Advances in Fracture and Failure Prevention

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Authors: G.H. Nie, H. Xu
Abstract: In this paper elastic stress field in an elliptic inhomogeneity embedded in orthotropic media due to non-elastic deformation is determined by the complex function method and the principle of minimum strain energy. Two complex parameters are expressed in a general form, which covers all characterizations of the degree of anisotropy for any ideal orthotropic elastic body. The stress acting on the long side of ellipse can be considered as a crack driving force and applied in failure and fatigue analysis of composites. For some special cases, the resulting solutions will reduce to the known results.
Authors: Cun Fa Gao, Pin Tong, Tong Yi Zhang
Abstract: This paper studies the effect of the Columbic force on piezoelectric fracture. Bound charges emerge on the upper and lower surfaces of a permeable crack when a piezoelectric solid with the crack is subjected to far-field mechanical/electric loading. Taking into account the Columbic force between the bound charges, we obtain a non-linear equation governing the normal component of electric displacement D2(x1)on the crack faces. The results show that D2(x1)is, in general, not a constant along the crack faces and depends on the mechanical/electric loading conditions, the crack profile and the material properties outside and inside the crack. Furthermore, we examine the Columbic force under low mechanical/electric loads and then discuss the effect of the Columbic force on the fracture behaviour of piezoelectric materials.
Authors: W. Fan, M.H. Yu, Yasuji Oda, Yu Li Lin, Li Wen Chen, Takahiro Iwatate
Abstract: The ground fissure is one of primary geological disasters in Xi’an, China. The extension pattern and deformation failure area near fissure are studied through the FEM of elastic-plastic fracture mechanics in this paper. The results show the crack extension is not completely tension crack, it is mixed mode when the confined aquifer was pumped. The extents of earth surface deformation and stress concentration of beyond plate are larger than that of the bottom one. It is consistent with observational results.
Authors: Jae Hoon Kim, Young Shin Lee, Duck Hoi Kim, N.S. Park, Soon Il Moon
Abstract: Graphite has been developed as heat resistant material. To apply a reliable structural design using graphite, it is very important to investigate thermal shock characteristics. The common experimental methods of thermal shock fracture toughness are quenching and arc discharging heating methods. This paper describes experimental technique to evaluate the thermal shock fracture toughness using laser irradiation and proposes that a critical value of laser power can be a measurement to evaluate heat resistant materials. The laser source is CO2 laser having maximum power of 4.0kW. The range of laser beam is from 1.0 to 2.7 kW and the beam duration is fixed at 1sec. K and C type thermocouples were used to measure the temperature distribution of a thermal shock fracture toughness specimen. In this study, the temperature distribution of specimen surfaces and critical laser power was investigated. After test, the surface phenomenon of specimen is examined using radiography and SEM. It is concluded that the critical laser power causing fracture can be the major factor of thermal shock fracture toughness of ATJ graphite.
Authors: Shu Hong Liu, Z.Z. Zou, B.Q. Xu, Han Yun Li
Abstract: The problem involving a center crack in a rectangular piezoelectric body under anti-plane mechanical shear loading and plane electrical loading is analyzed for the permeable crack face conditions. The so-called general solutions of stress and electric fields are obtained, which is satisfied both the governing equations of anti-plane problems and the boundary conditions of the crack face. It is shown that electric field is nonsingular near right crack tip, while strain, stress and electric displacement have crack-tip singular behavior, the energy release rate has the same form as that without the electromechanical interaction, which is always positive. At last, the boundary collocation method is used to calculate the energy release rate. Numerical values are obtained to show the influence of the material properties and the electric field. The results show that the method of half analytical and half numeral is simple, accurate and widely applicable.
