Cohesive Stress Research Articles

Interface characteristics of nanorope reinforced polymer composites

A shear-lag model is proposed to obtain interface characteristics of nanorope reinforced polymer composites using representative volume element (RVE) concept. In the axisymmetric RVE, the nanorope is modelled as a closed-packed cylindrical lattice consisting seven single-walled carbon nanotubes. In the model, rope is considered to be perfectly bonded with the polymer resin where the nanotubes are assumed to be chemically non-bonded with each other in the rope system. Since, nanotubes are considered to be non-bonded in the nanorope there must exist a van der Waals interaction in terms of Lennard-Jones potential. A separate model is also proposed to determine the cohesive stress caused by this interaction. Closed form analytical solutions are derived for stress components of rope, resin and individual carbon nanotubes in the rope system. Parametric study has also been conducted to investigate the influences of key composite factors involved at both perfectly bonded and non-bonded interfaces.

Numerical Method for Mixed-Mode I–II Crack Propagation in Concrete

A crack-propagation criterion is proposed in this paper for mixed-mode I–II fracture in concrete. In this criterion, crack propagation is initiated when the difference between the stress-intensity factor at the crack tip caused by the external force and that by the cohesive stress satisfies the crack-initiation equation. On the basis of this criterion, a numerical method is developed to simulate mixed-mode I–II crack propagation in concrete. To verify the criterion, three sets of experimental data are selected: one is obtained from the self-conducted test and the other two are collected from the literature. The numerical results show that the calculated load versus crack-mouth opening and sliding displacement curves and crack trajectories are in good agreement with experimental results. Therefore, once the mode I initial cracking toughness, elastic modulus, fracture energy, possion’s ratio, and tensile strength of concrete are available, the whole mixed-mode I–II fracture process in concrete can be predicted with reasonable accuracy.

Finite Element Modeling of Crack Tip Blunting for Estimation of Energy Release Rate Component of Mode I Crack near a Strength Mismatched Interface

The paper presents finite element modeling of crack tip blunting for numerical estimation of fracture parameter of a Mode I crack, in weak alloy steel, which is near and normal to the interface of elastically identical but stronger maraging steel. The bimetallic body is subjected to monotonic load in K dominated regime. Crack tip yield zone across the interface, treated as Dugdale’s cohesive zone, is isolated from the bimetallic domain and is modeled alone under the action of respective cohesive stress over yield zones for obtaining the contribution of mismatch between yield strength of the steels in crack energy release rate component, Jinterface. Effect of far field load on Jinterface is found separately from a theoretical model. Numerical and theoretical results of Jinterface are in good agreement.

Effects of shot peening on the nanoindentation response of Cu47.5Zr47.5Al5 metallic glass

Abstract The effects of shot peening (SP) on the nanoindentation response of Cu47.5Zr47.5Al5 metallic glass, both in the as-cast state and after thermally induced structural relaxation (i.e., after annealing close to the glass transition), are investigated. As expected, annealing causes an increase of hardness attributable to annihilation of free volume. Conversely, nanoindentation tests, carried out on a transverse section at different distances from the SP surface, reveal that SP induces mechanical softening. Both the hardness and the reduced Young’s modulus progressively decrease as the peened surface is approached. Finite-element simulations of the nanoindentation curves indicate that the cohesive stress and the Mohr–Coulomb friction coefficient also decrease after SP, an observation which is in line with the induced mechanical softening. Differential scanning calorimetry measurements reveal an increase of the relaxation enthalpy, particularly when SP is performed on the previously annealed metallic glass, consistent with deformation-induced creation of free volume.

Crack-tip fields in gradient enhanced elasticity

Nonlocal and gradient theories are capable of describing deformation of heterogeneous elastic materials better than classical elasticity theory. Crack-tip stress and strain fields in a gradient enhanced elastic material are derived on closed form. Physical requirements of finite stresses and strains at infinity and at the tip are applied to remove singularities. A fracture criterion is derived that links applied remote macroscopic stress via microscopic cohesive stress in the vicinity of the crack-tip to the Griffith’s energy. A comparison to a classical nonlocal theory by Eringen is made. It is believed that the solutions will help engineers to deal with fracture analyses in elastic brittle heterogeneous materials.

