Issue 33

D. A. Hills et alii, Frattura ed Integrità Strutturale, 33 (2015) 61-66; DOI: 10.3221/IGF-ESIS.33.08 62 stress into the ( K I <0,K II >0 ) quadrant, implying, as intuitively expected, firm closure of the contact corner, whereas the application of a shear force or tension drives the state of stress into either the ( K I >0,K II >0 ) or ( K I <0,K II <0 ) quadrants. These imply some degree of local opening [2, 3]. C HOICE OF TEST ll of the early ‘complete’ fretting tests conducted at Oxford were carried out on a modified version of the group’s general purpose fretting-fatigue apparatus, originally designed to conduct ‘incomplete’ (mainly Hertzian) fretting tests, which is shown in Fig. 1. One of the features which was thought to be very desirable was to use pad holders that could swivel, so intimate contact was made between the pad ends and the dogbone specimen faces. At the same time, the geometry was carefully arranged so as to attempt to ensure that the applied shear force was imposed strictly through the plane of the interface to ensure that there was no moment present tending to ‘tip’ the pads. Very large numbers of tests were carried out (over one hundred) on super-CMV steel, but the spread of results obtained was disappointing [4]. Figure 1 : Fretting-fatigue test rig at the University of Oxford for studying both incomplete and complete contacts [2]. One difficulty was that the usual increase in the coefficient of friction experienced during the first few cycles of loading typically increased the friction coefficient from about 0.4 to 0.7. In the case of Hertzian contacts, this simply has the result of reducing the width of the slip zones, and this happens in a smooth manner. But, in the case of sharp-edged contacts, it can make the difference between edge slip and a fully adhered contact, so the steady state problem is little more than a notch incorporating an interface. The interface, including asperities, is still likely to be a source of weakness, so the fatigue strength of the pair will be lower than that of a monolithic component incorporating a notch. A further feature of the contact pair is that, because all surfaces are machined prior to assembly, the contact corner is truly sharp, whereas, in the case of a notch, an internal corner is present which, no matter how careful the machining process, will always have some finite radius. A principal advantage of the pad-on-dogbone arrangement is that it is possible to control three separate external loads – the normal force, P , which is normally held constant; the shear force, Q ; and the bulk stress, T , which are both controlled by servo-hydraulic actuators as shown in Fig. 2. All three components of load excite both mode I and mode II loading, and the first really thorough derivation of the contact corner stress intensity factors was carried out by Flicek [5], with the following results A