Critical Stretch Research Articles

Evaluating a valid fracture specimen geometry for predicting geometry independent SZWc and JSZW value in SA333 Gr. 6 carbon steel material

To obtain geometry independent critical stretch zone width there is a need to understand its variation with fracture specimen thickness, initial crack size, fracture specimen geometries and the type of loading. It comes out that minimum tri-axiality quotient value on the un-cracked ligament and the plastic zone size contained within fracture specimen geometry are the two criteria to be considered while choosing the fracture specimen shape and its geometry for the evaluation of valid critical SZW value of the material. The present study also showed the possibility of further improving the minimum thickness criterion of fracture specimen.

Non-linear thermoelasticity with energy limiters

In this article we couple the previously developed theory of elasticity with energy limiters for modeling material failure with a description of thermal processes. We present a generalized formulation of the thermoelasticity with energy limiters. We illustrate the new theory via the analytically tractable example of uniaxial tension. The theory predicts the existence of the thermoelastic inversion point observed in experiments. Besides, it predicts the experimentally observed stiffening of rubberlike materials under heating. We find above all that the tensile strength increases as a result of the stiffening while the critical rupture stretch is almost unaffected by heating. It is hoped that the developed coupled theory can be useful for modeling failure in rubberlike solids.

The Effect of Size and Location of Tears in the Supraspinatus Tendon on Potential Tear Propagation.

Rotator cuff tears are a common problem in patients over the age of 50 yr. Tear propagation is a potential contributing factor to the failure of physical therapy for treating rotator cuff tears, thus requiring surgical intervention. However, the evolution of tears within the rotator cuff is not well understood yet. The objective of this study is to establish a computational model to quantify initiation of tear propagation in the supraspinatus tendon and examine the effect of tear size and location. A 3D finite element (FE) model of the supraspinatus tendon was constructed from images of a healthy cadaveric tendon. A tear of varying length was placed at six different locations within the tendon. A fiber-reinforced Mooney-Rivlin material model with spatial variation in material properties along the anterior-posterior (AP) axis was utilized to obtain the stress state of the computational model under uniaxial stretch. Material parameters were calibrated by comparing computational and experimental stress-strain response and used to validate the computational model. The stress state of the computational model was contrasted against the spatially varying material strength to predict the critical applied stretch at which a tear starts propagating further. It was found that maximum principal stress (as well as the strain) was localized at the tips of the tear. The computed critical stretch was significantly lower for the posterior tip of the tear than for the anterior tip suggesting a propensity to propagate posteriorly. Onset of tear propagation was strongly correlated with local material strength and stiffness in the vicinity of the tear tip. Further, presence of a stress-shielded zone along the edges of the tear was observed. This study illustrates the complex interplay between geometry and material properties of tendon up to the initiation of tear propagation. Future work will examine the evolution of tears during the propagation process as well as under more complex loading scenarios.

Peridynamic modeling of delamination growth in composite laminates

Delamination growth predictions in previous peridynamic models were based on the assumption of constant critical stretch for each interlayer bond interaction. This study presents a new approach to terminate the interlayer peridynamic bonds. The critical stretch of a bond is implicitly determined by using the measured critical energy release rate values for different modes of deformation, and can vary depending on the degree of deformation. The bond failure occurs when the amount of energy required to remove the bonds across a unit surface equals the critical energy release rate of the interface between the two layers. This approach is applied to model delamination growth in double cantilever beam (DCB) (Mode I) and transverse crack tension (TCT) (Mode II dominated) specimens. The peridynamic predictions correlate well with the numerical and experimental results available in the literature.

Buckling of a coated elastic half-space when the coating and substrate have similar material properties

This study investigates the buckling of a uni-axially compressed neo-Hookean thin film bonded to a neo-Hookean substrate. Previous studies have shown that the elastic bifurcation is supercritical if r ≡ μ f / μ s >1.74 and subcritical if r <1.74, where μ f and μ s are the shear moduli of the film and substrate, respectively. Moreover, existing numerical simulations of the fully nonlinear post-buckling behaviour have all been focused on the regime r >1.74. In this paper, we consider instead a subset of the regime r <1.74, namely when r is close to unity. Four near-critical regimes are considered. In particular, it is shown that, when r >1 and the stretch is greater than the critical stretch (the subcritical regime), there exists a localized solution that arises as the limit of modulated periodic solutions with increasingly longer and longer decaying tails. The evolution of each modulated periodic solution is followed as r is decreased, and it is found that there exists a critical value of r at which the deformation gradient develops a discontinuity and the solution becomes a static shock. The semi-analytical results presented could help future numerical simulations of the fully nonlinear post-buckling behaviour.

