Showing papers in "Experimental Mechanics in 2009"

Assessment of digital image correlation measurement errors: methodology and results

Abstract: Optical full-field measurement methods such as Digital Image Correlation (DIC) are increasingly used in the field of experimental mechanics, but they still suffer from a lack of information about their metrological performances. To assess the performance of DIC techniques and give some practical rules for users, a collaborative work has been carried out by the Workgroup "Metrology" of the French CNRS research network 2519 "MCIMS (Mesures de Champs et Identification en Mecanique des Solides / Full-field measurement and identification in solid mechanics, http://www.ifma.fr/lami/gdr2519)". A methodology is proposed to assess the metrological performances of the image processing algorithms that constitute their main component, the knowledge of which being required for a global assessment of the whole measurement system. The study is based on displacement error assessment from synthetic speckle images. Series of synthetic reference and deformed images with random patterns have been generated, assuming a sinusoidal displacement field with various frequencies and amplitudes. Displacements are evaluated by several DIC packages based on various formulations and used in the French community. Evaluated displacements are compared with the exact imposed values and errors are statistically analyzed. Results show general trends rather independent of the implementations but strongly correlated with the assumptions of the underlying algorithms. Various error regimes are identified, for which the dependence of the uncertainty with the parameters of the algorithms, such as subset size, gray level interpolation or shape functions, is discussed.

Characterization of Microvascular-Based Self-healing Coatings

Abstract: A protocol is described to assess self-healing of crack damage in a polymer coating deposited on a substrate containing a microvascular network. The bio-inspired coating/substrate design delivers healing agent to cracks in the coating via a three-dimensional microvascular network embedded in the substrate. Through capillary action, monomer flows from the network channels into the crack plane where it is polymerized by a catalyst embedded in the coating. The healing efficiency of this materials system is assessed by the recovery of coating fracture toughness in a four-point beam bending experiment. Healing results for the microvascular networks are compared to data for a coating containing microencapsulated healing agents. A single crack in a brittle epoxy coating is healed as many as seven times in the microvascular systems, whereas microcapsule-based healing occurs for only one cycle. The ability to heal continuously with the microvascular networks is limited by the availability of catalyst in the coating.

Full Field Strain Measurement in Compression and Tensile Split Hopkinson Bar Experiments

Abstract: The 3D image correlation technique is used for full field measurement of strain (and strain rate) in compression and tensile split Hopkinson bar experiments using commercial image correlation software and two digital high-speed cameras that provide a synchronized stereo view of the specimen. Using an array of 128 × 80 (compression tests) and 258 × 48 (tensile tests) pixels, the cameras record about 110,000 frames per second. A random dot pattern is applied to the surface of the specimens. The image correlation algorithm uses the dot pattern to define a field of overlapping virtual gage boxes, and the 3-D coordinates of the center of each gage box are determined at each frame. The coordinates are then used for calculating the strains throughout the surface of the specimen. The strains determined with the image correlation method are compared with those determined from analyzing the elastic waves in the bars, and with strains measured with strain gages placed on the specimens. The system is used to study the response of OFE C10100 copper. In compression tests, the image correlation shows a nearly uniform deformation which agrees with the average strain that is determined from the waves in the bars and the strains measured with strain gages that are placed directly on the specimen. In tensile tests, the specimen geometry and properties affect the outcome from the experiment. The full field strain measurement provides means for examining the validity and accuracy of the tests. In tests where the deforming section of the specimen is well defined and the deformation is uniform, the strains measured with the image correlation technique agree with the average strain that is determined from the split Hopkinson bar wave records. If significant deformation is taking place outside the gage section, and when necking develops, the strains determined from the waves are not valid, but the image correlation method provides the accurate full field strain history.

A New Procedure to Measure Near Yield Residual Stresses Using the Deep Hole Drilling Technique

Abstract: Mechanical strain relief techniques for estimating the magnitude of residual stress work by measuring strains or displacements when part of the component is machined away. The underlying assumption is that such strain or displacement changes result from elastic unloading. Unfortunately, in components containing high levels of residual stress, elastic-plastic unloading may well occur, particularly when the residual stresses are highly triaxial. This paper examines the performance of one mechanical strain relief technique particularly suitable for large section components, the deep hole drilling (DHD) technique. The magnitude of error is calculated for different magnitudes of residual stress and can be substantial for residual stress states close to yield. A modification to the technique is described to allow large magnitudes of residual stress to be measured correctly. The new technique is validated using the case of a quenched cylinder where use of the standard DHD technique leads to unacceptable error. The measured residual stresses using the new technique are compared with the results obtained using the neutron diffraction technique and are shown to be in excellent agreement.

