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Janis Varna - One of the best experts on this subject based on the ideXlab platform.
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Nonlinear viscoelastic viscoplastic material model including Stiffness Degradation for hemp/lignin composites
Composites Science and Technology, 2008Co-Authors: Erik Marklund, Johannes Eitzenberger, Janis VarnaAbstract:Abstract In repeating tensile tests with increasing maximum strain for every loading cycle the hemp/lignin composites clearly showed a nonlinear behavior and hysteresis loops in loading and unloading. The explanation for this behavior is the inherent viscoelastic nature for this type of material, but also noticeable Stiffness Degradation with increasing strain level. Creep tests performed at different stress levels revealed a nonlinear viscoelastic response and after recovery viscoplastic strain was detected for high stress levels. It is demonstrated that Schapery’s model is suitable to model nonlinear viscoelasticity whereas viscoplastic strain may be described by a nonlinear functional presented by Zapas and Crissman. In a creep test this functional leads to a power law with respect to time and stress. In order to include Stiffness reduction due to damage Schapery’s model has been modified by incorporating a maximum strain-state dependent function reflecting the elastic modulus reduction with increasing strain measured in tensile tests. A generalized incremental model of the constitutive equation for viscoelastic case has been used to validate the developed material model in a linear stress controlled loading and unloading ramp. The model successfully describes the main features for the investigated material and shows good agreement with test data within the considered stress range.
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nonlinear viscoelastic viscoplastic material model including Stiffness Degradation for hemp lignin composites
Composites Science and Technology, 2008Co-Authors: Erik Marklund, Johannes Eitzenberger, Janis VarnaAbstract:Abstract In repeating tensile tests with increasing maximum strain for every loading cycle the hemp/lignin composites clearly showed a nonlinear behavior and hysteresis loops in loading and unloading. The explanation for this behavior is the inherent viscoelastic nature for this type of material, but also noticeable Stiffness Degradation with increasing strain level. Creep tests performed at different stress levels revealed a nonlinear viscoelastic response and after recovery viscoplastic strain was detected for high stress levels. It is demonstrated that Schapery’s model is suitable to model nonlinear viscoelasticity whereas viscoplastic strain may be described by a nonlinear functional presented by Zapas and Crissman. In a creep test this functional leads to a power law with respect to time and stress. In order to include Stiffness reduction due to damage Schapery’s model has been modified by incorporating a maximum strain-state dependent function reflecting the elastic modulus reduction with increasing strain measured in tensile tests. A generalized incremental model of the constitutive equation for viscoelastic case has been used to validate the developed material model in a linear stress controlled loading and unloading ramp. The model successfully describes the main features for the investigated material and shows good agreement with test data within the considered stress range.
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transverse cracks in cross ply laminates 2 Stiffness Degradation
Mechanics of Composite Materials, 1998Co-Authors: Janis Varna, Andrejs KrasnikovsAbstract:From the results of stress analysis between two transverse cracks in cross-ply laminate [1], a model for the Stiffness reduction based on generalized plane strain assumptions has been developed. Simple analytical expressions are obtained for the longitudinal modulus and the Poisson's ratio as a function of the transverse crack density. Apart from the crack density, these expressions depend only on the elastic and geometrical properties of constituent laminae and the average crack opening displacement (ACOD) normalized in the proper way. Calculations of the ACOD are performed and analyzed with the FEM and analytical models used for the stress analysis in [1]. The predicting capabilities of approximate models are discussed in comparison with experimental data and FEM results. In order to predict the Stiffness Degradation for a wide variety of laminates, a simple procedure requiring only one FEM calculation for some “average laminate” with “average crack spacing” is proposed and has been proved effective.
Julian Carrillo - One of the best experts on this subject based on the ideXlab platform.
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Stiffness Degradation model of thin and lightly reinforced concrete walls for housing
Engineering Structures, 2018Co-Authors: Julian Carrillo, Diego Vargas, Martha L SanchezAbstract:Abstract The Stiffness of structural walls is a key parameter for seismic design because the distribution of shear forces depends on the relative Stiffness of each wall of the structural system. The study of Stiffness Degradation of low-rise reinforced concrete walls has been focused on the description of the deterioration of such structural property. This study aims at proposing a semi-empirical model for assessing the lateral Stiffness Degradation in terms of drift ratio of thin and lightly reinforced concrete walls for low-rise and low-cost housing subjected to seismic demands. The variables of the study are the height-to-length ratio of walls (0.5, 1 and 2), type of concrete (normal-weight, light-weight and self-compacting), steel ratio of web shear reinforcement (0.125% and 0.25%), and the type of web shear reinforcement (deformed bars and welded-wire meshes). The experimental program comprises 23 walls tested under quasi-static reversed-cyclic loading. Lateral Stiffness computed using measured response of walls are compared with Stiffness computed using recommendations prescribed by codes or reported in a literature review. The effect of geometrical and mechanical properties of walls on Stiffness is also assessed in the paper. In addition, the study proposes limit values of Stiffness Degradation for different limit states and performance levels.