ANÁLISE DO NÍVEL DE COESÃO DE GRUPO E DO ESTRESSE PSICOLÓGICO PRÉ-COMPETITIVO DE ATLETAS ADULTOS DE VOLEIBOL

This descriptive study was designed to analyze the level of group cohesion and pre-competitive psychological stress of adult volleyball athletes. The subjects consisted of 155 male and female athletes from the state of Parana who played in the JAPS/ 1st division (A), and 2nd division (B). The assessment instruments used were the Group Environment Questionnaire (GEQ) and the Volleyball Psychic Stress Test (V-PST). For data analysis, the following tools were used: Cronbach’s alpha, Kolmogorov-Smirnov, Mann-Whitney, one-way Anova, and Tukey’s post hoc (p<0.05). The results revealed that athletes of division A were more negatively influenced by stress factors than the athletes of division B; in terms of individual attraction to the social group (p=0.014) and tasks (p=0016) athletes of division B had higher levels of group cohesion than athletes from division A; athletes with low social cohesion were negatively impacted by factors of "inappropriate physical conditioning" and "excessive nervousness;" athletes with low levels of cohesion for the task were negatively influenced by the factors of "Pressure from other people to win" and positively by the factor "Behavior of the fans in the game outside." It was concluded that group cohesion is demonstrated to be an intervening factor in pre-competitive stress thereby demonstrating that the higher the dispute level, the higher the negative influence of stress.

Sculpting sandcastles grain by grain: Self-assembled sand towers

We study the spontaneous formation of granular towers produced when dry sand is poured on a wet sand bed. When the liquid content of the bed exceeds a threshold value W*, the impacting grains have a nonzero probability to stick on the wet grains due to instantaneous liquid bridges created during the impact. The trapped grains become wet by the capillary ascension of water and the process continues, giving rise to stable narrow towers. The growth velocity is determined by the surface liquid content which decreases exponentially as the tower height augments. This self-assembly mechanism (only observed in the funicular and capillary regimes) could theoretically last while the capillary rise of water is possible; however, the structure collapses before reaching this limit. The collapse occurs when the weight of the tower surpasses the cohesive stress at its base. The cohesive stress increases as the liquid content of the bed is reduced. Consequently, the highest towers are found just above W*.

Micromechanical-based criteria for the calibration of cohesive zone parameters

This paper presents a new micromechanical model for a collection of cohesive zone models embedded between each mesh of a finite element-type discretization. It aims to fully extend the previous linear results of Blal et al. (2012) [11] to the calibration of damageable cohesive parameters (cohesive peak stress, critical opening displacement, cohesive energy, etc). The main idea of the approach consists in replacing the underlying cohesive-volumetric discretization by an equivalent ‘matrix-inclusions’ composite. The overall behavior of this equivalent composite is estimated using homogenization schemes (Hashin–Shtrikman estimate and the modified secant method) and is given in a closed-form as function of both cohesive and bulk properties and the mesh density. In the particular case of a bilinear cohesive law a micromechanical damage model for quasi-brittle materials is derived. The corresponding local-to-global relationships are obtained for any overall triaxiality loading ratio.

Advanced Natural Stitched Composite Materials in Skin-Stiffener of Wind Turbine Blade Structures

In this paper the failure behaviour of natural stitched composite materials in the skin-stiffener of wind turbine blade structures are investigated. For this study, the laminated composite beams were stitched using Flax yarns before curing process. Two stiffener structures of T-beam and Box-beam are studied in this paper. These specimens were tested under quasi-static loading condition to compare the failure resistance of adhesive and stitched bonding methods. Furthermore, the cohesive zone modelling (CZM) which is known as a variation in the cohesive stresses with the interfacial opening displacement along the localised fracture process zone is used to predict bonding failure in the skin-stiffener of wind turbine blade structures.

