Numerical analysis of mechanical damage on concrete under high temperatures

Concrete is a widespread material all over the world. Due to this material’s heterogeneity and structural complexity, predicting the behavior of concrete structures under extreme environmental conditions is a very challenging task. High temperatures lead to microstructural changes which affect the macrostructural performance. In this context, computational tools that allow the simulation of structures may assist the analysis, by reproducing varied situations of thermal and mechanical loading and boundary conditions. In order to contribute to this scenario, this study proposes a numerical methodology to simulate the thermomechanical behavior of concrete under temperature gradients, through inverse analyses and a user subroutine implemented in Abaqus software. Thermal loading effects were considered as loading data for a damage model. Experimental data available in the literature was adopted for adjustment and validation purposes. The preliminary results presented herein encourage further improvements so as to allow realistic simulations of such an important aspect of concrete’s behavior.

Numerical studies on the heat dissipation process in elastomers under rotating loading direction

Elastomeric components such as car bearings and vibration dampers are subjected to dynamic loads with various amplitudes and loading directions during operation. To better understand the lifetime expectancy of these components it is required to implement a material model that sufficiently accounts for the material thermo-mechanical behaviour. This paper implements a finite viscoelastic model which includes heat dissipation and addresses the effect of inelasticity on the self-heating and the applied loading conditions. The material model is implemented in a user subroutine and finite element calculations are carried out on a simple shear loading with rotating directions. The self-heating effect and the resulting variation of the dissipation induced forces are shown and discussed. With the aid of the presented material model, thermo-mechanically coupled simulations can be performed. Based on the results, the required loading limits and boundary conditions for the mechanical fatigue tests can be defined to minimise the thermal fatigue effects.

Applications of a Semi-Crystalline Thermoplastic Constitutive Model to Mechanical Responses of Electronic Connector Structures

The retention force of electronic connectors, in general one of the essential specification requirements, is defined as a maximum force of metallic terminals withdrawn out of the corresponding plastic housing. Accurate prediction of the retention force is an important issue in the connector design stage; however, it is not an easy task to accurately assess the retention force based on the authors’ knowledge. A finite element analysis is performed in conjunction with a self-coded user subroutine accounting for relaxation/creep behaviors of semi-crystalline thermoplastic polymers under various loading conditions in order to appraise the mechanical performance of the plastic base structure. Material parameters adopted in the constitutive model are evaluated by utilizing the automated design exploration and optimization commercial software. Applications of the developed subroutine with several damage criteria to assess retention forces of two electronic connectors were conducted. Retention forces predicted by utilizing the current constitutive model agreed fairly well with the associated experimental measurements. A dramatic improvement of the underestimation of the retention force based on the approach commonly adopted in the industry is also demonstrated here.

Material Modeling of PMMA Film for Hot Embossing Process

This study provides an analysis of the hot embossing process with poly methyl methacrylate (PMMA) film. The hot embossing process engraves a fine pattern on a flexible film using a stamp, applied heat and pressure. As the quality of the embossing pattern varies according to various process variables, the mechanism of making the embossed shape is complicated and difficult to analyze. Therefore, analysis takes much time and cost because it usually has to perform a lot of experiments to find an appropriate process condition. In this paper, the hot embossing process was analyzed using a computational analysis method to quickly find the optimal process. To do this, we analyzed the embossing phenomenon using the finite element method (FEM) and arbitrary Lagrangian–Eulerian (ALE) re-mesh technique. For this purpose, we developed a constitutive model considering the strain, strain rate, temperature-dependent stress and softening of the flexible film. Work hardening, strain softening, and temperature-softening behavior of PMMA materials were well described by the proposed method. The developed constitutive model were applied in the embossing analysis via user-subroutine. This proposed method allowed a precise analysis of the phenomenon of film change during the hot embossing process.

Finite Element Modeling and Experimental Validation of Microstructural Changes and Hardness Variation During Gas Metal Arc Welding of Aisi 441 Ferritic Stainless Steel

Grain growth and hardness variation occurring in high temperature Heat Affected Zone (HAZ) during the welding processes are two thermal dependant aspects of great interest for both academic and industrial research activities. This paper presents an innovative Finite Element (FE) model capable to describe the grain growth and the hardness decrease that occur during the Gas Metal Arc Welding (GMAW) of commercial AISI 441 steel. The commercial FE software SFTC DEFORM-3DTM was used to simulate the GMAW process and a user subroutine was developed including a physical based model and the Hall-Petch (H-P) equation to predict grain size variation and hardness change. The model was validated by comparison with the experimental results showing its reliability in predicting important welding characteristics temperature dependant. The study provides an accurate numerical model (i.e. user subroutine, heat source fitting, geometry,…) able to successfully predict the thermal phenomena (i.e. coarsening of the grains and hardening decrease) that occur in the HAZ during welding process of ferritic stainless steel.

