Dynamic crushing of a one-dimensional chain of type II structures

The development of an analytical model based on a ‘bar–hinge’ idealisation is described. It is used to investigate the structural response of a one-dimensional chain of components each constructed from a pair of slightly bent elastic–plastic struts under axial impact loading. Each component (a typical type II structure) in the chain is modelled as four axially compressible, elastic–plastic, straight bars of infinite bending rigidity connected to each other by elastic–plastic hinges of finite length. A new approach to formulate the constitutive relation between the generalised force and displacement of the bars and hinges is developed. A self-contact algorithm is used for intra-component contact simulation. The ‘bar–hinge’ model is validated using the results from finite-element simulations using ABAQUS. Good agreement is achieved. The analytical model was then used to investigate the crushing features of the chain structure. The effects of the number of the components in the chain and the crookedness angle of the components on the crushing behaviour were studied. It was found that collapse of the components in the chain occurs at the proximal end of the chain first and that the component at the fixed distal end will also collapse at some later time. The single component structure is more ‘inertia sensitive’ than a chain structure of more than two components.     

Application of the substructured finite element/extended finite element method (S-FE/XFE) to the analysis of cracks in aircraft thin walled structures

The substructured finite element/extended finite element (S-FE/XFE) approach is used to compute stress intensity factors in large aircraft thin walled structures containing cracks. The structure is decomposed into a ‘safe’ domain modeled with classical shell elements and a ‘cracked’ domain modeled using three-dimensional extended finite elements. Two applications are presented and discussed, supported by validation test cases. First a section of stiffened panel containing a through-thickness crack is investigated. Second, small surface cracks are simulated in the case of a generic ‘pressure membrane’ with realistic crack configurations. These two semi-industrial benchmarks demonstrate the accuracy, robustness and computational efficiency of the substructured finite element/extended finite element approach to address complex three-dimensional crack problems within thin walled structures.     

Ballistic impact studies of a borosilicate glass

The ballistic impact properties of a borosilicate (‘pyrex’) glass was studied using mild steel rods accelerated using a light gas gun. High-speed photography at sub-microsecond framing rates was used along with schlieren optics to investigate the propagation of elastic shock waves and fracture fronts. Flash X-radiography was used to visualise the deformation of rods as they penetrated the comminuted glass normally. The rod was seen initially to dwell on the surface for at least 3 μs creating a Hertzian cone-crack. Later on, between 40 and 60 μs, self-sharpening of the projectile was observed as the ‘wings’ of the heavily deformed front end sheared off. After this event, the front of the rod speeded up. X-rays also showed that the pattern of fissures within the comminuted glass was observed to be very similar shot-to-shot. X-radiography was also used to examine the mechanisms occurring during oblique impact of rods at 45°. In oblique impact, bending of the rod rather than plastic deformation (‘mushrooming’) takes on the role of distributing the load over an area larger than that of the original rod diameter. High-speed photography of the rear surface of a glass block on which a fine grid had been placed confirmed that the comminuted glass moved as larger interlocked blocks. The experiments were modelled using the QinetiQ Eulerian hydrocode GRIM making use of the Goldthorpe fracture model. The model was found to predict well the transition from dwell to penetration.     

Upgradation of gravity load designed sub-assemblages subjected to seismic type loading

In the present study, a methodology based on strength hierarchy has been proposed for upgradation of original gravity load designed (GLD) reinforced concrete (RC) structures. Exterior beam–column joint of an RC structure has been considered as the target sub-assemblage and the target strength of the deficient sub-assemblage was decided from that of a seismically designed ‘Ductile’ one. Three different types of upgradation schemes were investigated where shear- and flexural-strengthening were provided by Fiber Reinforced Plastics (FRP) and weak joint region of ‘GLD’ sub-assemblage was upgraded by steel plate jacketing. The original (‘GLD’-, ‘NonDuctile’-, ‘Ductile’-) and upgraded-sub-assemblages were investigated under repeated reverse cyclic loading. It was observed that the ‘GLD’ specimen seized to function under reverse loading and subsequent improvements, though not optimal, were observed from ‘NonDuctile’ and ‘Ductile’ specimens. It was further found out that the upgraded specimens showed considerable improvement in strength deterioration, stiffness degradation and energy dissipation. Further, the upgraded specimens with adequate energy dissipation could even be able to shift the plastic hinge from the joint face into the beam which was not observed even in original ‘Ductile’ specimen. The upgraded schemes are simple, practically feasible and efficient as well.     

