Papers by Author: David Nowell

Abstract: 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.

Abstract: 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.

Abstract: Fatigue crack closure is an important phenomenon which needs to be taken into account in the development of models for crack propagation. This paper presents an overview of techniques for measuring crack closure. The moiré interferometry approach is described in more detail and some experimental results are presented and compared with the predictions of closure models.

Abstract: Residual stress can be found in engineering components as a result of non-uniform plastic strain introduced through a variety of manufacturing processes such as rolling, casting, hot forging, cold working, shot-peening, laser shock peening, welding, etc. The numerical simulation of the resulting residual stress field requires the use of sophisticated coupled microstructural and thermomechanical models that rely on profound understanding of the constitutive laws and detailed knowledge of material parameters. In practice this level of understanding is not generally available, leading to the use of simplified models that are unable to reproduce or predict reliably the real residual stress distributions. This leads to the necessity of using increased safety factors and utilising overly conservative designs. A rational approach to the description of residual stress states is proposed that relies on the use of eigenstrain distributions as sources of residual stress. The problem of residual stress evaluation can then be replaced by the problem of determining the underlying eigenstrain distribution. An approach to this problem is proposed based on a simple variational formulation. Some examples of its application are shown, and the difficulties and challenges that may arise are discussed.