Notch design

Generally, polyetherimide displays notch sensitivity and stress concentrators must be avoided when designing. Notched Izod impact strengths slowly decrease between room temperature and -30°C retaining, for example, 80-100% of the original value. 

Polyallomers have a brittleness temperature as low as 40°C and a heat-distortion temperature as high as 210°F at 66 psi. The excellent impact strength plus exceptional flow properties of polyallomer provide a wide latitude in product design. Notched Izod impact strengths run as high as 12 ft Ib/in. notch. 

To design notched components, knowledge of Ki is required. Therefore, empirical formulae have been determined that can be used to calculate for different geometries and load cases. They are collected in tables e. g., Peterson s Stress Concentration Factors [109] or Dubbel [18]. One example, a shaft with a circumferential notch under tensile load, is shown in figure 4.3. The dimensions in the figure are the outer diameter D, the diameter at the notch root d, the notch depth t (with 2t = D — d), and the notch radius q. 

The simple, first-order, filter approach may not always be adequate. In fact, if too large a time constant is chosen, it may interfere with control. A more sophisticated approach is to determine experimentally the noise-frequency spectrum and to design notch filters (i.e., band rejection filters) fijr the dominant frequencies. 

The presence of notches or sharp angles or of a few holes, voids, particle inclusions or small inserts tends to concentrate the stress. Different polymers vary in their notch sensitivity and this is presumably a reflection of how close they are to their tough-brittle transitions. The aim of the designer and processor must be to reduce such stress concentration to a minimum. 

The recommended maximum design stress in tension is one-quarter the ultimate tensile strength (for cast irons a value up to 185 N/mm (12 tonf/ in. )). The fatigue strength is one-half the tensile strength. Notched... 

Graphs such as Fig. 2.79 also give a convenient representation of the notch sensitivity of materials. For example it may be seen that sharp notches are clearly detrimental to all the materials tested and should be avoided in any good design. However, it is also apparent that the benefit derived from using generously rounded comers is much less for ABS than it is for materials such as nylon or PVC. 

The use of V-notches in a trough wall for overflow is more sensitive to leveling problems than the other designs, and for the same %- to Me-in. level tolerance produces a more severe non-uniform flow distribution. The quality of distribution from a V-notch is poor compared to the other types of trough distributor, but does have advantages in slurry systems [131]. It should not be used for critical distillation applications, but is good for heat transfer and where solids are in the system. 

While ideally structures should be designed and fabricated so that environment-sensitive cracking is avoided, in practice it is sometimes necessary to live with the problem. This implies an ability to detect and measure the size of cracks before they reach the critical size that may result in catastrophic failure. Such inspection has important implications for plant design, which should be such as to allow inspection at relevant locations. The latter are regions of high residual stress (welded, bolted or riveted joints) and regions of geometrical discontinuity (notches, crevices, etc.) where stress or environment concentration may occur. 

If we return to our notched beam analogy, as shown in Figure 15-5, we find that we numbered the notches, 1, 2, 3, . These numbers serve as natural identifying designations. They are the quantum numbers of the balance beam. 

One general method of improving the performance of plastic products in impact loading is to prevent, by design and handling, the formation of notched areas which act as stress... 

Patches are placed on the test subject at designated locations. According to Durham and Wolfe, one patch should be placed on the top of each shoulder one on the upper chest near the jugular notch one on the back of the neck at the edge of the collar one on each upper front leg (thigh area) one on each lower front leg (just below the knees) and one on the back of each forearm. Patches may also be placed on the front and back of a hat or cap to measure exposure to the face and neck area. When collecting the patch samples, one may prefer to combine the two shoulder patches as one sample both lower arm forearm samples as one sample both lower front leg samples as one sample and both upper front leg samples as one sample, in order to examine the entire area of the body that the two patches represent. 

In all blast-resistant structures (steel, concrete, or masonry) special attention should be given to the integrity of connections between structural elements up to the point of maximum response. For example, it is important to prevent premature brittle failure of welded connections to avoid stress concentrations or notches at joints in steel structures and to provide ductile reinforcement detailing in concrete/masonry structure connections. For all materials, it is recommended that connections be designed to be stronger than the connected structural members such that the more ductile member will govern the design over the more brittle connection. 

Impact tests are not required when the maximum obtainable Charpy specimen has a width along the notch of less than 2.5 mm (0.098 in.). Under these conditions, the design minimum temperature shall not be less than the lower of -48°C (-55°0 or the minimum temperature for the material in Table IX-1 A. 

Tests may include tensile elongation, sharp-notch tensile strength (to be compared with unnotched tensile strength), and/or other tests, conducted at or below design minimum temperature. See also para. GR-2.1.3(d). 

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