Calculating crack extension resistance of concrete based on a new crack propagation criterion

A crack propagation criterion was proposed for model I crack in concrete by using the initial fracture toughness KICini as an inherent material property. Based on this criterion, crack begins to propagate when the difference, between the stress intensity factors caused by the applied load KIP and that by the cohesive stress KIσ, exceeds KICini. Finite element analyses was then carried out to calculate the complete load vs. crack mouth opening displacement (P-CMOD) curve, the critical crack propagation length ΔaC and the unstable fracture toughness KICun for notched beams under three-point bending. It was found that numerical results showed a good agreement with the experimental ones. Based on this crack propagation criterion, crack extension resistance, in terms of stress intensity factor, KR being able to consider the variation of fracture process zone (FPZ) was employed for describing crack propagation in concrete. KR is composed of KICini and KIσ, which is actually equal to the driving action of crack extension. It was concluded that given the elastic modulus E, the uniaxial tensile strength ft, the fracture energy GF and KICini, the complete fracture process in concrete and the KR-curve of concrete can be calculated based on the numerical method. Finally, discussion was made on the effects of fracture process zone, GF and specimens geometries on KR-curve.

Evaluation of Fracture Parameters by Double-G, Double-K Models and Crack Extension Resistance for High Strength and Ultra High Strength Concrete Beams

This paper presents the advanced analytical methodologies such as Double- G and Double - K models for fracture analysis of concrete specimens made up of high strength concrete (HSC, HSC1) and ultra high strength concrete. Brief details about characterization and experimentation of HSC, HSC1 and UHSC have been provided. Double-G model is based on energy concept and couples the Griffith's brittle fracture theory with the bridging softening property of concrete. The double-K fracture model is based on stress intensity factor approach. Various fracture parameters such as cohesive fracture toughness (4), unstable fracture toughness (K-Ic(c)), unstable fracture toughness (K-Ic(un)) and initiation fracture toughness (K-Ic(ini)) have been evaluated based on linear elastic fracture mechanics and nonlinear fracture mechanics principles. Double-G and double-K method uses the secant compliance at the peak point of measured P-CMOD curves for determining the effective crack length. Bi-linear tension softening model has been employed to account for cohesive stresses ahead of the crack tip. From the studies, it is observed that the fracture parameters obtained by using double - G and double - K models are in good agreement with each other. Crack extension resistance has been estimated by using the fracture parameters obtained through double - K model. It is observed that the values of the crack extension resistance at the critical unstable point are almost equal to the values of the unstable fracture toughness K-Ic(un) of the materials. The computed fracture parameters will be useful for crack growth study, remaining life and residual strength evaluation of concrete structural components.

Disminución de la resistencia de una arena limosa no saturada

It is generally considered that the shear strength of fine soils continuously increases with suction; however, this is not the case for all types of soils. There are some soils whose strength reaches a maximum at certain suction and then reduces with further of suction. Nevertheless, such cases have not been completely documented and analyzed. This paper presents the results of a series of suction controlled triaxial tests made on silty sand. The tests were made for both the wetting and the drying path. Suction was controlled by circulating air at a constant relative humidity. The retention curve was also obtained for the wetting and the drying path. The variation of cohesive stress was determined using some of the existing equations and the solid-porous model proposed by Rojas (2008a y 2008b). These equations include Bishop’s χ parameter, evaluated theoretically and experimentally. The triaxial tests results show that the strength of the soil increases to a maximum at certain suction and then decreases when suction is further increased.