Effect of loading rates on pullout behavior of high strength steel fibers embedded in ultra-high performance concrete

In this paper single fiber pull-out performance of high strength steel fibers embedded in ultra-high performance concrete (UHPC) is investigated. The research emphasis is placed on the experimental performance at various pullout rates to better understand the dynamic tensile behavior of ultra-high performance fiber reinforced concrete (UHP-FRC). Based on the knowledge that crack formation is strain rate sensitive, it is hypothesized that the formation of micro-splitting cracks and the damage of cement-based matrix in the fiber tunnel are mainly attributing to the rate sensitivity. Hereby, different pull-out mechanisms of straight and mechanically bonded fibers will be examined more closely. The experimental investigation considers four types of high strength steel fibers as follows: straight smooth brass-coated with a diameter of 0.2 mm and 0.38 mm, half end hooked with a diameter of 0.38 mm and twisted fibers with an equivalent diameter of 0.3 mm. Four different pull out loading rates were applied ranging from 0.025 mm/s to 25 mm/s. The loading rate effects on maximum fiber tensile stress, use of material, pullout energy, equivalent bond strength, and average bond strength are characterized and analyzed. The test results indicate that half-hooked fibers exhibit highest loading rate sensitivity of all fibers used in this research, which might be attributed to potential matrix split cracking. Furthermore, the effect of fiber embedment angles on the loading rate sensitivity of fiber pullout behavior is investigated. Three fiber embedment angles, 0°, 20°, and 45°, are considered. The results reveal that there is a correlation between fiber embedment angle and loading rate sensitivity of fiber pullout behavior.     

Prediction of fiber orientation structure for injection molded short fiber composites

Structure of fiber orientation in injection molded short fiber composites is predicted by the numerical analysis. To analyze the packing stage as well as the filling stage, a compressible generalized Hele-Shaw model is adopted. A numerical scheme free from coordinate transformation is developed for three-dimensional shell-like geometry. Flow-induced fiber orientation can be predicted by solving evolution equations for the orientation tensor with a suitable closure approximation. Fibers are mainly oriented toward the flow direction near the top cavity wall due to high shear rates, while they are randomly oriented near the centerline of cavity where low shear rates prevail. Thus, the molded parts show the skin-core structure of orientation. Structure of fiber orientation continues to change during the packing stage due to additional velocity gradients – which are likely to align fibers more towards the flow direction. Electronic Publication     

Characterisation of a turbo-molecular pumps by a minimum of parameters

The design of complex vacuum systems including turbo-molecular pumps requires knowledge of the characteristics of the turbo-molecular pumps. Normally, such characteristics for commercially available turbo-molecular pumps are presented as graphs of pumping speed, compression ratio and so on as a function of inlet or outlet pressure. It is difficult to incorporate such information into a model for designing complex vacuum systems, especially when optimising the number of pumps, their pumping speed and choice of backing pump.Voss [Characteristics of the turbomolecular pumps. Vakuum in Forschung ung Praxis 2002;14(4)] has published fitting formulae for specific pumps as a way of describing pressure, pumping speed and compression at the inlet, by means of a number of parameters which are described in a table for each gas and throughput. These fitting formulae can be used in modelling, but the fitting parameters for an arbitrary pump are not in general available.A new approach is proposed based on ‘true zero-throughput’, which has been defined as the ratio of the probability of a gas molecule travelling from the pump inlet to the outlet to that of its travelling from the outlet to the inlet. This is different from the usually reported ‘zero-throughput’ data which are measured with zero gas injection, but not ‘zero gas load’ at the inlet (due to thermal outgassing of a measuring dome and the pump itself). Parameters in the formulae developed here are no longer simply fitting parameters, but have clear physical meaning. The dependence of the parameters for different gases may be included as a function of mass. The new formulae presented, allow us to model the performance of cascaded turbo-molecular pumps. As an example, the formalism is applied to the differential pumping stages of the KATRIN experiment [KATRIN Collaboration. KATRIN design report 2004. Forschungszentrum Karlsruhe Scientific Report # FZKA 7090, 2005].     

