Innovative Experimental Techniques in the Service of Restoration of Stone Monuments - Part I: the Experimental Set up

AbstractThe mechanical response of marble epistyles interconnected to each other by means of metallic connectors when subjected to shear loading is studied experimentally. The study is motivated by the need of the scientific group of the Athenian Acropolis to substitute damaged ancient connections by new ones. In this direction “I”-shaped titanium connectors are placed in the grooves sculptured by ancient stonemasons in the marble blocks and the empty space is then filled by a suitable cement-based material. Guided by the experience gathered from the inspection of failed connections, which clearly indicates that failure starts at the interior of the “titanium-mortar-marble” complex, along the material interfaces, an experimental protocol was improvised, aiming at pumping data from the interior of the interconnected epistyles. For this to be accomplished innovative sensing techniques like pressure stimulated currents, digital image correlation and acoustic emission were used in conjunction with traditional ones. In the first part of this short two-paper series the experimental set-up, the materials and the specimens’ geometry are described

Fracture analysis of typical construction materials in natural time

The acoustic emissions observed before the failure of typical construction materials like marble and cement mortar are analysed in natural time. The study of the variance κ1 of natural time, the entropy S, as well as the entropy S− under time reversal reveals the presence of two behaviours which can be understood either by the Burridge–Knopoff (BK) train model or the Olami–Feder–Christensen (OFC) earthquake model. The present results may be of practical importance for the identification of the time of failure of materials subjected to increasing stress. © 2019 Elsevier B.V

A biomechanical study of the role of sitagliptin on the bone characteristics of diabetic rats

An experimental protocol is described aiming to explore the influence of Type 2 Diabetes Mellitus on the biomechanical response of the bone tissue and, also, to quantify the potential beneficial role of a pharmaceutical treatment, based on sitagliptin, a diabetes drug that increases the levels of natural substances called incretins. Twenty eight male, 10-week old Wistar rats were used, divided into three groups, i.e., the control one, the group including the diabetic rats and, finally, the group including the diabetic rats which were treated using sitagliptin. The biomechanical study was based on a series of three-point bending tests of the femora of the sacrificed rats and the analysis of the experimental data was implemented in terms of the actual geometry of the fractured cross-section. It was concluded that diabetic bones undertake larger forces despite the fact that the “diameter” of their crosssection was somehow smaller. On the contrary, the slope of the load-deflection curve (corresponding to a measure of the stiffness) of diabetic bones is slightly lower compared to the control bones. Finally, it seems that treating diabetic animals with sitagliptin only partly reverses the effect of Type 2 Diabetes Mellitus on their bone tissue, at least concerning its strength and stiffness. © 2020, Gruppo Italiano Frattura. All rights reserved

Similarity of fluctuations in critical systems: Acoustic emissions observed before fracture

Here, we study the scaled distributions for the variance κ1 of natural time χ when analysing the energy time-series of acoustic emission recorded when marble and cement mortar specimens were subjected to mechanical loading until fracture. It is shown that the κ1 scaled distributions exhibit – for at least three orders of magnitude – a characteristic exponential tail like that previously found for the order parameter in equilibrium (e.g., two-dimensional Ising model), and in non-equilibrium (e.g., turbulent flow) critical systems as well as in seismicity. The presented findings, which are also compared to those coming from the non-conservative Olami–Feder–Christensen earthquake model and the Burridge–Knopoff train model, advocate for the use of κ1 as an order parameter for the acoustic events preceding failure in a fashion similar to that in the case of seismicity. © 2020 Elsevier B.V

Acoustic Emission Analysis of Cement Mortar Specimens During Three Point Bending Tests

Abstract This work discusses the experimental results of Acoustic Emission (AE) recordings during repetitive loading/unloading loops of cement mortar beams subjected to three point bending. Six repetitive loading cycles were conducted at a gradually higher load level until the failure of the specimens. The experimental results clearly show the existence and dominance of the Kaiser effect during each loading loop. Regarding the AE data, alternative analysis was conducted using the improved b-value, and the cumulative energy behaviour. Both quantities considered, show qualitative and quantitative characteristics that could be used as pre-failure indicators. In addition, a novel statistical physics analysis involving the AE interevent times was conducted by calculating the cumulative probability function P(>δτ) that follows a q-exponential equation. The entropic index q and the relaxation parameter βq of this equation show systematic changes during the various stages of the failure process. The last cycle led to a q value equal to 1.42, implying the upcoming fracture which is in good agreement with previous results obtained from a wide range of fractured materials

Modelling acoustic and electric signals emitted during structural tests in terms of log-periodic power-law models

An alternative way to represent data regarding internal damage processes, specifically the data provided by the acoustic emission (AE) technique and the pressure stimulated currents (PSCs), is attempted in this study, in terms of log-periodic power-law models. Both monitoring techniques have been already successfully considered widely, either independently or in juxtaposition to each other, in case of standardized laboratory experiments with specimens of relatively simple geometry and moderate dimensions made of a single material. The innovation of the present study is that the AE and PSC data modelled were recorded during structural tests with specimens of complicated geometry made of three different materials, ie, marble, cement paste/mortar, and titanium. It was clearly concluded that both AE and PSC data can be quite successfully modelled in terms of log-periodic power-law models. Moreover, it was indicated that in case of complex experimental set-ups, log-periodic power-law models approach experimental data according to a more accurately manner than pure power-law models do, thus opening up new possibilities of studying the internal damage processes and detecting perhaps useful pre-failure indicators. © 2020 John Wiley & Sons, Ltd