The effect of shear-thickening on liquid transfer from an idealized gravure cell

Gravure printing is an economical roll-to-roll processing technique with potential to revolutionize the fabrication of nano-patterned thin films at high throughput. In the present study, we investigated the impact of shear-thickening on the liquid transfer from an idealized gravure cell by a combination of experiments and numerical computations. We chose as a model system fumed silica nanoparticles dispersed in polypropylene glycol; these dispersions exhibit shear and extensional thickening as verified by steady shear and filament stretching extensional rheometry. Model gravure printing experiments were conducted using a linear motor to pick out the fluid vertically up from a truncated conical shaped idealized gravure cell cavity; the cell size is large enough that gravity is important, and therefore experiments were also conducted to pickout the fluid vertically down from the cavity-on-top. The amount liquid transfer from the cavity was studied with varying stretch velocities and dispersion concentrations. The filament profile evolution during the pickout process was examined using a high speed camera. Beyond a critical stretch rate, shear-thickening of the fluid, manifested by the formation of long stable filaments, exacerbates gravitational drainage during pickout. Beyond a second critical stretch rate, shear-thinning induces conical profile evolutions that result in pickout insensitive to stretch rate. All of these observations were qualitatively predicted by finite element computations using a generalized Newtonian fluid model, where the shear rheology was modeled explicitly. We showed that under the influence of gravity, wetting/de-wetting may be a critical phenomenon in determining pickout at low stretch rates.

Mechanics of dynamic fracture in notched polycarbonate

Fracture toughness of brittle amorphous polymers (e.g. polymethyl methacrylate (PMMA)) has been reported to decrease with loading rate at moderate rates and increase abruptly thereafter to close to 5 times the static value at very high loading rates. Dynamic fracture toughness that is much higher than the static values has attractive technological possibilities. However, the reasons for the sharp increase remain unclear. Motivated by these observations, the present work focuses on the dynamic fracture behavior of polycarbonate (PC), which is also an amorphous polymer but unlike PMMA, is ductile at room temperature. Towards this end, a combined experimental and numerical approach is adopted. Dynamic fracture experiments at various loading rates are conducted on single edge notched (SEN) specimens with a notch of radius 150μm, using a Hopkinson bar setup equipped with ultra high-speed imaging (>105fps) for real-time observation of dynamic processes during fracture. Concurrently, 3D dynamic finite element simulations are performed using a well calibrated material model for PC. Experimentally, we were able to clearly capture the intricate details of the process, for both slowly and dynamically loaded samples, of damage nucleation and growth ahead of the notch tip followed by unstable crack propagation. These observations coupled with fractography and computer simulations led us to conclude that in PC, the fracture toughness remains invariant with loading rate at Jfrac=12±3kN/m for the entire range of loading rates (J̇) from static to 1×106kN/m−s. However, the damage initiation toughness is significantly higher in dynamic loading compared to static situations. In dynamic situations, damage nucleation is quickly followed by initiation of radial crazes from around the void periphery that initiate and quickly bridge the ligament between the initial damaged region and the notch. Thus for PC, two criteria for two major stages in the failure process emerge. Firstly, a mean stress based defect initiation is suggested. The value of the critical mean stress for defect initiation under dynamic loading is found to be 115±5MPa, which is significantly higher than its static value of 80MPa. The critical normal plastic stretch needed for crazes to nucleate from the nucleated defect is estimated to be about 1.78±0.2.

