Impact testers come in different shapes and sizes, corresponding to different capabilities and areas of application: so how do you go about selecting the right one?
If your requirement is only to test to one or more recognised standards, then your task is clear and simple: browse the product pages on our websites to find a system that covers the standards that you require.
How High And How Heavy
But what if that is not the case? Where do you start? The most important parameter to consider is the range of energies that you want to test at. There are essentially two types of impact test: those where the specimen itself absorbs all the energy of the falling weight, and those where the specimen is expected to fail completely with the excess energy being taken up by shock absorbers. In the first case, the energy range required is defined by the testing regime, while in the second the maximum energy required is dependant on the maximum energy that the specimens are going to absorb before they fail (usually with a safety factor of an additional 50% to 100%). Make sure the shock absorbers have the capacity to handle the maximum impact energy – specimens can sometimes offer surprisingly little resistance.
Impact energy is given by:
- Energy = m g h
where m is the impact mass, g is 9.81 (if we are talking metric) and h is the height the mass is dropped from. If you are not concerned about the speed of the impact then you can juggle the variables of mass and height to provide the energy range you require: practical drop heights range from 50mm to 4m, and the minimum practical mass for an instrumented test is about 2kg.
It is rare, however, that the impact speed is not an issue. The impact response of a lot of materials (and arguably all the interesting ones) are to a lesser or greater extent rate dependant, so the measured values of various properties will depend on how fast the impact mass is travelling at the moment of contact. Velocity and energy are related by:
- Energy = ½ m v²
Now the juggling because more difficult: you have to find a range of heights and masses that give you both the energy range that you require, and the velocity range. If the masses are fairly low (10kg or less) you can use the acceleration option to get higher velocities from a lower height. We at Imatek Systems are jugglers of many years’ experience and would be happy to help.
After the impact mass and drop height, the next thing to consider is the impact geometry. This is specified by:
- The size and shape of the striker (the part of the falling mass that impacts the specimen).
- The form of the specimen, whether it is a prepared sample (such as a flat sheet, tensile specimen or cube), or whether you want to impact an assembled component (such as a steering column or a crash helmet).
- How the specimen is mounted: is it placed on a flat anvil? Is it held in place by clamps? Is it free to move along the axis of the impact?
If the impact mass is likely to rebound from the specimen you should also consider whether you need to prevent it being hit a second time. Second impact prevention is relatively straightforward for impact masses up to about 30kg. Above that it becomes more difficult.
As obvious as it sounds, the specimen and its support need to fit inside the impact area of the tester (and for safety reasons the doors have tor be shut). Impact areas can be made just about any size but you have to consider how conveniently the instrument can be operated. .
The design of the instrumentation will be led by considering what it is that you are trying to measure. Either force or acceleration, or sometimes both, will be involved. Force is commonly measured between the striker and the mass carriage, but there are other possibilities: the load cell can also be mounted on the specimen fixture, or occasionally on the specimen itself. Accelerometers are usually placed on the mass carriage. Other measurements can include displacement, strain and pressure or even sound level.
The final item to consider is the temperature at which the test is performed, which might make necessary the use of an environmental chamber. It is also possible (although far less common) to control the humidity.