Notch nominal stress

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) ... 

The results of the ESC experiments obtained with the notched specimen in air, water and phosphoric acid solution are depicted in Fig. 3. The TTF is plotted against the nominal stress, and the dashed lines at 8 and 276 hours denote the respective saturation and the time to hydrolysis (50% chemical degradation see Material and Hydrolysis ). The time to hydrolysis and the TTF cannot be directly compared because they are the result of two totally different test methods, i.e. a tensile test and a constant-load test. However the fracture stress as a function of the exposition time shows a sharp decline around the time to hydrolysis, while there is hardly any influence at shorter exposition times. Therefore a significant influence of only hydrolysis on the TTF can only be expected around and after the time to hydrolysis. 

Fig. 3 The nominal stress versus the time to failure in air, water and a phosphoric acid solution (pH = 1.6) at 80°C. Note that the local stress is much higher due to the notch. 2 denotes that there are two measurements at the given points, which overlap.

For pure bending, the nominal stress at the root of the notch (7n is given by (7n = M/I)y, where I is the second moment of area (= Bt / 2 for a rectangular beam) andy is the distance to the neutral axis. 

Using the linear stress assumption the maximum stress at the root of the notch is the product of the nominal stress and the stress concentration factor a]j. Calculations of for general shapes of notch are available in the literature, but when the crack length c is much greater than the notch tip radius p, ak reduces to the simple expression ak = 2 Jcjp. 

In the context of notches, the stress concentration is always related to the net-section stress stress concentration at the notch root has to be compared to the nominal stress far away from the notch or, more precisely, (Tnooi infinitely far away from the notch), the increase in stress due to the reduced cross section and the stress concentration at the notch root have to be multiplied. 

Fig. 4.4. Axial stress and strain in the notched cross section for two different materials Linear-elastic (dashed line) and elastic-plastic (solid line). The figure shows the nominal stresses and strains (Tnss, nss) and the maximum values in the linear-elastic (o-max,el, Smax.el) and elastic-plastic case (Umax, Smax)...

If we assume that the maximum stress amplitude in the component must not exceed the fatigue limit of a smooth specimen, cte, we should expect that the maximum nominal stress amplitude for a notched specimen is... 

Stress corrosion cracking A process of cracking which occurs by the action of a corrodent and a stress. Stress concentration factor It is the ratio of greatest stress in the region of a notch or discontinuity to the corresponding nominal stress. 

Equation (6.33) may be solved using an iteration technique that yields the coordinates ea and nominal stress stress value a 2, as shown in Eig. 6.22. This results in a change of the notch root strain As and stress Ac. The new intersection between the unloading ramp and the Neuber s hyperbola must be found, as shown in Fig. 6.22 by point B, using the hysteresis loop curve, instead of the cyclic stress-strain curve, as done before for the loading ramp. The origin of the axes is now at point A. The new equations to be used are Eq. (6.32) with A[Pg.333]

T being the nominal stress far from the notch section and, therefore, not affected by the notch itself. Alternatively, it can be introduced a concentration factor l< t defined as... 

A multiplying factor for applied stress that allows for the presence of a structural discontinuity such as a notch or hole K, equals the ratio of the greatest stress in the region of the discontinuity to the nominal stress for the entire section. Also called theoretical stress concentration factor. 

Figure 1 shows the (nominal) fracture stress of the notched specimen as a function of the exposition time in water or phosphoric acid solution, both at 80°C. 

The nominal or macroscopic stress on the section (here, uniaxial) is o = P/BW, where P is the load applied to the component, and B and W are shown in Fig. 7.98. In the limit of distances sufficiently removed from a notch or crack to no longer be influenced by it, a is the uniform cross-section stress in the material. However, in the vicinity of the... 

Nominal notch length, in. Net failure stress, iCP lb./in. Stress concentration factor K % initial Strength retained ... 

Ratio of the maximum stress in the region of a notch, or another stress raiser, to the nominal corresponding stress. SCF is a theoretical indication of the effect of stress concentration on mechanical behavior. Since it does not take into account the stress relief due to plastic deformation, its value is usually larger than the empirical fatigue notch factor or strength reducing ratio. External or internal cracks in a plastic caused by imposed stresses. 

Fig. 14.12 shows the stress—strain—normalized resistance plot for the specimen with 12-mm notch spacing. We know that the nominal strain at fracture for the composite material is around 0.0147. Due to the notches, the strain concentration would be three times that of the smooth specimen. Thus any damage around the notch should start at one-third of the applied strain on the smooth specimen. This is indeed the case as seen in Fig. 14.13. A sharp change in the slope of the stress—strain curve is seen at a nominal strain of 0.005 = l/3eu- Before this knee the resistivity variation is nonlinear with respect to the applied strain. With the onset of damage at e = 0.005 (at the edges of the notches) a sharp increase in resistivity is seen. After this point the resistivity response is linear with the applied strain. Another jump in resistivity can be seen, probably due to damage initiation at the other notch, however, the stress—strain diagram does not detect this. After the second jump the sensor responds very...

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