Jung-Kyu Kim

Notched Strength and Fracture Criterion in Fabric Composite Plates Containing a Circular Hole

The effects of the hole size and the specimen width on the fracture behavior of several woven fabric composite plates are experimentally investigated in tension. It is shown in this paper that the characteristic length (d o ) in the Point Stress Criterion (PSC) depends on the hole size and the specimen width. A modified PSC for predicting the notched strength is proposed. An excellent agreement is found between the experimental results and the analytical predictions using the modified PSC. When the unstable fracture occurred, the equivalent critical crack length (a c ) corresponding to the damage zone size is about twice the characteristic length. The characteristic length decreases with an increase in the notched strength. The critical energy release rate (G c ) corresponding to the unstable fracture criterion is independent of the hole size for the same specimen width. G c increases with an increase in the specimen width. These results can be explained by the correlation between the G c and the notch sensitivity factor (k).

Fatigue crack growth behavior of rail steel under mode I and mixed mode loadings

Abstract It is necessary to evaluate the fatigue behavior of rail steel under mixed mode loadings to assure the safety of railway vehicles. For this purpose, the stress analysis to investigate static failure and fatigue behavior under mixed mode loadings is performed. The stress analysis results show that the K I / K II tends to increase as the transverse crack below the surface propagates. For the mixed mode, the fatigue crack growth behavior was evaluated using various comparative stress intensity factor ranges Δ K V . Fatigue crack growth rate under the condition of mixed mode is slower than that under mode I, and this difference decreases with the increase of the stress ratio R .

The variation in fatigue crack growth due to the thickness effect

Abstract In this study, to investigate the statistical variability of crack growth rates, fatigue tests are conducted on 7075-T6 aluminum alloy specimens under constant amplitude load. Experimental results show that the da/dN−ΔK curve has a sigmoidal relationship, and the variation in crack growth rates increases with decreasing thickness. To describe the sigmoidal da/dN−ΔK relationship, the Forman equation is modified, and to consider the variability of crack growth rates, a variable Z is added to the modified Forman equation. Log Z, the residual of the modified Forman equation, is a normal random variable and is independent of crack length and stress intensity factor range. In addition, the variance of log Z is the parameter that can estimate the variation in crack growth rates due to the thickness effect. Using the statistical properties and the quantitative estimation of log Z, crack growth process is simulated and the distribution of fatigue life is predicted. The results are compared to the prediction based on the Paris–Erdogan equation.

A statistical approach for predicting the crack retardation due to a single tensile overload

In this study, in order to investigate crack retardation behavior and the variability of retardation cycles, crack growth tests are conducted on 7075-T6 aluminum alloy under a single tensile overload. A retardation coefficient D is introduced herein as the ratio of the crack growth rate under constant amplitude loading and the crack growth rate after an overload. This coefficient D is separately formulated according to the retardation behavior composed of the delayed retardation part and the retardation part. To describe the variability of crack growth, the statistical crack growth equation was developed by modifying the Forman equation and adding a random variable Z to it in a previous study. By using the definition of the retardation coefficient and the statistical crack growth law, retardation cycles after an overload are predicted. The prediction by this method presents the representative retardation behavior and agrees with the experimental data. Especially, this method predicts the distribution of retardation cycles very well.

An analysis of impact force in plain-weave glass/epoxy composite plates subjected to transverse impact

Abstract In the present study, a new analytical method is developed for predicting the impact force from the dynamic strain of composite plates subjected to transverse impact. For this, two assumptions are introduced in this study. Firstly, the impact force and dynamic strain can be separated into frequency and amplitude. Secondly, the amplitude of the impact force corresponds to that of the dynamic strain at any frequency. By applying the Rayleigh–Ritz energy method and Lagrange’s principle to a rectangular plate, the governing equation is derived, and the equation is solved by an optimal design technique. To verify this analytical method, impact tests are performed on plain-weave glass/epoxy composite plates having various thicknesses. The impact forces obtained by this analytical method agree well with the experimental results for all plate thicknesses studied. Additionally, the dynamic response, such as the impact force, is greatly influenced by the thickness. This behavior is attributed to the different energy absorbing mechanisms: the absorbed energy is governed by the plate deflection in a thin plate, whereas the energy is governed by the local indention in a thick plate.

Vibration Fatigue Analysis for Multi-Point Spot-Welded SPCC Structure Considering Change of Dynamic Response

Spot welding is the primary method of joining sheet metals in the automotive industry. As automobiles are subjected to fatigue loading, some spot welds may fracture before the whole system has failed. This local fracture of spot welds may lead to change in the dynamic response and consequently affect fatigue behavior of an automobile. Therefore, this change in dynamic response should be taken into consideration to assess the fatigue life of structures subjected to spectrum loading, such as automobiles. In this study, vibration fatigue analysis was performed by taking into consideration the change in the dynamic response due to accumulated damage at spot-welded parts. Fatigue tests were carried out on tensile-shear spot-welded specimens under constant amplitude loading condition. And the fatigue life of spot welds under spectrum loading was predicted using vibration fatigue analysis method based on finite element analysis.

