Break stress

Most solid surfaces are marred by small cracks, and it appears clear that it is often because of the presence of such surface imperfections that observed tensile strengths fall below the theoretical ones. For sodium chloride, the theoretical tensile strength is about 200 kg/mm [136], while that calculated from the work of cohesion would be 40 kg/mm [137], and actual breaking stresses are a hundreth or a thousandth of this, depending on the surface condition and crystal size. Coating the salt crystals with a saturated solution, causing surface deposition of small crystals to occur, resulted in a much lower tensile strength but not if the solution contained some urea. 

DMC supports may be used but limiting factor is cross-breaking stress hence only SMC supports must be used... 

Bnich-kupfer, n. scrap copper, -last, /. breaking load, -metall, n. broken metal, scrap metal, -modul, m. modulus of rupture, -probe, /. breaking test, breakdown test, -punkt, m. breaking point, -riss, m. (Meial.) failure crack, -silber, n. broken silver, scrap silver, -spaonung,/. breaking stress tensile strength, -stein, m. quarry stone broken stone, -stelle,/. broken place, place of fracture. -strich, m. (Math.) fraction stroke (between numerator and denominator), -stiick, n. fragment shred, -stiicke, pi. debris scrap, -teil, m. fraction, -zahl, /. fractional number. 

The mechanical properties can be studied by stretching a polymer specimen at constant rate and monitoring the stress produced. The Young (elastic) modulus is determined from the initial linear portion of the stress-strain curve, and other mechanical parameters of interest include the yield and break stresses and the corresponding strain (draw ratio) values. Some of these parameters will be reported in the following paragraphs, referred to as results on thermotropic polybibenzoates with different spacers. The stress-strain plots were obtained at various drawing temperatures and rates. 

Table 3. Breaking stresses for polypropylene with 30% of glass fibers in function of the angle between the fiber orientation and direction of load application [157]...

The breaking stress may be evaluated using the formula proposed by Pegeaut [159] ... 

The effect of these small cracks is to concentrate the stress at localised points within the specimen. Figure 7.4 illustrates how this happens, using lines to indicate the stress distribution in the sample. For the unnotched specimen, (a), the stress is uniformly distributed throughout the material. However, for the notched specimen, (b), the lines of stress can be seen to converge at the notch tip, this giving a local stress greater than the apparent applied stress. When this happens, the breaking stress, will occur in the material at an actual stress somewhat less than this. As a result, the material as a whole is weaker than predicted on the basis of is chemical composition. 

As a result of this additional work to create a surface over and above the thermodynamic surface energy assumed by Griffith, we need to rewrite his equation for the relationship between breaking stress and crack length. We modify equation (7.2) to ... 

It has been calculated that the strength of a weak plane in the transverse (vertical) direction must be less than one-fifth of the tensile breaking stress in order to induce a crack to turn sideways [24]. Lake et al. [25] considered the energy release rate Gs available for further growth of a sideways crack that is already long, and concluded that Gs is a rather small fraction of the value Gp for forward growth ... 

Figure 1. N ature of the relation between time of loading and breaking stress for glass. This property is of a statistical nature, and the particular curve shown can only be taken as representative...

The breaking stress in tension of an elastomer expressed in Mpa, kg/cm2 or lb/in2. It is calculated on the original (unstressed) area of cross-section of the test specimen. 

The dimensions of a crystal affect also its breaking stress this effect is referred to in Section III.3. 

For a crack, w cm wide, to advance by dx cm in a solid, a ribbon (or curtain) of this solid, dx cm thick and w cm wide, must be extended to its greatest possible elongation. If the curtain of length L can extend freely, the work of extension is, see Eq. (44), 2 Lw dx/2E ergs. The cohesion is equal to the breaking stress <7m (g/cm sec2) of the theoretical solid, otherwise identical with the real, but free of all defects. rather than am should be used here because dx is so small that no defect can find place in the curtain. The work per unit area corresponds to the hypothetical 7 . Hence,... 

Criterion of Explosiveness. The criterion of explosiveness is energy release. The atoms in the molecules of an explosive can rearrange in response to breaking stress (Described on pD 227-L) to form new combinations having... 

There is thus an inherent instability. The molecules are in a condition of strain which on release can transmit a "breaking stress to others. A destructive stress may be applied to any material, but will not be transmitted appreciably in materials which do not release energy in sufficient quantity or intensity to maintain it. Explosives, on the other hand, are distinguished by sensitivity to various influences, such as thermal, mechanical and electrical... 

Detonation is the progressive breaking of the valence bond by a stress, known as breaking stress, which is transmitted thru the material as an elastic wave similar to... 

The source of such a breaking stress may be release of strains already existing in the materials, nevertheless some assistance from an outside force is usually required. Such strains exist in endothermic compds, such... 

Landel and Fedors (53, 54) have recently explored the usefulness of extreme value statistics applied to the statistical distribution of rupture in various unfilled polymer specimens. Both breaking stress and breaking strain of natural rubber (47) and styrene butadiene elastomers (53, 54) may be described by the double exponential distribution... 

Fig. 4. The nominal breaking stress (Amsier testing machine) of aluminium alloy/epoxy single lap joints as a function of the hydroxyl content of the epoxy raised to the two-thirds power51 (Reprinted from Ref. 51, p.307, by courtesy of Society of Chemical Industry)...

The breaking stress, corresponding to load D. is seldom determined or reported. 

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