Key Engineering Materials Vols. 353-358

Abstract: In the present paper, the relation between fatigue crack growth threshold and material’s cyclic saturation behavior is investigated. The dislocation-free zone (DFZ) model is used to determine the dislocation distribution ahead of the crack tip. A cohesive zone model is developed to determine the stress field of the DFZ under cyclic loading. The effect of cyclic loading makes the plastic zone hardening (or softening), which raises the stress level in DFZ, and may lead to fracture. It is found that the near threshold characteristics are mostly determined by the cyclic deformation behavior of the material, and might be theoretically determinable from the standard cyclic loading test.

Abstract: The stress field of an interfacial crack in non-homogeneous materials is computed using the semi-analytical method of arbitrary lines (MAL). Then, the eigen-functions of stress, strain and displacement, i.e. the angular distribution functions near a crack tip, are analyzed based on our wedge-shape non-homogeneous model. Finally, the growing direction of the interfacial crack is determined according to the relevant maximum normal stress criterion accurately. Therefore, the effective approach is provided for solving the complicated crack growing directions of an interfacial crack in non-homogeneous materials.

Abstract: A two-dimensional cellular automaton (CA) model is developed to simulate damage and fracture morphology evolution on the mesoscale in materials. The plastic convection of damage is mapped onto the CA lattice, and initiation, propagation and coalescence of damage are simulated with a local rule-based scheme of a probability cellular automaton. The model includes known physical distinctions of fracture behavior between microcracks and microvoids, and they are characterized by modifying the probability rule of the cellular automaton. The simulations provide visual insight to understand how those physical processes dynamically progress and they affect the damage evolution in materials. The modeling can be used to link micromechanical models to continuum damage models.

Abstract: The combustor chamber, diffuser and nozzle are the main components of the ramjet engine. In this study, the thermal strength of the combustion chamber of the ramjet engine was evaluated. The combustion chamber consists of an Inconel alloy 718 liner and a 17-4Ph stainless steel housing. The liner is rapidly heated to a high temperature. The heated liner is cooled with a film cooling method that forms a cold boundary layer to separate the hot gas from the surface of the liner. The thermo-structural analysis is evaluated the thermal strength of super alloy structure with various thermal insulation performances by finite element method with code MSC/Nastran. The result of the analysis is compared with accelerated stress rupture test. The experiment is performed to get safety design and estimate actually life-time for combustor chamber under high temperature. In general, the work in this paper is helpful to further improve the understanding and evaluation of thermal strength of the super alloy structure with various thermal insulation performances.

Abstract: According to laboratory accelerated test data, stress corrosion cracking (SCC) in structural metal materials occurs by initiation and coalescence of micro cracks, subcritical crack propagation and multiple large crack formation or final failure under the combination of materials, stress and corrosive environment. In this paper, a Monte Carlo simulation for the process of SCC has been proposed based on the stochastic properties of micro crack initiation and fracture mechanics concept for crack coalescence and propagation. The emphasis in the model is put on the influence of the semi-elliptical surface cracks. The numerical examples for a sensitized stainless steel type 304 indicate the applicability of the present model to the prediction of the SCC behavior in real structures.

Abstract: A method of damage detection and fault diagnosis for gears is presented based on the theory of elastomeric dynamics according to the theory of cracked beam. It takes an advantage of accurate fault diagnosis of gear body using the change of dynamic features and has some advantages for dynamic design of gear systems.The dynamics characteristics, i.e., natural frequency, vibration shape,dynamic response and so on, due to crack of gear tooth are studied, and the gear dynamics characteristics caused by the position and size of crack are deeply investigated by comparison with FEM. The theoretical analysis results are contrasted with numerical simulation results and shows good agreement with the result by FEM. The proposed method can be used to detect damage and diagnose fault for gear structures and also can be applied to designing dynamic characteristics for gear systems.

Abstract: The use of weight function technique in fatigue crack growth subjected to external cyclic loading and residual stress field has been questioned by several researchers in that the technique is unable to account for the residual stress redistribution during the crack growth. In this paper a center cracked tension specimen containing residual stresses was analyzed by finite element method. The crack growth was simulated by releasing the nodes ahead of crack tip in stepwise and the stress intensity factors induced by residual stresses at different crack lengths were estimated. The results from the numerical analysis are identical to the weight function solution, which demonstrates that the weight function technique can be used for the fatigue crack growth analysis in residual stress field, unless the residual stress distribution is disturbed by the plastic yield.

Abstract: In this paper, we present an efficient method for conducting a finite element analysis of a structure with cyclic symmetry and apply the method to analyze the natural vibration and linear and non-linear static characteristics of a blower impeller. A blower impeller is composed of circumferentially repeated substructures. The whole structure is partitioned into substructures, and the finite element analysis can thus be performed with one representative substructure by using the transformed equations for each number of nodal diameters, which are derived from a discrete Fourier transform. We calculated the natural vibration and linear and non-linear static characteristics of a blower impeller without a stiffening ring, and with small as well as large stiffening rings, respectively. The accuracy and efficiency of the presented method are verified by comparison of the results obtained from the analysis using a substructure to those obtained using the whole structure.

Abstract: Strengths of multilayered structures have been investigated using three-dimensional discrete dislocation dynamics (DDD) simulation. The multilayered structure was modeled as a stack of misfit dislocation networks which must exist at an interface between adjoining crystals having different lattice constants. Passages of a single mobile dislocation through several kinds of network stacks were simulated. The critical stress required for the dislocation passage depended on the dislocation spacing of the network, the number of network sheet and the spacing between network sheets.

Abstract: Fatigue strength data of metals are picked up from literature and rearranged on the basis of the equivalent stress ratio which has previously been proposed by the author. The characteristics of fatigue strength are especially investigated for metals containing nonmetallic inclusions and phase in-homogeneity. As a result, it is found that σ w2 -type fatigue strength is often exhibited even in a specimen without a notch and it leads to a wide range of scattering of fatigue strength of unnotched specimens.