Charpy polymers

Since several different wear characteristics were noted for the materials tested (e.g.y charring, flow, and brittle behavior), it can be inferred that there is no unique mechanism associated with impact wear of polymer thin films. Further, because of this aspect and the probability that the same mechanisms do not occur under impact testing conditions (Charpy and Izod), it is reasonable to infer that there is little correlation between impact wear resistance and impact strength. This latter point may be illustrated by considering polycarbonate. Even though it has the highest impact strength of any unfilled polymer (4), it exhibits the poorest wear behavior in this study. 

It has been shown that fracture is a very complex process and the fracture performance depends on both the initiation and the propagation of a defect [6-10] in the material. Under impact, most polymers break in very distinct manners. Several types of fracture have been identified depending on the amount of plastic deformation at the crack tip and the stability of crack propagation. For each type, an appropriate analysis has been developed to determine the impact fracture energy of the material. These methods have also been verified in various plastics [11,12]. The different fracture behaviors in most polymers are illustrated in Figure 27.1, which shows a schematic drawing of the load-deflection diagram of Charpy tests on HIPS [13] under an impact velocity of 2 m/s at various temperatures. 

Fig. 35. Charpy and Izod impact data for polyethylene — energy to fracture as a functkrn of [Reproduced from Plati, E., Williams, J. G. Polym. Engn. Sd. 15 (6), 470 1975).[...

It is clear from this work that most of the published values of and Gjc for rubber-tou ened plastics refer to plane-stress fracture. Measurements have been made on a variety of pdymers using SEN, double cantilever beam (DCB) and Charpy impact specimens. Figure 13 shows the results of one such study by Kobayashi and Broutman l, who used DCB specimens to measure C/c in AMBS polymers over a range of cr k speeds. The two most prominent features of the results are the rapid rise in Qc with rubber content, and the fall in Gx( at high crack eeds. Both effects are predicted by Eq. (8) the yield stress decreases with increas-in% rubber phase volume, so that the size of the plastic zone at the crack tip increases similarly, increating crack speed (and therefore strain rate at the crack tip) increases yield stress and reduces the size of the plastic zone. Thus the yield stress is die link between rubber phase volume and fracture resistance. 

Both Izod and Charpy tests are in regular use for impact measurements on plastics. One disadvantage of the Izod test is that clamping forces introduce unknown stresses around the fracture zone, so that the test is less suitable than the Charpy test for research purposes. The Charpy test has been widely adopted for fracture mechanics measurements on both metals and polymers. 

Figure 8.20. Charpy notched impact strength of calcium carbonate filled polypropylene. [Adapted, by permission, from Jancar J, Dibenedetto A T, Dianselmo A, Polym. Engng. Sci., 33, No.9, 1993, 559-63.]...

For polymer sheets, the notched Izod impact strength is often reported (ASTM D256). In this test, as well as the Charpy impact test in the same ASTM standard, the polymer sample is subjected to an impact from a falling hammer, and the... 

Elastic stress concentrations cannot explain most product failures, since yielding nearly always occurs before crack initiation. However, they indicate locations where yielding is likely to occur first. Therefore, the failure stress in Charpy impact tests (Section 9.5.1) should not be calculated using the notch q value. Craze formation is another form of (localised) yielding, which also modifies the stress distribution in the product. Section 9.4.4 shows that craze breakdown may occur at a critical opening displacement, rather than at a critical stress. Hence, elastic-plastic analyses must be used for most polymer product failures. 

The 5 ms failure time in a Charpy impact test is 2000 times smaller than the 10 s failure time in a slow bend test. This increases the value of the yield stress, without necessarily increasing the stress for crazing. For some polymer/temperature combinations there may be a changeover from yielding to crazing and plane strain fracture. 

Striker and Charpy specimen strains, for a 2.8 ms impact on polycarbonate with a 0.5 mm rubber inter-layer (from Aggag, G. and Takahashi, K., Polym. Eng. Sd., 36, 2260, 1996) John Wiley and Sons Inc. reprinted with permission. 

Figure 8.12 shows the principal types of test. In the Charpy test a beam of the polymer is held at each end and is struck at its centre by a hammer with one or two knife edges, giving a three- or four-point impulsive bending stress, respectively. Figure 8.13 shows a diagram of a standard Charpy impact tester. In the Izod test the specimen is held at one end and struck at the other. For either test a notch is cut in the sample at the point where it is to break and the sample is placed in the tester with the notch pointing away... 

In some cases, lack of noticeable influence of electron beam radiation on the values of tensile and Charpy impact strengths of 24-23-21-15-17 LDPE-HDPE-PP-PS-PET blends (based on recycled polymers) compatibilized with 1% trimethylol propane trimethylacrylate (TMPTMA) were found. Zenkiewicz et al. ° explained this lack of influence by the protective action of aromatic rings of PS and PET that hindered cross-linking. In the same article, the addition of 10% of styrene-ethylene/butylene-styrene elastomer grafted with maleic anhydride (SEBS-g-MA) led to the great increase of both tensile and Charpy impact strengths. 

Charpy impact strength - data Polymer Solids and Polymer Melts B. Langer, W. Grellmann Table 4.8 Charpy impact strength values (unnotched and notched) of unfilled materials (number in brackets represents the hammer energy used). ... 

The following Tables show a summary of fracture mechanical data of basic polymers estimated at impact loading at the instrumented Charpy Impact test (ICIT). 

Nak Nakamura, Y., Yamaguchi, M., Okubo, M. Instrumented Charpy impact test of epoxy resin filled with irregular-shaped silica particles. Polym. Eng. Sci. 33 (1993) 279-284. 

Fig. 12.24 Charpy and Izod impact energy as a function of cross-sectional dimensions of the specimen bar for a medium-density polyethylene, including energy corrections (from Platti and Williams (1975) courtesy oi Polymer Engineering Science).

The most frequently applied impact tests are shown in Fig. 24.17 A and B. A pendulum (shown as a filled arrow) falls from a certain height the loss in the potential energy of the pendulum is assumed equal (with a correction for losses such as friction) to the energy Uo absorbed by the specimen see Eq. (24.1). The Charpy test is described by the ASTM D 256 standard method B, the Izod test by the same standard method A. We see (Fig. 24.17 A) that in the Charpy test there is a symmetry with respect to the center of the specimen. By contrast, in the Izod test (Fig. 24.17 B) the bottom half of the specimen remains untouched while the top part is broken off. We—and more and more laboratories around the world— perform now both tests with a sensor installed on the pendulum and connected to a computer. Thus, not only a single value of the energy but a whole curve is obtained. For convenience single values of impact strength (IS) for a number of polymers are listed in Tables at the end of this chapter. 

---