Ductility, defined

Dividing/ by the equivalent mass L renders its ordinate in the acceleration dimension see Fig. 19. This transformation permits the comparison of the capacity curve with the seismic demand represented by the response spectrum in the ADSR format of the actual site. In this format for an inelastic SDOF system with a target ductility ji, the spectral acceleration Sav at yield is plotted against the corresponding peak spectral displacement Sd (Fajfar 1999), as shown in Fig. 20a. Note that the ductility defines the ratio of the maximum imposed (inelastic) deformation to the deformation at onset of yield. 

The use of the single parameter, K, to define the stress field at the crack tip is justified for brittle materials, but its extension to ductile materials is based on the assumption that although some plasticity may occur at the tip the surrounding linear elastic stress field is the controlling parameter. 

Fracture Mechanics Methods These have proved very usebd for defining the minimum stress intensity K[scc. t which stress corrosion cracking of high-strength, low-ductihty alloys occurs. They have so far been less successful when apphed to high-ductility alloys, which are extensively used in the chemicm-process industries. 

At and near room temperature, metals have well-defined, almost constant, moduli and yield strengths (in contrast to polymers, which do not). And most metallic alloys have a ductility of 20% or better. Certain high-strength alloys (spring steel, for instance) and components made by powder methods, have less - as little as 2%. But even this is enough to ensure that an unnotched component yields before it fractures, and that fracture, when it occurs, is of a tough, ductile, type. But - partly because of their ductility - metals are prey to cyclic fatigue and, of all the classes of materials, they are the least resistant to corrosion and oxidation. 

It may be seen from Fig. 2.80 that some plastics experience the change from ductile to brittle behaviour over a relatively narrow temperature range. This permits a tough/brittle transition temperature to be quoted. In other plastics this transition is much more gradual so that it is not possible to attribute it to a single value of temperature. In these circumstances it is common to quote a Brittleness Temperature, rg(l/4). This temperature is defined as the value at which the impact strength of the material with a sharp notch (1/4 mm tip radius) equals 10 kJ/m. This temperature, when quoted, gives an indication of the temperature above which there should be no problems with impact failures. It does not mean that the material should never be used below Tb(1/4) because by definition it refers only to the sharp notch case. When the material has a blunt notch or is un-notched its behaviour may still be satisfactory well below Tb(1/4). 

The ultimate tensile strength (UTS) of a material refers to the maximum nominal stress that can be sustained by it and corresponds to the maximum load in a tension test. It is given by the stress associated with the highest point in a nominal stress-nominal stress plot. The ultimate tensile strengths of a ductile and of a brittle material are schematically illustrated in Figure 1.11. In the case of the ductile material the nominal stress decreases after reaching its maximum value because of necking. For such materials the UTS defines the onset of plastic instability. 

His mechanistic approach is further illustrated by the way he organized his treatment of the metals, arranging them in a sequence of decreasing densities rather than according to some chemical property, or some traditional sequence. Metals are defined entirely in terms of their physical properties, as the heaviest of fossils, fusible by fire, but cohering again in the cold, so as to become ductile and distendable under the hammer. 

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