Izod impact test specimens plastics

Figure 3-87. During injection molding, a double gate can cause weld lines, when two melt fronts meet in a cavity, and reduce impact strengths, as well as other properties. Single-gated and double-gated Izod impact-test specimens were molded using different plastics to provide impact-strength comparisons.

In common with most polymeric materials, adhesives and sealants are sensitive to the rate of applied force. There are several different types of impact test for adhesives. The most common test (similar in application to the Izod impact test for plastic materials) has been withdrawn without replacement however, the test was not widely used in a production situation due to the difficulty in achieving reproducible results. A more informative test for impact is the cleavage test in which the test specimen is subject to an impact force through a wedge (refer to Table 24). This can be initiated by a pendulum impact machine with an impact energy range of 50-300 J and an impact speed of 3-5.5 m/s. 

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... 

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. 

The Izod impact test is performed on notched and unnotched single edge clamped specimens and serves to evaluate the toughness behaviour of plastics under impact loading. It is standardized in ASTM D 256 and... 

Impact resistance n. ( ) Ability of a coating to resist a sudden blow. Ability to resist deformation from impact. (2) The relative durability of plastics article to fracture under stresses apphed at high speeds. A widely used ASTM impact test, www.astm.org, employs the Izod pendulum striker swung from a fixed height to strike a specimen in the form of a notched bar mounted vertically as a cantilever beam. The Charpy tester, an alternative in D 256, uses a specimen in the form of a horizontal beam supported at both ends. ASTM lists some several different impact tests for plastics and plastics products. 

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. 

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 popular Izod impact tester can use different size specimens depending on the type of plastic and their method of fabrication. The specimen is usually 1/8 in. x l/2 in. x 2 in. other sizes are also used. Specimens can be notched or unnotched. A notch is cut in a specified manner on the narrow face of the specimen. The sample is clamped in the base of a pendulum testing machine so that it is cantilevered upward with the notch facing the direction of impact. The pendulum is released, and the force expended in breaking the sample is calculated from the height the pendulum reaches on the follow-through. The speed of the pendulum at impact is controlled. 

A cracked specimen of transparent PVC is being tensile tested. Is it realistic to expect to see the craze at the crack tip with the naked eye The Kic value is 2.0 MN m and the craze stress (Jq = 60 MPa. Discuss whether the notched Izod impact strengths of 3.2 mm thick plastic sheet are of any value for design purposes. 

ASTM D256 Standard Test Methods for Determining the Izod Pendulum Impact Resistance of Plastics. Used to determine the resistance of a plastic specimen to impact by a pendulum-type hammer. Specimens contain a milled notch and depending upon the test method, failure may be brittle or ductile. Test results are reported in terms of energy absorbed per unit of specimen width or per unit of cross-sectional area under the notch. 

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. 

Impact is a catastrophic event that has become very important for plastics. While measurements of impact performance have always been made in the past, new applications in the automotive, electronic, and consumer appli-cance industries have placed considerable importance on the quantification of this behavior. Historically, the Izod test has been used routinely to characterize impact. A notched rectangular bar is clamped in a vise and broken by a sharp impact from a hammer attached to a moving pendulum. The test has been widely criticized for being unsuitable for plastics, but it remains the most common test for impact and failure characterization. The Charpy test, widely used in Europe, has seen better acceptance by the scientific community. Here the test specimen, similarly notched, is held in a flexural mode while it is subjected to the impact. 

Impact Strength n The quantitative measure of the ability of a material to withstand shock loading in a standard test. For plastics the test is usually either the Izod or Charpy test described in ASTM (www.astm,org), and the result is calculated as the energy expended (work done) in breaking a specimen, divided by its width or thickness. Specimens of both tests are usually notched on the wide opposite that where they are struck, though the notch position may be reversed and unnotched specimens maybe tested. In SI the convenient reporting unit is J/cm of notch width, or, for urmotched specimens, J/cm. To provide a much simpler stress distribution free of notch effects, researchers, developed the... 

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