PSI - Issue 3

Stavros K. Kourkoulis et al. / Procedia Structural Integrity 3 (2017) 316–325 S. K. Kourkoulis and I. Dakanali / Structural Integrity Procedia 00 (2017) 000–000

324

9

Fig. 12. Cumulative AE energy and load change versus crack opening. Fig. 13. The fractured epistyle. The sensing techniques assessed until now (DIC and Acoustic Emission) were proven very effective. Their out comes were in excellent mutual agreement and also in good accordance with the respective data of the clip-gauges and the LVDTs. Moreover, they were proven quite advantageous since they either provided data from the interior of the specimen (AE), something extremely important in case of complexes with internal interfaces (as it is the case of restored epistyles), or they gave full-field data about the displacement field (DIC) rather than data from specific points, selected more or less arbitrarily before the onset of the experiment. Along the same line it is to be emphasized that the data provided by the AE technique can be considered as pre-failure indicators since they are characterized by clear warning signs preceding the respective ones of the quantities recorded by the traditional sensing techniques. Concerning the technique of the Optical Fibers, it was concluded that it provides valuable data for the strain of the bars (which are not accessible by any other sensing technique), well in agreement with these of the other techniques. However, this agreement is terminated at relatively low load-levels. As it can be seen in Fig.14a, immediately after the separation of the fragments started at t=400s the indication of the fiber increased abruptly (obviously due to the extreme straining of the bar’s small portion between the two fragments, i.e. the portion that is actually elongated) and then it ceased functioning abruptly. It is not yet clear why and obviously additional detailed study is required before definite conclusions about the applicability of the specific sensing technique in similar applications are drawn. The conclusions are more encouraging for the fourth technique employed, i.e. that of the Pressure Stimulated Currents. Its outcomes are not only in good mutual agreement with these of the other techniques used but moreover they also provided excellent pre-failure indicators, as it is seen in Fig.14b. Taking into account the very low cost for the application of the specific technique in praxis it can be said that it may be soon proven an excellent tool for con tinuous Structural Health Monitoring or restored elements of monuments of Cultural Heritage. Due to its importance the data of the PSC technique will be analyzed further in a forthcoming paper (Kourkoulis et al. 2017). Concerning now the response of the epistyle it was found to be linear for almost three quarters of the maximum load attained, indicating that the restoration procedure adopted is successful at the first assessment. Moreover when the linearity and non-linearity portions of the load-displacement graphs were exhausted a characteristic plateau was observed, which was finally attributed to yielding of the reinforcing bars. The final failure mode (fracture of the bars, Fig.13) confirmed this hypothesis. The specific type of failure although it is not the worst scenario (which corresponds to fracture of the marble volumes) it is not, also, the optimum one (which is the gradual pull-out of the bars since in that case actions can be undertaken preventing final failure). It is thus concluded that the technique nowadays adopted for restoring fragmented structural elements, besides its numerous advantages and pioneering characteristics, should be further considered thoroughly in the direction of re-calculating the number of reinforcing bars and their diameter and the material they are made of. The long experience of scientists and technicians working in-situ for the resto ration of the Athens Acropolis monuments is “sine qua non” for the successful implementation of such an attempt. Acknowledgements The authors kindly acknowledge the most valuable contribution of Professor D. Triantis, Associate Professor I. Stavra kas and Dr E. Pasiou, in the experimental procedure. Also, the assistance of Mr I. Chiou, experienced technician of the Parthenon work-site, in all stages of the preparation of the specimen, is kindly acknowledged.