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damage index based on Stiffness Degradation of low rise rc walls
Earthquake Engineering & Structural Dynamics, 2015Co-Authors: Julian CarrilloAbstract:Summary Widely used damage indices, such as ductility and drift ratios, do not account for the influences of the duration of strong shaking, the cumulative inelastic deformation or energy dissipation in structures. In addition, the formulation and application of most damage indices have until now been based primarily on flexural modes of failure. However, evidence from earthquakes suggests that shear failure or combined shear-flexure behavior is responsible for a large proportion of failures. Empirical considerations have been made in this paper for evaluating structural damage of low-rise RC walls under earthquake ground motions by means of a new energy-based low-cycle fatigue damage index. The proposed empirical damage index is based on the results of an experimental program that comprised six shake table tests of RC solid walls and walls with openings; results of six companion walls tested under QS-cyclic loading were used for comparison purposes. Variables studied were the wall geometry, type of concrete, web shear steel ratio, type of web shear reinforcement, and testing method. The index correlates the Stiffness Degradation and the destructiveness of the earthquake in terms of the duration and intensity of the ground motions. The Stiffness Degradation model considers simultaneously the increment of damage associated to the low-cycle fatigue, energy dissipation, and the cumulative cyclic parameters, such as displacement demand and hysteretic energy dissipated. Copyright © 2014 John Wiley & Sons, Ltd.
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Damage index based on Stiffness Degradation of low‐rise RC walls
Earthquake Engineering & Structural Dynamics, 2014Co-Authors: Julian CarrilloAbstract:Summary Widely used damage indices, such as ductility and drift ratios, do not account for the influences of the duration of strong shaking, the cumulative inelastic deformation or energy dissipation in structures. In addition, the formulation and application of most damage indices have until now been based primarily on flexural modes of failure. However, evidence from earthquakes suggests that shear failure or combined shear-flexure behavior is responsible for a large proportion of failures. Empirical considerations have been made in this paper for evaluating structural damage of low-rise RC walls under earthquake ground motions by means of a new energy-based low-cycle fatigue damage index. The proposed empirical damage index is based on the results of an experimental program that comprised six shake table tests of RC solid walls and walls with openings; results of six companion walls tested under QS-cyclic loading were used for comparison purposes. Variables studied were the wall geometry, type of concrete, web shear steel ratio, type of web shear reinforcement, and testing method. The index correlates the Stiffness Degradation and the destructiveness of the earthquake in terms of the duration and intensity of the ground motions. The Stiffness Degradation model considers simultaneously the increment of damage associated to the low-cycle fatigue, energy dissipation, and the cumulative cyclic parameters, such as displacement demand and hysteretic energy dissipated. Copyright © 2014 John Wiley & Sons, Ltd.
El M Mansori - One of the best experts on this subject based on the ideXlab platform.
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modelling of chip separation in machining unidirectional frp composites by Stiffness Degradation concept
Composites Science and Technology, 2009Co-Authors: L Lasri, Mohammed Nouari, El M MansoriAbstract:Abstract Progressive failure of unidirectional glass fiber-reinforced polymer composites (FRP) was studied using finite element analysis in orthogonal machining. Chip formation process and damage modes such as matrix cracking, fiber–matrix debonding and fiber breaking were modelled by degrading the material properties. Damage analysis was carried out using Hashin, Maximum stress and Hoffman failure criteria. After damage was detected, selective Stiffness Degradation was applied to the workpiece material. The objective of this study is to better understand the chip formation process and to analyse the cutting-induced damage from initiation stage until complete chip formation. The effect of the fiber orientation on cutting forces and sub-surface damage was investigated with different failure criteria. The results were addressed in terms of cutting forces evolution and damage progression in the composite structure during machining. It was demonstrated that the use of the Stiffness Degradation concept with the appropriate failure criterion responds potentially in a predictable fashion to changes in chip formation process for machining of FRPs.
