Title page for ETD etd-10102005-131618
|Type of Document
||Harrison, Peter Newton
||Interlaminar stress analysis of dropped-ply laminated plates and shells by a mixed method
|Johnson, Eric R.
|Haftka, Raphael T.
|Hyer, Michael W.
|Kapania, Rakesh K.
- Laminated materials Testing
- Strains and stresses
- Shells (Engineering)
|Date of Defense
A mixed method of approximation based on Reissner's variational principle is
developed for the linear analysis of interlaminar stresses in laminated composites, with
special interest in laminates that contain terminated internal plies (dropped-ply
laminates). Two models are derived, one for problems of generalized plane deformation
and the other for the axisymmetric response of shells of revolution. A layerwise approach
is taken in which the stress field is assumed with an explicit dependence on the thickness
coordinate in each layer. The dependence of the stress field on the thickness coordinate is
determined such that the three-dimensional equilibrium equations are satisfied by the
approximation. The solution domain is reduced to one dimension by integration through
the thickness. Continuity of tractions and displacements between layers is imposed.
The governing two-point boundary value problem is composed of a system of both
differential and algebraic equations (DAEs) and their associated boundary conditions.
Careful evaluation of the system of DAEs was required to arrive at a form that allowed
application of a one-step finite difference approximation. A two-stage Gauss implicit
Runge-Kutta finite difference scheme was used for the solution because of its relatively
high degree of accuracy. Patch tests of the two models revealed problems with solution
accuracy for the axisymmetric model of a cylindrical shell loaded by internal pressure.
Parametric studies of dropped-ply laminate characteristics and their influence on the
interlaminar stresses were performed using the generalized plane deformation model
Eccentricity of the middle surface of the laminate through the ply drop-off was found to
have a minimal effect on the interlaminar stresses under longitudinal compression,
transverse tension, and in-plane shear. A second study found the stiffness change across
the ply termination to have a much greater influence on the interlaminar stresses.
Correlations between the stiffness ratio of the thick to the thin sections of the laminates
and the magnitude of a parameter based on a quadratic delamination criterion were found
to be surprisingly good for longitudinal compression and in-plane shear loadings. For
laminates with very stiff terminated plies loaded in longitudinal compression, inclusion of
a short insert of softer composite material at the end of the dropped plies was found to
significantly reduce the interlaminar stresses produced.
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