Chemical construction, mechanical characterization and durability of metal and composite structural joints

In this dissertation, Parameters that characterize interface fracture are defined and procedures to measure them discussed. The interface strength sigma o is measured by using a novel laser spallation experiment, which uses a laser-induced stress wave to separate the interface. The intrinsic ( Go) and total (Gc) fracture energies are measured using a double cantilever beam experiment performed at ambient and cryogenic temperatures, respectively. These measurements are used to extract the desired mechanical properties from a bonded pair by tailoring the chemistry and microstructure of the interfacial region, and to study the effects of extrinsic parameters (geometry/temperature/time/humidity) on sigma o, Go, (Gc) and their relations to the plastic work (Gp) which accompanies interface decohesion. This relationship allows prediction of interface reliability from a fundamental standpoint. Although the methodologies discussed are general, the results will be demonstrated on two interface systems. For interfaces between sputter-deposited polycrystalline Tantalum coatings and sapphire substrates, Go and sigmao were modified by changing the orientation of the sapphire surface (basal and prismatic), while (Gc) was varied by changing the test temperature and the thickness of the ductile Tantalum layer. These experiments were also used to obtain the relationships between G c and Go, and between sigma o and Go, for joints between stainless steel and E-glass composite sections for use in lightweight ship and repair patch technologies. The durability of the joints under moisture exposure was also studied. Joint chemistries, including use of self-assembled monolayers on steel substrate are disclosed which optimize the joint performance such that the failure is through the composite substrate. Besides providing values that have interest in their own right, the work presented here provides a general framework for designing interfaces in bonded structures and serves as a basis to develop atomistic and continuum interface fracture models.