Cold brittleness

The temperature region in which a polymer can be used, is limited at the low as well as at the high side. The most serious limitation at low temperatures is, as a matter of fact, the glass - rubber transition for rubbers, below Tg they lose their rubbery nature and pass into the glassy phase. Besides, for thermoplastics, the principle limitation is that they become brittle at low temperatures, and thus lose their impact strength. The other properties are not affected, but, in most cases, (E-modulus, tensile strength), improved upon decrease of temperature. First of all we shall consider at which temperature cold-brittleness appears. 

Amorphous, glassy polymers, used far below their Tg, are cold-brittle if no other mechanisms are active an example is PS. If a polymer has been improved in impact strength by the addition of a rubbery phase (high-impact PS or PVC, ABS etc.), then the cold-brittleness temperature is related to the Tg of the added rubber. If the polymer shows a secondary transition in the glassy region (such as PC), then this governs the brittleness temperature. 

Gravity studies, based on tracking spacecraft orbits, have revealed that the density of the outer 100-150 km is about 1 gm/cm3 (Anderson et al. 1997). Evidently, the H2O extends from the surface down to that depth. The outermost portion of this thick layer of water is a lithosphere , meaning that it is cold, brittle, elastic ice. The lithosphere is probably M km thick, although that value is somewhat uncertain, and also depends on the timescale for deformation the transition depth from elastic to viscous ice depends on the rate of strain as well as on the temperature. The viscous portion of the ice crust lies below the lithosphere, and extends to a depth of several km below the surface. A belief that the ice crust is 20 km thick or greater has been widely promoted, but the evidence cited for that model (Pappalardo et al., 1998) has not survived quantitative study (Greenberg et al. 2003b). If the ice were that thick, the ocean would be isolated from the surface. 

A great authority on iron metallurgy in Sweden (and Europe) was Sven Rinman (Chapter 8, Iron, section 8.7.2). In his Mining and Metallurgical Dictionary (Bergiverks-lexikon) he had described a method to determine whether a specific iron was Hkely to be brittle at about 200°C. This cold brittleness is caused by a high content of phosphorus in the iron. Sefstrom himself described the method in the Transactions ofThe Royal Swedish Academy of Sciences in 1830 ... 

Some years ago the inspector [bei mastare] Rinman described a method with which it is easy to detect the disposition for cold brittleness in iron. The method is founded on the fact that iron of that sort, after etching with hydrochloric acid, gives a black powder. 

The influence of temperature on the mechanical properties of solids manifests itself primarily in the macroscopic dependence of the breaking stress, and the yield stress, Ty. Typically, both parameters decrease with increasing temperature, and the decline in Ty is typically more rapid than the decline in 0. As a result of this competition, the and Ty curves may contain a crossing point, which corresponds to the brittle-to-plastic behavior transition temperature, (Figure 5.12). This transition is sometimes referred to as the cold brittleness. Because of this, ionic crystals, metals, and... 

Kq is a constant level of the strength at temperatures that are lower than the threshold of cold brittleness, that is, the minimum value of the strength is the temperature corresponding to the brittle-to-plastic transition, that is,... 

Along with metals, the threshold of forced cold brittleness is also observed in solids of all other kinds, that is, covalent crystals (e.g., in the system germanium-gold), ionic substances (e.g., sodium chloride in the melted aluminum chloride), and molecular crystals (e.g., naphthalene in liquid hydrocarbon). In the other words, there is only a limited interval of optimum temperatures in which the Rehbinder effect is observed. At temperatures that are too low, the effect is retarded by the excessive starting brittleness and the solidification of the medium, while at temperatures that are too high, it is retarded by the excessive plasticity of the solid. This temperature dependence is one of the principal features of the Rehbinder effect, which makes it very different from the chanical or corrosive action of the medium, both of which intensify as temperature increases. 

It is known that phosphorus exists in steel in the form of solid solution, which can significantly improve strength and hardness of steel at room temperature. However, die plasticity and toughness of steel, especially the low temperature plasticity is sharply reduced and leads to high ductile brittle transition temperature. This phenomenon is commonly known as cold brittleness of steel [1-3]. Hence, phosphorus should be removed as much as possible in the steehnaking process, because it is generally harmfiil to the iron and steel products. 

Lithosphere The rigid outer rock layer of the Earth consisting of the crust plus uppermost mantle. It is a thermal and mechanical boundary layer for the Earth separating cold brittle surface rocks, whose thermal and mechanical properties change rapidly with depth, from hot mantle rocks whose physical properties change slowly with depth. 

Low Temperature Brittleness. Brittleness temperature is the temperature at which polyethylene becomes sufficiently brittle to break when subjected to a sudden blow. Because some polyethylene end products are used under particularly cold climates, they must be made of a polymer that has good impact resistance at low temperatures namely, polymers with high viscosity, lower density, and narrow molecular weight distribution. ASTM D746 is used for this test. 

Dry Ice. Refrigeration of foodstuffs, especially ice cream, meat products, and frozen foods, is the principal use for soHd carbon dioxide. Dry ice is especially useful for chilling ice cream products because it can be easily sawed into thin slabs and leaves no Hquid residue upon evaporation. Cmshed dry ice may be mixed directly with other products without contaminating them and is widely used in the processing of substances that must be kept cold. Dry ice is mixed with molded substances that must be kept cold. For example, dry ice is mixed with molded mbber articles in a tumbling dmm to chill them sufficiently so that the thin flash or rind becomes brittle and breaks off. It is also used to chill golf-ball centers before winding. 

The impact forces on car couplers put high stresses in sills, anchors, and doublers. This may start fatigue cracks in the shell, particularly at the corners of welded doubler plates. With brittle steel in cold weather, such cracks sometimes cause complete rupture of the tank. Large end radii on the doublers and tougher steels will reduce this hazard. Inspection of older cars can reveal cracks before failure. 

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. 

Some materials, like glass, have low and K, and crack easily ductile metals have high Gf and and are very resistant to fast-fracture polymers have intermediate G, but can be made tougher by making them into composites and (finally) many metals, when cold, become brittle - that is, G and fall with temperature. How can we explain these important observations ... 

Titanium is a white metal and, when cold, is brittle and may be powdered. At a red heat it may be forged and drawn. The tensile strength of titanium is almost the same as that of steel, while its specific gravity (4.5) is almost two times less than that of steel. Hence, its specific strength (tensile strength/speciflc gravity) is 1000, which is considerably higher than that of 18/8 steel, which has a value of 700. Titanium is now available as plate, sheet and tube, and its use in chemical plant construction is considered common. 

Use materials that are applicable over the full range of operating conditions such as normal, startup, routine shutdown, emergency shutdown, and draining the system. For example, carbon steel may be acceptable for normal operating conditions but may be subject to brittle fracture at low temperatures under abnormal conditions (as in the case of a liquefied gas). Cold water, of less than 60°F, during hydrotest may cause brittle fracture of carbon steel. 

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