|
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.
Pendulum Testing
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.
» Learn more about Charpy
& Izod Testing
Drop Weight Impact Test
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.
- It is applicable for molded samples, molded parts,
etc.
- It is unidirectional with no preferential direction
of failure. Failures originate at the weakest point in the sample
and propagate from there.
- 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.
» Learn more about Gardener
or Gardner Testing
Instrumented Impact 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.
The New Standard - Multiple Benefits for All
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.
|