Nonlocal Peridynamic Modeling and Simulation on Crack Propagation in Concrete Structures

An extended peridynamic approach for crack propagation analysis in concrete structures was proposed. In the peridynamic constitutive model, concrete material was described as a series of interacting particles, and the short-range repulsive force and anisotropic behavior of concrete were taken into account in the expression of the interactive bonding force, which was given in terms of classical elastic constants and peridynamic horizon. The damage of material was defined locally at the level of pairwise bond, and the critical stretch of material bond was described as a function of fracture strength in the classical concrete failure theory. The efficiency and accuracy of the proposed model and algorithms were validated by simulating the propagation of mode I and I-II mixed mode cracks in concrete slabs. Furthermore, crack propagation in a double-edge notched concrete beam subjected to four-point load was simulated, in which the experimental observations are captured naturally as a consequence of the solution.

Inhibition of counterflow methane/air diffusion flame by water mist with varying mist diameter

In the present study, the effect of fine water mist on extinguishment of a methane–air counterflow diffusion flame was investigated to understand the underlying physics of fire extinguishment of highly stretched diffusion flame by water mist. Twin-fluid atomizers were used to generate polydisperse water mist of which Sauter mean diameters were 10, 20, 40, and 60μm. When water mist is not added, the critical stretch rate at extinguishment is 439s−1 as compared to the theoretical value of 460s−1. For the case with water mist addition, when the stretch rate is small enough, almost all the water mist evaporates within the flame zone. On the other hand, for high stretch rate case, large mist droplets pass through the flame zone and can reach the stagnation plane. However, no oscillatory motion was found around the stagnation plane. Critical stretch rate at extinguishment decreases monotonously with the mass fraction of water mist independently of the mist diameter within the range of D32 from 10μm to 60μm. On the other hand, with increase in the surface area parameter, the critical stretch rate at extinguishment decreases rapidly and becomes less sensitive at large surface area parameter, of which tendency is qualitatively in good agreement with theoretical predictions. For a constant surface area parameter, the critical stretch rate decreases with mist diameter because the mass fraction of water mist should increase in proportion to the mist diameter to keep the surface area parameter constant. When the water mist evaporates completely in the flame zone as in the present study, the mass fraction of the water mist is the dominant factor for fire extinguishment, rather than the surface area parameter. Therefore, an appropriate combination of stretch rate and water mist mass fraction should be provided to suppress effectively a given fire with a small amount of water mist.

Long-Range Force Transmission in Fibrous Matrices Enabled by Tension-Driven Alignment of Fibers

Cells can sense and respond to mechanical signals over relatively long distances across fibrous extracellular matrices. Recently proposed models suggest that long-range force transmission can be attributed to the nonlinear elasticity or fibrous nature of collagen matrices, yet the mechanism whereby fibers align remains unknown. Moreover, cell shape and anisotropy of cellular contraction are not considered in existing models, although recent experiments have shown that they play crucial roles. Here, we explore all of the key factors that influence long-range force transmission in cell-populated collagen matrices: alignment of collagen fibers, responses to applied force, strain stiffening properties of the aligned fibers, aspect ratios of the cells, and the polarization of cellular contraction. A constitutive law accounting for mechanically driven collagen fiber reorientation is proposed. We systematically investigate the range of collagen-fiber alignment using both finite-element simulations and analytical calculations. Our results show that tension-driven collagen-fiber alignment plays a crucial role in force transmission. Small critical stretch for fiber alignment, large fiber stiffness and fiber strain-hardening behavior enable long-range interaction. Furthermore, the range of collagen-fiber alignment for elliptical cells with polarized contraction is much larger than that for spherical cells with diagonal contraction. A phase diagram showing the range of force transmission as a function of cell shape and polarization and matrix properties is presented. Our results are in good agreement with recent experiments, and highlight the factors that influence long-range force transmission, in particular tension-driven alignment of fibers. Our work has important relevance to biological processes including development, cancer metastasis, and wound healing, suggesting conditions whereby cells communicate over long distances.

