The Charpy impact test
The Charpy impact test, also known as the Charpy V-notch test, is a standardized high strain-rate test which determines the amount of energy absorbed by a material during fracture. This absorbed energy is a measure of a given material's notch toughness and acts as a tool to study temperature-dependent ductile-brittle transition. It is widely applied in industry, since it is easy to prepare and conduct and results can be obtained quickly and cheaply. A disadvantage is that some results are only comparative.
The test was developed around 1900 by S.B. Russell (1898,
American) and Georges Charpy (1901, French). The test became known as the
Charpy test in the early 1900s due to the technical contributions and
standardization efforts by Charpy. The test was pivotal in understanding the
fracture problems of ships during WWII.
Today it is utilized in many industries for testing
materials, for example the construction of pressure vessels and bridges to
determine how storms will affect the materials used.
In 1896 S. B. Russell introduced the idea of residual
fracture energy and devised a pendulum fracture test. Russell's initial tests
measured un-notched samples. In 1897 Frémont introduced a test trying to
measure the same phenomenon using a spring-loaded machine. In 1901 Georges
Charpy proposed a standardized method improving Russell's by introducing a
redesigned pendulum, notched sample and generally giving precise
specifications.
Definition
he apparatus consists of a pendulum of known mass and length
that is dropped from a known height to impact a notched specimen of material.
The energy transferred to the material can be inferred by comparing the
difference in the height of the hammer before and after the fracture (energy absorbed
by the fracture event).
The notch in the sample affects the results of the impact
test, thus it is necessary for the notch
to be of regular dimensions and geometry. The size of the sample can also
affect results, since the dimensions determine whether or not the material is
in plane strain. This difference can greatly affect conclusions made.
The "Standard methods for Notched Bar Impact Testing of
Metallic Materials" can be found in ASTM E23, ISO 148-1 or EN 10045-1,
where all the aspects of the test and equipment used are described in detail.
The Charpy Impact Test is commonly used on metals, but is
also applied to composites, ceramics and polymers. With the Charpy impact test
one most commonly evaluates the relative toughness of a material, and as such,
it is used as a quick and economical quality control device.
Charpy Sample
The standard Charpy Impact Test specimen consist of a bar of
metal, or other material, 55x10x10mm having a notch machined across one of the
larger dimensions.
V-notch: 2mm deep, with 45° angle and 0.25mm radius along
the base
U-notch and keyhole notch: 5mm deep notch with 1mm radius at
base of notch
Sample sizes
According to ASTM A370, the standard specimen size for Charpy impact testing is 10 mm × 10mm × 55mm. Subsize specimen sizes are: 10 mm × 7.5 mm × 55mm, 10 mm × 6.7 mm × 55 mm, 10 mm × 5 mm × 55 mm, 10 mm × 3.3 mm × 55 mm, 10 mm × 2.5 mm × 55 mm. Details of specimens as per ASTM A370 (Standard Test Method and Definitions for Mechanical Testing of Steel Products).
According to EN 10045-1, standard specimen sizes are 10 mm × 10 mm × 55 mm. Subsize specimens are: 10 mm × 7.5 mm × 55 mm and 10 mm × 5 mm × 55 mm.
According to ISO 148, standard specimen sizes are 10 mm × 10 mm × 55 mm. Subsize specimens are: 10 mm × 7.5 mm × 55 mm, 10 mm × 5 mm × 55 mm and 10 mm × 2.5 mm × 55mm.
Quantitative results
The quantitative result of the impact tests the energy
needed to fracture a material and can be used to measure the toughness of the
material. There is a connection to the yield strength but it cannot be
expressed by a standard formula. Also, the strain rate may be studied and
analyzed for its effect on fracture.
The ductile-brittle transition temperature (DBTT) may be
derived from the temperature where the energy needed to fracture the material
drastically changes. However, in practice there is no sharp transition and it
is difficult to obtain a precise transition temperature (it is really a
transition region). An exact DBTT may be empirically derived in many ways: a
specific absorbed energy, change in aspect of fracture (such as 50% of the area
is cleavage), etc.
Qualitative results
The qualitative results of the impact test can be used to
determine the ductility of a material. If the material breaks on a flat plane,
the fracture was brittle, and if the material breaks with jagged edges or shear
lips, then the fracture was ductile. Usually a material does not break in just
one way or the other, and thus comparing the jagged to flat surface areas of
the fracture will give an estimate of the percentage of ductile and brittle
fracture.
WHY IS IMPACT TESTING IMPORTANT?
Impact resistance is one of the most important properties
for a part designer to consider, and without question, the most difficult to
quantify. The impact resistance of a part is, in many applications, a critical
measure of service life. More importantly these days, it involves the
perplexing problem of product safety and liability.
One must determine:
1.The impact energies the part can be expected to see in its lifetime
2. The type of impact that will deliver that energy, and then
3. Select a material that will resist such assaults over the projected life span
Molded-in stresses, polymer orientation, weak spots (e.g. weld lines or gate areas), and part geometry will affect impact performance. Impact properties also change when additives, e.g. coloring agents, are added to plastics.
DUCTILE VS. BRITTLE
Most real world impacts are biaxial rather than unidirectional.
Further complication is offered by the choice of failure modes: ductile or brittle. Brittle materials take little energy to start a crack, little more to propagate it to a shattering climax. Other materials possess ductility to varying degrees. Highly ductile materials fail by puncture in drop weight testing and require a high energy load to initiate and propagate the crack.
Many materials are capable of either ductile or brittle failure, depending upon the type of test and rate and temperature conditions. They possess a ductile/brittle transition that actually shifts according to these variables.
