Transition temperature, ductile-brittle

The temperature at which a sample changes from brittle to ductile can be called the brittle-ductile transition temperature (Tgg). 

FIG. 13.66 Rate of strain dependence upon brittle-ductile transition temperatures of various solids. According to Vincent (1962). 

There are several parameters that affect the brittleness and the brittle-ductile transition temperature, such as molecular weight, presence of cross-links, crystallinity and the presence of notches. A schematic way, following Fig. 13.75 to depict the influence of the various parameters, is shown in Fig. 13.76. 

On the other hand, it is expected that the strain rate also influences Tj. It has been found that while brittle fracture is hardly affected, the yield stress changes significantly with the strain rate. As shown in Figure 14.26, when the strain rate increases, Oy increases. Therefore the brittle-ductile transition temperature increases, as does the strain rate. This is easily illu-... 

The brittle-ductile transition temperature depends on the characteristics of the sample such as thickness, surface defects, and the presence of flaws or notches. Increasing the thickness of the sample favors brittle fracture a typical example is polycarbonate at room temperature. The presence of surface defects (scratches) or the introduction of flaws and notches in the sample increases Tg. A polymer that displays ductile behavior at a particular temperature can break in the brittle mode if a notch is made in it examples are PVC and nylon. This type of behavior is explained by analyzing the distribution of stresses in the zone of the notch. When a sample is subjected to a uniaxial tension, a complex state of stresses is created at the tip of the notch and the yield stress brittle behavior known as notch brittleness. Brittle behavior is favored by sharp notches and thick samples where plane strain deformation prevails over plane stress deformation. 

Moreover, the brittle-ductile transition temperature depends on the molecular structure and morphology of the polymer sample. The correlation between chemical structure and fracture behavior is not yet well understood. It is recognized that entanglements control the fracture behavior of glassy... 

At room temperature, PP is close to its Tg(0-25°C) and well above its normal brittle-ductile transition temperature ( -30°C). However the presence of surface cracks in the photo-oxidized film is apparently sufficient to promote brittle failure at room temperature. According to the Griffith crack theory, once a critical crack length has been exceeded, a critical crack velocity is required to propagate the crack. If this velocity is not exceeded, cold drawing of the amorphous zones ensues. 

However, the monolithic compounds possess a lack of room temperature ductility and toughness because of their complex lattice structures and sessile superdislocations with large Burgers vectors. The brittle-ductile transition temperatures of these silicides are quite high of the order of 800 to 1050 °C, respectively. 

Steels killed with silicon, such as ASTM A515 plates, tend to have a coarse grain structure usually with a silicon content of 0.15 to 0.30 wt%. They characteristically have relatively high brittle-ductile transition temperatures, making them unsuitable for applications requiring low-... 

Low-temperature embrittlement occurs in carbon and low-alloy steels at temperatures below their brittle-ductile transition temperature range. The effect is reversible when the alloy is heated above the transition range, ductility is restored. This embrittlement is avoided by following the Charpy impact test requirements of the relevant engineering codes. The need to test depends primarily on the material, its thickness and the minimum design temperature. 

Duplex stainless steels are susceptible to 885°F (475°C) embrittlement and to sigma-phase formation, and they are usually not selected for temperatures above 650°F (345°C). Because of their ferrite content, they are susceptible to low-temperature embrittlement. However, the duplex stainless steels tend to have relatively low brittle-ductile transition temperatures. The engineering codes typically require the duplex stainless steels to be qualified for low-temperature service by impact testing. They can be susceptible to hydrogen embrittlement, but are less susceptible than are the ferritic and martensitic stainless steels. 

Not all polymers show cold drawing there are requirements such as a minimum molecular weight for strain hardening, and this has been extensively discussed elsewhere (see for example Ref. 5, pp. 271 and 322). It is clearly necessary for the polymer to be above its brittle-ductile transition, but this is a necessary rather than a necessary and sufficient condition for cold drawing. It should perhaps also be emphasised that there is no immediately obvious relationship between the glass transition temperature and the brittle-ductile transition temperature Tb (see for example Ref. 6). 

The brittle-ductile transition temperatures of some commercial polymers, blends, and composites are given in Table 10.23. 

Impact Modifiers. Notched impact strength and ductility can be improved with the incorporation of impact modifiers, which can also lower the brittle-ductile transition temperature and give much improved low temperature toughness. Impact modifiers are rubbers (often olefin copolymers) that are either modified or contain functional groups to make them more compatible with the nylon matrix. Dispersion of the rubber into small (micrometer size) particles is important in order to obtain effective toughening (19). Impact modifiers can be combined with other additives, such as glass fiber and minerals, in order to obtain a particular balance of stiffness and toughness. Modified acrylics, silicones, and polyurethanes have also been proposed as impact modifiers. 

The transition of dispersed particles from the rubbery to glassy state defines the lowest temperature at which the incorporated rubber is able to reduce the matrix jdeld stress and accoimt for significant toughening (280,281). The effect of added rubber usually fades away at temperatures 10-20 K above its which is manifested as a sharp drop in the fracture energy (256). An equation was derived for the brittle-ductile transition temperature as a function of the particle... 

Creep rate spectroscopy was also successfully used as the method allowing one to estimate the ability of steels to be inclined to brittle fracture. The temperature position and height of the CR peak within the range between —60°C and 20 °C was the distinct characteristics of their comparative tendency towards the brittle fracture and was of use for prediction of the critical brittle-ductile transition temperature [336]. 

Wu, S. (1992). Secondary Relaxation, Brittle-Ductile Transition Temperature, and Chain Structure. J. Appl. Polymer Sci. 46(4), 619-624. 

The extremely low ductility values at ambient temperature and the increased ductility with increasing temperatures strongly influence the observed fracture mode. Tfensile and fatigue specimens indicate that the predominant fracture modes are cleavage at low temperatures due to dislocation pile-up and intergranular fracture above the brittle-ductile transition temperature (Ref 13-17). 

The experimental set-up is shown in Figure 12.16. A particular advantage of this technique is that yield behaviour can be observed in compression for materials, which normally fracture in a tensile test. In this case, PMMA was studied at room temperature, i.e. below its brittle-ductile transition temperature in tension. 

Brittle-ductile transition temperatures of thin oxide scales at different reduction rates (strain amounts) [107,139-140]. 

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