M.S. Thesis Presentation by Thomas de Vaulx
Monday, February 15, 1999

(Dr. Richard Neu, advisor)

"Determination of the Damage Threshold Criterion of a Glass/Epoxy Composite Material Using an In-Situ Tensile Test"

Abstract

The deformation and damage mechanism in composites are dependent in composites are dependent on the loading conditions and laminates orientations.  The aim of this study was to experimentally determine when damage first occurs in a glass/epoxy composite under a quasi-static tensile loading.  Various laminate orientations were investigated in this work: +/- 30, 40, 45, 50, 60, and 0/90 degrees.  Using this information, a general first damage failure criterion at the microscopic scale was obtained.

It is relatively easy to determine the fracture strength (by using the stress/strain curves for instance), but it is more difficult to determine when first damage occurs because it occurs locally and does not manifest itself in the macroscopic response.

In this work, damage was directly observed in a scanning electronic microscope during the loading.  The damage observed included decohesions, cracks, and delaminations between the plies. However, the first damage mechanism for all laminate orientation was decohesion between fibers and matrix.  The in-situ experimentation allows the damage to be quantified by counting the number of decohesions during loading.  The microscopic damage criterion was determined by relating the macroscopic normal and shear stress at the fiber/matrix interface.  The local stress was determined using classical laminate theory and Mori-tanaka averaging schemes.

The results of this work are compared to other test involving high-speed deformation.