ORTHOTROPIC DAMAGE MODEL USING TSAI-WU FAILURE CRITERIA

This paper presents a novel approach concerning the development of an orthotropic damage model, based on the original plane Tsai-Wu failure criteria. In its original formulation, the Tsai-Wu is a mode independent criterion only capable of acknowledging the existence of damage in a certain point of the material. It is not capable of identifying if the damage is located in the fiber, matrix or intralaminar zone. This work plans to fill this gap in knowledge by providing a simple method, based on equivalent stress and strains, that identifies the failure modes when the Tsai-Wu failure criteria is near the on-set of damage. Using this novel method, it is possible to implement classical damage evolutions constitutive laws based on the MTL formulation. At the moment the proposed damage formulation is based on plane stress space and Mode I fracture, but it is expected in the future to evolve in to a full 3D damage model. The damage model is implemented in the commercial finite element software ABAQUS using user-subroutine UMAT, and all numerical models are compared with the experimental results.

Prediction of Ductile Damage and Fracture in the Single and Multi-stage Incremental Hole-flanging Process Using a New Damage Accumulation Law

Incremental hole-flanging (IHF) is a process in which a sheet with a pre-cut hole is flanged by the single point incremental forming (SPIF) process. Fracture prediction in IHF, such as SPIF, is associated with many challenges due to the deformation mechanisms. The purpose of this paper is to overcome the existing limitations and challenges, and thus, to predict accurately failure in single, and multi-stage IHF processes. To this end, the modified Mohr-Coulomb (MMC) criterion was implemented using an appropriate user subroutine in a finite element method (FEM) model. The AA6061-T6 aluminum alloy sheet, which has low formability, and is fractured from its free edges in the IHF process, was examined as an example. Initially, a linear damage accumulation law, in which the prediction error is high due to the non-linear stress and strain states in the IHF, was used to predict the fracture. Therefore, in the next step, a non-linear damage accumulation function was utilized. While the non-linear accumulation accurately predicts the single-stage IHF fracture, it is not able to predict the fracture well in the multi-stage IHF. It was observed that in multi-stage IHF, the rate of damage accumulation decreases with increasing the number of forming stages. Accordingly, a new non-linear damage accumulation rule was developed. Experimental and numerical results indicated acceptable accuracy of the proposed non-linear accumulation in the fracture prediction in the single and multi-stage IHF process.

Implementation of a Neural Network into a User-Material Subroutine for Finite Element Simulation of Material Viscoplasticity

In this study, a neural network is trained to predict the response of a viscoplastic solder alloy based on a reduced data set. The model is shown to accurately describe the behavior of the material for the temperature range from 298°K to 398°K and the strain rate range from 2e-5 sec−1 to 2e-2 sec−1. The model is then implemented in the form of a user subroutine in the finite element code Abaqus to be used for simulations of the material behavior. The implementation requires that the weights and biases of the network are extracted and that its gradients (derivatives of the output with respect to the inputs) are calculated to be passed on to the user subroutine. FE simulations based on the implemented neural network are compared with those based on the physical viscoplastic model of Anand, showing an overall good agreement between both approaches. However, some limitations concerning the neural network ability to predict the transient effects during a strain rate jump or a temperature change are identified and discussed.

Soft Finger Modelling and Co-Simulation Control towards Assistive Exoskeleton Hand Glove

The soft pneumatic actuators of an assistive exoskeleton hand glove are here designed. The design of the actuators focuses on allowing the actuator to perform the required bending and to restrict elongation or twisting of the actuator. The actuator is then modeled using ABAQUS/CAE, a finite element modeling software, and the open loop response of the model is obtained. The parameters of the actuator are then optimized to reach the optimal parameters corresponding to the best performance. Design of experiment (DOE) techniques are then approached to study the robustness of the system. Software-in-the-loop (SiL) is then approached to control the model variables via a proportional-integral-derivative (PID) control generated by FORTRAN code. The link between the two programs is to be achieved by the user subroutine that is written, where the subroutine receives values from ABAQUS/CAE, performs calculations, and passes values back to the software. The controller’s parameters are tuned and then the closed loop response of the model is obtained by setting the desired bending angle and running the model. Furthermore, a concentrated force at the tip of the actuator is added to observe the actuator’s response to external disturbance.