Improvement of Hardening Stiffness Test as an Indicator of Environmental Stress Cracking Resistance of Polyethylene

Long term mechanical behavior of polyethylene (PE) is of great importance especially in cases where structural integrity is required. In order to predict characteristics of the mechanical behavior of PE, it is necessary to fully understand the molecular structure of the employed resins. In this study, evaluation of several micromolecular properties of PE is conducted. These properties influence an important performance indicator of PE for structural applications, namely, the environmental stress cracking resistance (ESCR). ESCR in PE resins occurs through a slow crack growth mechanism under low applied stresses and long periods of time. This property is usually assessed by unreliable and time consuming testing methods such as the notch constant load test (NCLT) on notched PE specimens in the presence of an aggressive fluid at elevated temperatures. In the work presented herein, relationships between molecular structure and material response characteristics, mainly between molecular weight properties and short chain branching content in relation to strain hardening behavior of PE resins, were investigated based on results from tensile experiments. Inter-lamellar entanglements are believed to be the main feature controlling slow crack growth of PE. Extent of entanglements and entanglement efficiency has been investigated by monitoring the strain hardening behavior of PE resins in solid state through a uniaxial tensile test. The hardening stiffness (HS) test for prediction of ESCR was refined and improved to cover a broader range of PE resins, along with easier sample preparation, and faster testing. The improved test offers a more reliable and consistent ESCR picture without the drawbacks of the subjective notching process and ad-hoc presence of aggressive fluids.     

Efficient sample sizes in stochastic nonlinear programming

We consider a class of stochastic nonlinear programs for which an approximation to a locally optimal solution is specified in terms of a fractional reduction of the initial cost error. We show that such an approximate solution can be found by approximately solving a sequence of sample average approximations. The key issue in this approach is the determination of the required sequence of sample average approximations as well as the number of iterations to be carried out on each sample average approximation in this sequence. We show that one can express this requirement as an idealized optimization problem whose cost function is the computing work required to obtain the required error reduction. The specification of this idealized optimization problem requires the exact knowledge of a few problems and algorithm parameters. Since the exact values of these parameters are not known, we use estimates, which can be updated as the computation progresses. We illustrate our approach using two numerical examples from structural engineering design.     

First-order strong variation algorithms for optimal control

Journal of Optimization Theory and Applications - This paper presents two demonstrably convergent, first-order, differential dynamic programming algorithms for the solution of optimal control...     

Low-energy Auger and autoionization emissions of clean and O2-exposed FeTi and Fe2Ti4O

Three distinct features in the low-energy Auger spectrum of the intermetallics FeTi and Fe₂Ti₄O, namely the Ti(M₂₃VV) transition, a transition involving titanium-oxygen bonding electrons, Ti(M₂₃V)TiO(V), and gain satellites recently identified as being due to autoionization emission, were found to furnish detailed information concerning chemical bonding at the surface of clean and oxygen exposed samples. Marked changes in these spectral features upon O₂ exposure indicate selective interaction of oxygen with the Ti atoms from the early stages of adsorption. The spectral changes at low oxygen exposure are discussed in terms of the population of interstitial oxygen sites to a critical local concentration, and the subsequent nucleation and growth of a stoichiometric surface oxide. At ～ 40 L O₂ exposure apparent saturation is reached with a TiO₂ overlayer formed on both of the compounds at 300 K. Subtle differences between the autoionization emission feature in pure and in alloyed Fe are noted and discussed.     

Energy transfer in collisions of highly vibrationally excited tetrahedral molecules

Model trajectory calculations of the energy transfer processes in collisions of Ar with highly vibrationally excited CH₄, CD₄, SiH₄ and CF₄ are performed. Special attention is payed to the calculation of the energy transferred to active (vibrational) degrees of freedom. The results support the diffusion model of excitation-dissociation and give the low pressure collision efficiency β_c which qualitatively agrees with experiment in magnitude and temperature dependence.     

Pressurized planar electrochromatography

Theoretical backgrounds, development, examples of separations, constructional details and principle of action of devices of pressurized planar electrochromatography (PPEC) are presented. Development of the mode is described in respect of operating variables (composition of the mobile phase, pressure exerted on adsorbent layer, mobile phase flow velocity, temperature of separating system, etc.) influencing separation efficiency (kinetic performance, repeatability, separation time). Advantages of PPEC such as high kinetic performance, short separation time and different separation selectivities, especially relative to conventional thin-layer chromatography, are described. Examples of two-dimensional separations are demonstrated to show high separation potential of the mode when combined with conventional thin-layer chromatography (TLC). The PPEC mode is in infancy stage of development, so its challenges are presented as well.     

Pd☎ Ions In NaCl Single Crystal: EPR and Optical Studies

EPR and optical absorption of Pd centres in as grown NaCl single crystal arc reported. The EPR spectra show that palladium forms the Pd^? centre with d_x2-y2 wave function in the ground state. This centre formation and the accompanying distortion give rise to a superhyperfine interaction of the Pd^? ion with the nuclei of the four Cl^? ligands in the plane perpendicular to the C₄ symmetry axis. The results of the fitting of axial spin Hamiltonian parameters to the experimentally observed spectra give the following values at 12K: g∥ =2.85, g ⊥ ? 2.15, A ∥29.8 × 10^?4cm^?1, A ⊥ = 5.9 7imes; 10^?4cm^?1. When the temperature increases, lines are getting broader, their amplitude decreases and a single line with g-value of g= 2.38 at g = 1/3(g∥ ?2g⊥) appears, which demonstrates the known Jahn-Teller behaviour of d⁹ ions EPR spectra.     

Finite-termination schemes for solving semi-infinite satisfycing problems

The problem of finding a parameter which satisfies a set of specifications in inequality form is sometimes referred to as the satisfycing problem. We present a family of methods for solving, in a finite number of iterations, satisfycing problems stated in the form of semi-infinite inequalities. These methods range from adaptive uniform discretization methods to outer approximation methods.The research reported herein was sponsored in part by the National Science Foundation Grant ECS-8713334, the Air Force Office of Scientific Research Contract AFOSR-86-0116, and the State of California MICRO Program Grant 532410-19900.