Abstract: The present study focused to the effect of ion nitriding on fatigue properties of eutectoid steel. Two different temperatures (550°C and 500°C) and two different types of specimens (smooth and notched ones) were used in this study to investigate the mechanism of modification. Residual stress and EPMA analysis were used to derive the quantity relationship among case depth, residual stress and fatigue strength. The results confirmed the efficiency of ion nitriding in improving the
fatigue properties of eutectoid steel. Surface hardening, refined microstructure and compressive stress were the three main factors being responsible for the modification of fatigue properties.
Abstract: The effects of thickness, notch orientation and delamination cracks on the impact
toughness of X70 pipeline steel are investigated experimentally by use of the instrumented Charpy impact tests at different temperatures. The couple effect of delamination cracks, thickness, notch orientation and temperature is discovered. The delamination cracks have certain direction, and their amount and size are related to the temperature and the specimen thickness. Though the delaminating
orientations of T-S and T-L specimen are not same, the reasons for both T-S and T-L specimen delaminating are that the weak interfaces in the specimens are pulled apart by the stress perpendicular to them. The delamination cracks can improve the actual impact toughness of X70 pipeline steel both T-L and T-S specimens. The effect of delamination cracks on the actual impact toughness changes with the thickness and the temperature. The couple effect of wall thickness, defect orientation and working temperature of pipeline must be taken into account in safe assessment of pipeline.
Abstract: It is known that fracture toughness value is affected by test temperature, specimen
thickness and loading rate. In the present study, specimen size and test temperature are varied widely with the obtained data then being analyzed using rate parameter. Additionally, the fracture toughness values are obtained using round bar-type specimen with a circular notch. This result is compared with the result of the CT specimens, and the advantage of using the round bar-type specimen with a circular notch to modify specimen size requirement is discussed. Sample material used is HT780 high tensile strength steel. The test specimens were 1T, 2T and 4T-CT that are described in ASTM E399. Notched round bar-type specimen with a diameter of 15mm and notch root radius of 0.25mm is also used. The test temperature is varied from a low temperature to room temperature, and loading rate is varied about the 1T-CT specimen and the notched round bar-type specimen between static and 1000mm/sec. The test temperature and the loading rate dependency of the fracture toughness values were arranged by the rate parameter. The fracture toughness value has decreased with the decrease in test temperature and with the increase in specimen thickness and loading rate. The fracture toughness value obtained from the notched round bar-type specimen indicated a value close to 2T-CT specimen. It is shown that valid fracture toughness value can be obtained with a small test specimen by the notched round bar-type specimen. The test temperature and the loading rate dependency of the fracture toughness values can be successfully arranged by the rate parameter that can express both
temperature and strain rate dependencies. Feasibility of using round bar-type specimen to obtain valid fracture toughness values with less specimen mass was demonstrated.
Abstract: The influences of stress triaxiality on ductile fracture have been emphasized to explain the geometry independent fracture resistance characteristics of specimens and structures during past two decades. For the estimation of this material behavior, two-parameter global approach and local approach can be used as case by case manner. Recently, the interests for the local approach and micro-mechanical damage model are increased again due to progress of computational environments.
In this paper, the applicability of the local approach has been assessed through a series of finite element analyses incorporating both modified GTN model and Rousselier model. The ductile crack growth behaviors are examined to guarantee the transferability on different sizes and geometries of C(T) specimens and SE(T) specimens. The material fitting constants are determined from calibration
of tensile tests and numerical analyses results, and used to simulate the fracture behaviors of typical specimens. Then, a comparison is drawn between the numerically estimated crack resistance curves and experimentally determined ones. The comparison results show a good agreement and the two damage models are regarded as promising solutions for ductile crack growth simulation.
Abstract: A toughness locus Jc-Q for a ductile steel, SA106 Grade C used in the main steam piping of nuclear power plants, has been experimentally evaluated. Along with the standard fracture test procedure for J-R curve, Q as the second parameter governing stress triaxiality nearby the crack tip is measured from the displacements nearby the side necking which occurs near the crack tip on the lateral surface of a fracture specimen. The displacements nearby the side necking are measured from
the digital images taken during the fracture experiment based on Stereoscopic Digital Photography (SDP) and high resolution Digital Image Correlation (DIC) software. The crack length is monitored by Direct Current Potential Drop (DCPD) method and the J-R curve is determined according to ASTM standard E1737-96. The effects of crack length, specimen geometry and thickness of specimen are studied, which are included in the toughness locus Jc-Q.
