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Published in Alva Peled, Barzin Mobasher, Arnon Bentur, Textile Reinforced Concrete, 2017
Alva Peled, Barzin Mobasher, Arnon Bentur
The width of the localization zone (hL) is equivalent to a development length needed for the force transfer from fiber to matrix in order to reach the critical stress necessary for matrix cracking (see Figure 8.13a). The characteristic damage state (CDS) is a strain level where no more cracks in the matrix can develop due to the inability of the fibers in transferring sufficient load back into the matrix and correlates with the final crack spacing. The role of bond parameters in the formation of additional cracks and slip-related multiple cracks are expressed in terms of hL and s representing the crack spacing. Parameters hL and s for 15 individual TRC and TRC-ARG specimens were measured using DIC, and their probability distribution functions are expressed as a two-parameter Weibull distribution (Mobasher et al., 2014): Pσ=1−exp−xλk wherex is the measured parameters (hL or s)λ is the reference or scaling value related to the meank is the Weibull modulus or shape parameterThe cumulative distribution function (CDF) of hL and s as shown in Figure 8.13b and c indicates that the mean value of hL decreased from 7.4 to 6.5 mm with the addition of short fibers (Figure 8.13b) as well as spacing s reduced from 10.3 to 8.4 mm (Figure 8.13c). This measurement confirms the role of short fibers in mitigating and bridging the microcracks in bond enhancement. At the microstructural level, short fibers improve the bond by means of active load transfer and cross-linking with hydration products, and thus a greater number of microcracks serve as nuclei for macrocrack formation (Butler et al., 2011). Addition of short fibers supports stress transfer across cracks as well as crack deflection mechanisms, both of which play a role in toughening. Therefore, stress relaxation of the matrix in the vicinity of cracks is less pronounced and a smaller development length is needed, hence cracks form more closely. As a result of narrower localization zones, finer crack pattern and smaller crack widths were obtained.
Characteristics of Thermally Sprayed Alumina-Titania Ceramic Coatings obtained from Conventional and Nanostructured Powders - A Review
Published in Australian Journal of Mechanical Engineering, 2023
Mohammed Thalib Basha G, Venkateshwarlu Bolleddu
The analysis of stable crack propagation in thermal-sprayed Al2O3 and Al2O3-ZrO2-TiO2 coatings were performed by Marc Neumann et al. (Wang et al. 2019b). This study represents the investigation of mechanical and structural properties of thermal-sprayed alumina coatings. The microstructure and phase analyses were carried out by SEM and XRD. It was found that the alumina rich materials possess better thermal shock resistance. The various compositions of alumina show an intense micro crack formation as shown in Figure 8. It was also found that the phase transition in pure alumina occurs above 1200°C. The phenomenon of raising crack resistance related to microstructure provides crack deflection which improves microcrack toughening.
Experimental investigation on the enhancement of Mode I fracture toughness of adhesive bonded joints by electrospun nanofibers
Published in The Journal of Adhesion, 2018
F. Musiari, A. Pirondi, A. Zucchelli, D. Menozzi, J. Belcari, T. M. Brugo, L. Zomparelli
Adhesive bonding is increasingly used in joining structural materials in several fields as it helps the fabrication of multi-material, light weight structures having high strength to weight ratios. [1] Epoxy structural adhesives have typically high modulus, high strength, low creep and good performance at intermediate to high temperatures. On the other hand, epoxy resins alone are relatively brittle. Toughening can be obtained, for instance, by addition of organic (rubber-like), inorganic (mineral, ceramic) particles or chopped fibers. Nevertheless, reinforcements such as carbon nanoparticles, clay or chopped fibers may have a limited efficiency due to problems of aggregation. The aggregation of particles or chopped fibers yields stress concentration in the adhesive layer, that can significantly decrease the joint strength. Carbon nanotubes (CNTs) have received a significant amount of attention as nanofillers in polymers to improve their properties, such as stiffness, strength and fracture toughness, besides electrical conductivity. However, the increase of strength and fracture toughness can be more or less pronounced, when not detrimental [2–8] depending on CNT content and type, surface treatment, dispersion, testing methodology and temperature.
Effect of TiO2 on mechanical and thermal properties of Al2O3-based coating via atmospheric plasma spraying
Published in Journal of Asian Ceramic Societies, 2023
Hansol Kwon, Yeon Woo Yoo, Youngjin Park, Uk Hee Nam, Eungsun Byon
After the thermal shock, the surface morphologies of the samples test displayed variations, as portrayed in Figure 9. The microstructural observations of the fractured sample revealed that all coating failures caused by the thermal shock occurred inside the top coat. Thus, the top coat constituted as a region most susceptible to thermal stress. For the pure Al2O3 top coat, bulk delamination occurred in only a few thermal shock cycles. In case of additional coatings with TiO2, the fracture mode differed from that of pure Al2O3. In the early stages of the thermal cycle, cracks were formed on the surface. Subsequently, the major failure mode involved chipping and spalling with small pieces. In particular, the spallation area increased with the number of thermal cycles. Prior studies reported that the thermal shock failure is most affected by the thermal stress inside the top coat [19, 22]. They revealed that thermal stress can be generated based on a temperature gradient, coefficient thermal expansion gradient and phase transformation. As the present temperature range (600°C) was insufficient to induce the transformation from γ-Al2O3 to α-Al2O3, the effect of phase transformation was negligible. During sample heating, a local temperature gradient is inevitable. Accordingly, the surface region of the sample is preferentially heated, which may produce a deviant thermal expansion inside the top coat. In case of rapid quenching, a temperature gradient existed between the surface and the interior of the top coat. This generated tensile residual stress inside the top coat, resulting in vertical crack formation, crack propagation, and failure. The outstanding thermal shock resistance of the A-40T sample can be attributed to the preexisting microcracks located inside the top coat. These microcracks could relieve the residual stress during deposition and under thermal cyclic conditions. In addition, the microcrack toughening effect is advantageous for suppressing crack propagation.