Utilization of lignocellulosic natural fiber (jute) components during a microbial polymer production

Lignocellulosic fiber (jute), a low cost natural complex carbon source, was introduced in a fermentation medium to observe the effects of its constituents on the production of a commercially potent bacterial extracellular polysaccharide (EPS) synthesized by Bacillus megaterium RB-05. It has been found that among all the fiber components the bacterium has utilized cellulose most for the EPS production. Maximum polymer yield of 0.297 g g^− 1 substrate was found after 72 h fermentation. Consumption of fiber components was typically driven by in situ bacterial enzyme activity as EPS production was found significantly (p < 0.05) accelerated from 36 h onwards with considerable cellulase activity. Utilization of fiber components during different fermentation hours were characterized using scanning electron microscopy, FT-IR spectroscopy, X-ray diffraction, tensile property, and contact angle measurement.     

A theoretical approach to the deposition and clearance of fibers with variable size in the human respiratory tract

In the study presented here, a mathematical approach for the deposition and clearance of rigid and chemically stable fibers in the human respiratory tract (HRT) is described in detail. For the simulation of fiber transport and deposition in lung airways an advanced concept of the aerodynamic diameter is applied to a stochastic lung model with individual particle trajectories computed according to a random walk algorithm. Interception of fibrous material at airway bifurcations is considered by implementation of correction factors obtained from previously published numerical approaches to fiber deposition in short bronchial sequences. Fiber clearance is simulated on the basis of a multicompartment model, within which separate clearance scenarios are assumed for the alveolar, bronchiolar, and bronchial lung region and evacuation of fibrous material commonly takes place via the airway and extrathoracic path to the gastrointestinal tract (GIT) or via the transepithelial path to the lymph nodes and blood vessels.Deposition of fibrous particles in the HRT is controlled by the fiber aspect ratio β in as much as particles with diameters <0.1 μm deposit less effectively with increasing β, while larger particles exhibit a positive correlation between their deposition efficiencies and β. A change from sitting to light-work breathing conditions causes only insignificant modifications of total fiber deposition in the HRT, whereas alveolar and, above all, tubular deposition of fibrous particles with a diameter ≥0.1 μm are affected remarkably. For these particles enhancement of the inhalative flow rate results in an increase of the extrathoracic and bronchial deposition fractions. Concerning the clearance of fibers from the HRT, 24-h retention is noticeably influenced by β and, not less important, by the preferential deposition sites of the simulated particles. The significance of β with respect to particle size may be regarded as similar to that determined for the deposition scenarios, while breathing conditions do not have a valuable effect on clearance.     

Multiple cracking response of plasma treated polyethylene fiber reinforced cementitious composites under flexural loading

The effects of low frequency cold plasma treatments on the microstructure and chemistry of Polyethylene (PE) have been investigated. PE plates and fibers were exposed to plasmas of argon and oxygen gases. The surface wettabilities of plasma-treated plates were monitored. Possible physical changes on fiber surfaces were observed by a scanning electron microscope (SEM) at micrometer scale and by an atomic force microscope (AFM) at nanometer scale after this process. The effects of plasma treatment on surface chemistry of PE fibers have been analyzed by using an X-ray photoemission spectroscope (XPS). The fibers modified by plasma treatments were used in prismatic cementitious composites. The flexural performance of samples were characterized at two different ages (28 days and 8 months). Results showed that plasma treatment caused significant modifications on fibers’ surface structure and composites’ performance. Proper plasma treatment conditions significantly leads to improvement of multiple cracking behavior of fiber reinforced composites.     

Gaussian function characterization of unnotched tension behavior in a carbon/epoxy composite containing localized fiber waviness

This study presents the results of analytical and experimental efforts performed on the tension behavior of a common aerospace grade carbon fiber/epoxy material system containing out-of-plane fiber waviness. The composite material selected for this study is one designated by the Federal Aviation Administration’s Advanced General Aviation Transportation Experiments program for use in aerospace structural applications. Unidirectional and multidirectional laminates were examined with different thicknesses containing three levels of waviness. The waviness morphology considered in this study is a ‘degrading’ type waviness, where a relatively sharp wave occurs on the first ply and is completely diminished at the other laminate face. To determine analytically the tensile stiffness, a Gaussian function is introduced to characterize the waviness observed and is used in a transformation-based classical lamination theory approach. The tension failure behavior is also reported for the waviness specimens tested.     

