Fretting corrosion contact load

Fretting corrosion is a combined wear and corrosion process in which material is removed from contacting surfaces when motion between the surfaces is restricted to very small amplitude oscillations (often, the relative movement is barely discernible ranging from a few tens of nanometers to a few tens of micrometers). Fretting occurs where low-amplitude oscillatory motion in the tangential direction takes place between contacting surfaces, which are nominally at rest. (Waterhouse) It is necessary that the load be sufficient to produce a distortion of surfaces. This is a common occurrence, since most machinery is subjected to vibration, both in transit and in operation. Figure 6.41 shows a typical example. Pressed-on wheels can often fret at the shaft/wheel hole interface.31... 

Anaerobic adhesives of appropriate viscosity are often used to augment and standardise the performance of shrink and interference-fitted components, where they can reduce hoop stress in the outer component and reduce its tendency to crack or display fretting corrosion. These benefits stem from the ability of anaerobics to fill the voids that are present in even the best-prepared surfaces. This spreads loads evenly over the whole of the faying surfaces instead of concentrating them on the engaged peaks, which represent only a small proportion of the theoretical contact area. 

Fretting corrosion refers to corrosion damage at the asperities of contact surfaces (Fig. 6.50). This damage is induced under load and in the presence of repeated relative surface motion, as induced, for example, by vibration. Pits or grooves and oxide debris characterize... 

Fretting corrosion occurs at contact areas Review Questions between material under load and subjected to... 

One major benefit of bonded joints is their superior fatigue resistance compared with mechanically fixed assemblies. The dynamic performance of anaerobic adhesives has been assessed. It has been proposed that, since the adhesive makes intimate contact with the metal surface, it fills all the micro-roughness in the surface, so ensuring that the bond line is completely filled. This means that loads, particularly compressive loads, are more evenly distributed over the joint area in comparison with mechanically assembled parts, especially when these are interference fitted. In the latter case there is rarely more than 20% of the available bond area actually in contact. In addition, the ingress of air is suppressed and fretting corrosion is eliminated. The net result is a superior performance under demanding dynamic conditions. 

Leakaae Modes -Across seats -Lowers overall plant efficiency -Could cause reverse rotation of Brayton unit -Erosion -Corrosion -Scoring, spalling, fretting, wear, cracks -Particle interference -Valve seat distortion -Improperly seating disc (misalignment) -Improper contact or contact loads... 

Velocity effects include erosion-corrosion, a form of attack caused by high velocity flow cavitation caused at even higher flow by the collapse of bubbles formed at areas of low pressure in a flowing stream and fretting that is caused by vibratory relative motion of two surfaces in close contact under load erosion-corrosion, cavitation, fretting). 

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