Notch radius

Fig. 2.79 Variation of impact strength with notch radius for several thermopiastics...

Likewise, dead sharp corners or notches subjected to tensile loads during impact may decrease the impact resistance of a product by acting as stress concentrators, whereas generous radii in these areas may distribute the tensile load and enhance the impact resistance. This point is particularly important for products comprised of materials whose intrinsic impact resistance is a strong function of a notch radius. Such notch sensitive materials are characterized by an impact resistance that decreases drastically with notch... 

PET. The behavior of crystalline PET at plane strain can be explained if its yield locus is similar to that of PS and PMMA (9, 10) where a craze locus intercepts the shear yield locus. The transition at plane strain to a craze locus would account for the brittleness. This transition, which takes place quite sharply at W/t = 23 (W/b = 8), is probably the cause for the low impact strength (< 1 ft-lb/inch) of the Vs-inch thick notched bars. The plane strain brittleness can be avoided if the geometric constraints can be removed, such as making the notch less sharp or making the test bar thinner. In fact, unnotched bars of PET, equivalent to having an infinite notch radius, are quite tough. The notch sensitivity of PET is typical of crystalline polymers. 

Notches act as stress raisers and redistribute the applied stress so as to favor brittle fracture over plastic flow. Some polymers are much more notch sensitive than others, but the brittleness temperature depends in general on the test specimen width and notch radius. Polymers with low Poisson ratios tend to be notch sensitive. Comparisons of impact strengths of unnotched and notched specimens are often used as indicators ofthe relative danger of service failures with complicated articles made from notch sensitive materials. 

The first part of the paper is devoted to the design of the notched specimen and the analysis of the resulting stress intensity factor fi om a finite element calculation. The second part deals with the practical preparation of the samples. In a third part, the evolution of the toughness and the analysis of the crack tip fields with the loading rate are presented for PMMA and PC. The influence of the notch radius is also considered. 

Fig. 5. Photoelasticity of PC for (a) 250 pm notch radius and (b) a sharp notch radius.

For a brittle glassy polymer like PMMA, the results for both sharp and blunted cracks are reproducible and show negligible scattering. The sharp cracks give a lower estimation of the toughness but this is not only related to a notch root effect. In the case of a blunt crack with notch radius of 250 micrometers, stress induced birefringence related to plasticity is observed... 

Elastic modulus and yield stress were measured by compression tests on 10x5x4 mm prisms cut out from hot pressed plates. The J-integral measurements (J-Aa curve) were performed from tensile tests on CT specimens of 48 x 40 x 4 mm size, with a closed loop controlled machine (Schenk-Trebel). The starting crack was made using a razor blade (notch radius < 50 pm) up to a depth ao of 10 mm. 

Figure 6 Effect of notch radius on the Izod strength of various engineering plastics. (From Ref. 33)...

Round, circular-notched tensile specimens (D = 9mm notch radius, Rh 0.05 mm). 

Kic = coefficient of stress intensity under conditions of plane deformation max = coefficient of stress intensity under maximum fracture load Kq = coefficient of stress intensity under the load corresponding to 5% secant line Rff = notch radius... 

Figure 8.28 Stress concentration factor as a function of notch radius for a single edge-notched beam. (Adapted from Rooke et al., 1981, reproduced courtesy of Elsevier Science Ltd, Kidlington, UK.)...

Table 1.11 Izod and Charpy Impact Notch Radius Options...

Notch Izod Notch Radius (mm) Charpy Notch Radius (mm)... 

Figure 1.46 Impact strength vs. notch radius for various plastics.

The effect of the notch-root radius on the apparent dynamic fracture toughness, K(j, is shown in Fig. 1.77. Kdynamic fracture toughness. A similar notch-radius effect is observed for AI2O3. Note that the static fracture toughness of Si3N4 increases at first, but then becomes constant (Fig. 1.77b). 

The aluminum alloys used in this investigation and their history and chemical analyses are listed in Table I. The materials were tested in the "as-received" condition without further heat treatment or working. The tensile specimens used in this investigation are shown in Fig. 1. All tensile specimens were inspected and individually measured for area determination. Notched specimens were inspected and measured by means of an optical comparator and all specimens out of tolerance were rejected. The stress concentration factor, as determined by [(V2 width between notches)/(radius of the notch) s 6.3 with tolerance limits of 5.7 to 7.1. 

Polymers may also be embrittled by part design variables. In Figs. 14.12 and 14.13, the embrittling effects of sharper notch radius and of increased part thickness on various materials are shown. Again, the PC-siloxane resins are shown to be superior to PC homopolymers in this regard. 

Figure 14.12 Iso Izod impact strength for PC-siloxane grades—notch radius study.

Fracture Toughness from Flexed-Beam Impact Tests. As stated above, the so-called impact toughness values obtained from Izod and Charpy tests are not material properties because they depend on specimen thickness, notch depth, notch radius, and other factors imrelated to material properties. These... 

Nylon" in 1976 resulted in the toughest plastic yet known, one that showed no dependence of toughness on thickness, orientation direction, notch radius, or type of impact test. ICI has recently announced a composition with dispersible long fibers. These are the kinds of innovations that keep nylon-66 young and predict continued healthy growth. 

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