Brittle-tough transition temperature

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. 

Plasticizers are particularly used for polymers that are in a glassy state at room temperature. These rigid polymers become flexible by strong interactions between plasticizer molecules and chain units, which lower their brittle-tough transition or brittleness temperature (Tb) (the temperature at which a sample breaks when struck) and their Tg value, and extend the temperature range for their rubbery or viscoelastic state behavior (see Figure 1.19). 

Fig. 11.25 Temperature of brittle-tough transition as a function of the weight-average particle size for blends of PA6 and 10 wt.% rubber, with different types of rubbers , EPDM -h, EPR X, LDPE A, Keltaflex ...

If a ferritic RPV steel (body-centered-cubic structure) specimen experiences brittle cleavage fracture prior to the full development of a resistance curve, the /-integral value at the onset of fracture, / , is used to calculate an equivalent stress intensity factor, Kj, shown in Table 10.1 and Eq. 10.4. The specimen types used to measure Kj are essentially the same as those shown in Rg. 10.1. The Kj parameter is also used to determine the parameter To, the temperature at which the median fracture toughness (Kq) of a minimum specified number of IT specimens (25.4 mm thick) is 100 MPa Vm. This parameter. To, defines the fracture toughness transition temperature using the Master Curve concept developed by Wallin (1984). The Master Curve concept has been further developed as a consensus test standard in ASTM E 1921 (ASTM, 2013f) and is described in greater detail below. 

Wu [175-177] proposed that the MLT, i.e., surface-to-surface inter-particle distance is a fundamental parameter. For effective toughening, mean matrix ligament thickness (x) should be less than that of the critical value (xj where brittle-tough transition occurs. The x is independent of rubber volume fraction, particle size and characteristics of the matrix alone at a given test temperature and rate of deformation. For blends with dispersed spherical particles, the x can be related to the rubber particle size and rubber volume fraction < >) by the following equation [177] ... 

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. 

PA6 EPDM-g-MA The brittle-tough transition goes to lower temperature with increased rubber content, decreased rubber particle size and decreased rubber phase modulus 39... 

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). 

Transition metal carbides and nitrides are not only inherently brittle at room temperature they are also difficult to manufacture. For engineering applications, the hard carbides (and nitrides) are therefore bonded or cemented by a ductile metal binder, usually cobalt or nickel (hence the name Cemented Carbides ). Additions of binder metal in the range of 5-20 wt% increase the toughness (transverse rupture strength) of the tools without seriously reducing hardness, rigidity, or compressive strength. Metallic binders also enable carbide products to be manufactured to full density. 

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. 

Choice of Polymer. The ideal polymer is a tough, amophorous, but not brittle thermoplastic with a glass transition temperature more than 50 °C above the expected use temperature. A high molecular weight is important. Commercial polymers made for injection molding have molecular weights in the 30000... 

---