Notch sensitivity test

The glass-fibre nylons have a resistance to creep at least three times as great as unfilled polymers. In the case of impact strength the situation is complex since unfilled nylons tend to break showing tough fracture whereas the filled polymers break with a brittle fracture. On the other hand the glass-filled polymers are less notch sensitive and in some tests and service conditions the glass-filled nylons may prove the more satisfactory. 

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 Izod impact test may indicate the need to avoid inside sharp corners on parts made of such materials. For example, nylon and acetal-type plastics, which in molded products are among the toughest materials, are notch-sensitive and register relatively low values on the notched Izod impact test. 

This test has possible advantages over the notched Izod test. The notch sensitivity factor is eliminated, and energy is not used in pushing aside the broken portion of the specimen. The test results are recorded in ft-lb/in.2 (kJ/m2). This allows for minor variations in dimensions of the minimum in cross-section area. 

PC/PE. In the case of PC/PE, plane strain alone does not produce significant changes in the yield stress and the deformation behavior. Its yield locus in the tension-tension quadrant is therefore either very nearly a quarter circle or similar to a Tresca locus. The exact shape of the locus can be determined only by much more elaborate biaxial tests. This material is not very notch sensitive compared with PC. The energy to break in a notched Izod impact test is 15 ft-lb/inch for Vs-inch thick bars and 11 ft-lb/inch for 4-inch bars whereas for PC the latter figure is about 2 ft-lb/inch. This reduction in notch sensitivity over pure PC appears to be related to the material s ability to void internally, probably relieving the plane strain. 

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. 

There can be no doubt as to the importance of plane strain conditions for the fracture of plastics especially where sharp notches and thick sections are concerned. Such conditions nearly always lead to brittle or semi-brittle fracture. Vincent has shown that the notch sensitivity in a braod range of amorphous and crystalline polymers is increased as the testing temperature is lowered and the loading rate is increased. Before fracture occurs, amorphous plastics often craze under these conditions. The complex questions of craze initiation, propagation and transformation into a crack have been treated extensively for amorphous polymers in the first three chapters of this book (see also The problem becomes more complicated when... 

Tests of notched specimens (Figure 26) and determination of notch sensitivity coefficient (K) show that, despite thickness of rolled semiproducts, the Kt coefficient (for a dendritic structure) is less than K2 (for a nondendritic structure) by 20—25% for room and cryogenic temperatures. Thus, K2 > 1.5 for plates and K2 = 1 for thick sheets, which indicates their full insensitivity to a notch. 

Materials may break in shear or in tension the former occurs in short (e.g., brittle), the latter in long (e.g., rubbery) materials. Another material property is notch sensitivity. In a test piece that is put under tension, notches can be applied, and in several materials the ensuing stress concentration greatly lowers the overall stress needed for fracture propagation. The notch sensitivity is smaller if the material contains defects of the size of a small notch, or if the bonds between structural elements are much stronger in the direction of the applied stress than in a perpendicular direction, as in many fibrous systems. 

The test may be carried out on plain rectangular bars, but most often a carefully defined notch is molded or machined into the face to be struck (Figure 12.8). The impact tests are often regarded as a means of assessing the resistance of a material to shock where notches or stress raisers generally are present. The ratio of impact strength of unnotched to that of notched specimen is sometimes regarded as a measure of the notch sensitivity of a material. 

The test can be performed using either unnotched or notched test pieces. Three types of notches arc standardized (Fig. 13). the preferred one having a radius at the noteh base of 0.25 mm (the type A notch). A blunt 1.0 mm (type B notch) and a very sharp 0.1 mm (type C) notch are also covered. Notches of different base radius are useful for more extensive characterization of plastics than a simple quality lest or data sheet entry, in that they enable an estimate of the notch sensitivity of the plastic to be investigated. The flatwise test can also be performed notched or unnotehed. except that here the notched test has two notches machined across the 4 mm direction and directly opposite... 

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. 

J. G. Kaufman and E. W. Johnson, The Use of Notch-Yield Ratio tq Evaluate the Notch Sensitivity of Aluminum Alloys, Am. Soc. Testing Materials Proc., 62 (1962). 

R. John, D. J. Buchanan, and L. P. Zawada, Notch-sensitivity of a woven oxide/oxide ceramic matrix composite, in Mechanical, Thermal and Environmental Testing and Performance of Ceramic Composites and... 

The presence of notches, sharp angles, voids, particle inclusions, and small inserts tends to concentrate stress on impact. One way of studying the sensitivity of a polymer or polymer compound to stress concentration is to carry out a series of tests with different notch angles or notch tip radii. Materials vary enormously in flieir notch sensitivity, as is shown in Figure 1.46. Materials, such as unplasticized PVC, nylon, polyacetals, and PCs (not shown in the diagram), are very sensitive to... 

Notch sensitivity n. The extent to which a material s tendency to fracture under load, particularly an impact load, is increased by the presence of a surface in homogeneity such as a notch or sharp inside corner, a sudden change in section thickness, a crack, or a scratch. Low notch sensitivity is usually associated with ductility, while brittle materials exhibit higher notch sensitivity. Most engineers and physical testers consider the notched Izod and Charpy impact tests to be as much measures of notch sensitivity as they are of pure impact strength. 

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