Papers by Keyword: Viscoplasticity

Abstract: Despite the growing use of biopolymers in automotive, packaging and structural applications, predictive modelling of their elastic–viscoplastic deformation remains limited. In this work, a micromechanically based constitutive model is proposed to describe the micro‑ to macroscopic behaviour of a semi‑crystalline PLA matrix reinforced with short hemp fibers. The formulation relies on a multiplicative split of the deformation gradient into elastic and viscoelastic–plastic parts, with elasticity governed by fiber and crystalline phases and time‑dependent deformation localized in the amorphous phase. High fiber content and strong fiber–matrix bonding enable the suppression of lattice crystalline anisotropy, leading to a compact model with a reduced number of internal variables. The model is calibrated and validated using uniaxial tensile tests on pellet‑extrusion 3D‑printed specimens with controlled porosity and plasticiser content, and reproduces nonlinear loading, unloading, creep and stress relaxation. In a second step, synthetic data generated by the constitutive model are used to train surrogate machine‑learning models, which are discussed as a perspective for accelerating long‑term simulations and parametric studies in forming applications.

Abstract: Noting that there is very little literature on the topic, a first analytical approach is proposed in this work for estimating the viscosity-like parameter of three-phase viscoplastic materials. In a first part, the conditions of application and the consequences of the three classical averaging equations involving the strain rates, the stresses and the power are reviewed for 2-phase mixtures and extended to three phases. The classical static and Taylor bounds as well as the heuristic Iso-strain rate assumption are analyzed. An extension of the Mori-Tanaka estimation to the three-phase case is then proposed for viscoplastic linear constituents. If the volume fraction of one of the phases (inclusions) is very low, in particular when its viscosity tends towards zero or infinity, fully analytical results are presented, which provides an extension of the classical dilute model.

Abstract: This paper shows some cyclic plastic properties of the lead free solder alloy Sn-3.5Ag-0.75 under fully reversed cyclic torsional loading. The low-cycle fatigue behavior of the material was evaluated too. The lead free solder material reveals a strong strain rate effect. A viscoplastic model was implemented into a commercial finite element program and calibrated using data from literature to can be used for simulations of all performed tests in a future work.

Abstract: In this study, development of a decoupled multiscale analysis method for woven composites is conducted. To this end, an elastic-viscoplastic macroscopic constitutive model which is able to express strong anisotropy of composites is introduced, and the material parameters in the constitutive model are determined based on the results of triple-scale homogenization analysis. Moreover, the constitutive model is implemented in the finite element analysis code LS-DYNA. The developed method is applied to 3-point bending analysis of plain-woven carbon fiber-reinforced plastic (CFRP) composites with various types of laminates configurations. It is shown that the present method can analyze their different behavior depending on the laminate configuration with greatly reduced computational costs.

Abstract: In this study, a two-scale thermoelastoviscoplastic analysis method for carbon fiber-reinforced plastic (CFRP) laminates is proposed based on a homogenization theory for time-dependent composites. For this, macroscopic and microscopic boundary value problems for CFRP laminates are derived to discuss the relation between the two problems. Using the relation, a two-scale thermoelastoviscoplastic analysis method is constructed, and then applied to the analysis of thermal behavior of an unsymmetric cross-ply carbon fiber/epoxy laminate. The laminate is subjected to a macroscopic temperature change from 180°C to 20°C. It is shown that quite high residual stress and strain occur both macroscopically and microscopically in the laminate, resulting in large macroscopic warpage of the laminate.

Abstract: The elasto-viscoplastic behavior of slanting-weft woven laminates, the fiber bundles of which are not crossed at a right angle, is investigated both macroscopically and microscopically. For this, an analysis model for the [±θ] slanting-weft woven laminate with a cross angle ±θ and its diamond-shaped unit cell are considered. Then, a basic cell, which is quarter of the unit cell, is defined as an analysis domain by considering the point-symmetry of the internal structure. For the basic cell, the homogenization theory for nonlinear time-dependent composites with point-symmetric internal structures is applied. Using the present method, the elasto-viscoplastic analysis of the [±θ] slanting-weft woven laminates subjected to an in-plane uniaxial tensile load is performed. From the analysis results, the macroscopic elasto-viscoplastic behavior and the microscopic stress and strain distributions of the laminates are investigated.

Abstract: The specimens (980 MPa-grade dual phase steel sheets) were stretched until the pre-defined strain was obtained. Then the specimens were held at the pre-defined strain and measured the change of stress durng holding. We investigated the effects of strain rate and strain at the starting time of holding and whether the stress change during holding could be described by Krempl model. The following results were obtained. First, the stress drop increased with increase of strain rate and the holding time. On the other hand, the stress drop was not affected by strain change at the starting time of holding. Second, initial stress relaxation rate increased with increase of strain rate. However, this strain rate dependency to stress relaxation rate diminished as the holding time became long enough roughly more than 100 s. Third, the stress change during holding obtained by Krempl model accurately agreed with experimental result. It was found that the stress change during holding could be well described by using Krempl model. This suggests that dislocation moves viscously. In addition, the strain rate dependency on stress change during holding could be described by change of the parameter A.

Abstract: Microdamage, viscoplastic and viscoelastic strain development in 90-layers of cross-ply laminates subjected to tensile loading is studied on unsymmetrical GF/EP laminates measuring the thermal curvature change. All three phenomena partially compensate for the effect of the thermal mismatch reducing the residual stress (specimen curvature). The viscoplastic strain contribution to curvature change is the largest whereas the effect of transient viscoelasticity is the smallest. Damage is included in the analysis through its effect on the effective transverse modulus of the 90-layer.

Abstract: A method for the virtual process design of combined quasi-static and electromagnetic forming processes based on a thorough process simulation is developed. Its flexibility is demonstrated by means of an identification problem for process parameters yielding a minimum bottom edge radius in round cup forming. Particularly, an optimum double exponential current pulse is identified. This class of pulses is parameterized as an example for pulses with mono-directed current employed to reduce the wear of the tool coil.

Abstract: A fully-modeled unit cell analysis is performed to investigate the macroscopic and microscopic elastic-viscoplastic behaviors of a quasi-isotropic carbon fiber-reinforced plastic (CFRP) laminate. To this end, a quasi-isotropic CFRP laminate and its microstructure composed of carbon fibers and a matrix material are considered three-dimensionally. Then, a hexagonal prism-shaped unit cell fully modeled with fibers and a matrix including interlaminar areas is defined. For this quasi-isotropic laminate, a homogenization theory for nonlinear time-dependent composites with point-symmetric internal structures is applied, enabling us to analyze both the macroscopic and microscopic elastic-viscoplastic behaviors of the laminate. The substructure method is introduced into the theory to reduce computational costs. The present method is then applied to the elastic-viscoplastic analysis of a quasi-isotropic carbon fiber/epoxy laminate subjected to an in-plane uniaxial tensile load, to investigate the macroscopic elastic-viscoplastic behavior of the laminate and the microscopic stress and strain distributions in them.