The role of polymer engineering in designing for reliability of plastic products

Designing of reliable plastic products is hardly possible without having a profound understanding of the structure-related properties of plastics. Fundamental research into several failure mechanisms [J. J. Horst, “Changes in glassfibre reinforced injection moulded polyamide due to fatigue loading,” in: A. J. Kinloch (editor),Deformation, Yield, and Fracture of Polymers, Institute of Materials, London (1994), p. 694] and [J. L. Spoormaker, “Reliable plastic products through applying failure analysis and basic principles,” in: V. V. Panasyuk and D. M. R. Taplin (editors),Selected Papers of the 8th Internat. Conf. on Fracture (ICF8, Kiev, Ukraine, June 1994), Elsevier (Pergamon), Oxford, UK (1994), pp. 555–560] is essential in obtaining a profound understanding of the failure behavior of plastic products. Failures related to the construction of the mold and processing of plastics are often very important and must never be neglected in designing for reliability of plastic products. The analysis and documentation of failed plastic products are essential for preventing designers from repeating the same errors all the time. Testing of speciments and products should be carried out extremely carefully because of the complex material behavior of plastics. Delft University, Delft, The Netherlands. Published is Fizyko-Khimichna Mekhanika Materialiv, Vol. 32, No. 4, pp. 17–22, July–August, 1996.     

Modelling hierarchical structure in functional brain networks

In this work, we focus on a complex-network approach for the study of the brain. In particular, we consider functional brain networks, where the vertices represent different anatomical regions and the links their functional connectivity. First, we build these networks using data obtained with functional magnetic resonance imaging. Then, we analyse the main characteristics of these complex networks, including degree distribution, the presence of modules and hierarchical structure. Finally, we present a network model with dynamical nodes and adaptive links. We show that the model allows for the emergence of complex networks with characteristics similar to those observed in functional brain networks.     

Modeling of surface microcracking caused by ultraviolet degradation in high-density polyethylene

The process of surface cracking of high-density polyethylene caused by ultraviolet degradation is modeled by using the percolation method. We describe the elements of the micromechanism of cracking of polymers subjected to ultraviolet degradation and the procedure of percolation modeling. It is shown that the geometric characteristics of the generated Voronoi decompositions and simulated clusters do not affect the dependence of the characteristic size of clusters on their capacity. At the same time, the rate of surface cracking is characterized by the presence of a strong size effect. Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv; Faculty of Design, Engineering, and Production, Delft University of Technology, the Netherlands. Published in Fizyko-Khimichna Mekhanika Materialiv, Vol. 35, No. 4, pp. 73–78, July–August, 1999.     

Mechanisms of fatigue in short glass fiber reinforced polyamide 6

An adaptation to existing failure models for fatigue fracture of short fiber reinforced thermoplastics is presented, based on results using some new experimental methods. These results lead to the following conclusion: Cracks in polyamide remain bridged (by plastically drawn matrix material and/or fibers) until just before final fracture. Important is the conditioning of the polyamide: conditioned to equilibrium water content, this mechanism occurs, but not when it is dry as molded. Fatigue damage measurements were done on thin foils cut from the fatigued specimen. When tensile tested, these foils show a change in both strength and fracture strain after fatigue. Further observations during the experiments and SEM fractography strengthen the conviction that fatigue damage initiates and grows in the form of bridged cracks. A correlation between tensile strength and fatigue strength was found; the degree of fiber alignment has a similar effect on both tensile and fatigue properties.     

Fatigue fracture mechanisms and fractography of short-glassfibre-reinforced polyamide 6

An adaptation to existing failure models for fatigue fracture of short-fibre-reinforced thermoplastics is presented. This was based on results using some new experimental methods. These results led to the conclusion that cracks in glassfibre-reinforced polyamide 6 (conditioned to equilibrium water content) remain bridged by plastically drawn matrix material and/or fibres until just prior to final fracture. In this article, emphasis will be on the fractographic evidence for the existence of this failure mechanism. Also some other phenomena in glassfibre-reinforced polyamide will be mentioned. Apart from the normal fractographic investigations, specimens were cryogenically fractured after fatigue, revealing the structure of damage, before failure. Both fracture surfaces were compared, showing that only a small fraction of the fibres is broken in fatigue; mostly the fibres are pulled out. The mechanism consists of the following steps: damage begins with void formation, mainly at fibre ends; these voids coalesce into small cracks. These cracks, however, do not grow into one full crack, but the crack walls remain connected at several points. This is contrary to the fracture mechanism for the dry as-moulded material. When the material is dry as moulded, the matrix material cracks, without showing much ductility, and no bridges are formed. This revised version was published online in November 2006 with corrections to the Cover Date.     

