Papers by Keyword: Toughening Mechanism

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Authors: Hideo Awaji, Seong Min Choi
Abstract: Intra-type nanocomposites, in which nanosized second-phase particles are embedded within matrix grains, generate dislocations around the dispersed nanoparticles. The intra-type nanostructure induces a thermal expansion mismatch between the matrix and the dispersed particles, which will yield nanoscale stress distribution around the particles and generate lattice defects, such as dislocations. The dislocations of ceramics can be generated at elevated temperatures, become sessile dislocations at room temperature, and serve as nanocrack nuclei in highly stresses fields, e.g. at a main crack tip. The frontal process zone size ahead of a crack tip is expanded due to creation of nanocracks and hence the fracture toughness is improved. Annealing after sintered nanocomposites is important in controlling the dislocation activities. Appropriate annealing will disperse dislocations into the matrix grains. However, dislocations are sensitive to temperature, and higher temperature or longer annealing time result in dislocation disappearance and cause the reduction of the strength and fracture toughness of nanocomposites. In this study, commercially available γ-alumina agglomerated powder with high porosity was used to create the intra-type nanostructure. Nickel nitrate solution was infiltrated into nanopores of the γ-alumina agglomerates in vacuum. The alumina/nickel composite powder following reduction in hydrogen atmosphere was sintered using a pulse electric current sintering method. The volume fraction of nickel was about 3 vol %. After appropriate annealing, the highest fracture toughness was obtained to be 7.6 MPam1/2, which is two times higher than that of monolithic alumina.
Authors: Yuan Lin An, Zhi Ming Liu, Gan Wang, Wen Jian Wu
Abstract: Nacre of molluscan shells is famous for its “brick and mortar” microstructure and possesses excellent mechanical properties. Three-point bending strengths of nacre parallel and vertical to the surfaces of platelets are tested and the different fracture surfaces were characterized. The result shows that the values of three-point bending strength in the two directions are approximately the same. Base on the microstructure of and toughening mechanism of nacre, the inspiration for architecture was put forward as follows: to redesign the building materials, toughen the architecture in several ways, and enhance the properties of architecture in all directions. The aim of the paper is to call out learning from nacre to create novel architecture and building materials.
Authors: D. Mc Auliffe, A. Karač, Neal Murphy, Alojz Ivanković
Abstract: In this study the adhesive joint fracture behaviour of a nano-toughened epoxy adhesive was investigated. Two experimental test methods were used; (i) the standard tapered double cantilever beam (TDCB) test to measure the mode I adhesive joint fracture energy, GIC, as a function of bond gap thickness and (ii) a circumferentially deep notched tensile test to determine the cohesive strength of the adhesive for a range of constraint levels. It was found that the fracture energy of the adhesive followed the well-known bond gap thickness dependency [1]. SEM analysis of the TDCB fracture surfaces revealed significant plastic void growth. Finally, numerical modelling of the experimental tests suggested that most of the fracture energy was dissipated via highly localised plasticity in the fracture process zone ahead of the crack tip.
Authors: Michal Kotoul, Petr Skalka
Abstract: The main drawback still impairing the use of bioactive glasses in load-bearing applications is their intrinsic brittleness. The addition of coating constituted by polyvinyl alcohol (PVA) and microfibrillated cellulose (MFC) PVA/MFC led to a 10 fold increase of compressive strength and a 20 fold increase of tensile strength in comparison with non-coated scaffolds. Crack bridging by polymer coating was identified by fractographic observations as a main toughening mechanism. In this contribution a detailed computational analysis of crack bridging due to coating film fibrils is presented and an improvement of fracture resistance of coated scaffolds is explained.
Authors: Shi Bin Li, Jie Guang Song, Hong Ying Ru, Xiao Bo Bai
Abstract: The shell structure YAG composite powders with different volume fraction (15, 20 and 25vol%) micro h-BN have been prepared by co-precipitation process. The bulk composites were performed by pressure sintering at 1600°C under a pressure of 30MPa in vacuum. The mechanical properties (elastic modulus, hardness, and fracture toughness) and relative density of the composites were investigated detailedly. Furthermore, phase composition and microstructure of the composites were analyzed thoroughly by X-ray diffraction, scanning electron microscopy. Meanwhile, good machinability is maintained due to the low hardness of the second phase.
Authors: Seong Min Choi, Takuya Matsunaga, Sung Ho Cheon, Sawao Honda, Shinobu Hashimoto, Hideo Awaji
Abstract: To improve fracture strength and fracture toughness in ceramic materials, we focused our attention on an intra-type structure of nanocomposites. We proposed new processing method for fabricating intra-type nanocomposites. In this work, Al2O3/Ni nanocomposites were fabricated using a soaking method and sintered by PECS(Pulse Electric Current Sintering) method. We also estimated seed effects on this system. Seeded nanocomposites showed high fracture strength and higher fracture toughness than non-seeded nanocomposites and monolithic alumina. The fracture strength of the seeded nanocomposites was more than 800MPa in all sintering temperature range. The maximum value of the fracture toughness was 5.5 MPa⋅m1/2 for the specimen sintered at 1350°C. The sintered specimens with high fracture strength and high fracture toughness were annealed from 800°C to 1000°C for 0 to 10 min. The specimen annealed at 800°C for 5 min showed the highest fracture toughness of 7.6 MPa⋅m1/2. This value is two times higher than that of the monolithic alumina.
Authors: Ai Kun Li, Li Ya Li, Yuan Dong Peng, Jian Hong Yi
Abstract: We present our findings of increased fracture toughness in high performance Sm2Co17-type magnet. The new Sm(Co 0.65 Fe 0.24 Cu 0.08 Zr 0.03)7.6magnet exhibits remanence of 11.13 kGs, maximum energy of 30.2 MGOe. This magnet shows not only a superhigh fracture toughness of 5.56 MPa m 1/2 but also distinguished yielding combined with an enhanced plastic plateau of 30 % to failure. It has been found that debonding, crack deflection, crac k branching and bridging are the major toughening mechanisms for the observed high toughness and long yield plateau. Long and straight screw dislocations observed in Sm rich precipitates accelerate the debonding of Sm rich grains.
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