Izod test

Impact tests are, however, used to try to compare the impact strength of different materials. Of these tests four require specific mention. These are the Izod test, the Charpy test, the falling weight tests and the tensile impact test. 

Of these the most well known is the Izod test. This consists of a bar, one end of which is held in a vice, the sample being held vertically. The bar is then struck by a striking device under controlled conditions at a specified point above the vice. The energy required to break the sample is noted. It is common to have a notch in the bar which is located during the test at the top of the vice and on the... 

Whilst Izod tests are, perhaps surprisingly, generally reproducible it has been found impossible to scale up the results and thus predict the energy required to break a bar of large cross-section from results obtained with a bar of smaller cross-section. This has led to some uncertainty in the method of quoting results and the following approaches are used ... 

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. ... 

Due to the polyether blocks, these polymers retain their flexibility down to about -40°C and only Grade 6333 breaks in an Izod test at this temperature (using specimens of thickness 3.2 mm). The materials generally show excellent resistance to crack growth from a notch during flexure some grades are reported... 

Methods employed to determine the impact resistance of plastics include pendulum methods (Izod, Charpy, tensile impact, falling dart, Gardner, Dynatup, etc.) and instrumented techniques. In the case of the Izod test, what is measured is the energy required to break a test specimen transversely struck (the test can be done either with the specimen notched or unnotched). The tensile impact test has a bar loaded in tension and the striking force tends to elongate the bar (Chapter 5, Impact Strength). 

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. 

The Charpy test and the Izodtest are both pendulum-type impact tests. The difference between these two tests is essentially the orientation of the sample. In the Charpy test, or simple-beam method, the sample is supported at both ends, but is not held down, as shown on the left in Figure 15.30. In the Izod test, or cantilever beam method, the sample is supported on one end in a vice, as shown on the right in Figure 15.30. In both tests the sample is at the bottom of the pendulum arc and the amount of energy absorbed by the sample is measured. 

FIGURE 15.30 Left, Charpy test. Right, Izod test. 

The test method may specify either a notched sample or an unnotched sample. If a notched specimen is specified, the dimensions of the notch are also given. Figure 15.31 is an example of these dimensions for a metal sample. In the Charpy test the notched sample is struck from behind the notch. In the Izod test the notch is facing the hammer. The notch, called a stress raiser, concentrates the stresses applied by the impact load. It makes the material brittle and increases the elastic limit in the notch area. Without the notch, many materials will bend without fracture, and the capacity to absorb energy will not be accurately determined. 

Izod Test. See Charpy and Izod Tests in Vol 2, p C154-L... 

Noninstrumented Izod Testing. The impact strength, reported in joules per meter of notch, was measured at temperatures between 25° and — 30°C. The samples were cooled to the test temperature using a Lauda Kryostat and equilibrated in the Kryostat for at least one and one half hours prior to testing. Five to seven seconds were required to remove a sample from the Kryostat, mount, and perform the Izod impact test. Ten samples of each composition were tested at each temperature. 

ABS usually contains about 20wt% rubber, about 25% acrylonitrile, and about 55% styrene, having a Tg value of about 105C, a tensile modulus of 2.5GPa, and an impact strength of about 4 J (12.7 mm)-1 on an Izod test. However, owing to the many variations possible, the properties can vary considerably. As an example, Super ABS can have a notched Izod impact strength of up to 8 J (25 mm)-1. Also some or all of one of the comonomers may be replaced. Methylmethacrylate may replace acrylonitrile, as in MBS and MABS, with improvement in... 

Most HIPS fail in the uniaxial loading of a cantilever impact (Izod) test only if the test specimens have been notched beforehand. In the notched impact test, the stress direction in injection molded specimens is the same as the preferred orientation direction. This increases the measured impact toughness and the flexural impact test and, therefore, serves primarily for comparing the toughness of different products. 

To design a resin with the property enhancements of AN without the cross-linking problem, it was found that SMA copolymers and terpolymers could be blended with ABS resins to form miscible blends with properties of HHABS. A fundamental look at the miscibility of SMA copolymers with SAN copolymers indicated that the optimum thermodynamic interaction occurs when the AN content of the SAN is nearly equal to the MA content of the SMA [72]. Kim et al. also found low impact strengths at all modifier levels when blending SMA with SAN-g-polybutadiene (GRC = grafted rubber concentrate) [73]. Blends of SMA with SAN and GRC (SAN + GRC = emulsion ABS) exhibited ductility behavior similar to HHABS. The impact strengths of the polymers were 2-5 ft-lb/in, in a notched Izod test at ambient temperature. 

Non toughened semi-crystalline PET is a very brittle polymer whatever the loading conditions are i.e., un-notched and notched tensile tests, dart test (impact of a hemispherical striker against a clamped plaque) and izod test (Fig. 1. to Fig.4.). Amorphous PET exhibits a more ductile behaviour except when notched. In such a case even amorphous PET is a brittle material at room temperature (Fig. 3. and 5.). 

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