The Critical Role of Specimen Dimensions and Notch in Charpy Impact Testing

Impact test methods like ASTM E23 outline impact testing to assess how metals behave under sudden force and multi-axial stresses due to a notch.

The Charpy V-notch impact test is widely used for mechanical testing of steel. It helps engineers study elastic to plastic deformation by analysing the test results and examining the broken specimen fracture surfaces.

The deformation behaviour of FCC metals and alloys like austenitic steels and non-ferrous metals can be evaluated using tensile properties. Ductility or brittleness correlates between tensile and impact tests; if brittle in tensile, it will likely be brittle in impact tests. However, for ferritic steels, tension tests do not predict notch conditions. Charpy and Izod tests are needed for this purpose.

?

1. Notch Effect

The presence of a notch induces a complex state of multi-axial stresses due to deformation occurring in directions perpendicular to the primary stress, leading to stress concentration at the notch base. A condition with severe notching is undesirable as it significantly increases the risk of crack initiation and sudden brittle fracture.

Certain metals can undergo ductile deformation even at low temperatures. In a brittle fracture, the cohesive strength is surpassed before plastic deformation occurs, resulting in a crystalline appearance of the fracture. In ductile failure, deformation happens before the final fracture, and the broken surface appears fibrous.

In some cases fracture occurs after a moderate amount of deformation and the fracture surface appears crystalline and fibrous in appearance.

When a notched bar is loaded, normal stress occurs across the base of the notch, initiating the fracture. Cohesive strength is the property that prevents cleaving and holds the material together. The bar fractures when the normal stress surpasses the cohesive strength. If this happens without the bar deforming, it is considered a condition of brittle fracture.

When an applied force creates normal stress that leads to shear stress at an angle above 45°, the normal stress ends elastic behavior once the shear stress exceeds the shear strength, resulting in plastic deformation and ductile failure.

If the notch is made sharper or more pronounced, the normal stress at the root of the notch increases relative to the shear stress, making the bar more susceptible to brittle fracture. As the speed of deformation increases, the shear strength also rises, increasing the likelihood of brittle fracture.

Increasing the temperature, while maintaining the same notch and deformation speed, lowers the shear strength, which causes the material to exhibit more ductile behavior.

2. Size effect

Increasing the specimen's width thickness increases the metal volume subject to distortion, absorbing more energy. However, thicker specimens can cause brittle fractures and reduce absorbed energy

a)????? Standard 2.0 mm ± 0.025 mm (0.079 in ± 0.001 in)

b)???? Standard 0.25 mm ± 0.025 mm (0.010 in ± 0.001 in)

3. Temperature Effect

The temperature significantly influences the notch's behaviour. By examining the fracture of the specimen and plotting the absorbed energy and shear fracture appearance against temperature, valuable insights can be obtained. At low temperatures, cleavage fracture starts to occur, leading to a noticeable decrease in absorbed energy. This decrease corresponds to a fracture with a crystalline appearance. The transition temperature, where the brittle effect occurs, is dependent on the size of the test specimen and the geometry of the notch

4. Testing Machine

The testing machine must be rigid enough to prevent excessive elastic energy loss when testing high-strength, low-energy materials. Ensure the wheel supporting the striker and the machine foundation board are securely tightened.

5. Velocity Straining

The velocity of straining is a variable that influences the notch behaviour of steel. Impact tests generally display higher absorbed energy values than static tests above the transition temperature

6. Correlation with Service

Although Charpy and Izod tests may not directly indicate the ductile or brittle behaviour of steel in large masses or as part of a large structure, they are useful for acceptance testing or distinguishing between different lots or types of steel.

Testing at a specific temperature other than room temperature may be required. The service or transition temperature of the full-scale specimen might not achieve the desired transition temperature for Charpy or Izod tests due to differences in size and notch geometry.

References:

• ASTM E23 - 2024
• ISO 148-1 - 2016
• IS 1757 - 2020