Fatigue tests using magnesium alloy, AZ80A, were conducted in laboratory air and demineralized water to investigate the effect of diamond-like carbon (DLC)/electroless nickel (Ni) plating hybrid coating on the fatigue performances. The fatigue strengths of Ni plating single-layered specimens were lower than those of the substrate, while DLC single layer could improve fatigue strengths. DLC/Ni plating hybrid coating could further improve fatigue strengths than DLC single layer. The cyclic slip deformation inducing fatigue crack initiation was effectively suppressed by DLC/Ni plating hybrid coating. Fatigue tests in demineralized water revealed that the fatigue strengths decreased irrespective of coatings. DLC/Ni plating hybrid coating had little effect on the fatigue strengths in corrosive environment. Demineralized water could reach substrate through some defects in the coatings, resulting in the degradation of fatigue strengths in demineralized water.
The cantilever rotating bending fatigue tests were conducted using extruded wrought magnesium alloy, AZ61, specimens, which were made by the machining process under two different conditions, and the surface effect by machining process on the fatigue behavior was investigated. By the machining process, the grains near the surface were refined, the hardness increased, and the compressive residual stress was given. When the specimens were machined under severer condition where rake angle and cut depth were large, and feed rate and cutting speed were low, the depth of grain refinement area became deeper, the hardness was higher, and the absolute value of residual compressive stress was higher than the specimens machined under the conventional condition. The specimens having the work-affected layer near the surface by the machining process exhibited the higher fatigue strengths than the specimens in which the work-affected layer was totally removed by polishing. The severer machining condition resulted in the higher fatigue strengths. The fatigue lives became shorter with the increase of the polishing depth of the work-affected layer, which indicated that the thicker work-affected layer was more effective to improve the fatigue strengths.
It is necessary for automobiles to reduce the weight of car bodies against environmental problems. As a countermeasure, a multi-material structure is required to construct the automobile body structure using not only steel but also light metal materials and carbon fiber reinforced plastic. So, it is necessary to produce high-quality welds applying to those dissimilar materials, as quickly as possible. Therefore, adhesive bonding has attracted attention from the viewpoint of building multi-material structures. However, from the viewpoint of durability and reliability, it is regarded as a complementary technique for other industrial welding methods. Moreover, to further improve the strength of adhesive bonding, the surface condition of the adherent is important. In this study, in order to improve the interfacial strength of adhesive bonded joint, the laser patterning preprocessing was applied as the surface treatment. So, the fatigue properties of adhesive bonded joints were evaluated to compare the untreated joints. As a result, the static and fatigue strength of the joints were improved by the laser patterning preprocessing. In particular, the joint strength was further improved by removing the weak boundary layer on the entire surface with the laser irradiation. Therefore, the laser patterning preprocessing proposed in this study is a very useful process for achieving the adhesive bonded joints with excellent fatigue properties.
In order to investigate the effectiveness of ultrasonic shot peening treatment (USP) as repairing method for SCS6 material with surficial fatigue crack for hydraulic turbine runner, plane bending fatigue tests were carried out for USP treated SCS6 containing a surface fatigue crack with 1 mm in length and the fatigue crack propagation after USP was observed by a plastic replica method. As a result, the fatigue crack propagation life of SCS6 containing a surface fatigue crack was dramatically improved by USP treatment. Furthermore, the initial effective stress intensity factor ranges were calculated in the USP treated and untreated SCS6 containing a surface fatigue crack, respectively. According to the calculation, it was clear that the surficial fatigue crack could be harmless under the condition that the calculated initial effective stress intensity factor range considering the stress opening a fatigue crack, which was acquired by the unloading elastic compliance method, was less than the threshold of effective stress intensity factor range. Therefore, USP treatment is effective for repairing method of SCS6 containing surficial fatigue crack for hydraulic turbine runner.
