Experimental Methods for Characterizing the Time-Dependent Behavior of Fiber Composites with Growing Damage
Authors: Bocchieri R.T. , Schapery R.A. 
Published: October 2008

Abstract
The offshore industry has shown great interest in using polymer composites for primary structural components due to their weight-saving and corrosion-resistive properties. Coiled tubulars and production risers are two primary structures where using carbon and glass fiber composites has been proposed. Both applications involve high stress conditions which will induce damage and creep in the material. Understanding the type of material behavior described in this paper will be needed for safe, reliable designs. The ply-level, nonlinear viscoelastic stress-strain behavior of a rubbertoughened carbon/epoxy composite is described. Emphasized are some practical aspects of carrying out a material characterization. For constant stress-rate conditions, it is found for a large portion of the loading curve that the shear and transverse compliances can be described by a quasielastic model with a single scalar function of stress state, a ratio of compliances, a time or rate exponent, and two elastic terms. This model offers a simple way to incorporate nonlinearity and time dependence in a material model for limited loading conditions. Special considerations for using off-axis coupons are examined and unique tabbing is recommended to derive material properties out to high stress levels. Results are then compared to those for a glass/epoxy composite with the same rubber-toughened resin system. Behavior of the two composite systems is compared and checked for consistency using a micromechanical model. Finally, damage progression in both materials is compared through acoustic emission monitoring.

Introduction
This paper summarizes a large portion of experimental work performed to characterize the nonlinear viscoelastic behavior of two polymeric composites. For more detail on many items see Ref. 1.
Glass and carbon fiber-reinforced plastics offer important structural advantages over conventional metals in many applications as a result of their high strength and stiffness, low density, tailorability of their properties and corrosion resistance, among others. However, questions on their durability have often inhibited engineers in their use. Here we are concerned with one aspect of the durability issue - how the deformation characteristics change due to the nonlinear viscoelastic behavior of the polymer matrix.
In most textbooks and in the preliminary design for most structural applications, unidirectional fiber composites (the building block of multidirectional fiber laminates) are modeled as anisotropic linear elastic solids. However, these materials have been shown to exhibit significant amounts of time dependence and nonlinearity in those properties dominated by the polymer matrix due to viscoelasticity and damage growth (e.g. Refs. 2 and 3). Under adverse environmental conditions, such as elevated temperature, moisture, or stress, the time dependence of the matrix properties is greatly enhanced. Even when the overall deformation of a fiber-dominated composite structure is essentially immune to matrix properties prior to significant levels of cracking such as delamination, the ply-level stresses are not. These stresses affect matrix cracking, softening and fiber-matrix debonding which can lead to premature failure of the fibers, shortening the life of the structure. In some circumstances, 'structural failure' may not mean fiber breakage but matrix cracking leading to leakage of some fluid, such as in pressurized composite tubulars.