Advances in Fracture and Damage Mechanics XVI

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作者: Sergio Baragetti, Nedunchezhian Srinivasan, B. Lalithkumar, Ravi Kumar
摘要: Diamond-like coating (DLC) of 2.5 micron thick coated on the aluminum alloy samples, were subjected to rotating bending fatigue test in methanol and air environments respectively. DLC coating is X-ray amorphous in nature as revealed by grazing incidence X-ray diffraction studies. Residual stresses generated due in the coatings were measured using Raman peak shift analysis of G band of carbon. Fractographic analysis of uncoated fatigue tested samples in methanol revealed severe damage in contrast to coated samples in methanol. Diamond polished and DLC coated samples revealed better coating adherence and fatigue performance than 1200 grit polished and fatigue tested samples.
作者: Giorgio Zattini, Laura Mazzocchetti
摘要: This study provides an overview of the effects of prolonged acid attack on the surface of PTFE gaskets, assessed using Fourier Transform Infrared Spectroscopy (FT-IR), Thermogravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC). The PTFE analysed originates from two manufacturers in form of Teflon adhesive tapes, used as gaskets in an ultrasonic tyre moulds cleaning plant (UMCS). The gaskets were exposed to various acid baths, temperature and ultrasonic waves, and shown anomalous damage and surface degradation in a relatively short operation time. The analyses here reported have been aimed at investigating this unusual behaviour. PTFE samples have been treated with different acid mixes to evaluate possible different aging effects. Both surface and core have been analysed after aging, providing evidence of the degradation phenomena. In particular, different acid treatments have highlighted different response of PTFE and differences among manufacturers.
作者: Marialaura Tocci, Lorenzo Montesano, Annalisa Pola, Marcello Gelfi, Giovina Marina La Vecchia
摘要: In the present work, the effect of Cr and Mn addition on corrosion resistance was investigated on AlSi3Mg alloy. Potentiondynamic corrosion tests in a 3.5 wt. % NaCl solution were performed on samples in different heat-treated conditions, and corrosion current density and potential were determined by Tafel method. Brinnel hardness measurements were also carried out in order to couple corrosion resistance with mechanical properties. It was interestingly found that Cr presence enhanced mechanical properties and corrosion resistance in comparison with the base alloy.
作者: Tamaz Eterashvili, G. Abuladze, L. Kotiashvili, T. Dzigrashvili, M. Vardosanidze
摘要: Crack propagation after low-cycle fatigue (LCF) deformation has been studied in the chromium martensitic structural steel. Although the study of a fundamental mechanism of fatigue crack growth has received much attention over the last decade, it still remains a sufficiently complex problem and needs full understanding. Moreover, the recent studies show that the cracks propagate discontinuously even on the millisecond timescale, and their growth rate significantly depends on a microstructure of the material. In the present work the boundaries of the former austenitic grains were revealed on the polished surfaces of the thermally treated samples, which subsequently were undergone low-cycle fatigue tests. The experimental studies show that fatigue macrocracks mainly grow along the boundaries of the former austenitic grains, and changetheir propagation direction when crossing the grain boundary, however, remain within 45 ̊ interval with regard the cycling axis. In particular cases, when the boundaries of a martensite packets and those of the former austenite grains lay along the length of a packet, the macrocrack is better formed and with regular borders. After a macrocrack reaches a definite length ~30-50μ, a microcrack is nucleated ahead of the macrocrack tip, and is oriented along the substructure element of the steel. Further deformation tests provide an increase in the length of the main crack via aggregation of microcracks initiated ahead of it during the LCF. In the cases when the macrocrack is deviated, slip bands are formed in martensitic structures along the boundaries of martensite packets (laths). A correlation is revealed between the microcrack components and the substructure elements of the steel as well. The same results were obtained by fractography of the tested and fractured samples. However, in the latter case correlation was established between the cleavage facets and the dimensions of packets.
作者: Sergio Baragetti
摘要: 7075-T6 aluminium alloy is under investigation in the last decades because of its sensitivity to corrosion and its poor surface behaviour. The alloy is widely used in many mechanical, aeronautical and structural engineering appliances because of its high strength-to-weight-ratio. Notwithstanding its strength and light weight advantages, its applications in corrosive environments, with fatigue applied loads, is still under research. The mechanical and chemical driving forces have still to be accurately evaluated and the role of each one identified. Results of tests on rotating bending fatigue (R = -1) 7075-T6 uncoated and PVD DLC coated specimens are reported in this paper under various stress levels and number of cycles to failure. Fracture surfaces were observed via SEM characterization in order to assess the role of the chemical aggressive environment, the residual stress field and the applied load.