MORPHOLOGIC MODELLING OF THE COASTAL EVOLUTION IN ADA, GHANA

A couple of years ago, the Ghanaian government decided to protect the coast in Ada. In total a stretch of about 16 km had to be defended with a combination of groynes and a beach nourishment. The project was split into two phases: first the groynes in the most critical stretch were built (and finished summer 2013), next the remaining groynes and the beach nourishment will be executed in the second phase (currently ongoing). This paper will focus on the design of the “soft” element of this coastal protection: the beach nourishment. In order to select the most suitable design, different kinds of morphological modeling have been undertaken: long term morphological modeling (with Litpack) to predict the effect of the groynes and nourishment on the long term, and shorter term modelling with XBeach to investigate the coastal processes and to select the best design for the beach nourishment. For this XBeach modelling, first the model was calibrated and validated based on the in-situ measurements of the hydrodynamic conditions (nearshore waves) and the coastal response (beach profiles and coastlines) over several months. The model proved to be able to reproduce the most important processes of beach erosion, both also overwash over the low lying dunes. Therefore it has been used in a next step to investigate the most suited beach profile, the interaction between groynes and the nourishment and thus also the lay-out for the nourishment. This paper will present some key steps during the construction of the model, and how it has been used to support the design of the coastal protection with groynes and a beach nourishment.

A peridynamic material model for the analysis of dynamic crack propagation in orthotropic media

A new material model for the dynamic fracture analysis of anisotropic materials has been proposed within the framework of the bond-based peridynamic theory. This model enables predicting complex fracture phenomena such as spontaneous crack nucleation and crack branching, curving and arrest, a capability inherited from the bond-based peridynamic theory. An important feature of the model is that the bond properties, i.e. the stiffness constant and critical stretch, are continuous functions of bond orientation in the principal material axes. This facilitates fracture analysis of anisotropic materials with random orientations, such as polycrystalline microstructures. Elastic and fracture behaviour of the model has been verified through simulating uniaxial tension of a composite plate and fracture of a cortical bone compact tension specimen, and making quantitative comparisons to analytical and experimental data. To further demonstrate the capabilities of the proposed model, dynamic fracture of a polycrystalline microstructure (alumina ceramic) has been simulated. The influence of the grain boundary and grain interior fracture energies on the interacting and competing fracture modes of polycrystalline materials, i.e. intergranular and transgranular fracture, has been studied.

Timing and magnitude of prestretch strongly affect cardiac muscle tension development (545.2)

Ectopic ventricular pacing leads to regional myocardial prestretch. In‐vivo studies suggest that the timing of prestretch determines the effect on pump function and can lead to regional wall remodeling. Here we measure the direct effects of prestretch timing on the magnitude of tension development in isolated cardiac muscle. The effects of prestretch were simulated using excised murine right ventricular papillary muscles. A servo motor was used to impose precisely timed stretches, while a force transducer measured force output and strain was monitored using a CCD camera and topical markers. A critical stretch timing interval was observed to statistically increase peak tension by as much as 500%, while shortening during this interval inhibited peak tension significantly by as much as 40%. A simple model consisting of experimentally obtained time varying elastance and the force‐velocity relation could not completely explain the results. An estimate of shortening deactivation was measured by performing quick stretches to dissociate crossbridges. The results showed that the timing of shortening deactivation could explain the remaining differences between the model and the experimental results. These evaluations have led us to the conclusion that prestretches performed at slow physiologic velocities cause alterations in twitch kinetics that cannot be explained solely by time varying elastance.

Compression of a hyperelastic layer-substrate structure: Transitions between buckling and surface modes

This paper studies the bifurcation behaviors of a hyperelastic layer bonded to another hyperelastic substrate of finite thickness subjected to compression. We aim at revealing some interesting transitions between different bifurcation modes as the geometrical parameters vary. A linear bifurcation analysis is carried out for obtaining the bifurcation condition in the framework of exact theory of nonlinear elasticity. This condition, in the form of a determinant with complicated elements, contains a few parameters, and here the task is to analyze it to determine different behaviors. From the critical stretch curves, it is found that there are two mode types for the layer: buckling mode and wrinkling mode. By further considering the eigenfunction, three types of modes for the substrate are identified, including buckling mode, buckling-surface mode and wrinkling-surface mode. A careful analysis is carried out to determine the parameter constraints for each type of modes. In particular, three critical thickness ratios and two critical aspect ratios of the layer are found. As a result, we manage to classify the plane of the aspect ratio of the layer and the thickness ratio into six domains for different mode types and whose boundaries determine where the transitions of mode types take place. Finally, an asymptotic analysis with double expansions for each unknown is carried out to give the explicit formulas for the critical mode number and the critical stretch (which also give an improvement on the existing results for a layer coated to a half-space). Also, simplified relations for those critical thickness ratios and aspect ratios are derived. The asymptotic results also reveal some interesting insights, e.g., why the Poisson’s ratio has little effect and in a wrinkling mode the critical stretch is almost independent of the layer thickness.