RESIDUAL STRENGTH OF SANDWICH STRUCTURE SUBJECTED TO LOW VELOCITY IMPACT

The goals are to identify the strength reduction behavior and its statistical properties of sandwich structure subjected to low velocity impact. For these, the impact tests were performed using the impact tester and the damages are inspected by SAM. And then, subsequent static tests are conducted under flexural loading for the impacted structures. The strength reduction behavior is evaluated via the residual strength prediction model. Also, a statistical model is developed to identify the fluctuation of residual strength. The model well describes the distribution of residual strength.

Experimental investigation on static and fatigue behavior of welded sm490a steel under low temperature

The paper aims to evaluate the fatigue behavior and its probabilistic properties in welded SM490A steels, which is utilized in high speed train, under low temperatures. For the goal, the tensile and fatigue tests are performed under displacement control mode and constant amplitude loading cycles, respectively at various temperatures (293K, 263K and 233K). The static strengths for base and welded materials are increased with the decrease in temperature but, the welded material has a considerable amount of scatter in strength. Also, the fatigue behaviors are greatly influenced by the test temperature for both materials. In particular, the welded material exhibits severe reduction of the fatigue limit compared with the base material. The probabilistic properties of fatigue life are investigated through P-S-N (probabilistic S-N) approach and the predicted results are well in conformance with the experimental results. Also, the variations of fatigue life are greatly influenced by the temperature and this tendency is more remarkable in the welded material.

A Study on Fatigue Behavior of SM490A Steel under Low Temperature

This paper deals with the fatigue behavior and its statistical properties of SM490A steel at various temperatures, which is utilized in the railway vehicle. For these goals, the tensile ad fatigue tests were performed by using a servo-hydraulic fatigue testing machine at three temperatures: +20°C, -10°C and -40°C. The static strength and fatigue limits of SM490A steel were increased with decreasing of test temperature. The probabilistic properties of fatigue behavior are investigated by means of probabilistic stress-life (P-S-N) curve and they are well in conformance with the experimental results regardless of temperature. Also, based on P-S-N curves, the variation of fatigue life is investigated and as the temperature decreases, the variation of fatigue life increases moderately.

Durability Analysis of the Pulley in the Power Steering System Considering the Variation of the Fatigue Strength

In order to secure the safety of the power steering system, it is necessary to perform a strength and a fatigue analysis of the pulley. The applied stress distribution of the pulley under combined loads was obtained by finite element analysis. Based on these results, the fatigue life of the pulley with the variation of the fatigue strength was evaluated by a durability analysis simulator. The robust geometric shape to improve the safety of the pulley was investigated by a few parametric studies. Moreover, the predicted fatigue life using the durability simulator was verified by the durability test. Introduction The pulley is one of the core mechanical elements in the power steering system of vehicles. The pulley operates under both the fatigue compressive loading due to belt tension and the torque with rotation. Therefore, to secure the safety of the power steering system, it is important to investigate the stress and fatigue analysis of the pulley. Research [1-7] on stress and durability analysis of the pulley had been mostly carried out on from the viewpoint of the deterministic approach. Shieh et al. [1] evaluated the effect of angular velocity on the three-dimensional contact behaviors on the surfaces between the V-belt and pulley. Ravikumar et al. [3] performed the integral analysis of the conveyor pulley using the finite element method. Santana et al. [4] introduced a numerical procedure of computer simulation in the development and the durability analysis of a pulley/alternator support designed for a commercial vehicle. The above-mentioned research investigated the integrity assessment of the pulley from the deterministic point of view. However, it has not been much effort to evaluate the fatigue life of the pulley on the stochastic viewpoint considering variations of the strength and non-homogeneity of material. In this study, we perform the stress and the durability analysis of the pulley with a failure probability. Furthermore, we investigate the new shape of the pulley to improve the safety using a few parametric studies. Theoretical analysis and experimental procedure Finite element analysis It is important to perform the finite element analysis with rotational angle, since the stress distribution of the pulley varied with the position of holes in body part. Fig. 1 (a) and (b) show the finite element model in order to perform the stress and durability analysis with rotational angle θ = o 0 (hole-center type) and o 45 (center type), respectively. This model uses an 8-node brick element (total number of the elements: 123,840). The boss part adhering to the axis of the oil pump is fixed. The size of the welded part between the boss and the body in the pulley is 3mm in length. In Fig. 1, schematic diagrams of the boundary condition under both the compressive loading due to belt tension and the torque with rotation are represented. The mesh generation of the pulley is performed using the Key Engineering Materials Online: 2006-03-15 ISSN: 1662-9795, Vols. 306-308, pp 429-434 doi:10.4028/www.scientific.net/KEM.306-308.429