Computational Analysis of a Cross-linked Actin-like Network

Abstract: Gels formed from G-actin or other filament-forming monomers exhibit a range of morphologies that differ widely in terms of pore size, fiber diameter, degree of isotropy, and frequency of cross-linking or branching. These characteristics are determined, in large part, by the nature and concentration of the proteins that form cross-links between single filaments, yet little is known how filament-forming monomers and cross-linkers assemble to generate a particular network morphology. Some of the important attributes of a cross-linker are the spatial and angular orientation of its two filament binding sites, its size, and stiffness to both rotation and extension. Here, we introduce a Brownian dynamics (BD) simulation model in three dimensions in which actin monomers polymerize and become cross-linked by two types of cross-linking molecules that form either parallel filament bundles or perpendicular cross-links. We analyze the effects of various system parameters on the growth and morphology of the resulting network. Some scaling behaviors emerge that are insensitive to the detailed choice of parameters. Our model thus has the potential as a base BD model that can be further refined for investigating various actin-related phenomena.

Mechanical Properties of Ballistic Gelatin at High Deformation Rates

Abstract: The characterization of soft or low impedance materials is of increasing importance since these materials are commonly used in impact and energy absorbing applications. The increasing role of numerical modeling in understanding impact events requires high-rate material properties, where the mode of loading is predominantly compressive and large deformations may occur at high rates of deformation. The primary challenge in measuring the mechanical properties of soft materials is balancing the competing effects of material impedance, specimen size, and rate of loading. The traditional Split Hopkinson Pressure Bar approach has been enhanced through the implementation of polymeric bars to allow for improved signal to noise ratios and a longer pulse onset to ensure uniform specimen deformation. The Polymeric Split Hopkinson Pressure Bar approach, including the required viscoelastic bar analysis, has been validated using independent measurement techniques including bar-end displacement measurement and high speed video. High deformation rate characterization of 10% and 20% ballistic gelatin, commonly used as a soft tissue simulant, has been undertaken at nominal strain rates ranging from 1,000 to 4,000/s. The mechanical properties of both formulations of gelatin exhibited significant strain rate dependency. The results for 20% gelatin are in good agreement with previously reported values at lower strain rates, and provide important mechanical properties required for this material.

Determination of the Johnson–Cook Material Parameters Using the SCS Specimen

Abstract: This note addresses the determination of the Johnson-Cook material parameters using the shear compression specimen (SCS). This includes the identification of the thermal softening effect in quasi static and dynamic loading as well as and the strain rate hardening effect in dynamic loading. A hybrid experimental–numerical (finite element) procedure is presented to identify the constitutive parameters, with an application to Ti6Al4V alloy. The present results demonstrate the suitability of the SCS for constitutive testing.

The Potential of MEMS for Advancing Experiments and Modeling in Cell Mechanics

Abstract: Response to mechanical stimuli largely dictates cellular form and function. A host of extraordinary yet unexplained responses have been identified within the hierarchical cell structure. As experimental and model-based investigations in cell mechanics advance, the underlying structure-function mechanisms dictating these responses emerge. Here we explore the potential of microelectromechanical systems (MEMS) for advancing understanding of cell mechanics. To motivate the discussion, existing experimental techniques are summarized. Interrelated model-based approaches, which aim to interpret or predict observed results, are also outlined. We then focus on a representative set of MEMS-based devices designed for investigations in cell mechanics and point to the fact that, while these devices have yet to maximize their functionality through higher levels of sensor/actuator integration, they are highly complementary to existing techniques. In closing, novel MEMS sensor and actuator schemes that have yet to materialize in this field are discussed to motivate the next generation of MEMS for investigations in cell mechanics.