作者: Sho Omori, Takayuki Morita, Koki Matsumoto, Asahiro Nagatani, Tatsuya Tanaka
摘要: There are many rubber products like tires, and reinforcing agents derived from fossil resources such as carbon black (CB) are used for them. However, in recent years, conversion to biomass resources has been demanded due to problems such as depletion of fossil resources. Therefore, we have studied the composite of natural rubber (NR) reinforced with cellulose nano-fibers (CNF), which has attracted attention as a next-generation biomass resource. It is very difficult to uniformly disperse CNF in the conventional kneading process. Therefore, it is preferable to agitate CNF in NR latex. Then, it is necessary to study the optimum agitation equipment of CNF. In this study, the reinforcing effect by CNF was investigated when the agitation equipment was changed. Agitation of NR latex and CNF was carried out by a hand, a homogenizer, a dispersion mixer, and planetary centrifugal mixer. Thereafter, agitated materials were dried and masterbatches were made. Furthermore, the masterbatch and vulcanizing agents were kneaded using an internal mixer. Tensile test and X-ray CT observation of the prepared vulcanized rubber sheets were carried out to evaluate the reinforcing effect and dispersion state of CNF. As a result, the planetary centrifugal mixer was most useful to uniformly disperse CNF.
作者: Ivo Šulák, Karel Obrtlík
摘要: The present work is focused on the study of crack initiation during low cycle fatigue (LCF) loading of the second generation nickel-based superalloy MAR-M247 treated with hot isostatic pressing. LCF tests were conducted on cylindrical specimens in symmetrical push-pull cycle under strain control with constant total strain amplitude and strain rate at 800 °C in air atmosphere. Selected specimens were electrolytically polished to facilitate surface relief observations. Crack initiation sites were studied by means of scanning electron microscopy (SEM) in dual beam microscope TESCAN LYRA 3 XMU FESEM equipped with focus ion beam (FIB). The microstructure of the material is characterised by coarse dendritic grains with numerous carbides and small casting defects. The average grain size was 2.1 ± 0.3 mm. Fractographic analysis revealed the fatigue crack initiation sites and their relation to the casting defects and material microstructure. Casting defects, carbide inclusions and interdendritic areas were found to be important crack nucleation sites. Specimens’ surface observations revealed the formation of pronounced surface relief with short worm-like markings. Fatigue crack initiation in these places is documented and discussed.
作者: Takuya Murakoshi, Hayato Sakamoto, Taichi Shinozaki, Ken Suzuki, Hideo Miura
摘要: The degradation process of the micro texture of Ni-base superalloys was observed under fatigue and creep-fatigue loading conditions at elevated temperatures higher than 700oC by applying electron back-scatter diffraction (EBSD) analyses. The local distribution of the crystallinity of a grain and a grain boundary was defined quantitatively by analyzing the Kikuchi pattern obtained from each electron-beam-irradiated area with a diameter of 50 nm. The calculated image quality value was used for the analysis. It was found that the crystallinity of grain boundaries degraded seriously under creep-fatigue loading conditions due to the acceleration of anisotropic strain-induced diffusion of component elements. Since the initial finely-controlled strengthened micro texture disappeared due to the anisotropic diffusion of component elements, this degradation was found to cause the drastic decrease of the strength of grains and grain boundaries and thus, lifetime of the material. The decrease of the strength of both grains and grain boundaries was measured by micro tensile test system in a scanning electron microscope by making a small sample from bulk specimen using focused ion beam. The strength of a grain and a grain boundary varied drastically depending on their crystallinity, in other words, the image quality value.
作者: S.M.J. Razavi, M. Peron, J. Torgersen, F. Berto
摘要: The problem of mixed mode (I+III) brittle fracture of polycrystalline graphite is investigated systematically here for the first time. The present study considers cylindrical specimens weakened by circumferential notches characterized by different acuities. A new complete set of experimental data is provided considering different geometrical configurations by varying the notch opening angle and the notch tip radius. The multiaxial static tests have been performed considering different values of the mode mixity ratio. A criterion based on the local Strain Energy Density previously applied by the same authors only to pure modes of loading is extended here to the case of tension and torsion loadings applied in combination.