Dynamic response of full-scale sandwich composite structures subject to air-blast loading

Glass-fibre reinforced polymer (GFRP) sandwich structures (1.6 m × 1.3 m) were subject to 30 kg charges of C4 explosive at stand-off distances 8–14 m. Experiments provide detailed data for sandwich panel response, which are often used in civil and military structures, where air-blast loading represents a serious threat. High-speed photography, with digital image correlation (DIC), was employed to monitor the deformation of these structures during the blasts. Failure mechanisms were revealed in the DIC data, confirmed in post-test sectioning. The experimental data provides for the development of analytical and computational models. Moreover, it underlines the importance of support boundary conditions with regards to blast mitigation. These findings were analysed further in finite element simulations, where boundary stiffness was, as expected, shown to strongly influence the panel deformation. In-depth parametric studies are ongoing to establish the hierarchy of the various factors that influence the blast response of sandwich composite structures

Uniaxial compression of single crystal and polycrystalline tantalum

A series of compression experiments characterising the elastic-plastic response of single crystal and polycrystalline tantalum from quasi-static to intermediate strain-rates (10^−3 – 10^3 s−1) over a range of temperatures (233–438 K) are reported in this paper. The single crystal experiments show significant differences in the response of the three principle crystal orientations of tantalum in terms of yield, work hardening and ultimate deformed shapes. Modelling is undertaken using a dislocation mechanics based crystal plasticity finite element model giving insight into the underlying microscopic processes that govern the macroscopic response. The simulations show the importance of the dislocation mobility relations and laws governing the evolution of the mobile dislocation density for capturing the correct behaviours. The inclusion of the twinning/anti-twinning asymmetry is found to influence [100] orientation most strongly, and is shown to be critical for matching the relative yield strengths. In general the simulations are able to adequately match experimental trends although some specific details such as exact strain hardening evolution are not reproduced suggesting a more complex hardening model is required. 3D finite element simulations approximating the tests are also undertaken and are able to predict the final deformed sample shapes well once the twinning/anti-twinning asymmetry is included (particularly for the [100] orientation). The polycrystalline data in both as-received and cold rolled conditions shows the initial yield strength is highest and work hardening rate is lowest for the cold-rolled material due to the increase in mobile dislocation density caused by the prior work. The general behavioural trends with temperature and strain-rate of the polycrystalline materials are reproduced in the single crystal data however the specific form of stress versus strain curves are significantly different. This is discussed in terms of the similar active slip systems in polycrystalline material to high symmetry single crystals but with the significant added effect of grain boundary interactions

Innervation of gonadotropin-releasing hormone neurons by peptidergic neurons conveying circadian or energy balance information in the mouse

Background: Secretion of gonadotropin-releasing hormone (GnRH) produced in neurons in the basal forebrain is the primary regulator of reproductive maturation and function in mammals. Peptidergic signals relating to circadian timing and energy balance are an important influence on the reproductive axis. The aim of this study was to investigate the innervation of GnRH neurons by peptidergic neurons. Methodology/Principal Findings: Immunohistochemistry and confocal microscopy were used to detect appositions of peptidergic fibers (NPY, β-endorphin, MCH) associated with energy balance and metabolic status in transgenic mice expressing a green fluorescent protein reporter construct in GnRH neurons. The frequency of these appositions was compared to those of vasoactive intestinal peptide (VIP), a hypothalamic neuropeptide likely to convey circadian timing information to the GnRH secretory system. The majority of GnRH neurons (73-87%) were closely apposed by fibers expressing NPY, β-endorphin, or MCH, and a significant proportion of GnRH neurons (28%) also had close contacts with VIP-ir fibers. Conclusions/Significance: It is concluded that GnRH neurons in the mouse receive a high frequency of direct modulatory inputs from multiple hypothalamic peptide systems known to be important in conveying circadian information and signalling energy balance. © 2009 Ward et al

Reaction forces of laminated glass windows subject to blast loads

Several blast trials on laminated glass windows have been performed in the past, using both full field 3D Digital Image Correlation and strain gauges located on the supporting structure to collect information on the glass pane behaviour. The data obtained during three blast experiments were employed to calculate reaction forces throughout the perimeter supports both before and after the fracture of the glass layers. The pre-crack experimental data were combined with finite element modelling results to achieve this, whilst solely experimental results were employed for post-cracked reactions. The results for the three blast experiments were compared to identify similarities in their behaviour. It is intended that the results can be used to improve the existing spring–mass systems used for the design of blast resistant windows

Recent African derivation of Chrysomya putoria from C. chloropyga and mitochondrial DNA paraphyly of cytochrome oxidase subunit one in blowflies of forensic importance

Chrysomya chloropyga (Wiedemann) and C. putoria (Wiedemann) (Diptera: Calliphoridae) are closely related Afrotropical blowflies that breed in carrion and latrines, reaching high density in association with humans and spreading to other continents. In some cases of human death, Chyrsomya specimens provide forensic clues. Because the immature stages of such flies are often difficult to identify taxonomically, it is useful to develop DNA-based tests for specimen identification. Therefore we attempted to distinguish between C. chloropyga and C. putoria using mitochondrial DNA (mtDNA) sequence data from a 593-bp region of the gene for cytochrome oxidase subunit one (COI). Twelve specimens from each species yielded a total of five haplotypes, none being unique to C. putoria. Therefore it was not possible to distinguish between the two species using this locus. Maximum parsimony analysis indicated paraphyletic C. chloropyga mtDNA with C. putoria nested therein. Based on these and previously published data, we infer that C. putoria diverged very recently from C. chloropyga

