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Izod Test – Determination of Impact Energy Using the Izod Test (Part – II)

The essential features needed to perform ASTM E23 are: (a) a suitable specimen (specimens of several different types are recognized), (b) an anvil or support on which the test specimen is placed to receive the blow of the moving mass, (c) a moving mass of known kinetic energy which must be great enough to break the test specimen placed in its path, and (d) a device for measuring the energy absorbed by the broken specimen.

The methods of impact testing referred to in ASTM E23 relate specifically to the behavior of metal when subjected to a single application of a load resulting in multiaxial stresses associated with a notch, coupled with high rates of loading and in some cases with high or low temperatures. For some materials and temperatures, impact tests on notched specimens have been found to predict the likelihood of brittle fracture better than tension tests or other tests used in material specifications.

The Charpy V-notch (CVN) impact test has been used extensively in mechanical testing of steel products, in research, and in procurement specifications for over three decades. Where correlations with fracture mechanics parameters are available, it is possible to specify CVN toughness values that would ensure elastic-plastic or plastic behavior for fracture of fatigue cracked specimens subjected to minimum operating temperatures and maximum in service rates of loading.

The notch behavior of the face-centered cubic metals and alloys, a large group of nonferrous materials and the austenitic steels can be judged from their common tensile properties. If they are brittle in tension they will be brittle when notched, while if they are ductile in tension they will be ductile when notched, except for unusually sharp or deep notches (much more severe than the standard Charpy or Izod specimens). Even low temperatures do not alter this characteristic of these materials. In contrast, the behavior of the ferritic steels under notch conditions cannot be predicted from their properties as revealed by the tension test. For the study of these materials the Charpy and Izod type tests are accordingly very useful. Some metals that display normal ductility in the tension test may nevertheless break in brittle fashion when tested or when used in the notched condition. Notched conditions include restraints to deformation in directions perpendicular to the major stress, or multiaxial stresses, and stress concentrations. It is in this field that the Charpy and Izod tests prove useful for determining the susceptibility of a steel to notch-brittle behavior though they cannot be directly used to appraise the serviceability of a structure.

Impact > Test Types

There are basically two types of impact tests: pendulum and drop weight. Izod, Charpy, and tensile impact are the most common of the pendulum type tests.

The first attempts at obtaining this value were done by means of a swing pendulum. A pendulum of a known weight is hoisted to a known height on the opposite side of a pivot point. By calculating the acceleration due to gravity (32.2 ft/sec2 or 9.8 m/sec2), the engineer knows that the weight falling from a set height will contain a certain amount of impact energy at the bottom of the swing. By clamping or supporting a specimen on the bottom, the sample can be released to strike and break the specimen. The pendulum will continue to swing up after the break event to a height somewhat lower than that of a free swing. The engineer can use this lower final height point to calculate the energy that was lost in breaking the specimen. Many pendulum machines will incorporate a pointer and energy reading device so that calculation is unnecessary.

A second method was to drop a weight in a vertical direction, with a tube or rails to guide it during the "free fall." Once again, with the height and weight known, impact energy can be calculated. In the early days, there was no way to measure impact velocity, so engineers had to assume no friction in the guide mechanism. Since the falling weight either stopped dead on the test specimen, or destroyed it completely in passing through, the only results that could be obtained were of a pass/fail nature.

Falling weight impact has several key advantages over other methods.

1.                  It is applicable for molded samples, molded parts, etc.

2.                  It is unidirectional with no preferential direction of failure. Failures originate at the weakest point in the sample and propagate from there.

3.                  Samples don't have to shatter to be considered failures. Failure can be defined by deformation, crack initiation, or complete fracture, depending on the requirements.

These factors make falling weight testing a better simulation of functional impact exposures, and therefore closer to real-life conditions. However, there are drawbacks to uninstrumented falling weight and Gardener or Gardner weight drop testing.

Simple impact tests such Izod, Charpy, and Gardner tests are useful but lack important information about what was happening to the test specimen during the impact event and can be misleading. For example, composites can fail internally but display no damage externally.

Much of impact testing is arguably at the stage where tensile testing was 50 years ago. Early day tensile testers provided a simple analog readout of the maximum tensile strength of the specimen. Today, we recognize that modulus, yield, peak and break strength and strain, % strain or load at preset points, energy, etc. are all important and critical material properties. More and more engineers and designers are realizing that their impact tests must also be upgraded in a similar manner.

This past decade has witnessed a significant expansion in the application of instrumented impact testing.

An instrumented impact test is an impact test where the load on the specimen is continuously recorded as a function of time and/or specimen deflection prior to fracture. All of the above impact tests can be retrofitted or designed with electronic sensing instrumentation.

The best systems record load vs. time or deformation for the entire period of the impact event. This gives a much more complete representation of an impact than a single calculated value. Another area of improvement with instrumentation is time. Test times can be reduced and automation can even be incorporated into the testing.

Instrumented drop weight and pendulum testing is considered to be the best general impact testing method presently available. By multiple testing at various rates, a very complete impact profile can be developed for a polymer. This approach can be useful in simulating functional impact resistance and running material comparisons. There is enough flexibility to simulate real life conditions, and also to perform audit inspections on parts or molded samples.

Compiled by Sanjib Das