Creep and Fatigue in Polymer Matrix Composites

Rui Miranda Guedes

• Publisher: Woodhead Publishing
• Year: 2010
• eBook ISBN: 9780857090430
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

Creep is the tendency of materials to deform when subjected to long-term stress, particularly when exposed to heat. Fatigue phenomena occur when a material is subjected to cyclic loading, causing damage which may progress to failure. Both are critical factors in the long-term performance and reliability of materials such as polymer matrix composites which are often exposed to these types of stress in civil engineering and other applications. This important book reviews the latest research in modelling and predicting creep and fatigue in polymer matrix composites.The first part of the book reviews the modelling of viscoelastic and viscoplastic behaviour as a way of predicting performance and service life. Part two discusses techniques for modelling creep rupture and failure. The final part of the book discusses ways of testing and predicting long-term creep and fatigue in polymer matrix composites.With its distinguished editor and international team of contributors, Creep and Fatigue in Polymer Matrix Composites is a standard reference for all those researching and using polymer matrix composites in such areas as civil engineering.- Reviews the latest research in modelling and predicting creep and fatigue in polymer matrix composites- A specific focus on viscoelestic and viscoplastic modelling features the time-temperature-age superposition principle for predicting long-term response- Creep rupture and damage interaction is examined with particular focus on time-dependent failure criteria for lifetime prediction of polymer matrix composite structures illustrated using experimental cases

Part I

Viscoelastic and viscoplastic modeling

1

Viscoelastic constitutive modeling of creep and stress relaxation in polymers and polymer matrix composites

G.C. Papanicolaou, University of Patras, Greece

S.P. Zaoutsos, Technological Educational Institute of Larissa, Greece

Abstract:

This chapter discusses the basic concepts of viscoelastic behavior of polymers and polymeric composites as well as respective modeling for both the linear and nonlinear behavior. The material is discussed in four main sections. The first discusses creep-recovery and stress relaxation experiments. The second section discusses the concept of linearity in viscoelasticity and a short presentation of the time–temperature, time–stress and time–temperature–stress superposition principles is made. The third section discusses the different linear viscoelastic models, from the basic viscoelastic elements up to the generalized models. Finally, the fourth section discusses the nonlinear viscoelastic behavior of polymers and polymeric composites and different predicting models and methods along with specific applications are presented.

Key words

creep

stress relaxation

viscoelastic modeling

linear viscoelastic behavior

nonlinear viscoelastic behavior modeling

1.1 Introduction

Viscoelasticity is the property of materials that exhibit both viscous and elastic characteristics when undergoing deformation. A viscous material exhibits time-dependent behavior when a stress is applied while under constant stress and deforms at a constant rate, and when the load is removed, the material has ‘forgotten’ its original configuration, remaining in the deformed state. On the other hand, an elastic material deforms instantaneously when stretched and ‘remembers’ its original configuration, returning instantaneously to its original state once the stress is removed. Viscoelastic materials have elements of both of these properties and, as such, exhibit time-dependent strain showing a ‘fading memory’. Such a behavior may be linear (stress and strain are proportional) or nonlinear. Whereas elasticity is usually the result of bond stretching along crystallographic planes in an ordered solid, viscoelasticity is the result of the diffusion of atoms or molecules inside an amorphous material.

Polymers are characterized by the fact that their behavior under load or deformation is, to a large extent, time dependent even at room temperature. Moreover, their response to a load or deformation will depend, in some cases, upon any previous load, deformation or temperature history. This time dependence manifests itself in several forms: two of these are creep, that is to say a progressive increase in deformation under a constant load; and stress relaxation, a gradual decrease in stress under a constant deformation. Both these phenomena infuence and, in many cases, limit the application of plastics for structural and load-bearing applications.

In a plastics pipe assembly, a pipe with its coupling may have to withstand a continuous internal pressure. Under these conditions both the pipe coupling and the pipe will slowly creep; unless the coupling creeps radially less than the pipe, a leak will occur. It is therefore a matter of design to ensure that the relative movement is kept to a minimum.

Sometimes a plastics component or part is deliberately deformed so that it should exert a mechanical force due to its elasticity. A spring will grip or support an article as long as the external forces exerted on the article are less than the force exerted on it by the grip or spring. If the grip or spring is made of a plastics material, stress relaxation will take place and the force exerted by it, when it is deformed, will progressively decrease with time until it is no longer able to oppose the external forces. A similar problem can be encountered with bottle closures; these remain in position by virtue of the fact that they are strained: if stress relaxation is present these closures may eventually fail. The property of stress relaxation does not preclude the use of plastics for this type of application but knowledge of the degree to which it occurs under various conditions is essential for design purposes.

In general, given suitable laboratory information on the behavior of poly...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Contributor contact details
• Part I: Viscoelastic and viscoplastic modeling
• Part II: Creep rupture
• Part III: Fatigue modeling, characterization and monitoring
• Index