Charpy V-Notch Testing
ASTM A370 / ASME SA-370
What is Charpy Impact Testing?
The Charpy impact test, also known as the Charpy V-notch test, is a dynamic test in which a notched specimen is struck by a single blow in a testing machine dedicated to this purpose. The test measures the amount of energy absorbed, the percentage shear fracture, and the lateral expansion opposite the notch.
The Charpy impact test is performed at defined temperature intervals, so as to measure the toughness of materials under impact load at those given temperatures. Targeted temperature values are specified by one or more of the following: material specs, end user design requirements, or welding code requirements.
What Does Charpy Impact Testing Do?
Charpy impact testing helps determine materials ability to absorb energy or impacts, also defined as the materials toughness.
When this testing is performed over a range of temperatures a ductile-brittle transition temperature (DBTT) may be derived where the material will exhibit ductile fracture on the warmer side, and brittle fracture on the colder side of the transition temperature.
What is the Charpy Impact Testing Process?
The specimen prepared to its required dimensions, including the v-notch, and is brought to the specified temperature.
The v-notch specimen is placed into the fixture of the impact testing machine with the notch facing away from the pendulum hammer.
The pendulum hammer is released from the loaded high point position.
The pendulum falls swinging from its pivot point, allowing the weighted hammer head to swing to the lowest point of its circular trajectory, thus striking the specimen retained in the fixture.
The sample fractures, absorbing part of the kinetic energy of the hammer.
The remaining residual energy causes the hammer to swing up to a measured height on its backswing.
The deformation energy and final height achieved depend on the toughness of the specimen.
Tough materials require more deformation before breaking, resulting in higher absorbed energy.
Brittle specimens break with minimal deformation and absorb less energy.
Charpy Impact Testing Equipment
A Charpy impact machine is one in which a notched specimen held in a fixture is broken by a single blow of a freely swinging pendulum.
Pendulum: The pendulum is held stationary and then released from a fixed height. Since the height to which the pendulum is raised prior to its swing, and the mass of the pendulum are known, the energy of the blow is predetermined.
Fixture: the fixture or anvil is built to support and hold the impact specimen while allowing it to be impacted by the pendulum. The fixture is typically located at the lowest point in the pendulum’s swing radius.
Readout: The equipment will have a digital or analog readout to indicate the energy absorbed in breaking the specimen.
How is a Charpy V-Notch Specimen Prepared?
The standard specimen is 55 mm long and has a square cross-section dimension of 10 mm X 10 mm.
The Charpy-V notch specimen has a 2 mm deep V-shaped notch with a tip radius of 0.25 mm machined on one face.
The notch provides a defined predetermined breaking point, generating a triaxial (stress in three dimensions) stress state in the notch base.
The notch must have regular dimensions and geometry for consistent results.