Tests of Knife Edge Toughness

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In an earlier article I wrote about the microscopic mechanisms by which chipping and micro-chipping occurs in edges. However, that article did not cover specific tests of edge toughness. Correlating conventional toughness tests with edge toughness is difficult for many reasons:

1. Fracture toughness tests do not incorporate the effect of crack initiation which is the primary mechanism by which micro-chipping occurs, and is part of the energy required for macro chipping. Unnotched and to some extent c-notch or u-notch impact tests do include crack initiation, however.
2. Toughness test specimens are generally much larger than the tip of a cutting edge. Therefore, the statistical occurrence of large carbides or inclusions that act as crack initiation sites is very different [1]. Other small microstructure features such as retained austenite may have different effects in such a small volume.
3. The small cross section of a thin edge means it is likely to deform rather than chip even at relatively high hardness (see the article on bending of steel for more information).
4. The loss of sharpness through micro-chipping and gross fracture through chipping likely occur through somewhat different mechanisms (see my article on chipping for more information). For micro-chipping it may be useful to perform actual sharpness tests to measure the degradation of the edge.
5. Measuring the effect of edge thickness, edge angle, shape, etc. is obviously not possible without a test of actual edges.

Factors that Affect Toughness

Higher hardness, impurities, retained austenite, larger grain size, greater carbide volume, larger carbides, and smaller spacing between carbides all reduce toughness. Thicker edges and more obtuse edges require more energy for deformation or fracture. However, determining the extent of influence of each of these different variables requires edge toughness testing.

Tests of Edges

Tool Steel Simplified

In Tool Steel Simplified they describe a process for impact testing of edges [2]. They used a 3/8″ piece of M2 with a 45° single bevel edge:

They then impacted the edge with a pendulum at different heights. As shown below the knife was held at an angle relative to the impact. At lower impact energies the edge would deform, they increased the energy until the edge would deform sufficiently to fracture. Greater height meant higher impact measured in inch-lbs:

They then produced a plot comparing edge toughness against tempering tempreature for the M2 steel austenitized at 2225°F:

Highest toughness correlated with the lowest hardness, and showed a great decrease in the initial secondary hardening range (800-1200°F) due to increase in hardness and the transformation of retained austenite (see my article on tempering). They compared the knife edge toughness against several other measures of mechanical properties:

The comparisons were made with torsion tests which are simply twisting a bar of steel until it fractures. The strength required to break is the torsional strength. The strength required to yield, or begin plastic deformation, is the elastic limit. Torsion impact toughness is a rapid twist of the steel so that toughness is measured. Poor correlation was found between the edge toughness test and torsional strength, torsional elastic limit, and torsional impact toughness, with a strong correlation with torsional ductility. Torsional ductility is the amount of twist a bar undergoes prior to fracture, and is similar to strain in a tensile test. However, there are other tests that they did not compare with, such as pendulum impact tests or fracture toughness. Fracture toughness, for example, shows a peak in toughness at a similar tempering temperature [3]:

I don’t know if they ever continued the use of this toughness test. It was removed from the 4th edition of Tool Steel Simplified, probably in part because all references to torsion testing were removed from the 4th edition. No other steels, edge angles, etc. are reported in the book.

Agricultural Blades

Impact tests have also been reported for agricultural blades [4][5] which are single bevel as well:

A test was performed with a pendulum impact tester using simple single bevel blades to mimic the cutting edges of the agriculture blades. The test overall is very similar to that described in Tool Steel Simplified. The hammer was dropped from an unspecified height and the degree of deformation was measured. I emailed the primary author of the study for more information such as the energy used of the impact tester, what type of impact hammer was used, orientation of the sample relative to the hammer, etc. but he wouldn’t tell me anything. However, it appears that the amount of deformation and/or chipping of the edges was relatively significant:

When comparing different steels it is apparent that hardness is an important parameter, presumably because higher strength would reduce the amount of deformation that occurs in the edge. However, there are still differences between steels independent of hardness:

Almost all of the steels that were tested are medium carbon steels meaning they are unlikely to have much in terms of carbides while 1.3243 is a high speed steel which does have some primary carbide making fracture initiation easier. Perhaps that is why XAR650 showed superior behavior to 1.3243 despite its lower hardness; the high speed steel fractures more easily due to carbides. However, none of that is confirmed or described in the referenced paper. 

The researchers also tested a range of different edge angles where, as expected, a more acute edge led to more deformation during the impact testing.

Edge Stability

Edge stability tests were designed by Roman Landes and reported in different publications but most prominently in his book [6]. The test was conducted by pressing a 2 mm Titanium Nitride coated rod into the knife edge with a load of 1 kg for 10 seconds. The initial tests were performed with 20° single bevel edges:

A series of 10 indentations were performed on each edge and the extent of deformation or chipping was measured. The tests were performed on a range of steels and heat treatments to test the effect of steel properties on “edge stability,” or the resistance to deformation and chipping. In the future I am going to write a couple articles on edge stability so I’m not going to go in depth on the edge stability test in this article. However, I think it was important to mention it because it would be strange not to bring it up.

Future Research

I think the impact tests of edges show promise for quantitatively characterizing edge toughness. I think doing the impacts in combination with a sharpness test would help to characterize the effect of impacts on the micro-scale. Perhaps tests could be separated into different regimes such as sharpness loss, deformation, chipping, etc. Tests would be necessary to determine where those dividing lines might be. With a well designed test it would be possible to explore what steel property and edge geometry parameters are most significant for edge toughness. Some have asked what is the point of toughness testing if the knives are designed simply for cutting and not for any heavy chopping. However, edges also lose sharpness due to micro-chipping or rolling, particularly in thin edges which are desirable for superior cutting ability. The combination of an impact test with a sharpness tester is appealing for measuring the effect of small impacts on sharpness. Also multiple low energy impacts could be used to simulate sustained use. I’m currently exploring my options for purchasing a small impact tester to start on edge toughness tests. With such tests we can determine what correlation there is, if any, between charpy impact tests and edge toughness testing. The number of knifemakers I have convinced to make charpy toughness specimens for me to test is still relatively small but maybe it would be easier to convince them to make simple knives since that is what they are used to producing. To see some of the charpy impact testing we have done you can read the article on Cruwear toughness and the article on CruForgeV toughness. 

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[1] Karagöz, Sadi, and Hellmut Fischmeister. “Microstructure and toughness in high speed tool steels: the influence of hot reduction and austenitization temperature.” Steel research 58, no. 8 (1987): 353-361.

[2] Palmer, Frank R. and George V. Luerssen. Tool Steel Simplified. Chilton Company, 1954.

[3] Horton, S. A., and H. C. Child. “Relationship between structure and fracture behaviour in 6W–5Mo–2V type high-speed steel.” Metals Technology 10, no. 1 (1983): 245-256.

[4] Rostek, Tim, and Werner Homberg. “Locally Graded Steel Materials for Self-Sharpening Cutting Blades.” Procedia Engineering 207 (2017): 2185-2190.

[5] Rostek, Tim, and Werner Homberg. “Grading technologies for the manufacture of innovative cutting blades.” In AIP Conference Proceedings, vol. 1960, no. 1, p. 100013. AIP Publishing, 2018.

[6] Landes, R. “Messerklingen und Stahl.” Aufl. Bad Aibling: Wieland Verlag (2006).