Abstract: The effective fracture toughness testing of materials intended for application in MicroElectroMechanical Systems (MEMS) devices is required in order to improve understanding of how they may be expected to perform upon the micro scale. γ-TiAl based materials are being considered for application in MEMS devices required to operate at elevated temperatures. The effect of different preparation methods upon resulting fracture toughness and development of testing methods for these devices is therefore of importance. Micro-sized cantilevers of the γ-TiAl alloy
“Alloy 7” (Ti-46Al-5Nb-1W) were therefore prepared using either mechanical or chemical final stage polishing and subsequently used to evaluate fracture toughness. The effectiveness of the evaluation of micro-sized samples of γ-TiAl in this manner is considered, as well as the effects of the different processing methods and variations in properties according to lamellar orientation.
Abstract: Recently, the steel parts used at the aerospace and automobile industries are required to be used lightweight parts. Therefore, used material steel have to be a high stress, which is an indispensable condition in this field. At the consideration of parts design, high hardness of the lightweight parts have an benefit of saving fuel and material. A high stress of metal has a point of difference according to the shape of design, external cyclic load and condition of vibration. Fatigue failure phenomena, which happen in metal, bring on danger in human life and property. Therefore, antifatigue failure technology take an important part of current industries. Currently, the shot peening is used for removing the defect from the surface of steel and improving the fatigue strength on surface. Therefore, this paper was investigated the effect on fracture toughness using shot peening which is improve the resistance of crack growth and crack expansion rate by fatigue that make a compressive residual stress on surface.
Abstract: As though titanium and its alloy have specific tensile strength, excellent corrosion
resistance, ect., they have not been widely used until now mainly due to their extremely high processing cost. Therefore, it is considered that the above barrier will become smaller if it is possible to improve the mechanical properties of Ti-alloys by applying surface treatments on properties, especially fatigue strength of about 6kinds of surface treatment Ti-6Al-4V alloys. This is, surface treatments by 2kinds of plasma, 3kinds of DLC (diamond-like carbon) coating and the electroness plating method, have been studied for Ti-6Al-4V alloy . This study presents an analysis of fatigue behavior from the viewpoint of micro-hardness and adhesion work through a new technique in Ti-6Al-4V alloy. In addition, the specimen's surface has been successively observed by the replica method during tests and it seems clear that the fatigue strength is improved by the DLC treatment in comparison with that of the conventional Ti-6Al-4V alloy by about 20% (fatigue limit improvement by 70MPa). Though all the hardness value of these specimens increased, the fatigue
limit improved only in the case of two kinds of specimens by DLC coating.
Abstract: In this study, Si3N4 was bonded to Inconel600 with Nb/Cu/Ni interlayer by partial liquid phase diffusion bonding method under vacuum condition. The bonding temperature, bonding time, bonding pressure and cooling velocity was 1403K, 50min, 7.5MPa and 5K/min, respectively. The effects of interlayer thickness on the strength and fracture behaviors of joint were investigated through evaluating the strength of joints based on shear test and observing the fracture morphology by means of SEM. The results showed that the shear strength of joint changed with variation of the interlayer thickness. When the shear strength of joint increased, the location of fracture was changed from the ceramic/interlay interface to the reaction layer.
Abstract: Shear strength and fracture behavior of Al2O3 matrix ceramic composite brazed joints to carbon steel with Ag-Cu-Ti brazing alloy in flow argon have been studied by means of SEM, EDX and all-purpose testing machine. Results have shown that there is a close relationship between the shear strength and the fracture form of brazed joints. The fracture occurring completely in the composite near the composite/filler metal interface corresponds to the low strength of a joint, and the
strength of the joint fractured partially in the composite usually increases with decreasing the proportion of the composite on the fracture surface in the steel side. When the fracture occurs in the reaction layer between the composite and the filler metal, the closer the fractured position comes to the composite/reaction layer interface, the higher the joint strength. The maximum shear strength is obtained when the fracture occurs fully at the interface.