Temperature brittle-ductile transition temperatur

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. 

Tihe brittle-ductile transition of metals as reported by Orowan (I) is explained on the basis that brittle fracture occurs when the yield stress exceeds a critical value. This is based on the Ludwik-Davidenkov-Orowan hypothesis that brittle fracture and plastic flow are independent processes yielding separate curves as a function of temperature and strain rate. Therefore, the operative deformation process is the one occurring at the lower stress. The intersection of the brittle stress and yield stress curves therefore defines the brittle-ductile transition. 

The fracture test on three-point-bend samples revealed that the fracture behavior of ABS remains ductile for temperatures above -80 °C at a crosshead speed of 5mm/min. The maximum fracture energy at crack initiation is also observed around -80 °C. Figure 27.20 shows that above this temperature, G, decreases continuously with increasing temperature. When the test speed is increased to lOOmm/min, the temperature at brittle ductile transition is shifted to about -40 °C as shown in Figure 27.21. At an impact velocity of 2.5 m/s, the brittle-ductile transition occurs at around -20 °C as shown in Figure 27.22. The results also confirm that the fracture energy at crack initiation is maximum at the brittle-ductile transition. 

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... 

Several cautions are, however, in order. Polymers are notorious for their time dependent behavior. Slow but persistent relaxation processes can result in glass transition type behavior (under stress) at temperatures well below the commonly quoted dilatometric or DTA glass transition temperature. Under such a condition the polymer is ductile, not brittle. Thus, the question of a brittle-ductile transition arises, a subject which this writer has discussed on occasion. It is then necessary to compare the propensity of a sample to fail by brittle crack propagation versus its tendency to fail (in service) by excessive creep. The use of linear elastic fracture mechanics addresses the first failure mode and not the second. If the brittle-ductile transition is kinetic in origin then at some stress a time always exists at which large strains will develop, provided that brittle failure does not intervene. 

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. 

Effect of moisture content of chocolate cookies on the transition temperature the glass transition, Tg, the critical sensorial transition, 7, and the brittle-ductile transition, Tm-... 

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. 

Other applications are wear resistant coatings and thin layers for diverse electronic and surface engineering purposes. The main disadvantage of the silicides is their intrinsic brittleness at room temperature up to the brittle-ductile transition at temperatures of 700 °C to 900 °C. However,... 

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. 

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