Daniel M. Mulvihill

Measurement of tangential contact stiffness in frictional contacts: the effect of normal pressure

The tangential contact stiffness of frictional interfaces affects both the vibration response and structural integrity of structures comprising frictional joints. Vibration and structural response of monolithic structures can be predicted very accurately; however, when assemblies of components involve frictional interfaces, additional damping and compliance are present due to these interfaces. These features make it more challenging to predict the vibration characteristics of assemblies with the same degree of accuracy as can be achieved for single components. If these interface properties can be determined, it should then be possible to significantly enhance current models of the vibration of engineering assemblies. Measurements of both force and displacement in the tangential direction are obtained from a series of in-line fretting tests involving flat pads with rounded corners clamped against the flat surface of a specimen which is oscillated by a hydraulic tensile testing machine. In order to measure the local displacement field very close to the contact interface, the digital image correlation (DIC) method is employed. The effect of normal contact pressure on tangential contact stiffness is investigated. Multiple experiments with the same parameters show good repeatability given the number of variables involved.

Investigation of the Friction Variation with Sliding which is Commonly Observed in Individual Fretting Test Cycles

Many researchers that have carried out fretting wear tests have presented results which show a variation (usually an increase) in the friction force during the sliding phase of individual cycles. This phenomenon is contrary to the Amontons/Coulomb model of friction which predicts a constant friction force as sliding proceeds. An in-line fretting test involving an abrupt increase in amplitude was used to show that the effect is a result of wear scar interaction effects. A pair of rotational fretting tests were then carried out to determine whether these interaction effects originate from interaction of the wear scar ends, or whether such interaction occurs throughout the nominal contact area. A new experimental rig designed to adapt conventional in-line test machines for rotational fretting was designed and built for this purpose. Results show that the friction variation occurs whether or not wear scar ends are present. After testing, the worn surface topography of each fretting pair was scanned using a focus variation microscope, and these surface images revealed the existence (and size) of many distributed local peaks and troughs distributed throughout the contact area. In conclusion, the friction variation commonly observed in the literature arises predominantly from the interaction of local wear scar features distributed over the contact region.

Three-dimensional in situ observations of compressive damage mechanisms in syntactic foam using X-ray microcomputed tomography

Syntactic foams with hollow glass microspheres embedded in an epoxy matrix are used in marine, aerospace and ground transportation vehicle applications. This work presents an in situ experimental study of failure mechanisms in syntactic foam based on X-ray microcomputed tomography with uniaxial compression. Under different levels of compressive strain, the material was scanned using X-ray microcomputed tomography to obtain three-dimensional (3D) images of its internal microstructure. Experiments with the same parameters were carried out to investigate repeatability. The microscopic observations have suggested that damage nucleation occurs at the weakest microspheres. When applied strain increases, shear collapse bands (SCBs) develop local to the fragmented microspheres due to stress concentration and bending deformation around SCBs occurs. After significant strain, the thickness of the SCBs increases owing to the accumulation of the broken microspheres. The relationship between the volume fraction of microspheres and applied bulk strain has been characterised.