(Dr. Steve Johnson, advisor)

"Fatigue Damage Mechanisms in Advanced Hybrid Titanium Composite Laminates"

Abstract

In recent years aerospace researchers have endeavored to meet the ongoing demands of both military and commercial aircraft to fly faster and longer then ever before.  Hybrid Titanium Composite Laminates (HTCL) are a type of hybrid laminate designed for operating at long lives (> 10,000 hours) and possessing superior fatigue resistance and damage tolerance at temperatures up to 350oF.  Comprised of varying layers of titanium alloy alternately bonded with layers of polymeric matrix composites (PMC), HTCL has proven to possess exceptional strength and fatigue resistance at ambient or elevated temperatures, making it prime candidate for many advanced, high-speed aerospace applications.  However, initial testing consistently revealed that when a fatigue crack forms in a titanium layer, the crack quickly leads to complete failure of that ply and delaminates from the laminate.  To limit or at least control such damage, extensive research was conducted to determine the optimal combination of PMC resin and titanium surface treatment to adequately strengthen the interfacial bond between the HTCL layers.

This thesis discusses the mechanical capabilities of an improved HTCL, particularly in fatigue damage propagation, where the results of this interfacial research are manifested.  Using a combination of PETI-5 polyimide for the PMC adhesive resin and an alkaline-perborate titanium surface treatment, this advancement in HTCL is compared to first generation HTCL testing to determine the effect of a stronger bondline in cyclic loading conditions.  The focus of the testing is to characterize the evolution of damage following single titanium ply failure in tension-tension and tension-compression constant amplitude fatigue.  The damage initiation, progression, and ultimate failure surfaces of the modified HTCL are closely analyzed, to ultimately determine these characteristic damage mechanisms that are present.