Notch sensitivity index

Peterson [12-14] introduced a notch sensitivity index q, or simply q factor, defined... 

Fig. 7.4 Notch sensitivity index q for steels and aluminum alloys versus notch root radius (modified from [15])...

Method D deals with the estimation of notch sensitivity by having the test carried out at two notch radii, 0.25 mm and 1.0 mm. The ratio of the difference in the two energy-values to the difference in notch radii is then taken as the index of notch sensitivity. Where the 1.0mm radius leads to test pieces which do not break, a 0.5 mm radius notch may be substituted. 

Figures 18.30(b) and (c) show similar data for temperatures of 0 and -20 °C, respectively. With the exception of the low melt index PP copolymer, the notch sensitivities are similar to those at room temperature but the marked drop in overall level brought about by a decrease in temperature is again clearly demonstrated.

Figures 18.31 and 18.32 show the combined data for room temperature and -20 "C for HDPE, and for ABS terpolymer and toughened PS, respectively. Of particular note is the very small effect of temperature on both the impact level and notch sensitivity of HDPE, although the effect of MW (or melt index) is again clearly demonstrated. This indicates the suitability of HDPE for low-temperature applications.

Figure 2.14 shows the relative notch sensitivities of some polyolefins. Among the polyolefins, the low-melt-index polypropylene copolymer (GMT 61) has the lowest notch sensitivity, whereas the higher-melt-index polyethylene and polypropylene homopolymers have the highest. 

FIGURE 2.15 Influence of temperature on Izod impact strength of various polymers of low and high melt indexes. GMT 61 is a low-melt-index and low-notch-sensitivity polypropylene copolymer. KMT 61 is a high-melt-index and low-notch-sensitivity polypropylene polymer. HDPE 65045 is a high-melt-index and high-notch-sensitivity high-density polyethylene. 

This actually means that if a notch sensitivity factor is introduced also for the fatigue finite life that, usually indicated as Idf, it will always be < Idfnotch sensitivity factor 4 at 1,000 cycles analogous to the q index already seen in Sect. 7.2 for the fatigue limit, so that it is... 

PEI is an amorphous thermoplastic resin with a rated continuous temperature of use up to 170°C. It has an limiting oxygen index (LOI) of 47, one of the highest in the commonly used engineering thermoplastics, coupled with a low smoke emission. PEI retains 41% strength retention at 190°C. Like PES it is a tough resin, but it is sensitive to notches and sharp corners. 

The index of sensitivity, cp, to stress concentrations, which is equal to the ratio of the Kic values, obtained by the SEVNB and SENB methods is shown in Table 2.3. In the tests of elastic materials,

deformation behavior of V-notched ceramic specimens with a cp value of about 0.6 produced linear load-deflection diagrams or diagrams with small nonlinearity, as seen in Fig. 2.11 of lines 1 and 3, which are the results of comparatively slow crack growth. 

Figure 2.15 shows the variation in standard Izod impact strength (1 mm notch tip radius) with temperature for the same range of materials. By far the most temperature sensitive is the low-melt-index polypropylene (PP) copolymer. It is interesting to compare the sharp fall-off in notched impact strength with this material between 23°C and 0°C with the steady values obtained for falling weight impact strength for the same temperature range (Figure 2.16). The removal of points of stress concentration is thus the most vital factor governing the impact performance at tanperatures of 90°C and below.

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