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Initiation of edge delamination of symmetric laminates consisting of angle and 90° plies was investigated. Five independent criteria were used to determine delamination initiation for a given set of m and n of angle and 90° plies. In Criteria 1 and 2, delamination moment rates (delamination moment coefficients or DMCs) relative to the applied average axial stress and strain in the laminates with intact 90° plies were maximized. The same approach was used in Criteria 3 and 4 except that the laminates had cracked 90° plies. Criterion 5 was based on establishing the maximum mismatch of Poisson's ratios of angle and 90° plies. The expressions derived on the basis of the criteria were numerically evaluated. The results of these studies indicated that the laminates designed on the basis of Criterion 5 had the minimum tendency to delaminate. In addition, it was found that an increase in thickness of laminates tended to increase the delamination tendency.
To confirm the results, four laminates were designed in accordance with Criteria 1, 2, and 5, respectively. All four laminates were analyzed using the finite element method. Besides analytical studies, four graphite-epoxy panels corresponding to the four laminates were fabricated, cut into specimens, and tested under uniaxial tensile loading to determine the onset of delamination.
Both finite element analyses and experimental data confirmed the delamination moment coefficient to be a valid quantitative measure of the tendency of the laminates to delaminate. This delamination moment coefficient concept can be used to determine the stacking sequence of laminates either to have maximum or minimum tendency to delaminate. The laminates designed to have maximum tendency to delaminate were observed to fail in mixed modes consisting of combined transverse cracking and edge delamination. This mixed mode can be avoided by limiting the strain in the loading direction to a value less than the cracking strain (0.4%) of 90° plies.
Aerospace engineer, Structural Integrity Branch, Structures Division, Flight Dynamics Directorate, Wright Laboratory, Wright-Patterson AFB, OH
Aerospace engineer, Structural Concepts Branch, Structures Division, Flight Dynamics Directorate, Wright Laboratory, Wright-Patterson AFB, OH
Stock #: CTR10211J