Izod and Charpy Tests

An alternative to the Izod test is the Charpy test in which a sample supported, but not gripped, at each end is subject to an impact in the centre. According to the test a notch may be present in the centre of the sample on the face opposite to that subjected to impact. In spite of the differences between the tests there is a surprisingly good correlation between Izod and Charpy test results as shown by Figure 9.5 based on tests on several thermoplastics. ... 

Although the Izod and Charpy tests are widely used for plastics, other types of test are also popular. These include tensile impact tests and flexural plate (falling weight) tests. The latter is particularly useful in situations where the effects of flow anisotropy are being assessed. In addition, arbitrary end-product tests are widely used to provide reassurance that unforseen factors have not emerged to reduce the impact performance of the product. 

In recent years impact testing of plastics has been rationalised to a certain extent by the use of fracture mechanics. The most successful results have been achieved by assuming that LEFM assumptions (bulk linear elastic behaviour and presence of sharp notch) apply during the Izod and Charpy testing of a plastic. 

Figure 14.37 (a) Schematic diagram of an impact test machine, (b) Geometry of the specimen in the Izod and Charpy tests. 

Measuring loads in high speed tests is difficult because of the short times involved and the presence of transient effects from stress waves. The more simple method which has been developed in both the Izod and Qiarpy tests is to break the spedmoi with a pendulum and measure the energy absorbed. Some form of scaling parameter from the specimen dimmsions is then applied and an impact number derived. The practical utility of these numbers is beyond question but they have many, weU known, drawbacks. In particular, the nund)ers are geometry dependent and do not agree, for example, between the Izod and Charpy tests. 

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. 

From the physical point of view, the Izod equipment is equivalent to that of Charpy. However, the main differences between the two are the clamping system, the notch, the hammer speed and its weight. Charpy adopted the keyhole form of notch. At such an early stage in the history of impact testing, Charpy found that correlations between static and dynamic tests were obtainable provided a notched bar was used. In 1925, the Izod and Charpy tests were extended to plastic materials and many results on plastics were pub-hshed a year later [Werting, 1926]. 

Correlation of results from one test to another for a given material becomes difficult because of different stress states of the specimen and the associated strain rates in different tests. In the tensile-impact test, the stress state is uniaxial and it measures the tensile property at a high strain rate. In Izod and Charpy tests, the presence of notch gives a triaxial state of stress. The falling-... 

The most common tests for impact strength are the Izod and Charpy tests. In these tests, a short beam specimen, which may be notched, is struck and broken by a swinging pendulum. The impact strength is defined as the energy needed to break the specimen. Both these tests are arbitrary in their specification. By changing the test conditions quite different results may be obtained and therefore the tests are usually used for quality control and comparison of different materials only. 

Drop weight tests are mainly used for quality control of sheet materials. These tests can be applied to FRP laminates and are more appropriate than Izod and Charpy tests since they allow failure criteria other than ultimate failure to be specified. 

Both Izod and Charpy tests are based upon a swinging pendulum, such as that shown in Figure 1.27. 

There are different ways to mount the test specimen, and there are different specimen sizes and preparation methods. The different sample mounting configurations for the Izod and Charpy tests are shown in Figure 1.28. Figure 1.29 shows the details of the notch. The sharpness of the bottom of the notch affects the test result. Table 1.11 shows the different notch radii possible. 

Impact tests are carried out at a fast rate of loading to promote a brittle failure. Izod and Charpy tests are used for isotropic and laminated materials and are not suitable for unidirectional material, or out-of-plane measurements with laminated materials, since notches are not always effective. 

The chip impact test is carried out in a similar way to the Izod test but without a notch. The test specimen is usually 25.4 mm long, 12.7 mm wide, and 1.65 mm thick. The tests are carried out using ASTM D4508 [79]. Because the test involves an un-notched sample, it reflects material toughness rather than the notch sensitivity measured for the Izod and Charpy tests. 

Standardized notched impact tests such as the Izod and Charpy tests (ASTM, ISO, DIN) are the most commonly used to characterize the impact strength of plastic materials. It is very difficult to use measured data from tests using idealized laboratory specimens to predict impact behavior of end-use polymeric material. The apparent lack of good correlation between measured impact fracture energy and end-use impact resistance is due to the extreme complexity of microscopic fracture processes. In particular, the influence of specimen geometry is sometimes poorly matched with the type of failure mechanism of defects present in the actual molded part subjected to end-use impact forces. 

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... 

DESIGNING WITH PLASTICS AND COMPOSITES A HANDBOOK Izod and Charpy Testing... 

The most common impact tests for plastics and metals are Izod and Charpy tests (see Figs. 3-77 and 3-78). A heavy hammer at the end of a pendulum arm swings down and strikes a cantilevered specimen (in the Izod test) or a beam specimen supported on both ends (in the Charpy test). The specimens are unnotched unless the data reports that they are to be notched for information on specimens and test procedures see ASTM D 256. 

The notch used in both the Izod and Charpy tests produces a severe stress concentration. For example, the standard Izod notch of 0.01 in. is sharp enough to be considered an artificial crack. Therefore, the energy measured in these stan-... 

The Izod and Charpy tests are the test methods usually employed for the determination of the toughness of a material and they are especially useful in comparing the toughness of a particular polymer-particle system. Fracture mechanic theories are the conunonly employed routes for the study and characterization of the toughness of, in particular, ductile polymer composites. 

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 unrelated to material properties. These values are therefore useful only for the comparison of different materials, and are useless for design calculations. By contrast, fracture toughness (ASTM D5045-99), which is a material property, is useful in design calculations. Consequently it is desirable to have fracture toughness values that are taken under impact conditions, and especially if they could be obtained from the Izod or Charpy test machines. 

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