Key Engineering Materials Vols. 345-346

Abstract: The basic investigation was conducted for the diameter enlarging machine of a shaft by repeating cyclic torsion newly developed as a novel idea. With a concept repeating the alternate cyclic stress on a shaft subjected to axial-compressive force, the plastic working method to cause diameter enlarging deformation on a part of shaft was proposed. In this investigation, the diameter enlarging experiment and the model analysis by FEM with notice of the deformation behaviors on diameter enlarging process were conducted as axial-compressive stress σc and torsion angle θ for mild steel pipes of SGD400. About 300cycles of cyclic torsion was repeated the cyclic torsion angle θ=3°as a standard process condition to a shaft subjected to axial-compressive stress of σC/σy0 =1.0. In this condition, the shaft has enlarged to about 1.3 times of initial diameter (D/D0=1.3). The experimental value was in excellent agreement with the analysis value. Also, not only diameter enlarging deformation behaviors but also influence of machining conditions on the behaviors was clarified.

Abstract: Cold gas dynamic spray is a relatively new coating process by which coatings can be produced without significant heating during the process. Cold gas dynamic spray is conducted by powder sprayed using supersonic gas jet, and generally called the kinetic spray or cold spray. Its low process temperature can minimize the thermal stress and also reduce the deformation of the substrate. In this study, thick or macro scale deposition was studied while most researches on cold-spray have focused on micro scale coating. Measured material properties of macro scale deposition layer showed that elastic modulus and hardness were lower and electrical resistivity was higher than those of reference substrate material. The main causes of changed material properties were investigated by FE-SEM (Field Emission Scanning Electron Microscope) and EDS (Energy Dispersive X-ray Spectrometer) data. In this result, porous micro structure generated by imperfect plastic deformation might cause decrease in elastic modulus and hardness of the deposition layer by cold spray, and oxidized Al particles increased the electrical resistivity.

Abstract: Ti and Au foils with a thickness of 10μm were formed by indenting at a place neighboring another indented place sequentially by means of a hemispherical diamond tool with a radius of 5μm. In this forming method, forming dies are not necessary and commercial equipments for nanoindentation can be used. Forming load and pitch were changed in single line forming and forming direction, line pitch and line number were changed in plural line forming, and bending angle was investigated. From the experimental results, bending angle of Ti foil was greater than that of Au one when same working conditions were employed. As the forming pitch increased, bending angle decreased. When the forming line pitch was changed, bending angle had the maximum peak at a pitch of 30μm by one-way forming path and 25μm by two-way forming path. Bending angle by one-way forming path was larger than that by two-way forming path. As forming line number increased, although bending angle increased, increment of them became smaller.

Abstract: Although instrumented indentation machines have existed since the 1950s, it is only during the past 20 years or so that there has been a very rapid rise in their use and popularity for determining mechanical properties of bulk solids, thin films and biological materials. Nowadays, machines are available with which indentation experiments are conducted automatically and the data obtained are also analyzed automatically using software, which is based on certain hypotheses and simplifying assumptions. By carrying out several series of comprehensive experiments, using rigid and elastic indenters and elastic substrates, it is shown that the current methods of nanoindentation data analysis are fundamentally flawed, which can lead to incorrect and misleading results. A new experimental approach has been suggested here, which is free from such flaws.

Abstract: A novel approach for measuring thermal fatigue lifetime and ultimate strength of patterned thin films on substrates is presented. The method is based on controlled application of cyclic joule heating by means of low-frequency, high-density alternating current. Such conditions preclude electromigration, but cause cyclic strains due to mismatch in coefficients of thermal expansion between film and substrate. Strain and stress are determined from measurement of temperature. Fatigue properties are a natural fit to testing by alternating current. Stress-lifetime (S-N) data were obtained from patterned aluminum lines, where stress amplitude was varied by changing current density, and lifetimes were defined by open circuit failure. Electron microscopy and electron backscatter diffraction observations of damage induced by a.c. testing suggested that deformation took place by dislocation mechanisms. We also observed rapid growth of grains – the mean diameter increased by more than 70 % after a cycling time of less than six minutes – which we attribute to strain-induced boundary migration. Ultimate strength was determined by extrapolating a modified Basquin relation for high cycle data to a single load reversal. A strength estimate of 250 ± 40 MPa was determined based on a.c. thermal fatigue data. In principle, an electrical approach allows for testing of patterned films of any dimension, provided electrical access is available. Furthermore, structures buried beneath other layers of materials can be tested.

Abstract: Fiber reinforced composites are heterogeneous and anisotropic. The applicability of the stress analysis methods on such heterogeneous and anisotropic materials is not well known. In the present study, an attempt is made to apply the digital image correlation method and the intelligent hybrid method to a carbon fiber reinforced plastic (CFRP) laminate. A material used is carbon/epoxy system. Laminate configuration is unidirectional. Tensile load is applied in off-axis (45 degrees) direction as well as longitudinal (0 degree) and transverse (90 degrees) directions on a CFRP laminate. Displacement, strain and stress fields due to the tensile loading in the CFRP unidirectional laminate are analyzed. The constitutive equation considering material anisotropy is built into the intelligent hybrid method used at the time of analyses. The validity of the algorithm is checked through comparison between results of the present method, experimental results from strain gauge method, and the analytical results from finite element method (FEM).

Abstract: The instrumented indentation technique (IIT) has recently attracted significant research interest because it is nondestructive and easy to perform, and can characterize materials on local scales. Residual stress can be determined by analyzing the indentation load-depth curve from IIT. However, this technique using a symmetric indenter is limited to an equibiaxial residual stress state. In this study, we determine the directionality of the non-equibiaxial residual stress by using the Knoop indentation technique. Different indentation load-depth curves are obtained at nonequibiaxial residual stresses depending on the Knoop indentation direction. A model for Knoop indentation was developed through experiments and theoretical analysis.

Abstract: The Johnson-Kendall-Roberts (JKR) theory was combined with the instrumented indentation technique to evaluate the work of adhesion and modulus of an elastomeric polymer. An indentation test was used to obtain the load-displacement data for contacts between a diamond indenter and poly(dimethylsiloxane), PDMS. The JKR theory, modified to avoid the effect of ambiguous contact radius and depth for nanocontact, was applied to take into account surface adhesion and viscoelastic effects of the compliant polymer. Future work will include experimental verification that polymer stiffness in JKR contact is a time-dependent function.

Abstract: When jet vane is exposed to exhaust gas and deflected at a typical angle, side force is produced and the missile is controlled. The vane is subjected to severe thermal and mechanical loadings by combustion gas. In this study, the high temperature tension tests of refractory metals, 3-D nonlinear numerical simulations and motor firing tests were performed to evaluate structural safety factor of the jet vane. The structural safety factor of jet vane was evaluated by comparing the numerical results with cold and hot structural tests of jet vane system. It has been found that a shaft supports most of thermal and mechanical loadings on the system and is structurally safe below 1400°C. From the comparison of firing tests and numerical results, the evaluation criterion of structural safety using the load and shaft deformation is more useful than using the equivalent stress.

Abstract: This paper presents a experimental study of the strength enhancement under impact loading of metallic cellular materials as well as sandwich panels with cellular core. A testing method using 60mm diameter Nylon Hopkinson pressure bars is used to investigate the rate sensitivity of various metallic cellular materials as honeycombs, foams. Finally, an inversed perforation test on sandwich panels with an instrumented pressure bar is also presented. Such a new testing setup provides piercing force time history measurement, generally inaccessible. Testing results show a notable enhancement of piercing forces, even though the skin aluminum plates and the foam cores are nearly rate insensitive.