VISCOELASTIC BEHAVIOR OF THE NON-HOOKE DEFORMATION BEFORE YIELDING IN UNIAXIAL DRAWING OF AMORPHOUS POLY(ETHYLENE TEREPHTHALATE)*

Viscoelastic behavior of the non-Hooke deformation of amorphous PET film before yield was investigated in thetemperature region 74--80.5℃ around the glass transition temperature. The film specimen was drawn to yield point followedby unloading to zero stress, then the residual deformation was held constant, while the subsequent evolution of the stress wasrecorded. An induction period was found in the course of stress evolution fol1owed by a stress step-increase. The inductionperiod decreases with increasing drawing temperature with an activation energy of 1.10 MJ/mol·K, which is attributed tothe time needed for the relaxation of rubbery deformation through cooperative internal rotations. At temperatures lower than74℃, there is no stress increase or the induction period becomes too long to be observed. Thus the nature of anelasticity inthe non-Hooke region before yielding is attributed to stress induced rubbery deformation. The experimental results areinterpreted in terms of Perez' rheological model of a series connected Hooke spring and a Voigt element consisting of aparallel connected elastic spring and a dashpot.     

Dynamic electrochemical impedance spectroscopy measurements of passive layer cracking under static tensile stresses

Electrochemical impedance spectroscopy allows the examination of corrosion susceptibility and resistance for different construction materials, in particular the determination of the properties of their passive films. This technique makes possible the analysis of electrochemical processes in time domain, including rapid phenomena such as changes in the properties of passive films, but it has never been used for passive layer cracking examination. In many cases, fracture of the passive film under tensile stresses leads to stress corrosion cracking. Therefore, investigations of passive layer cracking on austenitic stainless steels under tensile stresses facilitate the understanding of the mechanism of stress corrosion cracking in these common engineering materials. The effect of static tensile stresses on the passive film cracking behaviour of type 304L stainless steel immersed in 0.5 M NaCl solution at room temperature has been investigated. This paper presents the impedance spectra obtained for 304L stainless steel samples at different potential values.Contribution to the 3rd Baltic Conference on Electrochemistry, Gdansk-Sobieszewo, Poland, 23–26 April 2003Dedicated to the memory of Harry B. Mark, Jr. (28 February 1934–3 March 2003)     

Detection of stress whitening in plastics with the help of X-ray dark field imaging

The processing of thermoplastics can induce a wide range of defects such as stress whitening, cavitation and porosity, which can adversely affect the reliability of the final products. Hence, fast and effective non-destructive detection methods for such defects are highly important for quality assurance on production lines. In this paper, X-ray dark field imaging is presented as a new non-destructive testing method that allows the visualization of stress whitening or cavitation efficiently. The performance of the method is demonstrated for the case of an injection-moulded polyvinylidene fluoride part that exhibits stress whitening. Whereas the stress whitening could not be detected by conventional X-ray imaging, it was localized by an X-ray dark field image acquired within a few minutes. Once the precise location of the stress whitening was known, it was possible to verify the result by local micro X-ray computed tomography and by a micro section image.     

A rigid inclusion in an elastic space under the action of a uniform heat flow in the inclusion plane

A solution is presented for the three dimensional static thermoelastic problem of an absolutely rigid inclusion (anticrack) in the case when a uniform heat flow is directed along the inclusion plane. By using the potential method and the Fourier transform technique, the problem is reduced to a system of coupled two-dimensional singular integral equations for the shear stress jumps across the inclusion. As an illustration, a typical application to the circular anticrack is presented. Explicit expressions for the thermal stresses in the inclusion plane are obtained and discussed from the point of view of material failure.     