An elastic–plastic cohesive zone model for metal–ceramic interfaces at finite deformations

This study concerns an elastic–plastic cohesive zone model for metal–ceramic interfaces and the corresponding nonlinear finite element implementation for general boundary value problems that accounts for nonlinear traction separation constitutive relation including fine scale mechanisms of the bonded interfaces failure. The interfacial model, formulated in terms of geometrically nonlinear theory of plasticity, allows for localization of plastic deformation, ensuing damage and failure/fracture behavior of the interface at finite deformations. From the numerical simulations, fiber is modeled as a linearly elastic material while the constitutive relation of matrix material is modeled using large deformation of isotropic porous plastic material which includes the voids nucleate, grow and coalesce. Further, an extensive parametric study is conducted in which the effects of hardening/softening, cohesive yield stress and the damage displacement jumps in determining stress-strain response of the composite and fiber/matrix debonding process under quasi-static transverse loading. Moreover, the effects of initial yield stress and void volume fraction in the porous matrix are also considered. Results show there are significant effects of interface yield and failure/fracture as well as matrix porous plasticity on the overall deformational and failure response of composites.

Stochastic parameterization of dust emission and application to convective atmospheric conditions

Abstract. We develop a parameterization scheme of convective dust emission for regional and global atmospheric models. Convective dust emission occurs in the absence of saltation as large eddies intermittently produce strong shear stresses on the surface and entrain dust particles into the air. This dust emission mechanism has not been included in the traditional dust models. The scheme presented in this study is a new approach which takes account of the stochastic nature of convective dust emission. It consists of the statistical representations of soil particle size, inter-particle cohesion, and instantaneous surface shear stress. A method of determining the probability density function of the latter quantity is proposed. Dust emission is then estimated from the overlap of the probability density functions of the aerodynamic lifting and inter-particle cohesive forces. The new scheme is implemented into the WRF/Chem model and applied to dust modeling in the Taklimakan Desert. A comparison with lidar data shows that the model can reproduce the main features of the dust patterns and their diurnal variations. For the case studied, convective dust emission is typically several μg m−2 s−1 and at times up to 50 μg m−2 s−1.

Exploring the reduction of laboratory testing for the cohesive zone model for asphalt concrete

The cohesive zone model is a popular tool used to predict cracking in asphalt concrete. However, two of the three inputs required for the cohesive zone model, tensile stress and fracture energy, come from destructive laboratory tests. If the peak load from a fracture test (which supplies fracture energy) could be substituted for the tensile strength, the amount of laboratory testing could be halved. This research compared the peak loads from the single-edge notch beam, semi-circular bend and disk-shaped compact tension fracture tests as well as the tensile strength from the standard indirect tension (IDT) test. Even accounting for specimen geometry, the data did not indicate consistent correlation of the three fracture tests with the IDT test across various testing temperatures, loading rates or sample type (laboratory vs field samples). Therefore, no fracture test provided a sufficiently robust correlation to recommend replacing the tensile strength with the peak load of the fracture test for the cohesive zone model.

Fracture Analysis of High strength and Ultra high strength Concrete beams by using Finite Element Method

This paper presents the details of nonlinear finite element analysis (FEA) of three point bending specimens made up of high strength concrete (HSC, HSC1) and ultra high strength concrete (UHSC). Brief details about characterization and experimentation of HSC, HSC1 and UHSC have been provided. Cracking strength criterion has been used for simulation of crack propagation by conducting nonlinear FEA. The description about FEA using crack strength criterion has been outlined. Bi-linear tension softening relation has been used for modeling the cohesive stresses ahead of the crack tip. Numerical studies have been carried out on fracture analysis of three point bending specimens. It is observed from the studies that the computed values from FEA are in very good agreement with the corresponding experimental values. The computed values of stress vs crack width will be useful for evaluation of fracture energy, crack tip opening displacement and fracture toughness. Further, these values can also be used for crack growth study, remaining life assessment and residual strength evaluation of concrete structural components.