A Procedure for Accurate One-Dimensional Strain Measurement Using the Grid Method

Abstract: This paper deals with the accurate calculation of strain using the grid method. The strain field is first directly deduced from the fringe pattern without calculating the displacement field. This procedure is validated with two numerical examples. Two types of experiment are then carried out: a translation and a tensile test. It is observed that some additional fictitious strains appear in both cases. They are due to two main reasons which interact with each other: the grid defects and the displacement of the grid lines during testing. A suitable procedure is proposed to cancel out these fictitious strains. This procedure is successfully applied in two cases of fringe patterns.

The Fracture Process of Tempered Soda-Lime-Silica Glass

Abstract: This work presents experimental observations of the characteristic fracture process of tempered glass. Square specimens with a side length of 300 mm, various thicknesses and a residual stress state characterized by photoelastic measurements were used. Fracture was initiated using a 2.5 mm diamond drill and the fragmentation process was captured using High-Speed digital cameras. From the images, the average speed of the fracture front propagation was determined within an accuracy of 1.0%. Two characteristic fragments were found to form on each side of the initiation point and are named “Whirl-fragments” referring to the way they are generated. An earlier estimation of the in-plane shape of the fracture front is corrected and a hypothesis on the development for the fracture front is offered. The hypothesis is supported by investigations of the fragments using a Scanning Electron Microscope (SEM) which also revealed a micro scale crack bridging effect.

Stress Level Measurement in Prestressed Steel Strands Using Acoustoelastic Effect

Abstract: In-situ health monitoring of the tensioning components such as strands, tendons, bars, anchorage bolts, etc. used in civil engineering structures, namely bridges, dams, nuclear power plants, etc. is extremely important to ensure security of users and environment. This paper deals with a guided ultrasonic wave procedure for monitoring the stress levels in seven-wire steel strands (15.7 mm in diameter). For this purpose, simplified acoustoelastic formulations were derived from the acoustoelasticity theory and acoustoelastic measurements were performed. The results from acoustoelastic calibration tests and an anchorage block of seven-wire steel strands are presented and discussed. They show the potential and the suitability of the proposed guided wave method for evaluating the service stress levels in the prestressed seven-wire steel strands.

Deformation and Failure Modes of I-Core Sandwich Structures Subjected to Underwater Impulsive Loads

Abstract: This article reports an experimental study carried out with the aim of quantifying performance and failure modes of sandwich structures when subjected to impulsive blast loading. In particular, performance enhancement with respect to solid panels of equal mass per unit area is assessed. Likewise, the optimal distribution of the mass per unit area in the design of sandwich structures is investigated by comparing the behavior of sandwich structures with various distributions of face sheets thickness. By employing a previously developed FSI experiment, the study confirmed that usage of sandwich structures is beneficial and that performance enhancements, in terms of maximum panel deflection, as high as 68% are possible. The study also confirms theoretical and computational analyses suggesting that use of soft cores maximizes the benefits. Another interesting aspect revealed by this work is that the level of enhancement is highly related to the applied normalized impulse. The same distribution of mass per unit area between face sheets resulted in different normalized maximum deflection. A better performance enhancement was achieved at lower impulses. Here again, failure modes and their sequence seem to be the directly related to this finding. The work here reported clearly reveals a number of important features in the study of lightweight structures and points out to the synergies between structure geometry, materials, manufacturing methods, and threat levels as manifested by the strength of the impulse. Further theoretical and computational studies accounting for experimentally observed failure modes and its interdependence with the fabrication methods is needed to achieve additional predictive capabilities.

Study of a Crack at a Fastener Hole by Digital Image Correlation

Abstract: In this work the efficacy of using digital image correlation to determine stress intensity factors for a crack emanating from a fastener hole has been investigated. To this end a fatigue crack was grown in pure mode I from a 50 mm diameter hole in an Al 7010 alloy plate test-piece. This crack was then loaded elastically under several combinations of mixed mode (I + II) displacements. In each case, images of the sample surface before and after the deformation were recorded using a high resolution digital camera. The surface preparation consisted only of scratching the surface lightly with silicon carbide abrasive paper. The crack location and resulting displacements were then calculated using digital image correlation. The analytical displacement fields for a traction free crack under arbitrary loading conditions based on the Muskhelishvili’s complex function approach were fitted to the experimentally measured displacement fields and the mixed mode stress intensity factor was determined in each case. Good agreement with the nominal applied values was obtained. The uncertainty of the crack tip position has a major influence on the accuracy of the stress intensity factors and so the Sobel edge finding filter was successfully applied to experimental displacement fields to establish precisely the crack tip location.

