Notch maximum stress

In corrosion fatigue experiments, a notch is cut in the specimen at which maximum stress will occur. Crack initiation and growth thus occur at the tip of this notch and may be measured with the aid of a microscope. 

When smooth specimens of PS are cycled at 26.7 Hz in a tensile mode at a maximum stress of 17.2 MPa discontinuous crack growth bands similar to those shown in Fig. 8 are seen in the mirror region near the fracture source. These bands are also similar in morphology to those observed in testing notched PS specimens at a minimum stress that is maintained at one-tenth the maximum stress However, DCG bands were not seen when tests are carried out under a fully reversed stress of 17.2 MPa at... 

First, the elastic stress distributions of the un-notched specimens are obtained from a finite element analysis. For the PI un-notched specimen, the discrepancy between the finite element and the analytical result is very small (about 0.01%), thus validating the finite element calculation in terms of accuracy through the meshing and the type of element used. Therefore a similar calculation is conducted on the G1 un-notched specimen where the span to height ratio is smaller. The mismatch on the maximum stresses at the bottom and at the top of the beam between the finite element calculations and the analytical solution is 0.74% in tension and 0.79% in compression (and remains constant upon further mesh refinement). This estimation of the stress distribution is then used for the following evaluation of the stress intensity factor. 

In the blends, the maximum stress at low rates decreases, as expected, with increasing rubber concentration. At high rates, the 15 and 20% blends show an unexpectedly strong upswing in the maximum stress. The maximum stress before fracture can increase if the stress-concentration factor ahead of the notch is lowered. 

Figure 5. Notched tensile impact maximum stress versus piston speed for PA-6-EPR blends with different rubber contents at a constant rubber-particle size (0.3-0.4 pm). Key A, 0 wt% , 6.3 wt% +, 12.5 wt% , 18.4 wt% and O, 24.3 (13).

As the root radius (or radius of curvature) approaches zero, or as the elliptical notch is collapsed to approximate a crack, then the maximum stress should approach infinity (i.e., as p 0, am -> oo). 

Since valid Kic values could not be determined, the maximum load at failure was used to calculate the maximum stress intensity factor (ATmax) values. While ATmax is not a recognized measure of toughness, the values shown in Table VII support indications from the notch tensile and tear tests that the toughness of the 5083-0... 

Notch sensitivity is evaluated by considering a tensile specimen with a stress concentrating feature such as a hole. For an isotropic elastic tensile specimen with a circular hole of one half of the section width, as an example, linear elasticity theory predicts a maximum stress at the edge of the hole in the plate to be 2.15 times the average section stress... 

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. 

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. 

Figure 4.1 shows sketches of the stress trajectories near differently shaped notches. If we look at the stress trajectories at a cross section at the notch root, we see that they are not evenly distributed, but become more narrow at the notch root. Thus, there is a local stress concentration, with a maximum stress Crnax in the notch root as shown in figure 4.2. The shape and size of the... 

Imagine the shaft to be loaded in tension with a force of 1200kN. Far away from the notch, the stress is thus 152.8 MPa. Due to the smaller cross section at the notch position, the net-section stress is <7nss = 188.6 MPa. If the material is linear elastic, as we assumed so far in this section, and use Kt fa 2.7, the maximum stress is [Pg.121]

If cTmax exceeds the yield strength of the material, the material yields at the notch root and Hooke s law is no longer valid. As shown in figure 4.4, this increases e ax compared to the linear-elastic case. The maximum stress <7max, on the other hand, is reduced due to local unloading. Therefore, Kt,o- < Kt,e holds. The numerical values of Kt,o- and Kt.e are still unknown, though. 

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... 

The thread is in saw-tooth form as per DIN 2781 (see Table 19) with approximately the same fatigue-stress concentration factor as that of Whitworth threads consequently — 2.2 —3.2 for medium carbon steel. Let = 2.7, then the maximum stress at the notch is... 

Around mid 1950s, Siebel and Stieler [19] introduced a new parameter that they indicated with the symbol x and called relative stress gradient (RSG) at notch root. The fundamental concept is always the same that is to say that fatigue is governed by the stress gradient rather than by the maximum stress at notch tip. Stress gradient means distance over which a stress level is maintained. The x parameter takes the form... 

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