Monday, February 22, 1999

(Dr. Richard Neu, advisor)

"Determination of a Local Damage Threshold Criterion for a Glass/Epoxy Laminated under an Intermediate Rate of Loading"

__Abstract__

This investigation involves the study of the mechanical behavior of
a laminated composite material composed of R glass fibers in an epoxy matrix.
The mechanical loading was conducted at an approximate strain rate of 70
.s^{-1}, which represents an intermediate strain rate between that
of typical quasistatic experiments and the strain rate obtained by higher
rate dynamical systems such as the Hopkinson's bars (greater than 400.s^{-1}).
The laminate orientations tested included [0/90]_{s} , [+-30]_{s}
, [+-40]_{s} , [+-45]_{s} , [+-50]_{s} and [+-60]_{s}.
The goal of this investigation was to establish a general first damage
criterion for this intermediate strain rate.

Unique interrupted high speed tensile tests were performed on a series of specimens to track the evolution of damage. A threshold was found by extrapolating the evolution of damage to the undamaged condition. The damage observed included delamination between plies and decohesion between fibers and matrix. For all laminate orientation, though, the first damage involved only the decohesion phenomena.

In order to generalize the damage threshold for any laminate orientation geometry and loading, the damage criterion was made dependant on the local or microscopic scale stress. The local damage criterion was determined from the experimentally determined macroscopic damage threshold along with a computation model used to determine the local stress. The classical laminate theory was used to determine the ply stresses and the Mori and Tanaka model based on the Eshelby's inclusion problem was used to compute the stresses at the fiber/matrix interface within the ply.

Once the local damage criterion has been established, it is possible to determine whether failure occurs under more complex loading by determining the local stress and comparing it with the local damage criterion.