A coupled analysis of mechanical behaviour and ageing for polymer-matrix composites

doi:10.1016/S0266-3538(01)00146-4

Composites Science and Technology

Volume 62, Issue 4, March 2002, Pages 543-549

https://doi.org/10.1016/S0266-3538(01)00146-4 Get rights and content

Abstract

The use of organic-matrix composites for long lifetime aeronautics applications leads to a study of the coupling effects between thermo-mechanical stresses and thermal ageing. Taking into account the effects of oxidation in a behaviour law requires first, an understanding of every mechanism of degradation and the interactions between them. The objective of this paper is to present current studies of these subjects and, in particular, our own approach, in order to include oxidation in a macroscopic behaviour law. For the oxidation section, studies have led to establishment of kinetics of thermal-oxidation for several polymer matrixes. For the mechanical section, a visco-elastic model coupled with damage is presented and allows to obtain the multiaxial creep behaviour of any composite laminate from few short-time tests conducted on elementary laminates.

Introduction

This study takes place in the general context of long-term use of composite materials for supersonic applications (submitted to high temperature and severe mechanical loading). The aim is to take into account the effects of thermal ageing of polymer resins in a visco-elastic behaviour law coupled with damage at the macroscopic level. It is first necessary to consider the mechanical, physical and chemical effects separately. The aim of this article is to present the different studies on these subjects at ONERA and the process that will be henceforth followed to couple ageing with mechanical behaviour.

The first part describes the damageable visco-elastic model previously developed and the second part presents a computation of thermal-oxidation of polymer-matrix composites (PMCs) and their potential effects on laminate cracking without mechanical loading. The studied materials (carbon-fibre/polymer resin composites) belong to the materials which could be used for some structural parts of the future European supersonic civil aircraft plane: T800H/F655-2 (bismaleimid resin) and IM7/977-2 (epoxy resin).

Section snippets

Mechanical behaviour

The fundamentals of the visco-elastic model are established in the thermodynamic context which eases the introduction of the coupling of viscous flow with damage. The selected scale is the ply-scale and the model describes an anisotropic non-linear viscoelastic behaviour.

Damage is then integrated, in order to:

•
take into account the unilateral character of damage,
•
integrate the anelastic strains in the damage evolution law and thus consider the possibility of crack propagation during creep process,

OMC oxidation and effects on mechanical behaviour

Nowadays, thermal degradation is still a complex and little-known phenomenon. Indeed, over long periods of temperature exposure, ageing effects added to mechanical damage can considerably alter the material nature by chemical modifications or by micro-cracking appearance. Several models have already been established to understand ageing mechanisms. In this section, we will first describe the acquired knowledge about polymer matrix materials oxidation on an experimental as well as a theoretical

Conclusion

First of all, the understanding of different damage mechanisms occurring during long-time use of polymer composites without mechanical loading was investigated. This step was necessary to better understand how thermal ageing can considerably reduce composite lifetime by decreasing the cracking onset threshold. Thus, a first idea to introduce effects of ageing in the mechanical behaviour could be to take it into account through the cracking threshold. Nevertheless, the phenomenon induced by

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