Impact Tests
Impact tests are designed to measure the resistance to failure of a material to a suddenly applied force such as collision, falling object or instantaneous blow. The test measures the impact energy, or the energy absorbed prior to fracture. The most common methods of measuring impact energy are the:
• Charpy Test
• Izod Test
What is Impact Energy
Impact energy is a measure of the work done to fracture a test specimen.
When the striker impacts the specimen, the specimen will absorb energy until it yields. At this point, the specimen will begin to undergo plastic deformation at the notch. The test specimen continues to absorb energy and work hardens at the plastic zone at the notch. When the specimen can absorb no more energy, fracture occurs.
Brittle materials generally have lower impact stregths, while those registering higher impact strengths tend to to be tougher.
The Izod Test
The Izod test is has become the standard testing procedure for comparing the impact resistances of plastics. While being the standard for plastics it is also used on other materials.
The Izod test is most commonly used to evaluate the relative toughness or impact toughness of materials and as such is often used in quality control applications where it is a fast and economical test. It is used more as a comparative test rather than a definitive test. This is also in part due to the fact that the values do not relate accurately to the impact strength of moulded parts or actual components under actual operational conditions.
Izod Test Specimens
Izod test specimens vary depending on what material is being tested. Metallic samples tend to be square in cross section, while polymeric test specimens are often rectangular, being struck parallel to the long axis of the rectangle.
Izod test sample usually have a V-notch cut into them, although specimens with no notch as also used on occasion.
What Does the Izod Test Involve?
The Izod test involves striking a suitable test piece with a striker, mounted at the end of a pendulum. The test piece is clamped vertically with the notch facing the striker. The striker swings downwards impacting the test piece at the bottom of its swing.
|
|
|
Figure 1. Schematic of the Izod impact test.
|
Some Izod impact testers are equipped to be able to utilise different sized strikers, which impart different amounts of energy. Often a series of strikers may be used to determine the impact energy, starting with small strikers and working up until failure occurs.
Izod Tests at Different Temperatures
Tests are often performed at different temperatures to more closely simulate the actual service conditions. In the case of low temperature tests, specimens may are kept in a freezer until their temperature has equilibrated. They are then immediately removed and tested within seconds of removal from the freezer.
Determination of Izod Impact Energy
At the point of impact, the striker has a known amount of kinetic energy. The impact energy is calculated based on the height to which the striker would have risen, if no test specimen was in place, and this compared to the height to which the striker actually rises.
Tough materials absorb a lot of energy, whilst brittle materials tend to absorb very little energy prior to fracture.
ISO and ASTM Impact Strengths
ISO and ASTM standards express impact strengths in different units. ISO standards report impact strengths in kJ/m2, where the impact energy is divided by the cross sectional area at the notch. ASTM standards call for values to be reported in J/m, where the impact energy is divided by the length of the notch.
Factors Affecting Izod Impact Energy
Factors that affect the Izod impact energy of a specimen will include:
• Yield strength and ductility
• Notches
• Temperature and strain rate
• Fracture mechanism
Yield Strength and Ductility
For a given material the impact energy will be seen to decrease if the yield strength is increased, i.e. if the material undergoes some process that makes it more brittle and less able to undergo plastic deformation. Such processes may include cold working or precipitation hardening.
Notches
The notch serves as a stress concentration zone and some materials are more sensitive towards notches than others. The notch depth and tip radius are therefore very important.
Temperature and Strain Rate
Most of the impact energy is absorbed by means of plastic deformation during the yielding of the specimen. Therefore, factors that affect the yield behaviour and hence ductility of the material such as temperature and strain rate will affect the impact energy.
This type of behaviour is more prominent in materials with a body centred cubic structure, where lowering the temperature reduces ductility more markedly than face centred cubic materials.
Fracture Mechanism
Metals tend to fail by one of two mechanisms, microvoid coalescence or cleavage.
Cleavage can occur in body centred cubic materials, where cleavage takes place along the {001} crystal plane. Microvoid coalescence is the more common fracture mechanism where voids form as strain increases, and these voids eventually join together and failure occurs. Of the two fracture mechanisms cleavage involved far less plastic deformation ad hence absorbs far less fracture energy.
Ductile to Brittle Transition
Some materials such as carbon steels undergo what is known as a ‘ductile to brittle transition’. This behaviour is obvious when impact energy is plotted as a function of temperature. The resultant curve will show a rapid dropping off of impact energy as the temperature decreases. If the impact energy drops off very sharply, a transition temperature can be determined. This is often a good indicator of the minimum recommended service temperature for a material.
|