Dynamic Tensile Testing of Soft Materials

Abstract: Determination of dynamic tensile response of soft materials has been a challenge because of experimental difficulties. Split Hopkinson tension bar (SHTB) is a commonly used device for the characterization of high-rate tensile behavior of engineering materials. However, when the specimen is soft, it is challenging to design the necessary grips, to measure the weak transmitted signals, and for the specimen to achieve dynamic stress equilibrium. In this work, we modified the SHTB on the loading pulse, the equilibrium-monitoring system, and the specimen geometry. The results obtained using this modified device to characterize a soft rubber indicate that the specimen deforms under dynamic stress equilibrium at a nearly constant strain rate. Axial and radial inertia effects commonly encountered in dynamic characterization of soft materials are also minimized.

Experimental Testing of a Moving Force Identification Bridge Weigh-in-Motion Algorithm

Abstract: Bridge weigh-in-motion systems are based on the measurement of strain on a bridge and the use of the measurements to estimate the static weights of passing traffic loads. Traditionally, commercial systems employ a static algorithm and use the bridge influence line to infer static axle weights. This paper describes the experimental testing of an algorithm based on moving force identification theory. In this approach the bridge is dynamically modeled using the finite element method and an eigenvalue reduction technique is employed to reduce the dimension of the system. The inverse problem of finding the applied forces from measured responses is then formulated as a least squares problem with Tikhonov regularization. The optimal regularization parameter is solved using the L-curve method. Finally, the static axle loads, impact factors and truck frequencies are obtained from a complete time history of the identified moving forces.

Experimental Study of Open-Cell Cellular Structures with Elastic Filler Material

Abstract: Open-cell cellular structures have a high potential for use in automotive, railway, ship and aerospace industry as crash energy absorbers. This paper focuses on the influence of the second phase filler material as a way to further increase the capability of cellular material energy absorption. The behaviour of ductile (aluminium alloy) and brittle (polymer) cellular structures with regular topology with and without the pore filler (silicon rubber) under quasi-static and dynamic compressive loading conditions has been experimentally studied and evaluated. The base material properties of the aluminium alloy and the polymer were obtained with separate experimental testing. The use of second phase filler material resulted in significant changes in cellular material behaviour. It was observed that the pore filler material increases the capability of energy absorption and furthermore improves and stabilises the response of a brittle cellular structures.

Tailoring the Load Carrying Capacity of MWCNTs Through Inter-shell Atomic Bridging

Abstract: Recent studies have finally produced accurate measurements of the mechanical properties of carbon nanotubes, confirming the anticipated results computed from quantum and molecular mechanics. Several studies have also predicted an enhancement of these material properties as a result of electron irradiation. Here we prove conclusively through a rigorous TEM imaging study that this enhancement occurs as a result of multiple-shell load transfer through irradiation-induced crosslinks. Using a computational model of the system which mirrors the experimental setup, we show that intershell covalent crosslinks resulting from the irradiation are efficient atomic structures for inter-shell load transfer. A study of the correlation between number of defects and load transfer provides insight into the experimental results and quantifies the increase in load transfer with radiation dose. The combined experimental/computational approach therefore gives a complete understanding of the phenomenon and provides the means for tailoring the mechanical properties of 1-D nanostructures.

The Effect of Strain Rate, Specimen Geometry and Lubrication on Responses of Aluminium AA2024 in Uniaxial Compression Experiments

Abstract: The ability to observe and quantify intrinsic material response to loading at different rates of strain has been improved by reducing the errors of mechanical characterisation in uniaxial compression experiments. In order to perform comparisons of the results from uniaxial compression tests used to characterise mechanical properties of aluminium alloys at different strain rates, it is necessary to reduce errors resulting from factors such as specimen design. In this study, the effects of strain rate, specimen geometry and lubrication on the compressive properties of aluminium AA2024 alloy were quantitatively investigated by measuring the mechanical behaviour of this alloy as functions of strain rate, specimen aspect ratio and lubrication condition. Both the deformation history and the failure mode were identified using low and ultrahigh speed photography. The interaction of factors influencing the measured stress-strain response was quantified, and suitable specimen aspect ratios for compression tests at different strain rates were identified.