Experimental methods for obtaining fracture data from small specimens

Cette publication examine d'abord les problèmes liés à l'utilisation des méthodes d'impact pour obtenir la rupture rapide des polymères. Des jauges placées sur l'échantillon sont utilisées pour obtenir la distribution des contraintes de types transistoire et autres en différents points de l'échantillon. Une nouvelle méthode qui permet d'éviter les efforts transitoires et d'atteindre des vitesses de propagation des fissures dans les petits échantillons de polymères de plus de 300 m s-1 est ensuite présentée. La change nécessaire à juste assurer la propagation d'une fissure peut-être mesurée par cette méthode et la vitesse limite des fissures déterminée. Un aspect important de cette nouvelle méthode est que la tension dans le matériau peut être relaxée graduellement avant d'amorcer la rupture pour la simulation de ces conditions de travail.This paper first examines the problems of using impact methods to achieve fast fracture of polymers. On-specimen gauges are employed to obtain the distribution of transient and other stresses in different parts of the specimen. A new method is then introduced that avoids transient forces and achieves crack velocities in small specimens of tough polymers of over 300 m s-1. It provides for the measurement of load to just sustain a running crack and for determining the limiting crack velocity. An important point about the new method is that the material can be allowed to stress relax to various degrees before launching a crack to simulate the different working conditions

RELATED EXPERIMENTAL AND MODELLING METHODS OF ANALYSING CRACK INITIATION AND PROPAGATION CONDITIONS IN POLYMER MATERIALS

Une méthode d'essai évitant le recours à un dispositif par impact a été conçue pour amorcer et faire propager une fissure dans un polymère. Elle est basée sur les bonnes propriétés d'isolation thermique des polymères et des matériaux équivalents, et sur leur tendance à se fragiliser lorsqu'ils sont refroidis à basse température. L'amorçage d'une fissure dans une partie fragilisée d'une éprouvette est un processus simple, et il est facile d'en assurer la propagation au sein même de la partie active de l'éprouvette qui peut avoir plusieurs géométries et présenter aussi des caractéristiques variées telles que la présence de joints adhésifs par exemple. De la même façon on peut faire varier la contrainte de chargement de l'échantillon, selon les conditions réelles que l'on cherche à reproduire. Avec les dispositifs à choc qui s'appliquent à des éprouvettes de flexion trois points par exemple, ou encore avec d'autres conditions d'essais, il reste le problème de la détermination de la part prise par les forces transitoires lors du chargement dans l'initiation et la propagation de la fissure. Même si un appareil à choc débouche sur un essai très utile et réaliste, il peut être délicat de déterminer avec précision le point de rupture, ou d'autres spécificités auxquelles la méthode que nous proposons s'applique. Dans beaucoup de cas, l'essai par choc et la méthode proposé se complètent bien.A test method has been devised that initiates and propagates a crack in polymer materials that avoids the need of an impact device. This is to take advantage of the good thermal insulation properties of polymers and similar materials and their tendency to be embrittled when cooled to low temperatures. Crack initiation in an embrittled part of the specimen is a simple process and it can be arranged for the crack to then propagate into the main part of the specimen which can have a variety of different shapes and include bonded joints and other features. Also, the stress loading for the main part of the specimen can be varied to relate to the different conditions in which a material may be employed. With impact testers, that also apply dynamically a three-point bend or other test condition then a problem can be in determining how much the crack initiation and propagation is due to the impact transient forces relative to the three-point bending or other loading of the specimen. Although impact testing is a very useful and realistic test, it can be difficult to determine precisely the threshold of crack initiation and propagation and other key factors that the proposed test method achieves. In many ways, impact testing and the proposed method complement each other

Impact performance of composite sandwich structure under high velocity impact

Composite sandwich panels are well known for their relatively high stiffness over weight ratio and have been increasing utilized in various applications where the weight of the structure is a key design concern, e.g. in aircraft and aerospace components. However, these structures are vulnerable when subjected to a transverse impact loading. In this paper, the impact performance of composite sandwich structures with foam core is investigated. In particular, the idea of multi-layering the core by foam layers of different density and its effect on the energy absorption under low and high velocity impact is of interest. In this study, composite sandwich panels made of Glass Fibre Reinforced Polymers (GFRP) for skins and PVC foam for the core are used. Two different arrangements of foam core are considered: uniform core (80/80/80 kg/m3 ) and graded core (100/60/100 kg/m3 ). Both of these core arrangements have the same areal density. Low (up to 5 ms-1 ) and high (up to 200ms-1 ) velocity impact tests were performed using a drop tower and a gas gun respectively. In-plane and out-plane properties of composite sandwich samples were measured employing 2-D and 3-D Digital Imaging Correlation (DIC) methods. The results indicate that the composite sandwich structure with graded foam core has a better energy absorption capability compare to the one with uniform foam core