Assessment of the strain monitoring reliability of fiber Bragg grating sensor (FBGs) in advanced composite structures

Fiberoptic Bragg grating sensors were embedded in composite structures in order to be used as the detection system for structural health monitoring purposes. Firstly, optical fibers had been embedded in several locations in between the layers of composite material and for the typical tensile testing coupon configuration. It has been shown experimentally and with finite element analysis that embedding the optical fiber out of the neutral axis of the coupon, downgrades significantly the composite’s ultimate tensile strength. Secondly, a composite patch with embedded FBG has been used to restore a typical damage case on aluminum structure currently applied on advanced ‘aging’ vehicles. Data values taken from the FBG system during the experimental tension tests of the composite structures were calculated to axial strain values with the aid of an interrogator. Theoretical axial strain values of the composite patch had been calculated by exploiting a newly-developed finite element model. Discussion has been made on the correlation between experimental and theoretical results for the verification of sensing sensitivity and accuracy.     

Effect of fiber orientation on the structural behavior of FRP wrapped concrete cylinders

In this study, 27 concrete cylinders with a diameter of 152.4 and a height of 304.8 mm were prepared. Among them, 18 cylinders were wrapped using two layers of fiber reinforced polymer (FRP) with six fiber orientations; six cylinders were wrapped using four layers of FRP with fibers in axial or hoop direction only; the remaining three cylinders were used as control. The FRP used was E-glass fiber reinforced ultraviolet (UV) curing vinyl ester. Fifteen coupon specimens were prepared to experimentally determine the tensile strength of the FRP with fibers oriented at 0°, 45°, and 90° from the loading direction. Co-axial compression tests were conducted on the wrapped cylinders and control cylinders. The test results were compared with existing confinement models. It is found that the strength, ductility, and failure mode of FRP wrapped concrete cylinders depend on the fiber orientation and wall thickness. Fibers oriented at a certain angle in between the hoop direction and axial direction may result in strength lower than fibers along hoop or axial direction. A larger database is desired in order to refine the existing design-oriented confinement models.     

Monolayers of triaroylbenzene derivatives

Monolayers of triaroylbenzene derivatives bearing three octyl groups projecting away from the molecular core and terminated by hydroxyl, carboxylic acid, and methyl ester groups have been studied using surface pressure isotherm measurements and Brewster angle microscopy. The octyl derivative lacking a terminal hydrophilic group forms a monolayer of limited stability. The derivatives with hydrophilic end-groups on the octyl chains form stable monolayers. A reorientation at the water surface under compression from a ‘face-on’ to a distorted ‘edge-on’ arrangement appears likely. At intermediate molecular areas, a phase transition occurs and aggregate formation is observed. Studies of mixtures with methyl stearate exhibit a contrast inversion between the background phase and immiscible domains of methyl stearate supporting the concept of a reorientation of the triaroylbenzene derivatives during compression.     

Motorcycle and scooter speeds approaching urban intersections

Five urban, uncontrolled T-intersections known to be motorcycle crash ‘black spots’ were monitored using instrumentation and a roadside observer. Two sets of twelve-hour observations were collected for each site (N ≈ 100,000). Instrumentation recorded the ‘events’ of vehicles passing to measure, speed, direction, lane position, vehicle type (broadly characterised) and headway. Observers further recorded times of bicycle events, type of motorcycle (scooters or motorcycles), the behaviour of motorcycles and the use of ‘high conspicuity’ gear such as clothing or helmets. Results establish that motorcycles travel around 10% faster than the other traffic (car mean speed = 34.97 km/h), with motorcycles travelling on average 3.3 km/h faster than cars. Motorcycles were 3.4 times more likely to be exceeding the speed limit than cars. Similar results are described for scooters. Also examined are the influences on mean speeds such as the time of day, the presence of a car at the t-intersection, and the influence of free headway. The results are compared for robustness across locations and days. It is concluded that in urban areas motorcycles are travelling significantly faster than other traffic. These findings are discussed against a concern to reduce motorcycle crashes by improving conspicuity and previous research that implicates a ‘looked-but-failed-to-see’ effect for car drivers.     

A new electrospinning method to control the number and a diameter of uniaxially aligned polymer fibers

A novel electrospinning process of uniaxially aligned submicron fibers was developed. The number of the fibers was precisely controlled by changing biased collector, and the diameter of the fiber was varied by post-deposition stretching process. This method realized the formation of number-controlled aligned poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylenevinylene] (MEH-PPV)/poly(ethylene oxide) (PEO) fibers with the systematic control of the diameter ranging from micrometer to submicrometer. Significant improvement of the uniformity of the fiber diameter was also observed by the stretching process.     

