Brittle-to-ductile transition

When pressure tests are conducted at metal temperatures near the ductile-to-brittle transition temperature of the material, the possibility of brittle fracture shall be considered. 

The body-centered-cuhic (bcc) metals and alloys are normally classified as undesirable for low temperature construction. This class includes Fe, the martensitic steels (low carbon and the 400-series stainless steels). Mo, and Nb. If not brittle at room temperature, these materials exhibit a ductile-to-brittle transition at low temperatures. Cold working of some steels, in particular, can induce the austenite-to-martensite transition. 

The hexagonal-close-packed (hep) metals generally exhibit mechanical properties intermediate between those of the fee and bcc metals. For example Zn encounters a ductile-to-brittle transition whereas Zr and pure Ti do not. The latter and their alloys with a hep structure remain reasonably ductile at low temperatures and have been used for many applications where weight reduction and reduced heat leakage through the material have been important. However, small impurities of O, N, H, and C can have a detrimental effect on the low temperature ductihty properties of Ti and its alloys. 

Figure 4.18 Ductile to brittle transition diagram for a structural steel (Mager and Marschall, 1984)...

Materials with a high yield stress tend to go through the ductile to brittle transition at higher temperatures. This property has led to the assumption that true brittle fracture, unlike ductile fracture, is not accompanied by the motion of dislocations. The validity of this assumption is sometimes confirmed by the appearance of brittle fractures, which show essentially no ductility. 

The concept of a ductile-to-brittle transition temperature in plastics is likewise well known in metals, notched metal products being more prone to brittle failure than unnotched specimens. Of course there are major differences, such as the short time moduli of many plastics compared with those in steel, that may be 30 x 106 psi (207 x 106 kPa). Although the ductile metals often undergo local necking during a tensile test, followed by failure in the neck, many ductile plastics exhibit the phenomenon called a propagating neck. Tliese different engineering characteristics also have important effects on certain aspects of impact resistance. 

CA cellulose acetate (CAc) DBTT ductile-to-brittle transition... 

Wallin, Kim. Master curve analysis of ductile to brittle transition region fracture toughness round robin data. The EURO fiacture toughness curve. 1998. 58 p. 

The transition metal carbides do have a notable drawback relative to engineering applications low ductility at room temperature. Below 1070 K, these materials fail in a brittle manner, while above this temperature they become ductile and deform plastically on multiple slip systems much like fee (face-centered-cubic) metals. This transition from brittle to ductile behavior is analogous to that of bee (body-centered-cubic) metals such as iron, and arises from the combination of the bee metals strongly temperature-dependent yield stress (oy) and relatively temperature-insensitive fracture stress.1 Brittle fracture is promoted below the ductile-to-brittle transition temperature because the stress required to fracture is lower than that required to move dislocations, oy. The opposite is true, however, above the transition temperature. 

Figure 10.7 shows that the tensile strength is improved as polystyrene is incorporated. Data for conventional melt-blended samples (Fayt et al., 1989) are provided for comparison. We note that the ductile-to-brittle transition for our system is shifted toward much higher polystyrene content. Fayt and others have shown that conventionally prepared polyethylene/ polystyrene blends are relatively poor materials (Barentsen and Heikens, 1973 Wycisk et al., 1990). Blends of most compositions are weaker than polystyrene or polyethylene homopolymers because of the poor interfacial adhesion between the two immiscible polymers. The electron micrographs and the mechanical data for the blends described here indicate that poly-... 

It is most convenient to classify metals by their lattice symmetry for low-temperature mechanical properties considerations. The fee metals and their alloys are most often used in the construction of cryogenic equipment. Aluminum, copper, nickel, their alloys, and the austenitic stainless steels of the 18-8 type are fee and do not exhibit an impact ductile-to-brittle transition at low temperatures. Generally, the mechanical properties of these metals im-... 

Hardness and a ductile-to-brittle transition temperature (DBTT) have also been noted to follow a Hall-Petch relationship (Meyers, and Chalwa, 1984). Ductility increases as the grain size decreases. Decreasing grain size tends to improve fatigue resistance but increases creep rate. Electrical resistivity increases as grain size decreases, as the mean free path for electron motion decreases. 

In conclusion, the cohesive surface description presented in the foregoing sections appears suitable for capturing a ductile-to-brittle transition with increasing loading rate, and for predicting a toughening effect when the bulk is essentially elastic. These trends are reported experimentally and a calibration of the parameters used in the cohesive zone description is presented in [64],... 

Double-grooved specimens were used to study the failure of PC, PC/PE, PET, ABS, and HIPS during transitions from plane stress to plane strain. The yield behavior of PC is consistent with a von Mises-type yield criterion plane strain reduces its elongation. The yield behavior of PC/PE is consistent with a Tresca-type yield criterion plane strain appears to be relieved by voiding around the PE particles. PET undergoes a ductile-to-brittle transition its behavior is consistent with a von Mises-type yield locus intersected by a craze locus. The yield behavior of ABS and HIPS is not significantly affected by the plane-stress-to-plane-strain transition. Plane strain alone does not necessarily cause brittleness. 

X HE IMPORTANCE AND UTILITY of multiphase copolymer systems have been well documented in the literature (1-4), with emphasis on their unique combination of properties and their potential material applications. Or-ganosiloxane block polymers are a particularly interesting type of multiphase copolymer system because of the unusual characteristics of poly siloxanes, such as their stability to heat and UV radiation, low glass transition temperature, high gas permeability, and low surface energy (i, 2, 5). The incorporation of polysiloxanes into various engineering polymers offers an opportunity for many improvements, such as lower temperatures for the ductile-to-brittle transitions and improved impact strength. 

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