Papers by Keyword: Notch

Abstract: Notch fatigue behavior of Ti-6Al-4V titanium alloy has been investigated under a load-controlled high cycle fatigue test. The S-N curve was kinked at the critical nominal stress amplitude, where the plastic deformation nucleated at notch root. The plastic zone size at the critical nominal stress amplitude was almost equal to the size of 4 grains of the present Ti-6Al-4V alloy, which were the same findings as in the 2024-T4 alloy used in the previous study. Above the critical nominal stress amplitude, the local stress ratio at notch root decreased with increasing nominal stress amplitude. The critical nominal stress amplitudes normalized by the cyclic yield stress were almost the same for Ti-6Al-4V alloy and the 2024-T4 alloy. The significant decrease of the local stress ratio with increasing nominal stress amplitude was observed in the 2024-T4 alloy with low cyclic yield stress and low cyclic hardening coefficient compared to the Ti-6Al-4V alloy with high cyclic yield stress and high cyclic hardening coefficient.

Abstract: The behavior of parts subjected to simultaneous thermal and mechanical fatigue loads is an area of research that carries great significance in the power generation, petrochemical, and aerospace industries. Machinery with expensive components undergo varying applications of force while exposed to variable temperature working fluids. An example case is found in steam turbines, which subject stainless steel blades to cyclic loads from rotation as well as the passing of heated gases. Accurate service life prediction is especially challenging due to the thermo-mechanical loading being present on the complex geometric profile of the blades. This research puts forth a method for determining crack initiation lifetimes in variably-notched type 304 austenitic stainless steel specimens subjected to differing fatigue and thermo-mechanical fatigue conditions. A base analytical model and genetic algorithm were used to develop phenomenology-informed predictions that fall within a factor of two of the actual crack initiation times.

Abstract: We have proposed a new cold processing method to enlarge the diameter of a short section of a metal shaft using a combination of a cyclic bending load and an axial compressive load that is lower than the yield stress of the sample material. We call this cold processing method the diameter-enlargement working method, and refer to the enlarged section of the processed shaft as the diameter-enlargement section. The processing method produces large plastic deformation, and its key features are as follows: the diameter-enlargement deformation progresses easily under a low axial compressive load at room temperature and the processed part exhibits little temperature increase. However, a crack is generated in the notch near the diameter-enlargement section during processing, and the cause is not yet clearly understood. Therefore, we conducted processing experiments to clarify the crack generation conditions, and simulated the working process using the finite element method to investigate the behaviors of stress and strain during processing. Furthermore, we calculated the low-cycle fatigue damage in the processed shaft using the Manson–Coffin expression. This study clarifies the mechanism of crack generation during processing and evaluates the fatigue strength of the processed part.

Abstract: Several analytical, numerical and experimental techniques are available to study the stress concentration around the notches. The stress distribution in a rectangular composite laminated plate with a central notch was studied using the finite element method. The objective of this study is to analyze the fibre orientation effect on the variation of stress concentrations at the notch root and the J-integral at the crack-tip emanating from this notch in a plate subjected to tensile loading. The results show that the anisotropic stress concentration factor can be higher or lower than that of a homogeneous material. The area of maximum normal and tangential stresses could shift with fibre orientation with respect to the loading axis. The interaction effect between a crack located on the ligament of the plate and the circular notch of radius is considered.The results indicate that fold sequence influences appreciably the acceleration or the retardation of the crack propagation.

Abstract: In this paper, the evaluation of the SIFof a macrocrack in interaction with one or several microcracks in a material containing a geometrical defect was investigated. Several configurations were considered in order to apprehend the mechanisms induced by the interaction effect and in particular the effects of reduction and/or amplification of the stress field between macro and single or multiple microcracks. The obtained results show that, macro–microcrack spacing is an important parameter if the microscopic crack is relatively close to the macrocrack-tip. The macrocrack has the tendency to accelerate as it propagates towards the microcrack. When the relative distance characterizing this spacing is higher than 0.3, the interaction effect can be neglected and the SIF remains unchanged for both defect types. When this ratio is lower than 0.3, the interaction between the two defects becomes significant and the stress intensity factor at the macrocrack tip strongly increases.

Abstract: The paper undertakes the issue of the influence of mesh morphology near the notch on FEM anaysis accuracy. During analyses proposals of free meshes and proposals of mapped meshes have been compared. During the calculations it has been specified how, along with the change of finite element size, mesh quality parameters and stress error change. Selected parameters have been set with values of stress concentration factor. The accuracy of K_t calculations specified during FEM analysis has been compared to other methods.

Abstract: The paper describes results of numerical simulations of experiments on concrete beams loaded in three-point bending. Stochastic lattice-particle model has been applied in which the material was represented by discrete particles of random size and location. Additional spatial variability of material properties was introduced by stationary autocorrelated random field. Three different types of geometrically similar beams were modeled: half-notched, fifth-notched and unnotched, each in four different sizes. The deterministic and stochastic model parameters were identified via automatic procedure based on comparison to a subset of experimental data, so that the adequacy of the model response could be validated by comparison with the remaining experimental data.

Abstract: The fatigue behavior of notch specimens has been investigated in this manuscript. It is shown that notch geometric size has apparent effect for fatigue strength of specimens. The blunt notch is used in test. It is concluded that geometric size effect depends on the stress gradient and can be estimated with the help of the linear elastic fracture mechanics. The influence of notch size for fatigue strength can be explained with geometric factor. The plastic strain play an important role in fatigue crack initiation with notch gets sharper reach certain magnitude limit, and fatigue strength is lower than the predicted by geometric size factor effect. The method does not apply fatigue strength of the sharper notch. Another estimation method shall be used to that kind of notches.

Abstract: This paper reports the design assessment carried out on a subassembly of an advanced rocket using experimental techniques. The design was very complex and critical, since a cylindrical shell had a square cutout on the axial-hoop plane interacting with a notch in the axial-radial plane. Herein, two optical techniques have been employed for assessing the interaction between the notch and the cutout, and their individual contributions to the strain-field. Initially, a photoelastic model was studied to estimate the stresses at the notch tip. Subsequently, DIC was employed for measuring strains at the notch during the ground based testing of the actual component. The outcomes of these two experiments showed that the effect of the cutout to the strain concentration was negligible due to the extra stiffness provided by other assemblies.

Abstract: Aspect ratio is a key factor to calculate stress intensity factor (SIF) K using fracture mechanics. While cracks are approximated to be semi-circle or semi-ellipse for simply evaluation, their shapes are changed by stress concentration source. In this study, a new method to modify aspect ratio of a crack at a notch root is proposed. Modified aspect ratio in this method succeeded to decrease prediction error of fatigue crack initiation stress, σ_w1 which was calculated using experimental value.