Stress concentration factors in two-dimensional composites: effects of material and geometrical parameters

In the study of fracture processes in composite materials, the interactions between broken and intact fibers are of critical importance. Indeed, the redistribution of stress from a failed fiber to its unfailed adjacent neighbors, and the stress concentration induced in these, determine the extent to which a break in one fiber will cause more breaks in neighboring fibers. The overall failure pattern is a direct function of the stress concentration factors (SCFs). In this paper, we propose a new model for the SCFs in two-dimensional unidirectional composites containing broken fibers. A closed-form expression is derived for the SCF profiles as a function of material and geometrical parameters. The model differs significantly from earlier schemes, as the local effect of a fiber break on nearest neighbors is much milder than previously calculated, both as a function of the inter-fiber distance and of the number of adjacent broken fibers. Comparison with experimental results for silicon-carbide/epoxy composites demonstrates the validity of the proposed scheme. Since the overall fracture pattern in fiber composites is a direct function of the SCFs, the model may help shed light on fracture nucleation and growth in composites.     

Jamming in Wireless Networks Under Uncertainty

Jamming is a form of a denial of service attack in which an adversary can degrade the quality of the reception by creating interference. One can study jamming both in the purpose of protecting a wireless network against such attack or, on the contrary, in order to efficiently disrupt the communications of some adversary. In both cases jamming is part of a conflict for which game theory is an appropriate tool. Here we investigate the effect of partially available information in which the user does not even know whether or not the jammer is indeed present. Moreover, the user does not know how jamming efforts are distributed among sub-carriers, nor does it know the fading channels gains. As an object function for the user we consider the SINR. We show that the problem can be formulated as a zero-sum game. We study independent fading channel gains scenario as well as the case of dependent fading channel gains, both in a discrete and a continuous versions. We show that in all the scenarii the jammer equalizes the quality of the best sub-carriers for the user on as low level as its power constraint allows. Meanwhile the user distributes his power among these jamming sub-carriers. We find the equilibrium strategies in closed form and specify the range of sub-carriers where the user can expect the jamming attack.     

Implementing ISO 9000: performance improvement by first or second movers

When an organization implements a new managerial practice how should timing affect its decision? Should it be among the organizations that implement the new management practices early, i.e. first movers, or wait for others to implement and implement it a later time, i.e. second movers. The literature's findings with regard to many management practices, especially those that deal with quality, such as total quality management, suggest that while first movers implement a new management practice because of real needs and a high fit between what the practice suggests and their needs (technical efficiency), second movers implement the new management practice because of customer pressure and the fear of falling behind the competition (external pressure). Second movers just mimic first movers, and the new practice does not really fit with their operations. Thus, the new management practice achieves more for the first movers than the second movers. In this paper we ask whether this premise holds for the ISO 9000 quality standard, one which was specified in considerable detail by outside forces but was implemented in many different ways by organizations. Our results are based on a survey of 1150 quality managers who implemented ISO 9000. We find that learning is a more important factor than timing in explaining ISO 9000 performance. First movers achieve a high level of performance because they learn from their own experience, and second movers achieve a high level of performance because they learn from the experience of others. Whether an organization is a first mover or second, the ones that benefit from ISO 9000 are those who learn.     

Cure kinetics of bismaleimides as basis for polyimide-like inks for PolyJet™-3D-printing

Since polyimides are well known for their excellent chemical and thermal stability and outstanding mechanical properties there is increasing interest in developing polyimide-based inks to produce additively manufactured parts with properties superior to those of currently available materials. Usage of bismaleimides (BMI) as precursors allows polyimides to be fabricated via PolyJet™ printing (Stratasys Ltd., Rehovot, Israel). Characterization of the curing kinetics is a central part of process development, as fast curing initiated by UV light is desired. Here, a comprehensive study of thermal and UV curing of BMI oligomers with various molecular weights and chemical structures is presented. Fourier transform infrared spectroscopy serves as a tool for determining the curing degree. Furthermore, an estimation of the activation energy for thermal curing is performed. UV curing of the selected BMIs leads to highly crosslinked, thermoset polymers with excellent chemical resistance and thermal stability which are of great interest for PolyJet™ 3D printing. © 2018 The Authors. Journal of Applied Polymer Science published by Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47244.     

Investigation of channel hot electron injection by localized charge-trapping nonvolatile memory devices

A novel measurement method to extract the spatial distribution of channel hot electron injection is described. The method is based on characterization of localized trapped-charge in the nitride read-only memory (NROM) device. The charge distribution is determined by iteratively fitting simulated subthreshold and gate induced drain leakage (GIDL) currents to measurements. It is shown that the subthreshold and the GIDL measurements are sensitive to charge trapped over the n+ junction edge. Their characteristics are determined by the trapped charge width, density and location and the associated fringing field. Extremely high sensitivity of the GIDL measurement to localized charge over the n+ junction is demonstrated. The extracted charge distribution width is shown to be /spl sim/40 nm, located over the junction edge.     

Shock capturing by the spectral viscosity method

A main disadvantage of using spectral methods for nonlinear conservation laws lies in the formation of Gibbs phenomenon, once spontaneous shock discontinuities appear in the solution. The global nature of spectral methods then pollutes the unstable Gibbs oscillations over all the computational domain, and the lack of entropy dissipation prevents convergences in these cases. In this paper, we discuss the spectral viscosity method, which is based on high frequency-dependent vanishing viscosity regularization of the classical spectral methods. We show that this method enforces the convergence of nonlinear spectral approximations without sacrificing their overall spectral accuracy.     

A bipolar load CMOS SRAM cell for embedded applications

This paper presents a new SRAM cell concept which offers cell scaling without requiring complicated, specialized processing technology. The proposed cell utilizes a bipolar transistor in an open-base (base is floating) configuration as a simple means of realizing a high impedance load element. The Bipolar Transistor Load (BTL) is designed such that its open base current (the holding current) is always large enough to compensate for the NMOS pull-down transistor leakage current. The load holding current and the pull-down transistor leakage current are based on the same physical mechanism, namely thermal generation, as a result the load exhibits current tracking properties over varying process and temperature conditions. The cell size is 72 μm² with typical 0.8 μm design rules, which is about a 60% reduction as compared to a standard 6-T full CMOS cell. The operating properties of the BTL cell were studied analytically and characterized experimentally. The BTL SRAM module can be easily integrated as part of any CMOS process with minimal additional processing steps