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Impact

Introduction

There are two basic types of impact tests: Pendulum and falling weight.  Izod Impact and tensile impact are the most common of the pendulum type tests, and Gardner and Falling Dart are common falling weight type tests. There are variations on each method which helps contribute to the field of impact testing of polymers not being standardized. The present trend is toward instrumented testing. There are instrumented versions of all of the types of tests mentioned.

 Impact testing is extremely important. It is usually the most sensitive property and the first to be affected by mechanisms which degrade polymers.  Also, most polymer applications have impact requirements. The first commonly used impact test for polymers was the pendulum type. The Izod test was originated by a metallurgist of the same name for use in screening different metals for use in cutting tools. The test was adapted to polymers probably due to availability. Polymers are not nearly as uniform and predictable as metals, making pendulum impact testing of limited use in determining the actual impact strength of polymers.

 Izod Impact (ASTM D256)

 The most common version of Izod for polymers is the notched Izod test.  Samples, usually 12.7 mm (0.50 in) wide and either 3.2 mm (0.125 in), 6.4mm ( 0.250 in), or 12.7 mm (0.500 in) thick, are notched to a depth of 2.5mm (0.100 in). The angle of the notch is 45o. The radius of the notch is 0.25 mm (0.010 in). The notch concentrates the load on the specimen.  Most polymers would not fracture but simply bend if not for the notch.  Impact strengths are reported in units of energy per inch of notch. Therefore, on a 3.2mm (0.125 in) thick sample, the reading is multiplied by a factor of 8.

 In notched Izod testing, samples are mounted in a vice fixture with the notch facing the pendulum. A weighted pendulum, fixed at a point directly above the sample vice, is swung up and held stationary. This height and thus the speed of the pendulum at impact (11.3 ft/sec) is a constant for this test. When released, the pendulum swings through the path where the sample is fixed. As the sample breaks, energy is absorbed by the sample. The height the pendulum attains after impact is measured by an indicator on a fixed scale which reads in Joules (ft-lbs). The impact strength of the sample is the loss of momentum in the pendulum while breaking the sample off at the notch.

 This test configuration served as the standard of impact testing in the plastics industry for many years. The problems with the test involve several parameters which can drastically alter results if not strictly controlled. First, the radius of the notch is critical. It is meant to simulate conditions which might exist in applications where features such as an internal corner on an enclosure will act as a stress concentrator upon impact. In this test, the radius cannot be varied. The notch radius has a significant effect on the ability of a sample to absorb impact. Most polymers, especially polycarbonate and nylon have critical notch radii below which their impact strength falls off dramatically. In a fixed radius test, the data can give a false impression about the relative impact resistance of different polymers.

 The technique used to put the notch in the sample is also critical.  Generally a machine called a "notcher" (appropriate name) is used. In actual testing conducted on a polycarbonate resin, this author was able to produce impact strengths that varied from 2 ft-lbs to 16 ft-lbs, simply by changing the cutter speed on the notcher from approximately 1000 rpm's to 250 rpm's. There are several theories concerning this phenomena. One is that at higher speeds, the notcher blades overheat the polymer causing localized degradation in the base of the notch. This greatly decreases the strength of the molecules. When the impact event occurs, a crack initiates easily in the degraded area and propagates through the rest of the sample.  Another theory suggests that rapid heating and cooling due to friction induces high stresses in the notch. This reduces molecular mobility and results in lower energy absorption before failure.

 The feed speed of the samples into the cutter is also significant for the same reasons are cutter speed. Notch depth and sample thickness are two more variables which can significantly effect results. The test procedure allows various sample thickness to be used. The thinner samples, i.e. 3.2 mm (0.125 in) vs. 6.4 mm (0.250 in), generally give higher impact readings even when both are reported as energy per inch of notch. The thinner sample has a lower apparent modulus which allows material surrounding the notch to deform more easily. More energy can be absorbed before the critical stress is reached in the notch area and a crack is initiated. To optimize the repeatability of pendulum impact data, it is critical to control all of the variables mentioned above to the best degree possible. It is recommended that pendulum testing only be used for comparative analysis of polymers and not to simulate or draw conclusions about application functional performance.

 Charpy impact testing is a variation of Izod. It is covered under the same ASTM procedure as Method B. In Charpy, the sample is laid horizontal on two supports against an anvil. The sample spans a 95.3 mm (3.75 in) gap in the anvil. The sample is notched in the center and the notch side positioned away from the pendulum. When the pendulum swings through the gap in the anvil it impacts the center of the sample with a radiused hammer. The energy to break is measured and reported in the same way as with Izod. Charpy is not frequently used, therefore the significance of this test will not be addressed.

 PURPOSE

To determine the resistance of plastics to breakage by flexural shock as indicated by the energy extracted from pendulum-type hammers in breaking standard specimens with one pendulum swing. Results are reported in terms of energy absorbed per unit of specimen width (ft-lb/in). The excess energy pendulum impact test indicates the energy to break standard test specimens of specified size under stipulated conditions of specimen mounting, notching (stress concentration), and pendulum velocity at impact.

 SUMMARY

Test specimens are molded and notched in accordance with ASTM specifications.  Specimens are held as a vertical cantilever beam and are broken with a single swing of the pendulum with the line of initial contact at a fixed distance from the specimen clamp and from the centerline of the notch and on the same face as the notch. At least five determinations of impact value are made on each sample to be tested.  The average impact strength of the group of specimens is reported, but only values of specimens having the same nominal width and type of break may be averaged.