Study on the Use of Motion Compensation Techniques to Determine Heat Sources. Application to Large Deformations on Cracked Rubber Specimens

Abstract: This paper deals with the determination of the thermal response of elastomeric materials subjected to cyclic loading. In this case, the material undergoes large deformations, so a suitable motion compensation technique has been developed to track the material points and their temperature during the test. Special attention is paid to the Narcissus effect and to the detector matrix of the infrared camera used in the study. Heat sources are then derived from the temperature maps. The thermoelastic inversion phenomenon has been experimentally evidenced during a cyclic test performed on an elastomeric notched specimen. The heat source distribution close to the crack tip has also been deduced from the temperature maps, thus highlighting the relevance of the approach.

Quantitative Impact Testing of Energy Dissipation at Surfaces

Abstract: Impact testing with nanoscale spatial, force, and temporal resolution has been developed to address quantitatively the response of surfaces to impingement of local contact at elevated velocities. Here, an impact is generated by imparting energy to a pendulum carrying an indenter, which then swings towards a specimen surface. The pendulum displacement as a function of time x(t) is recorded, from which one can extract the maximum material penetration x max , residual deformation x r , and indentation durations t in and t out. In an inverse application one can use the x(t) response to extract material constants characterizing the impact deformation and extent of energy absorption, including material specific resistance coefficient Cin, coefficient of restitution e, and dynamic hardness H imp . This approach also enables direct access to the ratio H/E, or resilience of the deformed material volume, at impact velocities of interest. The impact response of aluminum was studied for different contact velocities, and the mechanical response was found to correlate well with our one-dimensional contact model. Further experiments on annealed and work hardened gold showed that dynamic hardness H imp scales with contact velocity and highlighted the importance of rate-dependent energy absorption mechanisms that can be captured by the proposed experimental approach.

Endurance Tests on a Colloidal Damper Destined to Vehicle Suspension

Abstract: Endurance tests on a colloidal damper destined to vehicle suspension are performed. Such absorber represents an ecological application of nano-damping; it employs the hysteresis which occurs when water is forced to penetrate and then naturally exudes from a nanoporous silica gel matrix, modified to become liquid-repellent. Damping performances decrease at the increasing of the number of working cycles, partially since the silica gel grains that undergo gradual fatigue fracture are able to escape at the packing used to seal the test chamber, and partially due to the fatigue fracture alone, which is accompanied by an enhancement of the hydrophilic silanol groups on the silica gel surface and a pore size redistribution. In order to augment damper’s life, silica gel is introduced inside of a tank that is separated by a filter from the main cylinder, in which only water is supplied. One discusses the influence of filtration on the colloidal damper performances and the variation of damper’s life versus the ratio of filter pore’s diameter to the mean size of the silica gel particles.

Evaluation of Residual Stresses Induced by Robotized Hammer Peening by the Contour Method

Abstract: Welded components suffer from high tensile residual stresses close to the weld beads. These stresses seem to be the origin of premature cracking which could result in a catastrophic rupture during operation and a reduction of the lifespan of these components. In this context, the Hydro-Quebec’s Research Institute (IREQ) developed a technique of residual stresses relaxation by robotized hammer peening which makes it possible to release stresses close to surface and preserve the mechanical and dimensional properties of manufactured components. Robotized hammer peening was used to induce compressive residual stresses on initially stress free samples of austenitic stainless steel 304L. Hammer peening layers from one to nine were performed and the resulting residual stresses were evaluated thanks to the contour technique. Complete 2D residual stress fields on samples cross sections were obtained. The ability of hammer peening to relax residual stresses within welded plates was then quantified on austenitic stainless steel 304L plates welded with a 308 steel and hammer peened. These tests show the efficiency of hammer peening as a method to relax tensile residual stresses and induce compressive ones to a depth of a few millimetres. Process parameters were optimized such as the number of hammer peening layers to be applied to reduce processing time and maximization of the intensity and spatial distribution of the compressive residual stresses.

