Contacting surfaces: A problem in fatigue and diffusion bonding

Contact between surfaces usually occurs at asperities under compression or at connecting ligaments, depending on how the interface is formed. This paper deals with the nondestructive evaluation of the topology of contact and with the use of this information to predict the effects that loads borne by these contacts have on mechanical properties. Two specific examples are discussed: a fatigue crack and a diffusion bond. Asperity contact along the fracture surface of a fatigue crack partially shields the crack tip from the externally applied driving force. Using information from acoustic experiments, the geometry of the asperities, the contacting stress, and the shielding stress intensity factor have been estimated. Acoustically, a diffusion bonded interface looks very similar to that joining the two sides of a partially closed crack. In this particular case, the acoustically determined geometry of well-bonded ligaments can be verified by fractography of destructively tested samples whose bond strength has also been determined. Models to determine the bond strength from the ligament geometry are being suggested.

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