Rotatable magnetron sputtering of aluminium in continuous and high power pulse modes using different strength magnetic arrays

This paper reports preliminary results of industrial size (152 mm target O.D.) rotatable magnetron sputtering of Al target in direct current (DC) and High Power Impulse Magnetron Sputtering (HIPIMS) modes using two standard commercially available magnetic arrays: standard strength array (as used for DC and AC processing) and a lower strength ‘RF’ array [i.e. as used for radio frequency (RF) magnetron sputtering]. A comparison of processes resulted in by combining the different magnetic arrays and power modes is made in terms of magnetic field distribution on the cathode surface, magnetron characteristics, process characteristics and deposition rates.Optical emission spectroscopy (OES) revealed enhanced sputtered Al flux ionisation in the HIPIMS discharge monitored 64 mm away from the target surface when using the ‘RF’ array. Importantly, the results of this work (at the processing conditions investigated) demonstrate that at the same average power the deposition rate of Al using HIPIMS in conjunction with the ‘RF’ array is substantially the same as that obtained for the ‘standard’ strength balanced array and DC power. This indicates that the magnetic field design of the ‘RF’ magnetic array affects favourably the sputtered flux transport perpendicular to the target surface by altering mass transport direction and minimising effects that reduce deposition rate (e.g. ion return effect). Arc rate is also reduced significantly (approximately ten times) if the low strength ‘RF’ array is used.     

Thinking outside the box: The time dependent behaviour of a reinforced embankment on soft soil

Applications of eco-friendly geotextiles are gaining a preference over traditional polymeric geotextiles as measures to reinforce earth embankments. Understanding the behaviour of these eco-friendly geotextiles sometimes known as limited life geotextiles (LLG) is in its infancy. This paper explains the behaviour of an embankment reinforced with Sisal fibre geotextiles constructed within a box. The diminishing need for geotextile is represented by an external load ‘outside the box’ which can be manually controlled depending on the rate of increasing foundation shear strength. The excess pore water pressure was observed ‘outside the box’ from the end of the construction of the embankment to the end of the consolidation by monitoring the height of the water in pipes ‘outside the box’.     

Towards optimization of crack resistance of composite materials by adjustment of fiber shapes

We investigate the evolution and propagation of cracks in 2-d elastic domains, which are subjected to quasi-static loading scenarios. In addition to the classical variational formulation, where the standard potential energy is minimized over the cracked domain under physical conditions characterizing the behavior of the material close to the crack (e.g. non-penetration conditions), we include a ‘cohesive traction term’ in the energy expression. In this way we obtain a mathematically concise set of partial differential equations with non-linear boundary conditions at the crack interfaces. We perform a finite element discretization using a combination of standard continuous finite elements and so-called cohesive elements. During the simulation process cohesive elements are adaptively inserted at positions where a certain stress bound is exceeded. In our numerical studies we consider domains consisting of a matrix material with fiber inclusions. Beyond pure crack path simulation, our ultimate goal is to determine an optimal shape of the fibers resulting in a crack path that releases for a given load scenario as much energy as possible without destroying the specimen completely. We develop a corresponding optimization model and propose a solution algorithm for the same. The article is concluded by numerical results.     

Elastic structural response of anisotropic sandwich plates with a first-order compressible core impacted by a Friedlander-type shock loading

The foundation of the non-linear theory of asymmetric anisotropic sandwich plates with a first order compressible weak orthotropic core under a Friedlander-type explosive blast is presented. The equations of motion are developed by means of Hamilton’s Principle. Within the theory, the face sheets are asymmetric while adopting the Love-Kirchoff model. In addition, the core layer is assumed to be compressible (extensible) in the transverse direction thereby capturing any wrinkling or global instabilities. The theory is then simplified and applied for the case of sandwich plates with symmetric unidirectional fiber reinforced laminated composite facings with the axes of orthotropy not necessarily coincident with the geometrical axes. The governing solution is developed using the Extended-Galerkin method resulting in two coupled non-linear second order ordinary differential equations which are then solved using the 4th-order Runge–Kutta method for a system of differential equations.