Average contact pressures

The contact resistance can be minimized by increasing the pull of the fasteners. Increasing the area of overlap may not reduce the contact resistance, unless the number of fasteners is also increased. It is mandatory to maintain a certain minimum contact pres.stire per unit area of the joint overlap. An average contact pressure at around 40-55 kg/cm is considered adequate. For the purpose of easy application, it is expressed in terms of bolt torque, depending upon the area of overlap and the number of fasteners, as specified in Table 29.1. 

Prediction of the restitution coefficient has been a challenging research topic for decades. Unfortunately, no reliable and accurate prediction method has been found so far. However, some useful simplified models with certain limits have been developed. One of them is the elastic-plastic impact model in which the compression process is assumed to be plastic with part of the kinetic energy stored for later elastic rebounding, with the rebound process considered to be completely elastic [Johnson, 1985]. In this model, it is postulated that (1) during the plastic compression process, a — r3/2a (2) during the compression process, the averaged contact pressure pm is constant and is equal to 3 Y and (3) the elastic rebound process starts when maximum deformation is reached. Therefore, the compressional force is... 

Pm Averaged contact pressure s Distance between the points of... 

The average contact pressure, taken over both contacting and noncontacting asperities (the population for which we wish to estimate AT), is defined as... 

It can be seen from Fig.3 that chromium films differ from molybdenum films in the mechanical behavior essentially. So, unloading curve for chromium at depth about 330 nm shows displacement discontinuity that, as is known, testifies to phase transition in silicon under loading (a metal phase of high pressure Si II [6]). On unloading curve for molybdenum the phase transition in silicon is not fixed. Besides average contact pressure in molybdenum film is lower, than in a chromium film more than in 2 times and this distinction increases with reduction of depth of contact. 

It can also be seen that both values of the friction coefficient are likely to decrease as the average contact pressure increases (Tsuya et al., 1991 Kawayu et al., 2000). 

The average contact pressures reached between the surfaces of the punch and the workpiece do not depend on the friction. On the other hand, the higher the width of the workpiece, the higher the contact pressures. Whereas the higher the height, the contact pressures are almost the same. Therefore, the contact pressures depend on the width of the workpiece while the height seems to have a lower relevance. 

Low-load approach and retraction measurements (up to 4 mN) were carried out by continuous motor travel, in part simultaneously with the friction measurements. For the high-load experiments described below, a manual approach with the micrometer screw was employed. The average contact pressure was calculated by dividing tile load by tiie contact area. This latter p mneter ch ged considerably during loading because of surface deformation, vshich varied from experiment to experiment as a result of differences in the thickness of mica and glue. Because of this deformation, the... 

Similar observations were also made on PLL(20)-g[3.5]-PEG(2) brushes mounted with a drop of polymer solution. Here, no friction could be detected up to loads of about 60 mN. At this load (corresponding to an average contact pressure of about 10 MPa), the film was compressed to 5.4 nm, which is one-third of its equilibrium film thickness. Film destruction, as described below, was never observed in the presence of excess polymer solution in the load range covered in this experiment. 

Number of scans 1024 Average contact pressure 34MPa... 

It is difficult to compare the applied loads reported in different studies, since often only the load is given but neither the contact pressure nor the contact area are reported. With applied loads ranging from 0.5 to 10 N in this work, average contact pressures in the range of 25-500 MPa were probed. 

The use of 90mm diameter rings sliding against a 130mm internal diameter liner produced a small elliptical wear scar. This simplified wear measurement and microscopy considerably. The 40N load gave average contact pressures at the end of the wear tests of between 4 and 8 MPa. 

The test procedure consisted of 10 minutes to establish a background level of hydrogen, then the motor was started and mn at light load 0.5 kN (Po=2.6 Gpa) for 5 minutes. This was then followed by the load being increased to 5.3 kN (Po=5.6 Gpa) for the remaining duration of the test. Po is the hertzian average contact pressure. The sampled gas was collected via a fine, trace-heated capillary into a quadrupole mass spectrometer at a rate of 20 cm min . The quadmpole mass spectrometer... 

The deformation behavior of bulk ZnO single crystals was studied by a combination of spherical nanoindentation and atomic force microscopy [101]. ZnO exhibited plastic deformation for relatively low loads (>4—13mN with a 4.2 mm radius spherical indenter). The average contact pressure hardness H and Young s modulus as a function of indenter penetration were determined by analyzing partial load-unload data. The hardness value of ZnO is measured to be 5.0 0.1 GPa at a plastic penetration depth of 300 nm. The Young s modulus remained essentially constant over the indenter penetration depth, with =111.2 4.7 GPa. Previous indentation studies performed mostly on polycrystalline ZnO have reported a wide range of H ( 1.5-12 GPa) and ( 40-120 GPa) values. However, it should be noted... 

When looking at Eq. (9.5), one might wonder why the coefficient of friction is really a constant and does not depend on load, i-f is a material constant, but the shear strength of a single contact is expected to depend on the load. However, for a multiasperity contact, the average contact pressure will simply take the value of H independent of load. An increase in load will not result in an increase in contact pressure but in This is different for the friction of single nanocontacts as we will... 

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