Contact of two plates one of which has a crack


The second plate (which has no cracks) can be in contact with the first plate (which has the crack). We assume that the plates remain at a distance (5 > 0 from each other in the stress free state, 5 = const (see Fig.3.3). They may be in contact due to exterior forces. The mid-surface of the second plate is precisely fl, which corresponds to the negative value of the coordinate By that the first plate is called the upper plate and the second one the lower plate.  [c.186]

Electric current conduction tests, in which current is introduced into test materials by direct electric contacts and potential drops are measured across specific zones of the test surface using additional contact electrodes, are used to measure effects of corrosion wall-thinning, as in petroleum product storage tank roofs, or to detect cracks in welds, metallic sheets and plate, and railroad rails (1,6). Modifications of such systems measure the resultant magnetic field distributions created by the current conduction, as for Hall effect semiconductor devices, moving pick-up coils, and even by thermal effects resulting from resistance heating within the test materials (1). Difficulties in applying direct electric contact testing include the possibiHty of sparks or bums created by passing high current through poor electrode contact areas, and the thermoelectric and triboelectric effects that may produce false signal voltages owing to short-circuit paths that the current may find through touching crack surfaces. Alternatively, thermoelectric or triboelectric voltage signals can be used to identify and sort metals and alloys where surface contamination effects do not interfere.  [c.131]

Down spouts (or up spouts) are best set flush with the plate from which they lead, with no weir as in gas-hquid contact. The velocity of the continuous phase in the down spout V, which sets the down-spout cross section, should be set at a value lower than the terminal velocity of some arbitrarily small droplet of dispersed phase, say, 0.08 or 0.16 cm i M or Mfi in) in diameter otherwise, recirculation of entrained dispersed phase around a plate will result in flooding. The down spouts should extend beyond the accumulated layer of dispersed phase on the plate.  [c.1480]

The study of the growth by fatigue of physically short cracks usually less than 0.1 to 1.0 mm deep is a topic of much current research interest. The study of environmental effects appears to have been confined so far to the influence of high temperature air oxidation of superalloys for aero-engines (see next section for more details) and to steels in salt-water environments. Even in the absence of reactive environments, short cracks grow considerably faster than long ones when expressed as a function of the linear elastic fracture mechanics parameter, A/T. This can be due to uncontained plasticity at the tip of the crack or microstructurally important features of similar dimensions to the crack size, both of which invalidate the representation of the crack-tip driving force by AK. One commonly applied technique to take account of reduced mechanical constraint at a short crack-tip is to plot the crack growth results from both short and long cracks (i.e. conventional fracture mechanics specimens in the second case) as a function of AK where a correction is made to the nominal AK to allow for the minimum stress intensity at which the crack closes. This point is often detected experimentally by electrical potential drop methods. When such corrections are made, short and long crack data are normally self-consistent as a function of AK . Similar successes of the AK approach have been achieved in the context of oxide blocking of cracks and pressure effects in viscous liquids. Another older method, though no less successful for low cycle fatigue, has been to express crack growth rates as a power law function of the applied plastic strain range.  [c.1305]


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