Instabilities of an electroelastic plate

In this paper the theory of nonlinear electroelasticity is used to examine diffuse modes of instability for two problems involving a thin dielectric plate subject to large deformations. The analysis is based on the equations governing linearized incremental deformations and electric displacements and accompanying boundary conditions superimposed on a known finitely deformed configuration in the presence of a known electric field (the underlying configuration). For each problem the underlying deformation is taken to correspond to an equibiaxial stretch with the electric field normal to the major faces of the plate. In the first problem the electric field is generated by equal and opposite charges applied to flexible electrodes on the major surfaces, while in the second problem there are no electrodes and the field is applied externally.Numerical results are obtained in respect of a simple model of an electroelastic material by way of illustration. The critical stretch corresponding to loss of stability of the uniform underlying configuration is obtained as a function of a dimensionless measure of the initial thickness of the plate for a series of values of the electric (displacement) field and of the parameters included in the material model. For the first problem, the results obtained for the critical stretch are compared with the results based on the so-called Hessian approach, which provides only a limited analysis of stability. It neither takes account of the plate thickness nor allows for non-homogeneous deformations, and significant differences in the predictions of the two methods are identified.

T-cell epitope prediction methods: an overview.

The scientific community is overwhelmed by the voluminous increase in the quantum of data on biological systems, including but not limited to the immune system. Consequently, immunoinformatics databases are continually being developed to accommodate this ever increasing data and analytical tools are continually being developed to analyze the same. Therefore, researchers are now equipped with numerous databases, analytical and prediction tools, in anticipation of better means of prevention of and therapeutic intervention in diseases of humans and other animals. Epitope is a part of an antigen, recognized either by B- or T-cells and/or molecules of the host immune system. Since only a few amino acid residues that comprise an epitope (instead of the whole protein) are sufficient to elicit an immune response, attempts are being made to identify or predict this critical stretch or patch of amino acid residues, i.e., T-cell epitopes and B-cell epitopes to be included in multiple-subunit vaccines. T-cell epitope prediction is a challenge owing to the high degree of MHC polymorphism and disparity in the volume of data on various steps encountered in the generation and presentation of T-cell epitopes in the living systems. Many algorithms/methods developed to predict T-cell epitopes and Web servers incorporating the same are available. These are based on approaches like considering amphipathicity profiles of proteins, sequence motifs, quantitative matrices (QM), artificial neural networks (ANN), support vector machines (SVM), quantitative structure activity relationship (QSAR) and molecular docking simulations, etc. This chapter aims to introduce the reader to the principle(s) underlying some of these methods/algorithms as well as procedural and practical aspects of using the same.

Selective apoptosis induction in MCF-7 cell line by truncated minimal functional region of Apoptin.

