Brittleness definition

By definition, a brittle material does not fail in shear failure oeeurs when the largest prineipal stress reaehes the ultimate tensile strength, Su. Where the ultimate eompressive strength, Su, and Su of brittle material are approximately the same, the Maximum Normal Stress Theory applies (Edwards and MeKee, 1991 Norton, 1996). The probabilistie failure eriterion is essentially the same as equation 4.55. 

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). 

The effect of temperature on the mechanical properties has definite place in specification of a material for any particular application. Some materials become dangerously brittle under arctic conditions, and all metals exhibit at elevated temperatures a phenomenon called creep. 

Comparison of the fracture surface of the opaque polyethylene layer between the "good" sample (Figures 65(a) and 65(b)) and the "bad" sample (Figures 65(c) and 65(d)) showed a definite difference in appearance. The "good" sample fracture surface showed evidence of ductility in the polyethylene layer. The polyethylene fracture surface of the "bad" sample showed less elongation and had a cleaner appearance that is characteristic of brittle failure. 

Maleate/vinyl ether formulations based on a model unsaturated polyester prepared from maleic anhydride and 1,5-pentane diol and triethylene glycol divinyl ether were studied. At molecular weights of less than about 10,000 the cured films were extremely brittle. When the equivalent weight of the unsaturated polyester was increased by replacing some of the maleic anhydride with succinic anhydride, measurable values for film elongation could be obtained but the cure speed was definitely slower. When either diethyl maleate or isobutyl vinyl ether were added as monofunctional diluents the cure dose needed to obtain 200 MEKDR was increased and the flexibility measured by pencil hardness increased as the amount of diluent was increased. A urethane vinyl ether was synthesized and used to replace DVE-3 and films with increased elongation were obtained at equivalent at dosages as low as 1 J/cm2. 

The - very fuzzy - definition of the brittle point is based on a more or less sudden reduction in the impact resistance or the flexibility. ITie indicated values must be carefully considered. 

Amorphous Selenium.—(1) Vitreous Selenium.—When molten selenium is cooled in not too protracted a manner, no definite solidification or crystallisation ensues, but the mass gradually hardens and the product really represents a strongly undercooled liquid like glass. Vitreous selenium is a brittle reddish-brown substance, exhibiting a conchoidal fracture. When finely powdered and viewed in thin layers it has a deep red colour. This form has an average density of 4-28 5 the value varies slightly, possibly owing to the presence of other allotropic modifications of the element. 

Figure 7. Average fluorescence spectra for four varieties of resinite from the Hiawatha Seam, Utah. Varieties 1, 2, and 3 have a definite shape and a brittle fracture while variety 4 occurs only as a void filling. The reflectance of the seam is 0.65%.

Using the experimental values for the width of the traveling wave front (portion be, Fig. 8), let us estimate the propagation velocity for the case of a thermal mechanism based on the Arrhenius law of heat evolution from the known relationship U = a/d, where a 10"2 cm2/s is the thermal conductivity determined by the conventional technique. We obtain 5 x 10"2 and 3 x 10-2cm/s for 77 and 4.2 K, respectively, which are below the experimental values by about 1.5-2 orders of magnitude. This result is further definite evidence for the nonthermal nature of the propagation mechanism of a low-temperature reaction initiated by brittle fracture of the irradiated reactant sample. 

When a liquid supercools (i.e., does not crystallize when its temperature drops below the thermodynamic melting point), the liquidlike structure is frozen due to the high viscosity of the system. The supercooled liquid is in a so-called viscoelastic state. If the crystallization can be further avoided as the ten ierature continues to drop, a glass transition will happen at a certain temperature, where the frozen liquid turns into a brittle, rigid state known as a glassy state. A well-accepted definition for glass transition is that the relaxation time t of the system is 2 X10 s or the viscosity / isio Pas (an arbitrary standard, of course). 

Although for some applications, particulady those involving wear resistance, the hardness of martensite is desirable in spite of the accompanying brittleness, this microstmcture is mainly important as starting material for tempered martensite stmctures, which have definitely superior properties for most demanding applications. 

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