Amar Khennane

Thermoplasticity model for concrete under transient temperature and biaxial stress

In the past, the theory of thermoplasticity has been confined to metal type materials exhibiting an elastic-perfectly-plastic behaviour. This paper describes the application of this theory to modelling the response of a nonlinear hardening material (concrete in the present case) under transient temperature and stress. The difficulties arising from the application of the theory of thermoelastoplasticity to modelling the behaviour of concrete at elevated temperatures are discussed, together with the inadequacy of the existing algorithms that were proposed for perfectly plastic materials, to cope with a nonlinear hardening case. An integration scheme derived from the Euler backward scheme is used to integrate the rate equations. The resulting model is used to analyse existing biaxial data and investigate the effect of a sustained load on the deformational response of concrete under biaxial loading and elevated temperature.

Plasticity model for the biaxial behaviour of concrete at elevated temperatures, part I: failure criterion

The mechanical response of concrete at elevated temperatures under a generalized state of stress is extremely nonlinear, with both the plastic behaviour and mechanical properties highly temperature dependent. In this and a companion article the plastic response is modelled as possible. Initially, a general analytical form of the failure surface is developed from the available biaxial test data. The envelope is written in terms of stress invariants as a function of temperature, and shows good agreement with experiment. For a given temperature, the loading surface is found by scaling the failure envelope using a hardening parameter. The model is used in the sequel to develop incremental elastic-plastic relationships, and to study deformations in a range of biaxial tests.

Analysis of CFRP-strengthened timber beams

The aim of this study is to develop a numerical procedure to simulate the flexural behaviour of carbon fibre-reinforced plastic-strengthened timber beams. Since wood exhibits complex failure modes, its behaviour can only be captured through the use of multidimensional failure criteria, a local approach based on the coupling of orthotropic elasticity and anisotropic plasticity described with the quadratic criterion of Hill is presented to model this behaviour. The theoretical and numerical aspects of the constitutive equations are presented in detail. The resolution of the resulting system of equations is carried out via a VUMAT user material, using ABAQUS/Explicit finite element code. The obtained results show that the proposed formulation can efficiently capture the global response with acceptable accuracy.

Plasticity models for the biaxial behaviour of concrete at elevated temperatures, part II: implementation and simulation tests

The thermo-mechanical behaviour of concrete at elevated temperatures under multiaxial stress is extremely nonlinear. In Part I we developed a general analytical failure criterion based on the available biaxial test data. Here, the loading surfaces are used to derive the elastic-plastic incremental stress-strain relationships for a plasticity approach, and the model is used to predict experimental strain measurements under biaxial stress. Agreement is shown to be very good. The predictions are also used to suggest possible variations in material parameters such as Poissons ratio, for which scant data exist.

Manufacture and Testing of a Hybrid Beam using a Pultruded Profile and High Strength Concrete

Abstract An improved design concept for a hybrid FRP-HSC (fibre reinforced polymer – high strength concrete) beam is presented. To reduce the initial cost of the beam, an “off the shelf” pultruded FRP profile and CFRP (carbon fibre reinforced plastic) laminate are combined with HSC. The HSC block and the CFRP laminate are bonded to the pultruded profile. The bonded system is then wrapped with a GFRP (glass fibre reinforced plastic) laminate to mitigate any debonding of the concrete, and hence create a full composite action between the concrete and the profile. The major feature of the proposed design is that it does not mimic that of reinforced concrete as reported previously. The CFRP laminate is not designed to fail first to serve as a warning of imminent failure, but rather to enhance the stiffness of the beam by compensating for the lack of stiffness of the GFRP profile. Experimental results have shown that this approach is successful. The much sought after pseudo ductility is fully achieved through the brittleness of the HSC block instead of that of the CFRP laminate, as reported in previously published similar works.

Effect of Transient Creep on Behavior of Reinforced Concrete Beams in a Fire

When concrete is subjected to elevated temperatures under a state of compressive stresses, it experiences transient creep. Although it is particularly relevant to the behavior of columns that most of the time are under a state of compression, its effects on the behavior of beams has hardly attracted any interest. Yet, in a real fire situation, it is possible for a beam to be subject to intensive heat on the compressive face. Using a previously developed finite element code, which is capable of considering transient creep either explicitly as an additional strain rate component or implicitly through the deformation properties of concrete, a systematic study on its effects on the behavior of beams is presented. It was found that transient creep does not have an effect on simply supported beams heated on the tensile face. On the other hand, when both the load and the heating are applied on the compressive face, transient creep has a major effect on the deformational behavior of the beam.

Numerical Analysis of the Cutting Forces in Timber

This paper presents an incremental approach based on the theories of continuum damage and anisotropic plasticity to model the forces that develop during timber cutting. The model used is based on the thermodynamics of irreversible processes with state variables and takes into account the coupling among orthotropic elasticity, anisotropic plasticity, and damage. The aim is to contribute to the development of simulation tools needed to optimize industrial cutting processes. Validation of the model, implemented using existing finite-element (FE) software, is carried out by simulating the forces that develop during the cutting of wooden planks and tree trunks of different timber species. It was found that there is good agreement between the predicted and measured cutting forces.

A micromechanics model for environmental stress corrosion in GFRP

Understanding the mechanisms of environmental stress corrosion is very important for assessing the durability and damage tolerance predictions of composites using glass as the main reinforcement. A mechanistic model describing these mechanisms for unidirectional GFRP in tension is described. The model is based both on the chemical behaviour of glass, and in particular that of glass flaws, and on more recent models of stress corrosion. These were combined with fracture mechanics, the shear lag theory, and a probability model for flaw size. The results are very encouraging. The model shows that it is possible to obtain mechanisms of GFRP breakdown, which corresponds to observed experimental behaviour.

Design and analysis of a composite beam for infrastructure applications - Part I: Preliminary investigation in bending

This study aims to contribute to the development of a composite beam for use in civil engineering systems. Based on the limitations in existing concepts, a new beam design is proposed and its behaviour studied. Using the classical beam theory, the Timoshenko beam theory, the Timoshenko plate theory, as well as the transformed section approach, borrowed from reinforced concrete, a simplified analytical approach, which could be used in design, is developed to conduct first and second order analysis of the proposed beam in order to achieve a rational sizing of its section before a rigorous testing regime is carried out. Finally, to validate the analytical model and gain confidence in the design, the analytical and experimental results are compared to a rigorous nonlinear finite element solution. The analytical model agreed relatively well with the experiments and the FE analyses, giving confidence in the validity of the underlying assumptions.

Filament winding processes in the manufacture of advanced fibre-reinforced polymer (FRP) composites

Abstract: Despite considerable potential and many advantages over conventional materials, composites are making limited progress in the field of infrastructure applications, where the only niche market for composites is in FRP deck construction over steel girders and externally bonded FRP repair. The reasons, of course, are to be found in their high initial cost compared with conventional materials. This can only be addressed through the use of large-volume automated processes such as pultrusion and filament winding, which have the potential to lower the cost of raw materials and technologies for all applications. This chapter summarises the current level of applications of filament winding in the infrastructure industry.