Serrated curves

Figure 10.11. Schematic view of a bread crumb specimen ready for tensile testing and typical tensile force eformation curves. Notice that the serrated curve is a record of successive tearing of cell walls.

Serrated stress-strain curves, similar to those occurring in metals, have also been observed in ceramics. Such stress-strain curves are shown in schematic Fig. 4.47 and experimentally observed serrated curves in alumina are illustrated in Figs. 4.48 and 4.49, formed during deformation at two temperatures and at the... 

The serration curves in ceramics are analogous with those observed in HCP and BCC metals during low-temperature deformation. Serration is formed by the movement of partial dislocations this motion converts part of a crystal to a twin orientation. It is believed that dislocations are involved in twinning, but the mechanism is not yet clear. 

In this section, the formation of deformation and annealing twins were discussed with the resulting stress-strain relation characterized by serrated curves. As indicated here, there is no basic difference in twin formation between ceramics and metals. 

One sometimes observes alternating (serrated) variations of activity (zig-zag curves) according to whether the number of atoms of carbon is even or odd. Such an example is found for antimalarials derived from methoxy-6-amino-8-quinoline (Figure 14.14). 

Forceps serrated and curved with 0.7 mm wide tips. 

Description Herbaceous perennial, with a thick caudex. Stems 10-60 cm tall, densely leafy. Caudex leaves scale like stan leaves alternate, sessile, linear-lanceolate, 2-6 cm long, 3-15 mm wide, slightly serrate. Inflorescences cymose, dense, compact. Flowers unisexual or occasionally bisexual. Sepals linear or triangular, 1.5-3 mm long. Petals 3-A mm long, brownish-red, pink or yellow. Fruits paired elongate follicles with curved apical beaks. 

The aspect of the flow curves in Fig. 9.6 indicates the predominant condition of the test. For instance. Fig. 9.6a shows uniform serrations (stick-shp) associated with a continuous, step-by-step process of fiber slipping away off the polymeric block. This exclusive pullout mechanism occurred only in the first part of the curve depicted in Fig. 9.5c. In this case, the fiber was intact after the test and, in some cases, covered with a layer of polyester. By contrast. Fig. 9.6d shows a smooth curve up to fracture. In this case, no pullout process occurred, and the fiber had undergone tensile rupture in association with the last horizontal part of the curve. The intermediate condition of both pullout and fiber rupture are shown in Fig. 9.6b, c. 

Figure 8.10 shows a schematic of the CBR test for a geotexthe. The specimen is clamped between the serrated surfaces of two steel rings. A plunger of 50 mm diameter is advanced at a constant rate onto the centre of the specimen and perpendicular to it. The push-through force, and push-through displacement are recorded and a force-displacement curve plotted. 

Fig. 4.47 A schematic illustration of the yield drop and serration in the load elongation curve...

Fig. 4.56 Serrated stress versus strain curves in a 24 mol% YzOs-ZrOz specimen, com nessed at an imposed strain-rate of = 1.6x10 s between 1310 and 1450 °C. All the tests are performed in easy glide conditions [21]. With kind permission of Elsevier...

A 10 /i. The Liiders bands seen in Fig. 4.79 are parallel to the 001 slip planes. The large yield drop corresponds to the first Liiders band formation while the formation of additional Liiders band is associated with the serration of the stress-strain curve. Many specimens tested gave the same stress-strain curves with Liiders bands formation. 

After the specimen was mounted into the test chamber and cooled to the desired test temperature the diameter-indicating dial was adjusted to read the calculated diameter at the test temperature. This was necessary since contraction of the four caliper fingers could indicate a false diameter reduction. For tensile tests, where load displacement records were obtained, the specimen gage length was scanned before and after each serration, and the minimum diameters of the several necks were noted and recorded on the curve. 

Tensile stress-strain curves below 319 F and in some cases above -319°F are marked with load serrations. The reasons advanced for these serrations are many and varied [3-6],... 

A clue to the reason for this behavior can be found from the load-diameter-extension data. At temperatures above which load serrations do not occur, the load increases uniformly as the diameter decreases until maximum load occurs, and further deformation is restricted to one neck. At the lower temperatures a different behavior occurs and the load extension curve is serrated. Each load drop corresponds to a diameter decrease at one or several necks, which have formed before "maximum load." Further, it must be realized that the rising load portion of each serration corresponds to an essentially elastic extension, whereas plastic deformation occurs during the rapidly decreasing load portions. During this decreasing load, at about constant strain, elastic strains are relieved and "exchanged" for an equivalent amount of plastic strain. Finally, consideration must be given to the fact that in these steels austenite is metastable and transforms under stress to yield martensite. 

The derivative DCP [52] can be successfully applied only if the current serrations are eliminated. Satisfactory derivative curves are obtained if the current is sampled at very slow sweep rate, e.g., 1-2 mV per a single drop-life [53]. Then the derivative dE/dt can be replaced by the ratio zlE/id. Since the derivative DCP curve is peak-shaped (see Fig. 16) the resolution of the recorded signal is improved. The peak height of the first derivative. Ip, is a linear function of the concentration of A ... 

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