Microplane Model for Concrete: Part I - State of the Art

The fundamental concepts of the microplane theory for non-linear modeling of concrete are presented in this paper. The basic idea behind the model is that the relationships between stress and strain are defined on planes of various orientations that represent microstructural damage planes, such as contact layers between aggregate pieces. Starting from the pioneering idea formalized as the slip theory of the plasticity, several models have been proposed in literature with the aim of formulating a general procedure for damage and fracture in concrete, under different loading combinations. The literature models are critically discussed with the aim to introduce an application presented in a companion paper [1]: the application regards CFRP-confined concrete elements modeled at macro and meso-scale.

Shape Memory Effect Behavior of NiTi Torque Tubes in Torsion

In order to simulate the torsional behavior of NiTi torque tubes, two different 3D thermo-mechanical constitutive models are utilized. Firstly, an available incremental constitutive model is used in which a return mapping algorithm is implemented to numerically calculate the strains for any applied stresses. Secondly, Microplane theory is employed based on which 1D constitutive laws are considered for associated stress and strain components on any arbitrary plane passing through a material point followed by a homogenization process to generalize the 1D equations to a 3D macroscopic model. Both of the constitutive models are implemented in ABAQUS by developing UMAT. In order to compare the two approaches, torque-angle of rotation and shear stress-shear strain responses for torsion of thin-walled Nitinol torque tubes with different thicknesses are studied. The numerical results of these two approaches show to be in a good agreement indicating the capability of Microplane theory in constitutive modeling of shape memory alloys. This theory provides explicit relationships to calculate strains in terms of stresses, and this makes it very beneficial in obtaining the SMA responses in a fast and easy manner.

Numerical Implementation of a Thermomechanical Constitutive Model for Shape Memory Alloys Using Return Mapping Algorithm and Microplane Theory

In this work, a return mapping algorithm is utilized to implement the model into a finite element program and then Microplane theory is employed. A numerical procedure is also developed to implement the model as a user material subroutine for ABAQUS-Standard commercial code. Uniaxial tension test under a constant axial stress is simulated in order to study the behavior of shape memory alloys. A very good agreement is seen between the results obtained by the two approaches indicating the capability of microplane theory.