Advances in Fracture and Materials Behavior

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Authors: Yan Li, Yan Ping Hu, Chun Jing Hu, Ye Hong Yu
Abstract: Natural fibers are excellent substitute materials for man made fibers in making fiber reinforced composites due to their high specific strength and modulus, low density, low price, easy availability in some countries, recyclable and degradable properties. They have raised great attentions among material scientists and engineers in the past decade. Many researches have been conducted to study the mechanical properties, especially interfacial properties of natural fiber reinforced composites. However, the properties, such as mechanical performances, moisture absorption behaviors, et. al of natural fibers themselves have been seldom investigated. Knowing the relationship between microstructures and properties of natural fibers are important for understanding the bulk properties of natural fiber composites and also good instructions for designing bio-mimic materials. In this study, four kinds of natural fibers which were extracted from different plant sources were investigated. The microstructures of these natural fibers were revealed with the aid of optical microscopy. Microstructure models were thereof set up and mechanical properties for the representative volume element were assumed. Fiber bundle fracture models together with probability statistics analysis were employed to calculate the mechanical properties of natural fibers. The results were compared with the experimental measurements. Different mechanical behaviors of natural fibers which were functioned differently in the nature were clearly explained by the above studies
Authors: Qing Hua Qin, Tie Jun Wang
Abstract: A unified yield criterion is proposed in this paper, which is valid for the metallic sandwich sections with various core strength and geometrical dimensions and can reduce to the classical yield criteria for solid monolithic section and sandwich section with weak core, respectively. Then, the unified yield criterion is used to derive the analytical solution for the large deflection of fully clamped metallic sandwich beam subject to a transversely concentrated load, in which the interaction of bending and stretching is considered. Comparisons of the present solutions with experimental results are carried out and good agreements are found. It is seen that the axial stretching induced by large deflection has a significant effect on the deflection of sandwich structure in the post-yield regime, and the load carrying capacity of metallic foam core sandwich beam may be underestimated as the core strength is neglected in analysis.
Authors: Hui Min Li, Qin Zhi Fang, Tie Jun Wang
Abstract: As a direct and simple method, essential work of fracture has been widely used for fracture measurement of ductile polymers. In this paper, fracture toughness of PC and PC/ABS alloy is experimentally investigated. A series of double edge-notched tension (DENT) specimens and essential work method are employed to measure the fracture toughness of PC and PC/ABS alloy. By the way, the critical crack tip opening displacement is obtained. Moreover, the fracture surfaces of DENT specimens are examined by using a scanning electron microscope (SEM).
Authors: Ying Jun Jin, Tie Jun Wang
Abstract: The objective of the work is to numerically study the meso-scale deformation of amorphous glassy polymer. A molecular polymerization algorithm is employed to generate three-dimensional (3D) random networks of polymer, in which the micro-parameters and meso-structures, such as entanglement point, sliding point, etc. are considered. Then, 3D unit cell model and finite element method are used to calculate the stress and deformation relations of polymer. The evolution of meso-structure of glassy polymer with deformation is obtained simultaneously. Further, variation of the orientation of polymer chain with deformation was quantitatively studied.
Authors: Jeong Seok Oh, W.K. Ju, Yi Qi Wang, Tae Gyu Kim, Jung Il Song
Abstract: The static and dynamic properties on the hoist hook of a vessel are necessary since they are affected by the damages of a static and dynamic load. Al-Si-Mg casting alloy (AC4C-T6) is widely used due to its good mechanical properties as well as the light weight and good casting with complex geometries. This study accomplished a static tension test and an impact test. Based on the test results and fracture surface analysis, we found that there are great differences between the fracture strain and yield stress in the different extracted regions of specimen. In tensile test, yield stress were 205 MPa at a low strain rate of 5 mm/min and 220 MPa at a high strain rate of 25mm/min. In Charpy impact test, impact properties of AC4C aluminum alloy were analyzed by impacting loading versus displacement and impacting energy versus displacement. Compared the fracture strains in different strain rates, maximum fracture strain of low strain rate was mainly 10 % higher than that of high strain rate. There were more than 20 % differences in the strain rate. The ductile and brittle behaviors were showed in low strain rate and high strain rate in static tensile test, respectively. The impact energy reached high when they were extracted from a plane region in the mold. But impact energy reached low when they were extracted from a curved and edge region. It is demonstrated that mechanical properties and impact energy of the samples where were extracted from a curved and edge region was lower than that of the samples where were extracted from a plane region.