A Singular Finite Element on the Mixed-Mode Bimaterial Interfacial Cracks

A singular finite element is presented to study the mixed-mode Dugdale-model-based bimaterial interfacial cracks. Firstly, the bimaterial interfacial crack problem is led into the symplectic space, and the symplectic dual equation is obtained and solved analytically. The cohesive stresses of the Dugdale model are treated as special solutions. Subsequently, the analytical solution is employed to develop a novel singular finite element, which depicts accurately the characteristic of displacements and singular stress fields near the crack tip. Finally, combining the singular finite element and conventional finite element method, the length of plastic zone, crack tip opening, and/or sliding displacement can be solved by iteration. Numerical examples are given to illustrate the validity of the present method.

Incremental displacement collocation method for the evaluation of tension softening curve of mortar

The tension softening curve (TSC), showing the relationship between the cohesive tensile stress and crack opening displacement, is the constitutive law of the cohesive crack model. Due to the difficulties in measuring local deformations around the crack tip, the TSC is usually determined inversely from the global responses such as load–deflection curve or load–crack mouth opening displacement curve of pre-notched specimens. However, the use of global responses alone in the inverse analysis usually causes problems that may affect the reliability and accuracy of the TSC which is basically a local material property. To overcome these limitations, an incremental displacement collocation method (IDCM) that is able to evaluate the TSC in a step-by-step manner is proposed in this paper. Both global and local responses of a pre-notched mortar beam, which are measured using an electronic speckle pattern interferometry technique, are used in the displacement collocation process. Furthermore, the finite element model (FEM) is utilized to simulate the response of the beam. The TSCs evaluated in this study are verified through the comparisons of the global and local displacements as well as the fracture energy. A tri-linear curve was found to be the best approximation of the TSC of mortar.

Neighborhood social cohesion and posttraumatic stress disorder in a community-based sample: findings from the Detroit Neighborhood Health Study

Posttraumatic stress disorder (PTSD) is common and debilitating. Although research has identified individual-level risk factors for PTSD, the role of macro-social factors in PTSD etiology remains unknown. This study tests whether perceived neighborhood social cohesion (NSC), measured at the both the individual and neighborhood levels, plays a role in determining past-year risk of PTSD among those exposed to trauma. Data (n = 1,221) were obtained from an ongoing prospective epidemiologic study in the city of Detroit. Assessment of traumatic event exposure and PTSD was consistent with DSM-IV criteria. Generalized estimating equations (GEE) and logistic regression models were used to estimate the association of neighborhood-level perceived NSC with the risk of PTSD, adjusting for individual-level perceptions of NSC and other covariates. The odds of past-year PTSD were significantly higher among those residing in a neighborhood with low social cohesion compared to high (OR = 2.44, 95 % CI: 1.58, 3.78), independent of individual sociodemographic characteristics, number of traumas, and individual-level perceptions of NSC. The odds of past-year PTSD were not significantly associated with individual-level perceptions of NSC. These results demonstrate that social context shapes risk of PTSD and suggest that changing the social context may shift vulnerability to this disorder.

Fabrication and thermal aging behavior of skutterudites with silica-based composite protective coatings

Silica-based composite coatings with dispersion of glass or alumina granular particles were fabricated on n-type Yb0.3Co4Sb12 and p-type CeFe3CoSb12 skutterudite (SKD) thermoelectric (TE) materials using hybrid silica sol as raw material which was synthesized through hydrolysis and condensation under acid condition process. The dispersion of either glass particles or alumina particles restrained the formation of micro-cracks during solidification due to relaxing tremendous cohesive stress in the gel. The high temperature stability for both the coating materials and the coated skutterudite matrix were investigated through thermal aging at 873K in vacuum. Inter-diffusion of Sn from glass toward SKDs and Sb from SKDs toward coating layer was observed in the glass particle dispersed silica composite coating, which resulted in the failure of protecting function of the coating layer. In the alumina particle dispersed silica coating, neither diffusion nor reaction between the coating and SKD materials was observed after thermal aging at 873K. The sublimation of antimony is suppressed in the SKDs overlaid by the alumina dispersed silica composite coating.