The Doctor has always been intrigued by the question of how to compare the impact values obtained by Izod testing methods with those of Charpy testing methods. A great deal of data, albeit older (mid-20th century), has been collected using Izod testing, and it would be of great value to be able to use it today in some fashion. The time has come to find out how. Let’s learn more.

In the studies presented here, a cautionary emphasis must be placed on interpreting them. While we are dealing with specimens of the same cross section, the size of the cross section under the notch may be different as well as the stress intensity of each given notch geometry. The information in these investigations can be very helpful in rank ordering materials with individual processing conditions/properties and making educated estimates.[7]

English Steel Study

The English Steel Corporation[7] performed one of the most comprehensive comparison studies. Izod data was compared with Charpy data from four different types of Charpy specimens (V notch, Mesnager U notch 2 mm deep, DVM U notch 3 mm deep, and the commonly used ISO U notch 5 mm deep), two of which are presented here (Figs. 1-2). The study also emphasized that a definite relationship between methods does not exist, the graphs must be considered approximate relationships and the data should not be used for comparing or compiling specifications.[7]

TimkenSteel Study

A study was conducted in the 1940s when both Izod and Charpy keyhole specimens were used extensively. It is unique in that the data are one-to-one values obtained on the exact same orders/heats of steel and heat treatment (liquid-quenched and tempered). For several months when an order was produced requiring one of the test types, both specimen types were machined and tested. When examining this data (Fig. 3) one may ask, Why would the Charpy U-notch data have a lower value than the Charpy V-notch data? The answer lies in the volume of material being tested – the U-notch being 5 mm deep while the V-notch is only 2 mm deep. The reader is directed to ASTM E 23 for more in-depth discussion. Note also that under brittle conditions up to 13.5-20.4 J (10-15 foot-pounds), the two methods were nearly identical.[9]

4340-Type Steel Study

TimkenSteel also conducted a one-off study of the effect of tempering temperature on the toughness of 4340-type steel using Charpy V test data. They then compared it to Izod data that had been published in the Alloy Digest. This graph was published in the Aerospace Structural Metals Handbook[9] and is reproduced here (Fig. 4). In this instance, the data were highly correlated, but one must keep in mind that the data represents different heats of steel produced years apart (1982 vs. 1954) and heat treated in separate processing equipment.

Special Note

Discussion within ASTM Standard E 23[7] notes the following in regard to the comparison of Izod and Charpy data, namely: “General correlation between the energy values obtained with specimens of different size or shape is not feasible, but limited correlations may be established for specification purposes on the basis of special studies of particular materials and particular specimens.

Final Thoughts

A few of the comparative studies between Izod and Charpy Impact values have been described. As the graphical representations show, the data will generally not fall in a one-to-one relationship. There are many reasons for the variances that can and will likely be experienced. Nonetheless, the data are both interesting and quite useful when applied with appropriate caution and consideration. The reader is urged to consult ASTM E 23 (latest revision) for additional detail and study the references provided (in particular numbers 8 and 10), which are outstanding sources for a tremendous variety of mechanical-property data.

 

References

  1. Mr. Craig Darragh, AgFox LLC, technical and editorial contributions and private correspondence
  2. Dieter Jr., George E., Mechanical Metallurgy, McGraw-Hill Book Company, 1961
  3. Wilby, A. J. and D. P. Neale, “Defects Introduced into Metals During Fabrication and Service,” Vol. III, Materials Science and Engineering, Encyclopedia of Life Support Systems
  4. Reed-Hill, Robert E., Physical Metallurgy Principles, D. Van Nostrand Company, Inc., 1964
  5. Herring, Daniel H., “Understanding Component Failures Parts 1 & 2,” Industrial Heating, July/August 2013
  6. Herring, Daniel H., Atmosphere Heat Treating, Volume II, BNP Media Group, 2015
  7. ASTM E23 (Standard Methods for Notched Bar Impact Testing of Metallic Materials)
  8. Woolman, J. and Mottram, R. A., The Mechanical and Physical Properties of The British Standard EN Steels (B.S.970-1955), Elsevier Ltd. 1966
  9. Resume of Investigations on Steels for High Temperature, High Pressure Applications 1963-1965, The Timken Roller Bearing Company, Steel and Tube Division (now TimkenSteel Corporation), Canton, Ohio, USA, 1965, p. 127-128
  10. Aerospace Structural Metals Handbook –Ferrous Alloys – Code 1206 (4340 Steel), CINDAS LLC , West Lafayette, Indiana, USA, section Code 1206 page22. (Note: the ASMH is now a portion of the Aerospace Structural Metals Database available from CINDAS LLC)
     

Part one and two of Dan Herring’s “Impact Testing” are archived on the Industrial Heating website:

Impact Testing (part 1)

Impact Testing (part 2)