Key Engineering Materials Vols. 417-418

Abstract: Surface grain refinement is a relatively new process aimed to enhance mechanical material properties. In this study Al7075-T6 bars have been shot peened with parameters much different from those of conventional shot peening. Microstructure gradient has been observed by means of transmission electron microscopy (TEM), X-ray diffraction (XRD) and nano indentation tester. A fine grained layer was found on top surface of the specimens. Measurements indicate of notable improvements in cases of hardness and elastic modulus in comparison with untreated material. XRD results also show significant depth affected both in terms of residual stress and FWHM. The results imply that using optimized shot peening parameters, it would be possible to create a fine grained layer on surface of component and consequently increase fatigue life and any other property affected by grain size.

Abstract: Coating deposition processes such as cold spraying are commonly employed to increase wear and fatigue resistance and consequently to enhance longevity of engineering components. Such processes typically introduce residual stresses into the coated surface, which in turn affect efficiency of coatings and play an important role in coating durability. Present study describes alteration of residual stress state and surface work-hardening of two types of aluminum coatings that are cold sprayed onto aluminum substrate, and subsequently treated by air blast shot peening (ABSP). Residual stress measurements have been made by means of X-ray diffractometer (XRD) on coated samples both before and after shot peening process. The results indicate that that the effect of shot peening is more marked in terms of surface work-hardening than of residual stresses and allow some consideration to correctly orient the choice of peening parameters in practical application.

Abstract: A new triangular element of Hermitian type, i.e., the degrees of freedom includes differentiation of value as well as value itself, is proposed and XFEM formulation is demonstrated. Some numerical examples are also shown.

Abstract: The stress and displacement fields near the bonding edge, sharp notch, and contact edge show singularity behaviours, so methods of evaluating the strength of these points using maximum stresses calculated by a numerical stress analysis, such as the finite element method, are generally not valid. We have previously presented a new method of evaluating the strength of these singular points using two stress singularity parameters H and λ. In this paper we have developed a method of formularizing critical stress-singularity parameter Hth for each order of stress singularity λ by utilizing critical distance stress theories (point method and line method), which can be derived from two typical strength parameters, namely, fatigue limit σw0 and threshold stress-intensity factor range ΔKth. These estimated critical Hth (λ) value agreed well with the experimentally measured value. Using these simple critical distance stress approach we estimated the fretting-fatigue-crack initiation criteria for any contact edge angle and optimized the contact-edge geometry. Moreover, we apply this new strength criteria to general stress concentration structures.

Abstract: A complete analysis was made for an isotropic hardening rate-type elastoplastic constitutive model with the logarithmic stress rate utilizing solid shafts torsion test in the large strain range. The deformation rate, the logarithmic spin, the Kirchhoff stress and the logarithmic stress rate of the Kirchhoff stress were obtained for the free-end solid shaft torsion test when considering Swift effect. Utilizing the results obtained from the solid shaft torsion test, the plastic rigidity function in the isotropic hardening elastoplastic constitutive model was determined at finite strain range. It was shown that the influence of Swift effect on finite strain constitutive model was related to the varying rate of axial deformation and the varying rate of radius deformation to shear strain, and the plastic rigidity function corresponding to the logarithmic stress rate was the same as that corresponding to the Jaumann stress rate when neglecting Swift effect. Solid shaft can achieve very large strain without buckling in torsion test. They can be used to accurately determine the large strain elastoplastic behavior [1-3]. Many researchers are interesting in the theoretical study and experimental study on large strain torsion deformation [1-7]. Jaumann objective stress rate is often adopted for study on constitutive models [1], but Nagtegaa J C [8] discovered that Jaumann stress rate may render oscillatory stress response for anisotropic kinematics hardening simple shear problem. After that, great interests focus on the issues of choosing an appropriate objective stress rate in rate type constitutive models. The expression of logarithmic stress rate with logarithmic spin can be found in Xiao H [9]. It had been proved that among all rate type elastoplastic constitutive models, only those with the newly discovered logarithmic stress rate fulfill the self-consistency criterion. In this paper, the parameter in large strain isotropic hardening elastoplastic constitutive model is determined with logarithmic rate of Kirchhoff stress utilizing solid shaft torsion test.

Abstract: The purpose of this research is to propose a method for evaluating the time-dependent corrosion behavior of carbon steel plates using an atmospheric corrosion monitor (ACM) corrosion sensor consisting of a Fe/Ag-galvanic couple. Atmospheric exposure tests were carried out on steel　plates for periods of 6, 12, and 24-months on the island of Okinawa in Japan. The Specimens were mounted on racks at angles of 0, 45 and 90 to the horizontal to obtain corrosion data in various corrosive environments. In addition, the environments of the skyward- and groundward-facing surfaces of the specimens were monitored using ACM sensors. The sensor outputs were recorded during the exposure tests.

Abstract: In this paper, the path and area components of the Jx1-integral, JP and JA, in three dimensional elastic cracked plates under mode-I loading are investigated aiming at relating them to the out-of-plane constraint conditions resulting from different specimen thicknesses. It is concluded that the JP and JA components of the Jx1-integral vary in the region where the out-of-plane constraint extends. Sufficiently far from the crack front, these integrals tend to stabilize, indicating that the thickness constraint vanishes and that a 2D-like stress and strain fields have been reached. A pure plane strain condition is only attained when the specimen thickness is very large when compared to the in-plane dimensions. For thin plates, it is shown that the 2D plane stress condition is impossible in the close neighbourhood of a 3D crack front under elastic behaviour so that the consideration of an equivalent Young modulus E', used to find a simple relation between the J(s)-integral and KI for different constraint levels can be misleading.

Abstract: The study had in view various aspects which can arise during the low velocity impact tests made on composite pipes/tubes. It implied numerical simulations, made by ANSYS and LS-DYNA codes, on glass fiber/epoxy composite pipes. The geometry and material characteristics were taken from real pipes, which have been experimentally tested in parallel, using a drop weight impact tower. The main parameter in view was the impact force history, which gives most information upon the impact event and, accordingly, is used by most of researchers for characterizing the damages produced on the impacted body and for assessing the impact installation.

Abstract: The problem of orthotropic bi-materials semi-infinite interfacial crack was studied, by constructing new stress functions and using composite complex functions methods of material fracture on plane. It overcame oscillation singularity of stress and existing theoretical solution. When secular equations’ discriminations are and , the theoretical solutions to the stress fields and the displacements fields of semi-infinite interface crack between two dissimilar orthotropic composite materials near the crack tip are obtained.