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Fretting fatigue crack initiation in titanium alloy, Ti−6Al−4V, was investigated experimentally and analytically by using finite element analysis (FEA). Various types of fretting pads were used in order to determine the effects of contact geometries. Crack initiation location and crack angle orientation along the contact surface were determined by using microscopy. Finite element analysis was used in order to obtain stress state for the experimental conditions used during fretting fatigue tests. These were then used in order to investigate several critical plane based multiaxial fatigue parameters. These parameters were evaluated based on their ability to predict crack initiation location, crack orientation angle along the contact surface and the number of cycles to fretting fatigue crack initiation independent of geometry of fretting pad. These predictions were compared with their experimental counterparts in order to characterize the role of normal and shear stresses on fretting fatigue crack initiation. From these comparisons, fretting fatigue crack initiation mechanism in the tested titanium alloy appears to be governed by shear stress on the critical plane. However, normal stress on the critical plane also seems to play a role in fretting fatigue life. At present, the individual contributions/importance of shear and normal stresses in the crack initiation appears to be unclear; however, it is clear that any critical plane describing fretting fatigue crack initiation behaviour independent of geometry needs to include components of both shear and normal stresses.

The purpose of this study was to investigate the fretting fatigue crack initiation behaviour of titanium alloy, Ti–6Al–4V. Fretting contact conditions were varied by using different geometries of the fretting pad. Applied forces were also varied to obtain fretting fatigue crack initiation lives in both the low‐ and high‐cycle fatigue regimes. Fretting fatigue specimens were examined to determine the crack location and the crack angle orientation along the contact surface. Salient features of fretting fatigue experiments were modelled and analysed with finite element analysis. Computed results of the finite element analyses were used to formulate a shear stress‐based parameter to predict the fretting fatigue crack initiation life, location and orientation. Comparison of the analytical and experimental results showed that fretting fatigue crack initiation was governed by the maximum shear stress, and therefore a parameter involving the maximum shear stress range on the critical plane with the correction factor for the local mean stress or stress ratio effect was found to be effective in characterizing the fretting fatigue crack initiation behaviour in titanium alloy, Ti–6Al–4V.

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The proper selection of electrical contact materials is one of the critical steps in designing a metal contact microelectromechanical system ͑MEMS͒ switch. Ideally, the contact should have both very low contact resistance and high wear resistance. Unfortunately this combination cannot be easily achieved with the contact materials currently used in macroswitches because the available contact force in microswitches is generally insufficient ͑less than 1 mN͒ to break through nonconductive surface layers. As a step in the materials selection process, three noble metals, platinum ͑Pt͒, rhodium ͑Rh͒, ruthenium ͑Ru͒, and their alloys with gold ͑Au͒ were deposited as thin films on silicon ͑Si͒ substrates. The contact resistances of these materials and their evolution with cycling were measured using a specially developed scanning probe microscope test station. These results were then compared to measurements of material hardness and resistivity. The initial contact resistances of the noble metals alloyed with Au are roughly proportional to their resistivities. Measurements of contact resistance during cycling of different metal films were made under a contact force of 200-250 N in a room air environment. It was found that the contact resistance increases with cycling for alloy films with a low concentration of gold due to the buildup of contamination on the contact. However, for alloy films with a high gold content, the contact resistance increase due to contamination is insignificant up to 10 8 cycles. These observations suggest that Rh, Ru, and Pt and their gold alloys of low gold content are prone to contamination failure as contact materials in MEMS switches.

This study investigated the effects of embedding piezoelectric lead zirconate-titanate (PZT) sensors on the tensile strength and fatigue behavior of a quasi-isotropic graphite/epoxy laminate as well as the embedded sensor's voltage degradation under these loading conditions. For this, AS4/3501-6 laminates were fabricated with a [0/±45/90] S lay-up where PZT was inserted into a cut-out area in the two middle 90 • plies. Monotonic tensile tests showed that both the average ultimate strength and Young's modulus of the tested laminate with or without PZT were within 4% of each other. The fatigue lives with and without PZT were very close to each other as well. Overall, the sequence of damage in this study agreed with previous investigations of the damage mechanisms for [0/±45/90] S quasi-isotropic laminates. The ranges of modulus reduction in both cases, with and without PZT, were within 5 to 15% of each other during fatigue loading. Delamination growths in both cases during most of the fatigue life were also very comparable to each other. Further, this study showed that the embedded PZT would maintain a steady voltage output indefinitely when mechanically cycled within its operational strain limit. It thus appears that the embedment of PZTs in a cut-out area of 90 • plies of quasi-isotropic graphite/epoxy laminates would not affect their monotonic tensile and fatigue behavior.

This study investigated the integrity of the host graphite/epoxy laminate as well as of the embedded active PZT sensor/actuator under monotonic and fatigue loads. For this, graphite/epoxy (AS4/3501-6) laminates were fabricated where the commercially available piezoelectric device in the pre-packaged form was embedded. Two lay-ups were tested: [0/±45/90] S or [0/0/±45/0/0/90] S . The piezoelectric actuator/sensor was embedded in two ways: one method involved placing them into a cut-out area in the two middle 90 • plies, and the second one involved insertion between the two middle 90 • plies without any cut-out. Ultimate tensile strength and Young's modulus of the tested laminates were not affected due to insertion of the piezoelectric actuator/sensor using either of the embedding techniques. There was also no degradation in the fatigue strength/lives of the tested laminates due to insertion of the piezoelectric actuator/sensor. Furthermore, these were not affected by the embedding method (i.e. cut-out versus simple insertion method). Also, the integrity of the embedded piezoelectric actuator/sensor was preserved when mechanically fatigued or loaded monotonically to the maximum stress level equal to its operational design limit.

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