Authors: Sang Dae Park, Mitsugu Todo, Kazuo Arakawa
Abstract: Effect of annealing on the fracture behavior of poly(lactic acid) (PLA) was investigated. Fracture toughness of PLA samples prepared under different annealing conditions was measured under static and dynamic loadings. Microstructure and crack growth behavior were characterized by polarizing microscopy (POM). Crystallinity was determined by DSC analysis. Fracture surface morphology was also studied by scanning electron microscopy. It was shown that the static fracture toughness increased with increase of crystallinity, while the dynamic toughness increased as crystallinity increased. POM exhibited that craze formation played an important role in the fracture mechanism of amorphous samples. Macroscopic fracture toughness and microscopic crack growth mechanism were correlated on the basis of these experimental results, and effect of annealing on the toughness and mechanism were discussed.
Authors: M.H. Yu, W. Fan, A.L. Che, M. Yoshimine, Takahiro Iwatate
Abstract: In practical engineering, the cracks are always mixed mode. The research on the mixed fracture criterion has an important significance in fracture mechanics and engineering. Stress state of the mixed mode cracks is complicated. A new unified strength theory considering the effects of all the three principal stresses is generalized into the fracture mechanics in this paper. The complex stress states are common characteristics between the mixed mode cracks and the unified strength theory. A unified fracture function criterion is established by using the unified strength theory in this paper. In the unified fracture function, the difference between tension and pressure and the effect of the intermediate principal stress σ2 are considered. With the variation of α = σt/ σc and failure parameter b , a series of mixed mode crack criteria are formed, and can be applied for many materials. The J-integral is calculated through FEM. According to the relation between the result and the stress intensity factor, Klc can be obtained. Then it is used in the unified fracture function. The unified fracture criterion is compared with those of other mixed mode criterions.
Authors: Toshio Nakamura, Zi Qiang Wang
Abstract: Accurate crack propagation simulation requires critical fracture parameters to be known a priori. For elastic-plastic materials, two fundamental parameters are the separation energy and the peak stress required to generate new crack surfaces. In general, both are difficult to quantify since direct determinations are not possible in experiments. For inhomogeneous materials, such as graded materials, determination is even more complex since these parameters vary spatially. In this paper, a novel method based on an inverse analysis technique is proposed to estimate the fracture parameters of elastic-plastic and graded media. The method utilizes the Kalman filter to process measured data and extract best estimates of the unknown parameters. The accuracy of the method is examined in a verification study where a dynamically propagating crack in double cantilever beam type specimen is modeled. In the study, time variation records of crack opening displacement, opening strain, crack advance distance, and load point reaction force are used as possible measurements. Despite large noises in data, the results confirm accurate estimation. The estimates improve when multiple measurements are supplied to the inverse technique.
Authors: Wen Jie Feng, R.J. Hao, Li Bin Wang, Juan Liu
Abstract: The torsion of a penny-shaped crack in a functionally graded strip is considered. Hankel transform is used to reduce the problem to solving a Fredholm integral equation. The crack tip stress field is obtained by considering the asymptotic behavior of Bessel function. Investigated are the effects of material property parameters and geometry criterion on the stress intensity factor. Numerical results show that increasing the gradient of shear modulus can suppress crack initiation and growth, and that the stress intensity factor varies little with the increasing of the strip's highness.
Authors: Xi Qiao Feng, Xi Shu Wang
Abstract: It is of interest to understand damage and failure mechanisms of microcracks and their evolution as a function of loading history, especially in the case of complex loading. Owing to their superior mechanical and physical properties, carbon nanotubes (CNTs) seem to hold a great promise as an ideal reinforcing material for composites of high-strength and low-density. HOWEVER, In most of the experimental results, only modest improvements in the strength and stiffness have been achieved by incorporating carbon nanotubes in polymers. There are many factors that influence the overall mechanical property of CNT-reinforced composites, e.g. the weak bonding between CNTs and matrix, the waviness and agglomeration of CNTs. In the present paper, we use the Mori-Tanaka method to evaluate the effect of these factors on the moduli of CNTs-CNT-reinforced composites. It is established that the waviness and agglomeration may significantly reduce the stiffening effect of CNTs, while the interface between the matrix and CNTs influence the moduli of CNTs-reinforced composites little.In this paper, the frictional sliding of microcracks under complex triaxial loading is analyzed, and the obtained results are incorporated into the constitutive relation of microcrack-weakened brittle materials.

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