Contact loading

Survey of acoustic emission results at contact loading... 

The AET was used at standard tests of numerous structural materials, above all steels and cast iron, prepared are ceramic samples. Part of tested samples had qjecial sur ce layer treatments by laser, plasma nitridation and similar. Effect of special surface treatment the authors published already earlier [5,6]. In this contribution are summed up typical courses of basic dependencies, measured by the AET at contact loading. 

Mutual comparison of here introduced facts enables relatively effective comparison of material behaviour at different contact loading, or at same loading but different surfece treatments (Fig-8). 

Fig.9 Distribution of emission counts number in individual levels (structural steel) a) higher contact load (4000 MPa) b) lower contact load (2500 MPa).

The lubrication requirements for single-screw type compressors are not severe, but in oil-flooded rotary units, the oxidizing conditions are extremely severe because fine droplets of oil are mixed intimately with hot compressed air. In some screw-type air compressors, the rotors are gear driven and do not make contact. In others, one rotor drives the other. The heaviest contact loads occur where power is transmitted from the female to the male rotor here the lubricant encounters physical conditions similar to those between mating gear teeth. This arduous combination of circumstances places a great demand on the chemical stability, and lubricating power, of the oil. 

Microindentation hardness normally is measured by static penetration of the specimen with a standard indenter at a known force. After loading with a sharp indenter a residual surface impression is left on the flat test specimen. An adequate measure of the material hardness may be computed by dividing the peak contact load, P, by the projected area of impression1. The hardness, so defined, may be considered as an indicator of the irreversible deformation processes which characterize the material. The strain boundaries for plastic deformation, below the indenter are sensibly dependent, as we shall show below, on microstructural factors (crystal size and perfection, degree of crystallinity, etc). Indentation during a hardness test deforms only a small volumen element of the specimen (V 1011 nm3) (non destructive test). The rest acts as a constraint. Thus the contact stress between the indenter and the specimen is much greater than the compressive yield stress of the specimen (a factor of 3 higher). 

Voevodin, A. A., Walck, S. D., andZabinski, J. S., Architecture of Multilayer Nanocomposite Coatings with Super-Hard Diamond-Like Carhon Layers for Wear Protechtion at High Contact Load, Wear, Vol. 203-204,1997, pp. 516-527. 

The fiber modulus and matrix shear modulus are also required for the analysis. The fiber s coordinates are recorded directly from the stage controllers to the computer. The operator begins the test from the keyboard. The x and y stages move the fiber end to a position directly under the debonder tip the z stage then moves the sample surface to within 4 yum of the tip. The z-stage approach is slowed down to 0.04 jan/step at a rate of 6 steps/s. The balance readout is monitored, at a load of 2 g the loading is stopped, and the fiber end returned to the field of view of the camera. The location of the indent is noted and corrections are made, if necessary, to center the point of contact. Loading is then continued from 4 g in approximately 1 g increments. Debond is determined to have occurred when an interfacial crack is visible for 90-120° on the fiber perimeter. The load at which this occurs is used to calculate the interfacial shear stress at debond. 

The establishment of the interrelationships between polymer fracture properties and contact fatigue behaviour requires some knowledge of the contact stress field. The specificity of fretting loading is that, depending on the applied contact load and imposed relative displacement, two different contact conditions can be achieved within the interface [54,55] (Fig. 5) ... 

The occurrence of either partial slip or gross slip condition is dependent on the material mechanical properties, the magnitude of the coefficient of friction and the contact loading parameters (normal load, imposed displacement). When dealing with non-adhesive elastic materials, the effects of these... 

Fig. 18 Calculated values of the mode I (Ki) and mode II (Kn) stress intensity factors within a cracked epoxy at various stages of a cyclic contact loading. Two cracks 350 nm in length and oriented at 10° with respect to the normal to the epoxy surface have been considered. For symmetry reasons, only the results corresponding to one crack (denoted A) have been represented. The tangential cyclic loading has been divided into eight successive steps...

A number of investigations have been made into the influence of contact load on the frictional properties of molybdenum disulphide. Puchkov and Pashkov used a technique which they claimed to differentiate between shear stress and surface friction. They studied the effect of varying compressive stress on the resistance to... 

The input speed to the gear train was varied from 1500 to 4500 rpm, the torque from 0.025 to 0.5 Nm, and the contact load on the idler gears from 0.5 to 30N. The peripheral speeds of the gears were up to 3.3 ms , the relative slip 1 ms and the maximum contact stress 900 MPa. The test temperature was varied between 20°C and 250 C, and the chamber pressure was 10 Torr. 24 lubricating gears containing different proportions of copper, silver and molybdenum disulphide were tested, and the best performance was given by a composite of 87% copper, 5% silver and 8% molybdenum disulphide. 

Other industrial applications still exist, but the use of dispersions is cleaner and more convenient. Burnishing of molybdenum disulphide films applied by means of dispersions can be carried out in exactly the same way as for free powder, and the resulting burnished coatings have similar properties, but there are no detailed reports about them other than those of Matsunaga described in Chapter 6. Films from dispersions will also be burnished in use by the effects of sliding under contact load, and their eventual form and behaviour are likely to be similar in all respects to those produced from loose powder. Similar burnished films are likely to be the end-product of many of the softer bonded coatings, and these will be discussed further in Chapter 11. 

In practice there is a continuous range of coating hardnesses varying from very soft to very hard, and the hardness is determined not only by the binder but by the concentrations of molybdenum disulphide and other solid components. The definition of hardness is also relative to the service contact loads. A coating of intermediate hardness may behave like a hard film under low contact pressures or like a soft film under high contact pressures. As a result, it is very difficult to compare the quoted load-carrying capacity and wear lives of different films tested under different conditions by different research groups. 

Some useful generalisations can be made about friction and wear behaviour. The coefficient of friction decreases with increasing contact pressure although the relationship will vary with the type of coating and other test conditions. The effects of contact load and speed on wear rate or wear life are less easily defined, although the general patterns are as shown in Figures 11.3 and 11.4. However, as has been... 

For the case of a noncontinuous fluid layer, i.e., partial pad-wafer contact, contact between the pad and the wafer will occur at the pad asperities (Figure 4.25). As the pressure increases, more asperities will come into contact with die surface. The contact properties of a given asperity are derived from Hertz s equations. The contact area, a, and contact load, /, are given by ... 

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