Erik Marklund - One of the best experts on this subject based on the ideXlab platform.
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nonlinear viscoelastic viscoplastic material model including Stiffness Degradation for hemp lignin composites
Composites Science and Technology, 2008Co-Authors: Erik Marklund, Johannes Eitzenberger, Janis VarnaAbstract:Abstract In repeating tensile tests with increasing maximum strain for every loading cycle the hemp/lignin composites clearly showed a nonlinear behavior and hysteresis loops in loading and unloading. The explanation for this behavior is the inherent viscoelastic nature for this type of material, but also noticeable Stiffness Degradation with increasing strain level. Creep tests performed at different stress levels revealed a nonlinear viscoelastic response and after recovery viscoplastic strain was detected for high stress levels. It is demonstrated that Schapery’s model is suitable to model nonlinear viscoelasticity whereas viscoplastic strain may be described by a nonlinear functional presented by Zapas and Crissman. In a creep test this functional leads to a power law with respect to time and stress. In order to include Stiffness reduction due to damage Schapery’s model has been modified by incorporating a maximum strain-state dependent function reflecting the elastic modulus reduction with increasing strain measured in tensile tests. A generalized incremental model of the constitutive equation for viscoelastic case has been used to validate the developed material model in a linear stress controlled loading and unloading ramp. The model successfully describes the main features for the investigated material and shows good agreement with test data within the considered stress range.
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Nonlinear viscoelastic viscoplastic material model including Stiffness Degradation for hemp/lignin composites
Composites Science and Technology, 2008Co-Authors: Erik Marklund, Johannes Eitzenberger, Janis VarnaAbstract:Abstract In repeating tensile tests with increasing maximum strain for every loading cycle the hemp/lignin composites clearly showed a nonlinear behavior and hysteresis loops in loading and unloading. The explanation for this behavior is the inherent viscoelastic nature for this type of material, but also noticeable Stiffness Degradation with increasing strain level. Creep tests performed at different stress levels revealed a nonlinear viscoelastic response and after recovery viscoplastic strain was detected for high stress levels. It is demonstrated that Schapery’s model is suitable to model nonlinear viscoelasticity whereas viscoplastic strain may be described by a nonlinear functional presented by Zapas and Crissman. In a creep test this functional leads to a power law with respect to time and stress. In order to include Stiffness reduction due to damage Schapery’s model has been modified by incorporating a maximum strain-state dependent function reflecting the elastic modulus reduction with increasing strain measured in tensile tests. A generalized incremental model of the constitutive equation for viscoelastic case has been used to validate the developed material model in a linear stress controlled loading and unloading ramp. The model successfully describes the main features for the investigated material and shows good agreement with test data within the considered stress range.
Shuai Liang - One of the best experts on this subject based on the ideXlab platform.
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Stiffness Degradation of shear walls under cyclic loading experimental study and modelling
Bulletin of Earthquake Engineering, 2019Co-Authors: Xiangyong Ni, Yizhu Li, Shuai LiangAbstract:Stiffness is an important parameter for the seismic design of shear walls, which is related to the shear force distribution of each wall member. The Stiffness Degradation occurs in shear walls under the effect of earthquakes, and the effect of Stiffness Degradation is considered by a constant reduction factor in many design codes. However, the constant reduction factor cannot consider the whole Stiffness Degradation process of shear walls. The aim of this paper is to experimentally study and model the Stiffness Degradation of shear walls under cyclic loading. Five wall specimens were tested under cyclic loading to study the influence of reinforcing bar strength, axial load ratio, failure mode, and cross-section type on the Stiffness Degradation. Based on the experimental Stiffness Degradation curves of rectangular shear walls failing in flexure, a four-line Stiffness Degradation model controlled by crack, yield, peak, and ultimate points was proposed, and the calculation methods for the values of each point were established. The four-line Stiffness Degradation model was used to obtain the analytical Stiffness Degradation curves of the wall specimens, which were compared with the experimental Stiffness Degradation curves. Study shows that increasing reinforcing bar strength decreases the initial Stiffness, and increasing axial load ratio significantly improve the initial Stiffness while accelerating the Stiffness Degradation rate with drift ratio. The Stiffness of shear walls failing in flexure is more fully degenerated than that of shear walls failing in shear. The Stiffness of T-shaped shear walls is larger loaded in the positive direction than that loaded in negative direction. The flange improves the Stiffness while accelerates the Stiffness Degradation rate with drift ratio. The analytical and experimental Stiffness Degradation curves were in the reasonable agreement.