Compressive fatigue in glass and graphite reinforced composites
Conners, Jeffrey Durnell
McGarry, Frederick J.
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The characteristics of compressive fatigue failure are described for unnotched laminates, with the following rank ordering by compressive strengths determined: [0 degrees/90 degrees] Graphite Epoxy, [ 0 degrees/+ 45 degree/90 degrees] Graphite Epoxy, [0 degrees/45 degrees] Graphite/Epoxy, [0 degrees/90 degrees] Glass Epoxy, [0 degrees/+45 degrees] Glass/Epoxy (0 degrees/+45 degrees/90 degrees] Glass Epoxy, Style 61 E Glass Woven Rovings/Polyester, and 1.5 ounce chopped E Glass Mat/Polyester. A normalized slope parameter with unites of percent decrease of ultimate compressive stress per decade of applied cycles is used to characterize the fatigue resistances of the materials, with the following rank order resulting: [0 degrees/90 degrees] Graphite Epoxy, [ 0 degrees/+ 45 degree/90 degrees] Graphite Epoxy, [0 degrees/45 degrees] Graphite/Epoxy, [0 degrees/90 degrees] Glass Epoxy, [0 degrees/+45 degrees] Glass/Epoxy (0 degrees/+45 degrees/90 degrees] Glass Epoxy. The degradation by water immersion of the Graphite/Epoxy systems is shown to be a loss static compressive strength and a decrease in normalized slope on the order of 10 percent. Multiple Regression Analysis used to quantify the effects of varying unidirectional ply laminate orientation shows that the compressive fatigue resistances of advanced composites cannot be satisfactorily explained only by ply orientation characterizations.. Various physical theories concerning the causes and processes of ultimate failure during compressive fatigue are postulated, based on the microstructures of the specimens and phenomena observed during testing.
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