Impact tests Izod notched

Izod Impact Test Izod impact is a test designed to determine the resistance of a plastic to the impaet of a suddenly applied force. It is a measure of impaet strength determined by the difference in energy of a swinging pendulum before and after it breaks a notched specimen held vertically as a cantilever beam. 

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. ... 

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. 

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). 

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. 

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. 

The Izod and Charpy impact tests are mostly used. A defined pendulum strikes the specimen sample, notched or un-notched, clamped with a defined device. The absorbed energy is calculated and expressed ... 

Figure 5.79 Izod (notched) impact test. Reprinted, by permission, from F. Rodrignez, Principles of Polymer Systems, 2nd ed., p. 235. Copyright 1982 by Hemisphere Pnblishing Corporation.

Polyolefins. Ultra-High-Molecular-Weight Polyethylene.TIus is easily made by conventional low-pressure coordination polymerization. Hercules Hi-fax 1900 has a molecular weight of 2.5-5.0 million. Its outstanding properties are low coefficient of friction (0.11) abrasion-resistance superior to nylon, polyurethane, and steel unbreakable in the Izod notched impact test and high resistance to most inorganic and many organic chemicals. 

IZOD impact test—method for determining the behavior of materials subjected to shock loading. Specimen supported as a cantilever beam is strack by a weight at the end of a pendulum. Impact strength is determined from the amount of energy required to fracture the specimen. The specimen may be notched or unnotched. 

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. 

Fig. 22 Evolution of the E-modulus and Izod notched impact strength of composites based on PP (MFR 4.2 dg min 1) and increasing amounts of talc. Injection molded samples tested according to ASTM D790. Data taken from Fujiyama [40]...

In the notched Izod impact test, with the exception of some rubber modified plastics, the fracture surface of a specimen that has failed ductilely exhibits significant inward collapsing on the sides of the impact bar near the notch, indicating that a large amount of plastic flow has... 

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. 

Block polymer B differs substantially in its failure characteristics from BP A polycarbonate. For the block polymer a mixed failure mode predominates in three-point bend tests of notched specimens from —100°-90°C. In the mixed mode craze breakdown and plane strain fracture occur first inside the specimen subsequently shear failure occurs in the surface regions of the specimen. Shear lips (11) are formed as a result. Shear lips are also found on the notched Izod impact fracture surfaces of block polymer B, implying that the same mixed mode of failure occurs under high speed loading conditions. 

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. 

Izod. An impact-testing device of the notched-bar type. 

The materials and morphological parameters are usually studied as function of test temperature and test speed. The Izod method as function of temperature (9,10) is standard. Also studied is the Charpy impact behavior, both as a function of test temperature and test speed (12). With a notched tensile impact test, both test speed and test temperature can easily be varied (I, 2,13). 

Figure 3. Results of notched Izod impact tests versus temperature for PA-6-EPR blends with different rubber contents at a constant rubber-particle size (0.3-0.4 pm). Key , 0 vol% , 6.3 vol% +, 12.5 vol% A, 18.4 vol% and O, 24.3 vol% (13).

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