True rupture stress

Fibres, containing derivatives of bis-aroilenbcnzimidazole, were subjected to ultra-violet light action of the lamp PRK-2. Mechanical properties of the fibres were determined depending on cross section area. Value of rupture stress (o) is the mean value of true rupture stress obtained from indexes of tearing machine. 

Fig. 3.79 a yield strength Oy, ultimate [Pg.181]

One of the simplest criteria specific to the internal port cracking failure mode is based on the uniaxial strain capability in simple tension. Since the material properties are known to be strain rate- and temperature-dependent, tests are conducted under various conditions, and a failure strain boundary is generated. Strain at rupture is plotted against a variable such as reduced time, and any strain requirement which falls outside of the boundary will lead to rupture, and any condition inside will be considered safe. Ad hoc criteria have been proposed, such as that of Landel (55) in which the failure strain eL is defined as the ratio of the maximum true stress to the initial modulus, where the true stress is defined as the product of the extension ratio and the engineering stress —i.e., breaks down at low strain rates and higher temperatures. Milloway and Wiegand (68) suggested that motor strain should be less than half of the uniaxial tensile strain at failure at 0.74 min.-1. This criterion was based on 41 small motor tests. 

It should be noted that the values of cross-section of the specimen, but in the course of deformation the specimen cross-section decreases. As Fig. 3.13 shows, the value of the true stress at rupture of the polymeric films decreases with increasing rubber content to a lesser extent than (To- At large rubber content, [Pg.151]

For a system with a sharp interface and weak intermolecular interaction, as in the nonpolar-polar polymer system mentioned above, failure exactly at the interface is a distinct possibility (or even probability). Thus, interfacial separation may be expected when interfadal strength is weaker than the bulk strength of the bonded materials. As we have seen, if the intermolecular interactions across the interface are more specific (including chemical bond formation) or if significant interpenetration of polymer chains occurs, rupture at the interface becomes less likely. In some cases, the locus of failure may depend on the rate at which stress is applied rapid application leads to cohesive failure and very slow apphcation, tending more toward true adhesive failure (since slow application of stress gives more time for the entangled molecules to slide past one another). 

True tensile strength n. The maximum tensile stress expressed in force per unit area of the specimen at the time of rupture. 

The resilience, denoted and expressed in Joules (J), is the ability of a solid material to absorb elastic energy and release it when unloaded (e.g., rebound, springback). In practice, the absorbed elastic energy can be calculated from the true stress-strain plot (S - e) by integrating the surface area under the curve between the true yield strength and the origin. This area represents the amount of elastic work per unit volume that can be done on the material without causing it to rupture ... 

A material s toughness is its ability to absorb energy in the plastic range. It is commonly measured by the modulus of toughness, U, that is, the amount of work stored per unit volume without causing rupture. As for the modulus of resilience, several mathematical approximations for the area under the true stress-strain curve can be used ... 

To test the valid it this relationship Voorhees rai and lowenad the stress ieyels on 18 conventional unnotched haum. On averaging the results for t e 18 tests, he found that the additions of the fractions of rupture Ufe checked with the experimental observations V ithin 1% (204). Voorhees points out that this rule of addibility of rupture-life fractions can not reasonably be expected to bold true if appreciable structural alterations occur. 

Rupture strength The true value of rupture strength is the stress of a material at failure based on the original ruptures cross-sectional area. 

Figure 2.54. Illustration of a true stress vs. strain curve and comparison of stress-strain curves for various materials. UTS = ultimate strength, and YS = yield strength. The tensile strength is the point of rupture, and the offset strain is typically 0.2% - used to determine the yield strength for metals without a well-defined yield point.Reproduced with permission from Cardarelli, F. Materials Handbook, 2nd ed.. Springer New York, 2008. Copyright 2008 Springer Science Business Media.

An approximate sketch of the stress-strain diagram for mild steel is shown in Fig. 2.8(a). The numbers given for proportional limit, upper and lower yield points and maximum stress are taken from the literature, but are only approximations. Notice that the stress is nearly hnear with strain until it reaches the upper yield point stress which is also known as the elastic-plastic tensile instability point. At this point the load (or stress) decreases as the deformation continues to increase. That is, less load is necessary to sustain continued deformation. The region between the lower yield point and the maximum stress is a region of strain hardening, a concept that is discussed in the next section. Note that if true stress and strain are used, the maximum or ultimate stress is at the rupture point. 

Note Property values such as those listed in this table vary widely and should not be used for design purposes without validating by testing tbe exact polymer to be used. ASTM Standard testing procedures offer reliable experimental protocols for such experiments. Mechanical properties of polymers can also be found in reference handbooks such as The Polymer Handbook (2006) and other textbooks such as Rodriguez, 1996 (p. 696-710) as well as various online databases such as plasticsusa.com. Variability of polymer properties can be seen for example in Fig. 3.7, where the true stress and strain at rupture for polycarbonate differ from the values tabulated here. 

The strength level of a material has a significant influence on its selection for a given application. This is especially true at elevated temperatures where the yield and ultimate strength are relatively low and the creep and rupture behavior may control the allowable stress values. In the ASME Code, VIII-1, the criteria for allowable stress at elevated temperatures take into account both the creep and rupture behavior as discussed in Section 2.4. In applying the ASME criteria for allowable stress as given there, the following procedures are used. 

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