Richard A. Schapery

Time-Dependent and Nonlinear Effects in Composites for Deepwater Application

Robert Bocchieri (PhD student), Robert Jenkins & Javier Lopez (UG students)

The overall objective is to develop constitutive equations for composite laminae which account for effects of stress and moisture histories. These equations are needed for durability analysis of laminated composite risers and other marine structures. Current emphasis is on theoretical model development and experimental characterization of the carbon/epoxy theme material in dry and moisture-saturated states and subjected to constant load rate and to cyclic loading.

This research supports three components of the Strategic Plan. It is a portion of the composite materials evaluation and environmental testing activities, and it provides constitutive equations needed in the composite structural analysis models.

A unidirectional composite layer is the basic material element used in this constitutive equation development. In the most general case, it is modeled as an orthotropic, nonlinear viscoelastic material with growing damage. The damage may be matrix microcracks and shear banding, fiber-matrix microdebonds as well as transverse cracks which span a full layer thickness. The theoretical approach is based on a combination of continuum mechanics, thermodynamics with internal state variables and fracture mechanics. Various simplifying features are introduced using material symmetry considerations and experimental information. The experimental work consists of testing unidirectional coupons with various fiber angles relative to the applied load (primarily 0, 90 and 15 degrees) and a ±45 degree angle-ply laminate at dry and saturated states. These tests provide the material constants and functions which enter the layer constitutive equations.

Studies of nonlinear viscoelastic behavior of fiber-reinforced plastics are underway at several institutions around the world (including Univ. of Washington, Purdue Univ., Univ. of Tennessee and the Free Univ. of Brussels), although most are concerned with high-temperature, aerospace applications The only study the P.I. is aware of which is directly concerned with marine applications is at the Univ. of Tennessee; Dr. Weitsman is the P.I. on this ONR-sponsored project, and we communicate regularly with him.

As part of the durability analysis of laminated composite structures it is necessary to predict the mechanical state of each layer. This state changes with time because it depends on creep, damage growth, moisture diffusion, as well as the external loads; it must be known because it influences the overall deformation and failure behavior. Although it is common practice to use linear elasticity theory for at least preliminary structural design purposes, this theory is not sufficiently accurate for durability analysis.

January 1993 to September 1999

1) The five elastic constants for the transversely isotropic, unidirectional layers in the carbon/epoxy and glass/epoxy theme composites.
2) Creep compliances for the unidirectional carbon/epoxy and glass/epoxy layers in the dry and wet (saturated) states and the associated viscoelastic constitutive equations for use in estimating the effects of creep and nonlinearity compared to linear elastic structural response.
3) Analytical model for damage growth.
4) Accelerated conditioning and test methods for characterization of long-term mechanical behavior of composites.

During 1996, our research results have been presented at the annual OTRC Industry Workshop in Houston;Texas Fracture Workshop, SWRI, San Antonio; University of Houston; Cabot Corporation, Boston; Gordon Conference on Polymers, Plymouth State College; Conference on Damage and Inelasticity, St. John's Newfoundland; Penn State University; Society of Engineering Science, Arizona State University; ASME Winter Annual Meeting, Atlanta.