, thus allowing to determine the stiffness and damping properties of the material. The relation between the modal parameters (structural parameters) and the material parameters, is obtained by using a numerical model of the specimen in combination with the modal strain energy method.In the first part of this paper, the complex moduli are introduced as measures for both material stiffness and damping and the relation between these complex moduli and the modal parameters of a thin plate specimen is derived. Next, the practical procedure of the mixed numerical experimental method is presented, followed by a procedure for estimating the reliability of the obtained results. Finally, two examples are discussed in which all the independent material damping properties are identified as functions of frequency.

A mixed numerical/experimental technique for the nondestructive identification of the stiffness properties of fibre reinforced composite materials

Abstract The characterization of the stiffness properties of fibre reinforced composite (FRC) materials presents more difficulties than the characterization of traditional isotropic materials. This paper first describes the difficulties that can arise and next presents a nondestructive method that offers a possible solution to the problems. The proposed method is a so-called ‘Mixed Numerical/Experimental Technique’ (MNET). From an experimental point of view, the method requires the measurement of resonant frequencies of freely suspended rectangular test plates. From the numerical side, an accurate model of the test plate must be available. The stress field associated with the different resonant frequencies is multi-axial. The measurement procedure however is very simple and requires little specimen preparation. The equipment is inexpensive and the measurement procedure can be controlled by a PC program. The proposed MNET allows the simultaneous determination of all the anisotropical properties and provides an estimation of their statistical distribution.

Comparison of techniques for modal analysis of concrete structures

Abstract This paper describes different experimental techniques for obtaining modal parameters of structures. Attention is focused on those techniques that may be applicable to in situ concrete structures (e.g. bridges). In a first stage, experiments are made on reinforced concrete beams of 6 meters length. The beams are excited using three types of excitation methods: impact hammer excitation and two different electromagnetic shaker signals: pseudo-random and swept-sine signals. The modal parameters are determined either by performing curve-fitting procedures on series of measured frequency response functions or by applying the stochastic subspace identification technique to the time response signals of the structure. The influence of the non-linear behaviour of the concrete beams is investigated by performing measurements at different excitation amplitudes. It appears that modal parameter estimates are affected by excitation techniques, data acquisition parameters and processing methods. The main cause of this is the non-linear behaviour which is observed even at very low vibration amplitudes. However, the influence on resonant frequencies and mode shapes is negligible. This is not the case for the modal damping ratios, so that the estimation of these parameters is unreliable.

The modelling of moisture absorption in epoxies : effects at the boundaries

Abstract When studying moisture absorption and desorption in polymer-matrix composites, it is very often tempting to use Ficks law, due to its inherent simplicity and straightforwardness. The present paper shows that this approach is sound, provided that modelling of both the initial and boundary conditions is carefully investigated. Different approaches are considered, each being coupled with an interpretation that attempts to explain the relation between the chosen boundary conditions and underlying physical phenomena. Comparison is made with experimental results on a cured epoxy matrix.

A simple model for moisture sorption in epoxies with sigmoidal and two-stage sorption effects

Abstract In the present paper the moisture absorption in an epoxy is considered for two cases, very often present simultaneously: (i) the sorption is sigmoidal with a considerable deviation from Fickian behaviour for small values of time and (ii) two-stage sorption takes place for large values of time. A simple mathematical model for the sorption process is proposed, which seems to cover satisfactorily the experimental data. It is based on the idea that besides the diffusion, fast and slow changes in the properties of material are taking place due to the interaction between the epoxy and the fluid. So the model utilises three constants: (i) the average diffusion coefficient, (ii) one characteristic time for the fast processes and (iii) one for the slow processes. The values of the parameters are determined as a function of the relative humidity with a general curve fitting type procedure. The model also offers a generalisation of the standard Fickian determination of the average diffusivity from the sorption curve for the case of small time sigmoidal behaviour.

Anisotropic Material Identification Using Measured Resonant Frequencies of Rectangular Composite Plates

In this chapter a method is presented which determines the elastic properties of a composite material plate using experimentally measured resonant frequencies.

Effects of the environment and curing on the strength of adhesive joints

—The influences of four parameters - pretreatment of the adherend, curing cycle, time before testing, and humidity — on the strength of adhesively-bonded single lap joints were investigated for aluminium 2024-T3 adherend and modified epoxy FM73M adhesive. The importance of conditioning the aluminium surface prior to bonding was confirmed. Furthermore, the pretreatment should be attuned to the environmental conditions under which the joint will be used. We found that roughening the aluminium surface gave a poorer joint strength. Cooling joints in normal environmental conditions after the curing procedure gave satisfactory strengths if a relaxation period of 30 days was allowed before loading. The fitting of the experiments to a statistical distribution was examined to allow use of the experimental results for finite element techniques with probabilistic models. The same statistical behaviour as that for joints submitted to cyclic loading was found for joints submitted to humidity.

Design and optimization of roof trusses using morphological indicators

The adequacy of a structure in strength, stiffness and stability can be evaluated using morphological indicators. This article establishes these indicators for volume, displacement and buckling, for roof trusses. Easy to use graphs then allow to take design decisions at the early stage of conceptual design. Although less precise than computer driven optimization methods, morphological indicators are a simple tool to choose an appropriate typology. In this article roof trusses are added to the morphological indicator theory.

Optimisation At The Conceptual Design StageWith Morphological Indicators:Design For Strength Or Design For Stiffness?

Within the framework of sustainable development we strive for constructions with a minimum volume of material. When we only consider criteria on resistance and buckling, at the stage of conceptual design a clear hierarchy among the different structural topologies can be established with Morphological Indicators (MI). MI are dimensionless numbers that represent a property of a structure (e.g. volume) and depend only on a small number of dimensionless variables (in its most simple appearance only the slenderness of the structure). We define the slenderness as L/H, L being the horizontal and H the vertical dimension of the rectangle that inscribes the structure. This allows one to compare the efficiency of structures objectively, with only a reduced number of variables to consider. Hence MI allow a very quick shape and topology optimisation at the conceptual design stage if only element strength (resistance) and buckling are to be considered. Though a lot of design problems, especially for lightweight structures, are characterized by stiffness constraints: global instability, upper limit on static displacements, acceptable vibrations... In such cases one of the main assumptions of the theory of MI is not valid, since fully stressed design does no longer guarantee the optimal section distribution. Therefore, problems subjected to stiffness related constraints have to be analyzed by different methods allowing the optimisation of the sizing of the different elements. Recent research enables determination of whether one has to deal with a design for strength or a design for stiffness problems. This paper presents schematically the theory of MI and the method to distinguish a strength characterized problem from a stiffness characterized one at the conceptual design stage. An example illustrates the design process. The principal aim is not to criticize the efficiency of the theory of MI but to point out when the theory can be applied and when not.