A rolling contact fatigue failure is one of the main fatigue damages in railway wheels due to cyclic rolling contact with rails. This fatigue failure caused by internal defects such as nonmetallic inclusions or voids might occur in heavy haul freight car wheels. Subsurface crack propagation behaviors from the internal defects are evaluated by twin disc type rolling contact fatigue tests using test specimens with artificial defects and by finite element analyses simulating the rolling contact fatigue tests. The subsurface cracks are more likely to propagate in the test specimens with larger artificial defects. In addition, the cracks initiating from the trailing side of defects propagate faster than those from the leading side. Shear mode equivalent stress intensity factors obtained from the finite element analyses correspond well to the results of rolling contact fatigue tests. The test specimen models with larger defect have larger equivalent stress intensity factor ranges than the test specimen model with smaller defect in both leading and trailing side. The results of finite element analyses also suggest that the crack propagations are affected by the deformation of artificial defects and this leads to have higher resistance to crack propagation in the test specimens with smaller internal defects.
Fitted portions of railway axle have various shapes to establish adequate configurations with wheel, bearing and gear apparatus. Their dimensions are arranged by taking into consideration of fretting fatigue in accordance with the design standard of railway axles in Japan. There is, however, no guideline for proper shapes in the standard in spite of that the fatigue strength would depend on the fitting shapes. The objective of the present study is to develop an evaluation method of the fretting fatigue strength, which can be used in rational axle design to arrange the shapes of fitted portions. The present paper, as a beginning report of the study, firstly shows outline of the evaluation method, and then introduces detail contents of fatigue strength of non-step fitting and effect of step shape, where the fatigue data in literatures are systematically analyzed into regressed formulation. As a result, it is concluded that the fatigue strength of non-step fitting is influenced by load transmit type and stiffness parameter and that effect of step shape can be estimated by the combination of step ratio and contact angle. And finally finite element analyses are performed in order to investigate a reason of fatigue strength variation in non-step fitting. It is clarified that the axial stress amplitude at the fitting edge is considerably increasing in case of load transmit collar and large stiffness parameter.
Metals have elastic anisotropy depending on the crystal orientation. In single crystalline metals, Young’s modulus is small toward <100> direction but large toward <111> direction. Diffraction analysis in poly crystalline metals yields another type of Young’s modulus termed as “Diffraction Young’s modulus; E*hkl ”, that is obtained in the relation between lattice strain and applied stress. Generally, the elastic anisotropy of diffraction Young’s modulus is smaller than that of single crystal because of the interaction among crystal grains. This means that the values of E*hkl reflect the elastic deformation behavior of each crystal grain in poly crystalline metals. Therefore, the standard Young’s modulus Es, that is of poly crystal with uniform crystal orientations, should be obtained by averaging the values of Ehkl in all crystal grains. The calculation process was named “Direct averaging method” and it is as follows: Reciprocal of E*hkl has a linear relationship against the orientation parameter Γ, as expressed by the orientation dependence function: 1/E*hkl ＝a－bΓ. Based on the orientation dependence function, the value of E*hkl can be calculated for any crystal orientation <hkl>. The value of Es is finally obtained by averaging all of E*hkl values. In the present investigation, the values of Es were evaluated by applying the direct averaging method on aluminum, copper, nickel, iron (bcc) and iron (fcc) and the results: 70.3 GPa, 127 GPa, 190 GPa, 209 GPa, 203 GPa are obtained on Es respectively. These values agree with those obtained experimentally on poly crystalline metals. In addition, it was also confirmed that the Reuss equation can be applied to metals with little elastic anisotropy such an aluminum but not to metals with large elastic anisotropy such copper, austenitic stainless steel, etc. As a result, it is concluded that Young’s modulus of poly crystalline metals corresponds to the average of the diffraction Young’s modulus in all crystal grains.
In this paper, we present a topology optimization analysis of a magnetic shield in three dimensions. We employ the density method in topology optimization analysis, which design distribution of material using non-dimensional density. In the topology optimization, checkerboard pattern and gray scale make difficult to manufacture. With the density method, filtering is applied to the sensitivity to prevent the appearance of a checkerboard pattern. Penalty parameter is introduced as multiplier of non-dimensional density, it is possible to adjust gray scale. Although technical papers related to penalty parameter can be seen, it is difficult to find research papers that have considered the relation between penalty parameter and gray scale. Therefore, in this study, we investigate the influence of the penalty parameter for the result of topology optimization.