The role of failure analysis in establishing design rules for reliable plastic products

Plastic products can fail in a large variety of modes because of errors in the design, processing and application. The designer must take into account these errors in order to prevent these errors rather than notice them.Without having knowledge of the complex mechanical behaviour of plastics and the structure related properties, adequate design of reliable products is impossible. The designer must also be aware of the specific failure modes, failure causes and failure mechanisms and he can achieve this by failure analysis. Important information from analyses of failed products is the handling of users in ways not foreseen by him. A classification of failed plastic products based on failure causes and failure mechanisms is presented and should be used in carrying out Failure Modes and Effects Analysis.     

Mechanical and morphological properties of stabilized and non-stabilized HDPE films versus exposure time

Non UV-stabilized and UV-stabilized (a UV-absorber) HDPE films were photo-oxidized in a Xenotest. The concentration of carbonyl and the UV-stabilizer, the molecular weight, the crystallinity and the density were studied versus exposure time. Furthermore, the mechanical behaviour was studied at different strain rates by tensile testing. The number of chain scissions was found equal to the number of carbonyl groups. The density of the film increased from 950 to 970 kg/m³ and the crystallinity from 70 to 82%. The increase in density is explained by the increase in crystallinity and by oxygen uptake. The rate at which carbonyl groups are formed, was found to accelerate with exposure time. This acceleration finally resulted in profiles, which could be measured by FT-IR and verified by computer simulations. After 14 days of exposure the specimen failed in a brittle manner. An activation volume was calculated and found to decrease. It is thought that a change in molecular mobility is responsible for the lower strain at yield values. The change in molecular mobility can also explain the increase in Young's modulus with the strain rate and exposure time. The Takayanagi and the Halpin/Tsai models were used to describe the increase of modulus of elasticity.     

Increased sleep pressure reduces resting state functional connectivity

Object

In humans, even a single night of partial sleep deprivation (PSD) can have a negative impact on cognition and affective processing, suggesting that sleep pressure represents a basic physiological constraint of brain function. Among the spontaneously fluctuating resting state networks, the default mode network (DMN) and its anticorrelated network (ACN) hold key functions in segregating internally and externally directed awareness. Task fMRI after sleep deprivation has revealed altered activation patterns in both networks. We hypothesized that effects of PSD in these intrinsically coupled networks can be detected by resting state fMRI.

Methods

We obtained 6-minute echoplanar imaging time series (1.5 Tesla) during eyes-closed, wakeful-resting experiments from 16 healthy volunteers after normal sleep and after PSD. We used independent component and cross-correlation analysis to study functional connectivity (fc), focusing on the DMN and ACN.

Results

After PSD, focal reductions of auto-correlation strength were detected in the posterior and anterior midline node of the DMN and in the lateral parietal and insular nodes of the ACN. Cross-correlation analysis confirmed reduced cortico-cortical connectivity within and between the DMN and ACN.

Conclusions

Increased sleep pressure is reflected in reduced fc of main DMN and ACN nodes during rest. Results have implications for understanding perceptual and cognitive changes after sleep deprivation and are relevant to clinical studies on conditions in which increased sleep propensity is present.     

Large-scale functional brain networks in human non-rapid eye movement sleep: insights from combined electroencephalographic/functional magnetic resonance imaging studies

This paper reviews the existing body of knowledge on the neural correlates of spontaneous oscillations, functional connectivity and brain plasticity in human non-rapid eye movement (NREM) sleep. The first section reviews the evidence that specific sleep events as slow waves and spindles are associated with transient increases in regional brain activity. The second section describes the changes in functional connectivity during NREM sleep, with a particular focus on changes within a low-frequency, large-scale functional brain network. The third section will discuss the possibility that spontaneous oscillations and differential functional connectivity are related to brain plasticity and systems consolidation, with a particular focus on motor skill acquisition. Implications for the mode of information processing per sleep stage and future experimental studies are discussed.