Measurement of Dynamic Properties of Viscoelastic Materials

Abstract: An improved method to measure the dynamic viscoelastic properties of elastomers is proposed. The method is based on the analysis of forced oscillation of a cylindrical sample loaded with an inertial mass. No special equipment or instrumentation other than the ordinary vibration measurement apparatus is required. Upper and lower surfaces of the viscoelastic material sample were bonded to a load disc and a rigid base plate, respectively. The rigid base plate was subject to forced oscillations driven by a vibration exciter. Two accelerometers were attached to monitor the displacement of the base plate and the load disc. The recorded magnitude ratio and the phase difference between the load disc and the base plate vibrations represent the axial, dynamic deformation of the sample. The data are sufficient to obtain the dynamic properties of the sample, oscillation properties of vibration exciter, whereas the sensitivity of gauges having no effect on the calculation results. For accurate calculation of the properties, a two-dimensional numerical model of cylindrical sample deformation was used. Therefore, a form factor, which takes into account the sample sizes in one-dimensional models, is not required in this method. Typical measurement of the viscoelastic properties of a silicone rubber Silastic® S2 were measured over the frequency range from 10 Hz to 3 kHz under deformations (ratio of vibration magnitude to sample thickness) from 10−4% to 5%. It was shown that the modulus of elasticity and the loss tangent fall on a single curve when the ratio of load mass to sample mass changed from 1 to 20. When the sample diameter was varied from 8 to 40 mm, the modulus of elasticity fall on the same curve, but the loss tangent curves showed some degree of scatter. Studied temperature dependence and nonlinear behavior of viscoelastic properties is found not to be associated with this effect.

Design of Inserts for Split-Hopkinson Pressure Bar Testing of Low Strain-to-Failure Materials

Abstract: In the present study a new insert design is presented and validated to enable reliable dynamic mechanical characterization of low strain-to-failure materials using the Split-Hopkinson Pressure Bar (SHPB) apparatus. Finite element-based simulations are conducted to better understand the effects of stress concentrations on the dynamic behavior of LM-1, a Zr-based bulk metallic glass (BMG), using the conventional SHPB setup with cylindrical inserts, and two modified setups—one utilizing conical inserts and the other utilizing a “dogbone” shaped specimen. Based on the results of these computational experiments the ends of the dogbone specimen are replaced with high-strength maraging steel inserts. This new insert-specimen configuration is expected to prevent specimen failure outside the specimen gage section. Simulations are then performed to validate the new insert design. Moreover, high strain-rate uniaxial compression tests are conducted on LM-1 using the modified SHPB with the new inserts. An ultra-high-speed camera is employed to investigate the changes in failure behavior of the specimens. Additional experiments are conducted with strain gages directly attached to the gage section of the specimens to determine accurately their dynamic stress–strain behavior.

Error Analysis in Trifilar Inertia Measurements

Abstract: Accurate calculation of the moment of inertia of an irregular body is made difficult by the large number of quantities which must be measured. A popular method is to use a trifilar suspension system to measure the period of oscillation of the body in the horizontal plane. In this paper, some sources of error are discussed with particular attention given to the alignment of the test object’s center of mass on the trifilar platform. The procedure is described, the necessary calculations are derived and the relative importance of accuracy in different measurements is assesed. It is determined that the accurate alignment of the centre of mass of the body being tested with the centre of the trifilar plate is insignificant compared to the accuracy of the other measurements required in the calculations.

Dynamic Deformation Behavior of a Golf Ball during Normal Impact

Abstract: The normal impact between a golf ball and a rigid steel target was studied to examine ball deformation and the contact force during the impact. Using high-speed video images, the normal and tangential compression ratios of the ball were measured to analyze the ball deformation quantitatively. In addition, the inbound and rebound ball velocities, contact time, and coefficient of restitution were determined as basic parameters of the impact. As the inbound ball velocity increased, the maximum normal compression ratio increased while the maximum tangential compression ratio, contact time and coefficient of restitution decreased. The ball center displacements during the impact were measured to determine the ball center velocity and acceleration, and the contact force was calculated by the product of the mass and acceleration. The contact force increased almost linearly with the inbound ball velocity, and its relationship agreed well quantitatively with the results from a load-cell, and also agreed well qualitatively with Hertz contact theory.

