Stress nominal

Stress concentration. Stress concentration refers to physical discontinuities in a metal surface, which effectively increase the nominal stress at the discontinuity (Fig. 9.7). Stress-concentrating discontinuities can arise from three sources ... 

The difficulty in accurately estimating the degree of local concentration remains one of the principal reasons susceptibility to SCC in a specific environment or circumstance is difficult to predict. Measurement of nominal stresses or levels of corrodent in the bulk environment can be quite misleading as predictors of SCC susceptibility. 

The plastic behaviour of a material is usually measured by conducting a tensile test. Tensile testing equipment is standard in all engineering laboratories. Such equipment produces a load/displacement (F/u) curve for the material, which is then converted to a nominal stress/nominal strain, or cT l , curve (Fig. 8.10), where... 

The nominal stress at yielding. In many materials this is difficult to spot on the stress-strain curve and in such cases it is better to use a proof stress. 

This equation is given in terms of true stress and true strain. As we said in Chapter 8, tensile data are usually given in terms of nominal stress and strain. From Chapter 8 ... 

In other words, on the point of instability, the nominal stress-strain curve is at its maximum as we know experimentally from Chapter 8. 

Assuming that a diamond-pyramid hardness test creates a further nominal strain, on average, of 0.08, and that the hardness value is 3.0 times the true stress with this extra strain, construct the curve of nominal stress against nominal strain, and find ... 

Sketch curves of the nominal stress against nominal strain obtained from tensile tests on (a) a typical ductile material, (b) a typical non-ductile material. The following data were obtained in a tensile test on a specimen with 50 mm gauge length and a cross-sectional area of 160 mm. ... 

Nominal stress Crack depth Crack growth rate... 

Fig. 28.13. A sharp change of section induces stress concentrations. The local stress can be many times greater than the nominal stress.

The parameter (1 -f l ajr ) is commonly termed the stress concentration factor K,) and for a hole where a = r then K, = 3, i.e. the stresses around the periphery of the hole are three times as great as the nominal stress in the material. 

Fig. 8.94 Nominal stress-extension curves for mild steel in oil giving ductile failure, and in 4n NaN03 producing stress-corrosion failure, at the same test temperature (104°C)...

In the majority of cases, the tests are conducted using a dead-weight lever-arm stress-rupture rig with an electric timer to determine the moment of fracture, but a variety of test rigs similar to those shown in Fig. 8.89g are also used. The evaluation of embrittlement may be based on a delayed-failure diagram in which the applied nominal stress versus time to failure is plotted (Fig. 8.103) or the specimen may be stressed to a predetermined value (say 75% of the ultimate notched tensile strength) and is considered not to be embrittled if it shows no evidence of cracking within a predetermined time (say 500 h). Troiano considers that the nature of delayed fracture failure can be described by four parameters (see Fig. 8.103) ... 

Feed ratio of OC2H5 TEOS groups to OH chain ends. b Volume fraction of polymer present at swelling equilibrium in benzene at room temperature. c Elongation at initial upturn in modulus. d Ultimate strength as represented by the nominal stress at rupture. Energy required for rupture... 

Figure 2 Nominal stress-strain curves calculated for three monodisperse polyethylenes of M = 1900 (a), M = 9,500 (b) and M = 250,000 (c).

The ultimate tensile strength (UTS) of a material refers to the maximum nominal stress that can be sustained by it and corresponds to the maximum load in a tension test. It is given by the stress associated with the highest point in a nominal stress-nominal stress plot. The ultimate tensile strengths of a ductile and of a brittle material are schematically illustrated in Figure 1.11. In the case of the ductile material the nominal stress decreases after reaching its maximum value because of necking. For such materials the UTS defines the onset of plastic instability. 

Figure 7 Typical dependence of nominal stress against elongation for two unimodal networks having either all short chains or all long chains, and a bimodal network having some of both.

Figure 11. Typical plots of nominal stress against elongation, for tetrafunctional PDMS networks at 25°C (15). All but three networks are bimodal.

The weld joint strength reduction factor is the ratio of the nominal stress to cause failure of the weld joint to that of the base material for the same duration. In the absence of more applicable data (e.g., creep testing), the factor shall be taken as 1.0 at temperatures equal or colder than 510°C (950°F), and 0.5 at 815°C (1,500°F) for all materials. The strength reduction factor shall be linearly interpolated for intermediate temperatures. The designer is responsible for determining weld joint strength reduction factors for temperatures warmer than 815°C (1,500°F). 

Note The term engineering or nominal stress is often used in circumstances when the deformation of the body is not infinitesimal and its cross-sectional area changes. 

The failure of materials can be associated with a number of parameters. Two major causes of failure are creep and fracture. The tensile strength is the nominal stress at the failure of a material. Toughness is related to ductility. For a material to be tough, it often takes a material having a good balance of stiffness and give. 

It should be considered that in the case of plotting 1 = a/ X — X 2) against inverse extension ratio (X 1), the nominal stress a is defined as the force divided by the undeformed cross-sectional area of the sample and X is the extension ratio, defined as the ratio of deformed to the undeformed length of the sample stretched in the uniaxial direction (as shown in Fig. 3). For PTFE powder, the intrinsic strain is deduced from (2) by defining X - 1 ... 

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