Incipient crack .

The crossover temperature must satisfy an overall heat balance around the furnace system, including transfer line heat exchangers and feed preheaters, and must not exceed certain limits to avoid incipient cracking. 

More recently it has been possible to examine used hip cups which have been removed from the body. Electron microscopy of hip cup sections after permanganic etching reveals a layer of polymer, at least 10 pm thick extending inwards from the wear surface, which showed incipient cracking and appeared to have been embrittled by exposure to the body environment (Jordan et al, 2000). 

In equation 13, t y is the time to failure, Uq is interpreted as the activation energy for chain scission, to is interpreted as the inverse of the molecular oscillation frequency, and y is a structure-sensitive parameter. Experimentally he (47) observed the formation of platelet-shaped nanoscopic voids, which he referred to as incipient cracks, that form very early in the polymer once a tensile load is applied. Both the incipient cracks and their number density were measured from small-angle x-ray scattering studies on samples that had tensile loads applied to them. The characteristic sizes and number densities of the incipient cracks are included in Table 2. Zhurkov showed that the incipient cracks formed at the very early stages and the remaining life was associated with the coalescence of these incipient cracks to ultimately cause failure. 

The fact that the interior of metals may emit sound is utiHzed in a modem method for materials testing, called acoustic emission (AE). When cracks are initiated in a structure under stress, a rapid release of energy occurs and high-frequency elastic waves are generated. Their sound is generally not detectable by ear. Tin cry is an exception. Instead piezoelectric sensors are used, which record the acoustic emission and transform it to optical signals. In this way it is possible to hear an incipient crack formahon in the material of for instance, nuclear reactors. 

Corrosion tests of clad and unclad coupons from the plates resulted in cracking or splitting down the center in the two unclad samples and in one clad sample after 24 hr at 500F, while the other clad sample withstood about 200 hr before separating cleanly at the bond lines. The unclad samples showed incipient cracks before corrosion testing. 

Note 8—Automatic vacuum controllers tend to mask the initial rise in pressure that indicates incipient cracking. 

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