A Tensile Split Hopkinson Bar for Testing Particulate Polymer Composites Under Elevated Rates of Loading

Abstract: A tensile split Hopkinson bar apparatus is developed for testing high strain rate behavior of glass-filled epoxy The apparatus uses a specimen gripping configuration which does not require fastening and/or gluing and can be readily used for castable materials Details of the experimental setup, design of grips and specimen, specimen preparation method, benchmark experiments, and tensile responses are reported Also, the effects of filler volume fraction (0–30%) and particle size (11–42 μm) are examined under high rates of loading and the results are compared with the ones obtained from quasi-static loading conditions The results indicate that the increase in the loading rate contributes to a stiffer and brittle material response In the dynamic case lower ultimate stresses are seen with higher volume fractions of filler whereas in the corresponding quasi-static cases an opposite trend exists However, the absorbed specific energy values show a decreasing trend in both situations The results are also evaluated relative to the existing micromechanical models The tensile response for different filler sizes at a constant volume fraction (10%) is also reported Larger size filler particles cause a reduction in specimen failure stress and specific energy absorbed under elevated rates of loading In the quasi-static case, however, the ultimate stress is minimally affected by the filler size

On the Occurrence of Portevin–Le Châtelier Instabilities in Ultrafine-Grained 5083 Aluminum Alloys

Abstract: The Portevin–Le Châtelier (PLC) instability is commonly observed in Al–Mg alloys and is manifested in serrated flow within the stress–strain response We investigate the persistence of this instability with reduction in grain size by studying an ultrafine-grained (ufg) aluminum alloy (Al5083) and a conventional grain size Al5083 Micro-scale tensile tests combined with digital image correlation (DIC) reveal strength anisotropy and heterogeneity of the deformation in the three material directions (extrusion, rolled, and transverse) For the same applied displacement rate, the PLC effect in ufg-Al5083 is observed only over a small strain range immediately following the yield, while the coarse-grained Al5083 exhibits serrated flow over nearly the entire plastic strain range These observations are explained using the stability analysis of Hahner (Acta Mater 45:3695–3707, 1997), and implications for nanocrystalline (nc) alloys are discussed

Planar Strain Measurements on Wood Specimens

Abstract: Wood specimens have been tested for compressive loading in the longitudinal direction. Planar deformation was recorded by means of video extensometry on the specimen surfaces. A post processing routine was developed to calculate stress and strain values from the sampled data. The routine made use of mathematical framework used in the finite element method. Material parameters were detected by means of an optimization algorithm, and the determined linear elastic parameters were in general found to be in good agreement with values given in literature. The utilized method offers simultaneous average values for active, passive and shear strains from the measured area. Moduli of elasticity, Poisson’s ratios and shear deformation can thus be evaluated. In addition, the variation of the three strain components over the area is measured. The results can therefore be used for quantification of material inhomogeneity and are further suitable for direct comparison with numerically computed strains comprising non-uniform strain fields. Since video extensometry does not require any physical contact with the specimen, measurements can be undertaken until failure. The present method offers thus an efficient and relatively accurate way to measure and evaluate the material characteristics of anisotropic and inhomogeneous materials like wood.

Quantifying Forces Mediated by Integral Tight Junction Proteins in Cell–Cell Adhesion

Abstract: Cellular adhesion and barriers formed by intercellular adhesion proteins [tight junctions (occludin and claudins) and adherens junction (E-cadherin)] are important in maintaining tissue homeostasis. However, disruption of these junction proteins is associated with diseases in the organ systems such as multiple sclerosis, diarrhea, asthma, and gastro-intestinal tract carcinomas among others. In this paper, the separation force needed to separate two cells expressing some of these proteins was measured using the dual micropipette assay. Results show that L-fibroblasts transfected with claudin-1 and claudin-2 exhibit higher separation force (~2.8 nN and 2.3 nN, respectively) as compared to control cells or cells transfected with occludin (~1 nN). Furthermore, the separation force was not affected on addition of calcium chelating agent (ethylene diamine tetra acetic acid, EDTA). The separation force was, however, significantly decreased on treating cells with the actin disrupting agent Cytochalasin-D. These results show that the dual micropipette assay is a simple and useful experimental technique for quantifying cell–cell adhesion.