BackgroundChicken Anemia Virus (CAV) VP3 protein (also known as Apoptin), a basic and proline-rich protein has a unique capability in inducing apoptosis in cancer cells but not in normal cells. Five truncated Apoptin proteins were analyzed to determine their selective ability to migrate into the nucleus of human breast adenocarcinoma MCF-7 cells for inducing apoptosis.MethodsFor identification of the minimal selective domain for apoptosis, the wild-type Apoptin gene had been reconstructed by PCR to generate segmental deletions at the N’ terminal and linked with nuclear localization sites (NLS1 and NLS2). All the constructs were fused with maltose-binding protein gene and individually expressed by in vitro Rapid Translation System. Standardized dose of proteins were delivered into human breast adenocarcinoma MCF-7 cells and control human liver Chang cells by cytoplasmic microinjection, and subsequently observed for selective apoptosis effect.ResultsThree of the truncated Apoptin proteins with N-terminal deletions spanning amino acid 32–83 retained the cancer selective nature of wild-type Apoptin. The proteins were successfully translocated to the nucleus of MCF-7 cells initiating apoptosis, whereas non-toxic cytoplasmic retention was observed in normal Chang cells. Whilst these truncated proteins retained the tumour-specific death effector ability, the specificity for MCF-7 cells was lost in two other truncated proteins that harbor deletions at amino acid 1–31. The detection of apoptosing normal Chang cells and MCF-7 cells upon cytoplasmic microinjection of these proteins implicated a loss in Apoptin’s signature targeting activity.ConclusionsTherefore, the critical stretch spanning amino acid 1–31 at the upstream of a known hydrophobic leucine-rich stretch (LRS) was strongly suggested as one of the prerequisite region in Apoptin for cancer targeting. Identification of this selective domain provides a platform for developing small targets to facilitating carrier-mediated-transport across cellular membrane, simultaneously promoting protein delivery for selective and effective breast cancer therapy.

Peridynamic analytical method for progressive damage in notched composite laminates

A new peridynamic model for fiber reinforced composite laminate was proposed and applied on the analysis of the progressive damages in composite laminate with notch or open hole. The mechanical characters of composite were described by bonds whose performance was depending on the distance and relative position of this couple of particles. In order to refine the description of composite’s anisotropy, the concept of transverse modulus in classic laminate theory was introduced into the definition of bonds. Peridynamic parameters such as micromodulus and critical stretch were established following the similar concepts in elasticity theory and mechanics of composite. In this peridynamic model three types of damages in composite laminate could be analyzed: fiber fracture, matrix fracture and delamination. The mesh independency was also examined for this approach. In the examples of laminate with unidirectional and multidirectional layup, the progressive damages and failure modes were successfully analyzed under a tensile load and had a good agreement with experiments. Moreover, much more details in the procedure of damage initiations and propagations, including fiber broken, matrix fracture and delamination, could be revealed. It had shown that this peridynamic model had a great potential for composite analysis.

Flow‐enhanced Crystallization Kinetics of iPP during Cooling at Elevated Pressure: Characterization, Validation, and Development

Using dilatometry combined with shear flow at conditions comparable to realistic processing conditions, the flow‐induced crystallization of polymers is modelled. The model describes the kinetics of quiescent nucleation, flow‐enhanced point nucleation, fibrillar growth, and the time evolution of the dimensions of the resulting crystalline structures. The growth rate of nuclei is coupled to the backbone stretch of a mode with a relaxation time representative of the average of the molecular weight distribution. The eXtended Pom‐Pom (XPP) model is used to calculate the backbone stretches from flow conditions. Three important model parameters are determined over a broad range of temperatures, pressures, and shear rates for a fixed shear time; a prefactor to the creation rate of flow‐induced nucleation, a prefactor to the shish growth rate, and the critical molecular stretch defining the transition between flow‐enhanced nucleation and flow‐induced crystallization of oriented fibrilar structures. Excellent agreement is obtained between calculated and experimentally determined crystallization kinetics of iPP. Moreover, the extended experimental dataset leads to an important adaption of the model, i.e., a new criterion for the initiation of shish growth.

Experimental real-time tracking and diffusion/mechanics numerical simulation of cavitation in gas-saturated elastomers

The effect of a gas decompression after saturation on rubbers was investigated with both an experimental and a numerical approach. Experimental results have been obtained with a commercial transparent rubber under hydrogen and a tensile machine fitted with a pressure chamber that allows a spatial and temporal tracking of damage. The influence of the decompression rate and cavity radius have been studied and compared with a numerical model based on the theory of the hollow sphere and the implementation of a Fickean law to have gas diffusion in the material. This multi-scale model was used to temporally predict the response of a spherical cavity in a hyperelastic incompressible material submitted to a coupled gas/mechanical loading. The aim of the study was to understand the gas exchanges between the cavity and the material and to temporally predict with a simple critical stretch ratio criterion when a small cavity can growth to a visible size. The influence of decompression rate, pressure level, cavity radius and position in the sample were discussed and compared with experiments.