A comparative first-principles study on electronic structures and mechanical properties of ternary intermetallic compounds Al8Cr4Y and Al8Cu4Y: Pressure and tension effects

An investigation into the bulk properties, elastic properties and Debye temperature under pressure, and deformation mode under tension of Al₈Cu₄Y and Al₈Cr₄Y compounds was investigated by using first principles calculations based on density functional theory. The calculated lattice constants for the ternary compounds (Al₈Cu₄Y and Al₈Cr₄Y) are in good agreement with the experimental data. It can be seen from interatomic distances that the bonding between Al1 atom and Cr, Y, and Al2 atoms in Al₈Cr₄Y are stronger than Al₈Cu₄Y. The results of cohesive energy show that Al₈Cr₄Y should be easier to be formed and much stronger chemical bonds than Al₈Cu₄Y. The bulk modulus B, shear modulus G, Young's modulus E and Poisson's ratio ν can be obtained by using the Voigt–Reuss–Hill averaging scheme. From the results of elastic properties, Al₈Cr₄Y has the stronger mechanical behavior than Al₈Cu₄Y. Our calculations also show that pressure has a greater effect on mechanical behavior for both compounds. The ideal tensile strength are obtained by stress-strain relationships under [001](001) uniaxial tensile deformation, which are 15.4 and 23.4 GPa for Al₈Cu₄Y and Al₈Cr₄Y, respectively. The total and partial density of states and electron charge density under uniaxial tensile deformations for Al₈Cu₄Y and Al₈Cr₄Y compounds are also calculated and discussed in this work.     

Surface effects in the deformation of an anisotropic elastic material with nano-sized elliptical hole

We examine the effect of surface energy on an anisotropic elastic material weakened by an elliptical hole. A closed-form, full-field solution is derived using the standard Stroh formalism. In particular, explicit expressions for the hoop stress, normal, in-plane tangential and out-of-plane displacement components along the edge of the hole are obtained. These expressions clearly demonstrate the effect of elastic anisotropy of the bulk material on the corresponding field variables. When the material becomes isotropic, the hoop stress agrees with the well-known result in the literature while both the in-plane tangential and out-of-plane displacements vanish and the normal displacement is constant along the entire boundary of the elliptical hole.     

Electromigration reliability of open TSV structures

Through silicon vias are the components in three-dimensional integrated circuits, which are responsible for the vertical connection inside the dies. In this work we present studies about the reliability of open through silicon vias against electromigration. A two-step approach is followed. In the first step the stress development of a void free structure is analysed by means of simulation to find the locations, where voids due to stress are most probably nucleated. In the second step, voids are placed in the through silicon vias and their evolution is traced including the increase of resistance. The resistance raises more than linearly in time and shows an abrupt open circuit failure. These results are in good agreement with results of time accelerated electromigration tests.     

Determination of Ti-6242 α and β slip properties using micro-pillar test and computational crystal plasticity

The properties and behaviour of an α−β colony Ti-6242 alloy have been investigated at 20 °C utilising coupled micro-pillar stress relaxation tests and computational crystal plasticity. The β-phase slip strength and intrinsic slip system strain rate sensitivity have been determined, and the β-phase shown to have stronger rate sensitivity than that for the α phase. Close agreement of experimental observations and crystal plasticity predictions of micro-pillar elastic-plastic response, stress relaxation, slip activation in both α and β-phases, and strain localisation within the α−β pillars with differing test strain rate, β morphology, and crystal orientations is achieved, supporting the validity of the properties extracted. The β-lath thickness is found to affect slip transfer across the α−β−α colony, but not to significantly change the nature of the slip localisation when compared to pure α-phase pillars with the same crystallographic orientation. These results are considered in relation to rate-dependent deformation, such as dwell fatigue, in complex multiphase titanium alloys.     

Spray pyrolysis deposition of undoped SnO2 and In2O3 films and their structural properties

In this paper the results of structural analysis of the SnO₂ and In₂O₃ films deposited by spray pyrolysis are presented. The main goals of this analysis are summarizing the results obtained in this field, highlighting a correlation between parameters of film deposition and the material structure and formulating some general regularities, typical for metal oxides. Peculiarities and mechanisms of pyrosol deposition as well as advantages and disadvantages of this technology for deposition of the films with required parameters were also discussed. It is shown that this technology has great potential for controlling structural parameters of metal oxides such as thickness, the grain size, texturing, roughness, the grain faceting and the porosity.     

Optimal intervention under stress scenarios: A case of the Korean financial system

We consider optimal intervention methods under budget constraints when financial systems face economic shocks. We propose two policies formulated by mixed-integer linear programs where regulators inject cash into institutions. One is to minimize systemic losses, and the other is to minimize the number of defaulting institutions. Using publicly available data on the Korean financial system, we construct its entire network and apply stress scenarios to the system to compare the performances of intervention strategies and derive insights on their workings.