Philippe Duffour

A study of the vibro-acoustic modulation technique for the detection of cracks in metals

One implementation of the vibro-modulation technique involves monitoring the amplitude modulation of an ultrasonic vibration field transmitted through a cracked specimen undergoing an additional low frequency structural vibration. If the specimen is undamaged and appropriately supported, the two vibration fields do not interact. This phenomenon could be used as the basis for a nondestructive testing technique. In this paper, the sensitivity of the technique is investigated systematically on a set of mild steel beams with cracks of different sizes and shapes. A damage index was measured for each crack. The correlation obtained between the crack size and the strength of the modulation is fairly poor. The technique proved extremely sensitive to the initial state of opening and closing of the crack and to the setup due to the modulating effects of contacts between the specimens and the supports. A simple model is proposed which explains the main features observed and approximately predicts the level of sideband obtained experimentally.

Adaptive control of structures under dynamic excitation using magnetorheological dampers: an improved clipped-optimal control algorithm

This paper investigates the performance of magnetorheological (MR) dampers to mitigate the effect of earthquake loading on civil engineering structures. MR dampers are semi-active fluid dampers containing an MR fluid. They can generate large controllable damping forces by tuning the magnetic field applied to the fluid, which changes its viscosity. Their fast response time and low electric power requirements make them attractive for potential applications in earthquake engineering. In this study the behavior of a prototype MR damper was simulated computationally using the Bouc-Wen model. A new clipped-optimal control strategy, the improved clipped-optimal algorithm (ICA) control model, was developed and aims to reduce the acceleration response of a structure. It was implemented for a scaled three-story building in Simulink. The structural response to seven earthquakes was simulated and compared with two other established control algorithms as well as passive damping. The ICA control algorithm generated the largest reduction in acceleration and displacement response while keeping low the electric power requirements and the force generated by the damper.

Thermodynamic Behaviour of the Solar Updraft Tower: A Parametric Model for System Sizing

The solar updraft tower (SUT) concept is an exciting renewable technology with the potential to deliver high power output. A comprehensive SUT sizing computer model has been developed to determine power output and thus appropriate system dimensions for different ambient conditions. The efficient thermodynamic model performs steady-state macro-scale simulations incorporating radiation and natural convection heat transfer mechanisms. The solar collector is simulated as a discretised, axisymmetric, radial system composed of thermal components. A set of linear simultaneous equations describes the heat exchanged between these components and is solved by matrix inversion. The short computation time of the model makes it ideal for parametric analysis of SUT plants across a range of dimensions. The thermodynamic performance of the collector proves to be a limiting factor of system power output. Results from the model show that for given chimney dimensions, there is a maximum collector size beyond which no further useful heat is added to the air as the system has reached thermal equilibrium. Therefore the only way to increase power output further is to increase chimney height and diameter as well as extending the collector diameter.

A new approach for the prediction of the thermosonic signal from vibration records

In this study a set of cracked steel beams was tested using the thermosonic method of NDT. Each beam was instrumented with one strain gage for the measurement of the vibrations excited. The heat released at the crack was predicted from the measured vibration and an experimental estimate of the additional damping introduced in the specimens by each crack. Hence the local surface temperature rise could be predicted as a function of time during the excitation and compared with the infrared camera measurements.

Energy and Cost Assessment of Adaptive Structures: Case Studies

AbstractThis paper demonstrates how adaptive design (details published elsewhere) can be employed to save, on average, 70% of whole-life energy on a range of spatial structures, the whole-life ener...

Sensitivity Analysis of Different Capacity Spectrum Approaches to Assumptions in the Modeling, Capacity and Demand Representations

Several capacity spectrum assessment methods exist for determination of structural performance of building models subjected to earthquake loading. The repetition of such analysis for earthquakes of increasing intensity will result in the derivation of analytical fragility functions. A comparison of three capacity spectrum assessment approaches (N2, SPO2IDA and FRACAS) has been carried out, highlighting the advantages and limitations of the approaches. Two experimental case studies have been chosen to evaluate the IM-EDP (Sa-Sd, ISDmax%) estimates obtained from the three different capacity spectrum procedures, as well as from non-linear time-history analyses (NLTHA). It is found that all three approaches perform well in estimating the response of a simple steel frame, but that FRACAS provides the best estimate of the response of an irregular reinforced concrete frame. It is concluded that further comparisons of the capacity spectrum approaches with large-scale experiments on structures are required to draw more general conclusions.

The relationship between crack damping, vibration amplitude and temperature rise in thermosonics NDT

In this study a set of cracked steel beams was tested using the thermosonic method of NDT. Each beam was instrumented with a strain gage for the measurement of the vibrations excited. The heat released at the crack was calculated from the vibration records and an experimental estimate of the crack damping. Thus, the local surface temperature rise could be predicted and compared with the infrared camera measurements. The relationship between crack size and level of vibration required for reliable crack detection was discussed.

Comparative Sensitivity Study of Vibro-Acoustic Modulation and Damping as NDT Techniques

The sensitivities of the conventional damping test and a particular implementation of the emerging vibro‐acoustic modulation (VAM) NDT technique have been compared on a set of mild steel beams cracked in the laboratory. The comparison showed very similar performances on the specimens used. Both techniques work best for lightly damped specimens and in setups such that the influence of the support can be minimized, but they also show a poor sizing performance. A preliminary explanation for this latter point is given.