DUCTILE-BRITTLE

From this therefore it is evident that the failure stress, ductile/brittle transitions which may be observed in plastics. According to line B, as the fiaw size decreases the failure stress tends towards infinity. Clearly this is not the case and in practice what happens is that at some defect size ([Pg.132]

Fig. 2.69 Effect of varying stress field on flaw size for ductile/brittle transition (AT = constant)...

With the precipitation hardening types, high strengths can be obtained with good toughness. A feature of these steels is that the ductile-brittle transition is less sharp, although low impact values are obtained at very low temperatures. Properties for a typical example are shown in Tables 3.17 and Fig. 3.13. 

Forming The fabrication of molybdenum is largely dictated by the ductile-brittle transition temperature. Most operations, except those on thin sheet or wire, are carried out warm and it is often necessary to heat not only the workpiece but also the die. 

Steels are normally ductile at ambient temperatures, although they are often close to brittle behaviour, as is indicated by the ductile-brittle transition temperature. If the conditions at the tip of a sharp crack are considered, it can be seen that brittle fracture will occur if it is easier to break the atomic bond at the tip of the crack than it is to emit a dislocation to blunt the crack (see Thompson and Lin ). As dislocation emission is more temperature sensitive than the bond strength it becomes more difficult at low temperatures and brittle fracture occurs. The very severe effects of hydrogen on the performance of steels can be attributed to its role in allowing brittle fracture... 

The early study of brittle failures, notably those of the Liberty ships, indicated a temperature dependence. This can be illustrated by plotting both fracture stress (of) and yield stress (Oy) against temperature (Fig. 8.81). Below a certain temperature some materials exhibit a transition from ductile to brittle fracture mode. This temperature is known as the ductile-brittle transition temperature DBTT. 

Earlier work had shown that lower-molecular weight compatibilizers have a more significant effect on reducing the ductile-brittle transition than higher-molecular weight ones. Eurther, the brittle-ductile temperature. I m, increases with the increase in loading velocity. 

Dual nickel, 9 820—821 Dual-pressure processes, in nitric acid production, 17 175, 177, 179 Dual-solvent fractional extraction, 10 760 Dual Ziegler catalysts, for LLDPE production, 20 191 Dubinin-Radushkevich adsorption isotherm, 1 626, 627 Dubnium (Db), l 492t Ductile (nodular) iron, 14 522 Ductile brittle transition temperature (DBTT), 13 487 Ductile cast iron, 22 518—519 Ductile fracture, as failure mechanism, 26 983 Ductile iron... 

Low Test Temperature. The possibility of brittle fracture shall be considered when conducting leak tests at metal temperatures near the ductile-brittle transition temperature. 

To avoid brittle fracture during operation, maintenance, transportation, erection, and testing, good design practice shall be followed in the selection of fabrication methods, welding procedures, and materials for vendor furnished steel pressure retaining parts that may be subjected to temperature below the ductile-brittle transition point. 

Thermoplastic structural foams with bulk densities not less than 50% of the solid resin densities are considered. Cellular morphology, uniform-density cell behaviour, the I-beam concept in designing, core-density profile and the role of the skin, mechanical properties, and ductile-brittle transitions are discussed. 63 refs. 

NOTE Good design practice should be followed in the selection of fabrication methods, welding procedures, and materials for vendor-furnished steel pressureretalning parts that may be subject to temperatures below the ductile-brittle transition temperature. The published design-allowable stresses for metallic materials in internationally recognised standards such as the ASME Code and ANSI standards are based on minimum tensile properties. Some standards do not differentiate between rimmed, semi-killed, fully killed hot-rolled and normalised material, nor do they take into account whether materials were produced under fine- or course-grain practices. The vendor should exercise caution in the selection of materials intended for services between 0 °C (-20 °F) and 40 °C (100 °F). 

However, at lower constant loads the rate of crystal plastic deformation decreases and (at 80 °C) disentanglement becomes competitive leading to the development of isolated planar craze-like defects extending perpendicular to the tensile axis (Fig. 15). The ensuing concentration of stress will further localize most of the sample deformation in such creep crazes and lead to a macroscopic ductile-brittle transition—in this material observed at 20 MPa (Fig. 14 [67]). 

Figure 10.1 Time-temperature map. Shape of main boundaries for linear or network polymers. (I) Glassy brittle domain B, ductile-brittle transition. (II) Glassy ductile domain G, glass transition. (Ill) Rubbery domain. The location of the boundaries depends on the polymer structure but their shape is always the same. Typical limits for coordinates are 0-700 K for temperature and 10-3 s. (fast impact) to 1010 s e.g., 30 years static loading in civil engineering or building structures. Fpr dynamic loading, t would be the reciprocal of frequency. For monotone loading, it could be the reciprocal of strain rate s = dl/ Idt.

Yielding and fracture are two very important properties of materials, particularly for thermosets. Both aspects can be associated by considering the ductile-brittle transition temperature, TB (Fig. 12.5). 

Figure 12.5 Effect of strain rate on ductile-brittle transition temperature — low speed — high speed.

At higher temperatures, the failure occurs with yielding, which is now the predominant deformation mechanism. From an experimental point of view, domains define what is called the ductile-brittle transition temperature, TB, which is a very important characteristic for polymers. The ductile-brittle transition is also associated with a stiffness-toughness balance. Note that it is also possible to determine a ductile-brittle strain rate transition varying k at a given temperature. 

Thus, the understanding of thermosets fracture needs the complete description of the yielding and the influence of both experimental variables, (T, e), on the one hand, and the relationship with structural parameters, on the other hand. Unfortunately, few results are available in the literature dealing with the ductile-brittle transition of thermosets. Very often it is stated that thermosets are more brittle than thermoplastics but this depends only on the location of the test temperature compared with the ductile-brittle transition temperature. 

For many years, several authors have tried to explain and predict the yield stress of polymers (crosslinked or not), as a function of the experimental test parameters (T, e) and/or structural parameters (chain stiffness, crosslinking density). These models would be very useful to extrapolate yield stress values in different test conditions and to determine the ductile-brittle transition. 

Eyring s equation may be regarded as a good phenomenological description of yield stress as a function of test parameters (T, e), but it cannot be related to physical processes at the molecular scale. The equation can be used at high e for impact properties and for the prediction of the ductile brittle transition temperature. Eyring s equation can be modified with two sets of parameters if two relaxations are involved in the range of temperatures and strain rates (Bauwens-Crowet et al., 1972). 

The linear dependence of [Pg.375]

The fracture energy cannot be related to the failure of chemical bonds which may contribute only with a few Jm-2. Furthermore, the possibility of crazing is not allowed in thermosets because fibrils cannot exist due to the high crosslink density. So, in the case of high-Tg cross-linked materials the main source of energy absorption before failure is the yielding of the network. This assumption is obviously valid only above the ductile-brittle transition temperature (Fig. 12.5), where yielding is temperature-dependent. ... 

Impact tests can be performed at various temperatures, especially at low temperatures (where there is a combination with the high speed), in order to determine the ductile-brittle transition. This transition is very important for characterizing the polymer behavior, and is determined usually at a constant speed and changing the temperature. Although it is less usual, it is possible to fix the temperature and to vary the speed. 

The ductile-brittle transition is clearly related to the yielding behavior of the thermoset in static experiments, (see Fig. 12.5). 

At room temperature, well below Tg, a brittle failure is generally observed. The ductile behavior appears when yielding becomes a competitive mechanism of deformation. At high speeds the brittle stress is not too much affected but ductile-brittle transition to higher temperatures. 

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