Charpy impact test

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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.[1]

The test was developed around 1900 by S. B. Russell (1898, American) and G. Charpy (1901, French).[2] The test became known as the Charpy test in the early 1900s due to the technical contributions and standardization efforts by Georges Charpy. The test was pivotal in understanding the fracture problems of ships during WWII.[3][4]

Today it is used in many industries for testing materials used in the construction of pressure vessels and bridges and to determine how storms will affect materials used in them.[3][5] [3]

Contents

Definition

A vintage impact test machine.

The 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 a the fracture (energy absorbed by the fracture event).

The notch in the sample affects the results of the impact test,[6] 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.[7]

The "Standard methods for Notched Bar Impact Testing of Metallic Materials" can be found in ASTM E23[8], ISO 148-1[9] or EN 10045-1[10], where all the aspects of the test and equipment used are described in detail.

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 and the yield strength. 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.[1]

Qualitative results

The qualitative results of the impact test can be used to determine the ductility of a material.[11] 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.[1]

Sample sizes

According to ASTM A370,[12] the standard specimen size for Charpy impact testing is 10mm×10mm×55mm. Subsize specimen sizes are: 10mm×7.5mm×55mm, 10mm×6.7mm×55mm, 10mm×5mm×55mm, 10mm×3.3mm×55mm, 10mm×2.5mm×55mm. Details of specimens as per ASTM A370 (Standard Test Method and Definitions for Mechanical Testing of Steel Products).

According to EN 10045-1,[10] standard specimen sizes are 10mmx10mmx55mm. Subsize specimens are: 10mmx7.5mmx55mm and 10mmx5mmx55mm.

According to ISO 148,[9] standard specimen sizes are 10mmx10mmx55mm. Subsize specimens are: 10mmx7.5mmx55mm, 10mmx5mmx55mm and 10mmx2.5mmx55mm.

See also

Notes

  1. ^ a b c Meyers Marc A, Chawla Krishan Kumar (1998). Mechanical Behaviors of Materials. Prentice Hall. ISBN 978-0-13-262817-4. 
  2. ^ Siewert
  3. ^ a b c The Design and Methods of Construction of Welded Steel Merchant Vessels: Final Report of a (U.S. Navy) Board of Investigation (July 1947). Welding Journal. 26. Welding Journal. pp. 569. 
  4. ^ Williams, M. L. and Ellinger, G. A (1948). Investigation of Fractured Steel Plates Removed from Welded Ships. National Bureau of Standards Rep. 
  5. ^ Siewert, T. A., Manahan, M. P., McCowan, C. N., Holt, J. M., Marsh, F. J., and Ruth, E. A (1999). Pendulum Impact Testing: A Century of Progress, ASTM STP 1380. ASTM. 
  6. ^ Kurishita H, Kayano H, Narui M, Yamazaki M, Kano Y, Shibahara I (1993). "Effects of V-notch dimensions on Charpy impact test results for differently sized miniature specimens of ferritic steel". Materials Transactions - JIM (Japan Institute of Metals) 34 (11): 1042–52. ISSN 0916-1821. 
  7. ^ Mills NJ (February 1976). "The mechanism of brittle fracture in notched impact tests on polycarbonate". Journal of Materials Science 11 (2): 363–75. Bibcode 1976JMatS..11..363M. doi:10.1007/BF00551448. 
  8. ^ ASTM E23 Standard Test Methods for Notched Bar Impact Testing of Metallic Materials
  9. ^ a b ISO 148-1 Metallic materials - Charpy pendulum impact test - Part 1: Test method
  10. ^ a b EN 10045-1 Charpy impact test on metallic materials. Test method (V- and U-notches)
  11. ^ Mathurt KK, Needleman A, Tvergaard V (May 1994). "3D analysis of failure modes in the Charpy impact test". Modeling and Simulation in Materials Science Engineering 2 (3A): 617–35. Bibcode 1994MSMSE...2..617M. doi:10.1088/0965-0393/2/3A/014. 
  12. ^ ASTM A370 Standard Test Methods and Definitions for Mechanical Testing of Steel Products

External links