This module presents a methodology for the structural diagnosis of column type elements based on the analysis of wave propagation inside them.
The defined procedure consists of two phases. In the first phase, a detailed experimental study of the structural assembly under analysis will be carried out, performing seismic tomography in different sections in order to characterize the material, define possible interior defects and the threshold values of wave propagation velocity that would indicate changes in the behavior of the element (fig. 1).
In phase 2, after the characterization of the structural elements, an application is created for the user, in which, from the ultrasonic pulse velocity measurement taken in three sections of the column and two perpendicular directions, the state of conservation of the element can be established in a simple way (fig. 2). If the velocities obtained are within the threshold, no further action is required. In case they have changed significantly, a new measurement campaign similar to the one established in phase 1 would be necessary to analyze these changes and their influence on the structural behavior.
Seismic and/or ultrasonic tomography allows to determine the degree of deterioration of the material, as well as the existence of internal defects that could influence the structural behavior of the element.
The user application, consisting of the analysis of the propagation of ultrasonic waves, is intended to be used for the conservation of structural elements, allowing the conservation teams of architectural heritage complexes to perform periodic and simple measurements on the elements and thus check the state of structural performance that these have and identify changes.
In phase 1, an expert team takes measurements in situ on all the structural elements. This data is collected measuring the acceleration in a series of points (Ai ) in several sections of each column, after the sections are struck with an instrumented hammer at another series of points (Gi ) in such a way that different wave propagation paths are taken inside the section (figure 3.a). In addition, these tests are complemented with the measurements of the ultrasonic pulse velocity in different routes (figure 3.b).
From these measurements tomographic images are extracted of the different sections: in these images the wave propagation speed in the complete section is represented (figure 4). The speed of propagation, as well as the attenuation of the wave, makes it possible to determine the integrity and quality of the material, since they are directly correlated with the material elastic properties: the propagation velocity of longitudinal waves is directly related to the square root of the elastic modulus and inversely related to the square root of the density. Thus, areas with lower velocities coincide with areas of higher density and lower stiffness relative to areas of higher velocity. After the definition of properties, reference wave propagation velocities are established for two perpendicular directions in the column that will serve as the basis for the study in phase 2 (table 1).
In phase 2, users work with ultrasound equipment consisting of two transducers that emit longitudinal waves of 54 kHz frequency. One of the transducer is a pulse generator (emitter) that emits the waves that are received by a second transducer (receiver). Ultrasounds are high frequency mechanical waves (greater than 20 Hz) that are used in many fields related to diagnosis. In the case of structural engineering it is presented as a non-destructive technique that allows determining the transit time of ultrasound pulses through the material. Knowing the distance between the emission points and the receiver, the propagation velocity (UPV) can be calculated.
For each column, two perpendicular directions (N-S and E-W) in three sections of each column are measured. These 6 values are entered in the application made for this purpose. The values are compared with the reference values (obtained in phase 1) and an answer is obtained as to whether or not any type of intervention is required.
Among the many advantages are identifying material quality, voids, cracks and other defects. In addition, the speed is related to the mechanical properties of the material.
The acceleration measures along the column height, due to different impacts along the same height, can be used to perform an Experimental Modal Analysis, that would provided averaged elastic properties of each column.
This methodology allows the state of conservation of a structural element to be established in a simple way by using the non-destructive UPV technique.
In the event that the velocity is lower than the threshold previously defined in phase 1, additional tests would be required.
These techniques are of great interest for the human teams dedicated to the conservation of heritage elements.
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