Authors: Yi Hui Zhang, Xin Ming Qiu, Dai Ning Fang
Abstract: Lattice structures have ranges of thermo-mechanical properties that suggest their implementation in ultralight structures, as well as for impact/blast amelioration systems and heat dissipation media. Considering that proper anisotropy of structure could increase load efficiency, two kinds of 2-D lattice materials designable in specific stiffness and strength of arbitrary direction have been brought forward: variational thickness cell and variational direction cell. The mechanical properties of variational thickness Kagome cell have been analyzed, including effective elastic modulus, yield strength and elastic buckling strength in arbitrary directions. Since the shear buckling of 2-D lattice materials is an important collapse mode especially when relative densities are low, shear buckling strength of various 2-D lattice materials have also been calculated. It is found that compared with the diamond cell, the variational thickness Kagome cell of thickness ratio, m=0.5, possesses the same elastic modulus and yield surface, and higher buckling strength.
Authors: Yue Guang Wei, Tie Ping Li, Hai Ou Xie
Abstract: The mechanical behaviors of the ceramic particle-reinforced metal matrix composites are modeled based on the conventional theory of mechanism-based strain gradient plasticity presented by Huang et al. Two cases of interface features with and without the effects of interface cracking will be analyzed, respectively. Through comparing the result based on the interface cracking model with experimental result, the effectiveness of the present model can be evaluated. Simultaneously, the length parameters included in the strain gradient plasticity theory can be obtained.
Authors: Pierre Vialettes, Lina Zhou, Bing Pan, Zhuo Zhuang
Abstract: The school of aerospace of Tsinghua University has started a project to develop new kind of stratospheric balloons for earth observation. These balloons will be designed to stand in the same position during a few days at 20 kilometers of altitude. To reach this goal, the first step is to select adapted materials for the balloon envelope. The materials for stratospheric balloons application should have specific properties adapted to the environment such as mechanical, thermo-optical, and permeability properties. Thus, we have asked a material manufacture company to develop and manufacture a material adapted to our application. In this paper, we will present the first results concerning the mechanical properties of the material. Experimental facilities have been developed to test the mechanical behavior of the materials at room and low temperature. The future development of the project will be to identify behavior law adapted to our material that fits with the experimental results.
Authors: Wang Cheng, Tian Bao Ma, Jian Guo Ning
Abstract: At long standoff, a shaped charge with small cone angle will produce a jet with a high tip velocity and a low tail velocity, causing it to stretch and break up, so its penetration ability into multilayer metal targets is very weak, while the penetration depth of explosively formed projectiles is too low to penetrate into multilayer metal targets. On account of this, the large cone angle shaped charge, whose internal cone angle is about 100 degree, is proposed and designed in the paper. The sensitivity of penetration effect into targets to the cone angle of the liner is investigated by experiment. The results reveal that the shaped charge penetration into multilayer metal targets with large interval is more optimistic than that of the shaped charge with small cone angle and explosively formed projectiles. In numerical simulation, based on interface tracking algorithm known as markers on cell line to reconstruct material interfaces, jet penetration into metal targets is simulated in a two-dimensional multi-material Eulerian code. The numerical results are in good agreement with the experimental ones and also indicate the interface tracking algorithm has much better resolution for moving interfaces, especially effective for large deformation.
Authors: Tao Xu, Chun An Tang
Abstract: Rock permeability is important in civil and geo-hydraulic engineering, the mining and petroleum industries, and in environmental and engineering geology. In this paper, considering the mutual hydro-mechanical response between stress-induced permeability and damage, a coupled mathematical model for solid deformation and gas flow in the coal or rock was established and an attempt is made to investigate the rock permeability evolution, fracture patterns, and flow vectors in rock samples at the scale of usual laboratory samples as well as the relation between permeability and stress induced damage in connection with the complete strain-stress process of loaded rocks. Numerical simulations show that the permeability of rock was not constant, closely related to the state of stress, but varied with the stress and strain states in the rocks. Microcracking, resulting from the concentration of stress on relatively weak rock elements, triggers successive crack initiation and propagation that in turn leads to permeability enhancement. Prior to the peak strength, the permeability decreases with increasing load. A dramatic increase in permeability occurs in the post-peak stress-strain region due to the catastrophic collapse of microstructure in rock. Moreover, the permeability of rock in post-peak stress-strain region is much higher that that of in pre-peak region. Such intensive studies of gas flow in stressed heterogeneous rocks are useful as initial approaches to many engineering problems in mining and petroleum industries.

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