In this study, the interface between different types of bacteria-embedded self-healing polylactic acid capsules (PLA) and cement paste is investigated. Particularly, the changes in microstructure and ...mechanical properties of the interface with respect to bulk cement paste were studied. First, nanoindentation was performed to obtain maps of hardness and elastic modulus in the interfaces. Lattice modeling of uniaxial tensile test on the mapped locations was performed then to obtain the overall tensile strength and stiffness of the interface. Moreover, hydrates assemblage and chemical composition around the PLA particles were studied through Backscattering Electron images and Energy Dispersive X-ray Spectroscopy. The ratios between resulting tensile strength and elastic modulus of the interface with respect to bulk paste were obtained for each PLA type. The results suggest that PLA can be tailored to optimize the physico-mechanical properties of the interface and hence, the mechanical behavior and triggering efficiency of the self-healing system.
In this study, Computer Vision and Phase-Based Motion Magnification (PBMM) are validated for continuous Structural Health Monitoring (SHM) purposes. The aim is to identify the exact instant of ...occurrence for damage or abrupt structural changes from video-extracted, very low amplitude (barely visible) vibrations. The study presents three experimental datasets: a box beam with multiple saw cuts of different lengths and angles, a beam with a full rectangular cross section and a mass added at the tip, and the spar of a prototype High-Aspect-Ratio wing. Both mode-shape- and frequency-based approaches are considered, showing the potential to identify the severity and position of the damage as well A high-definition, high-speed camera and a low-cost commercial alternative have been successfully utilised for these video acquisitions. Finally, the technique is also preliminarily tested for outdoor applications with smartphone cameras.
Cracks induced by rebar corrosion are among the most important causes of performance deterioration in reinforced concrete elements. Even though several methods have been developed to detect the ...amount and location of corrosion-induced cracks, the sensitivity to small closed cracks located well below the concrete surface is still an issue to be faced. In this paper, we show the high sensitivity of nonlinear ultrasonic measurements performed using the Scaling Subtraction Method to point out that, under certain conditions, nonlinear indicators might prove to be very efficient and suitable to identify small variations in the element microstructure, such as those due to corrosion crack initiation.
A crucial aspect in damage evaluation of masonry structures is the analysis of long-term behaviour and for this reason fatigue analysis has a great influence on safety assessment of these structures.
...Acoustic Emission (AE) technique is very effective in identifying micro and macro-defects and their temporal evolution due to fatigue in several materials. This technique permits to estimate the velocity of ultrasonic waves propagation and the amount of energy released during fracture propagation to obtain information on the criticality of the ongoing process.
By means of AE monitoring, an experimental analysis on two different sets of reinforced masonry walls under fatigue loading has been carried out. During these tests, the AE signals were recorded. The AE signals were analysed using Fast Fourier Transform (FFT) to examine the frequency distribution during the micro- and macro-cracking process. The evolution of the wavelength of the shear component of the AE signal was evaluated through the two characteristic peaks in the AE signals spectrum and the wave speed of the P or S waves. This wavelength evolution can provide information on the micro-crack and macro-crack progress. This procedure permits to estimate the characteristic fracture dimension in several loading conditions and for different masonry reinforcing methods.
An experimental study was conducted on different concrete cylinders damaged in compression. The evolution of damage was followed by means of linear and nonlinear ultrasonic methods, with the purpose ...to provide a better understanding of mechanical degradation processes in concrete and highlight the potentialities and limitations of the Non-Destructive Techniques used.
The presence of discontinuity surfaces in concrete structures, i.e. two or more layers in contact, may be an existing situation with evident relapses on damage formation and progression. Differences ...occur depending on the type of discontinuity, which could be a thin weaker layer or a pre-existing crack. The behavior of pre-existing interfaces is here studied by means of the Scaling Subtraction Method, a Nonlinear Ultrasonic Non-Destructive Technique, that revealed to be effective in describing the mechanical evolution of concrete samples with discontinuity surfaces under the effects of compressive loads.
A laboratory procedure aimed at generating a progressive deterioration of the interface between brick and mortar layers in controlled experimental conditions was developed at the Non Destructive ...Testing Laboratory of the Politecnico di Torino. It is intended as a preliminary stage in the design of a pre-qualification procedure to be applied to repair mortars for restoration of historical masonry buildings. Indeed, assessing the durability of repair products is a major concern because of the potential occurrence of de-bonding phenomena due to insufficient compatibility between original and repair materials, in terms of their mechanical characteristics. Therefore, the study of the long-term mechanical interaction between repair mortars and historical masonry substrate turns out to be crucial for the design of durable repair works.
In this direction the evolutionary phenomenon of mortar de-bonding generated in the lab is analyzed here through the cohesive crack model. The numerical simulation of the laboratory tests was shown to be able to describe the experimental evidence correctly, thus allowing us to characterize the mechanical behavior of the interface. It is therefore possible to use the analysis presented here to predict de-bonding phenomena in problems with different boundary conditions by changing the simulation parameters properly.
The definition and measurement of the nonlinear elastic properties of a sample is of great importance for a large number of applications, including characterization of material performances and ...damage detection. However, such measurements are often influenced by spurious effects due to a combination of nonlinearity and nonequilibrium phenomena. We will present experimental data to show how nonlinearity due to small cracks in concrete samples increases as a consequence of conditioning, i.e., after having perturbed them with a constant amplitude excitation. In addition, our experimental data highlight “memory effects,” i.e., they show that when the excitation is removed, the elastic modulus does not return instantaneously to the initial value.
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