Hardness versus Fatigue Strength

In the previous article we examined the relationship between hardness and tensile strength for the AISI fatigue database iterations through 141. As the hardness increased the tensile strength also increased for a majority of the steels and the relationship was linear. However, with the high hardness, high carbon, carburized case steels there was a significant variation in strength in the 600-750 Brinell hardness range. The variation in strength was 881-2227 MPa, which is a difference of 2.5:1.

In this article we examine the relationship between hardness and fatigue strength for the same samples. The fatigue strength is defined as the stress level where one million cycles is achieved. The data is shown in Figure 1. The curve looks much like the previous hardness versus strength curve. There is a linear correlation between hardness and fatigue strength to 600 Brinell. However, there seems to be more scatter or variation in the hardness versus fatigue strength curve. At a hardness of 600-750 Brinell there is a large variation in fatigue strength primarily due to the carburized case samples. The fatigue strength range is 235-819 MPa, which is a difference of 3.5:1 between the high and low iterations.

SMDI Blog 36 Figure 1Figure 1

The fatigue ratio is the fatigue strength divided by the ultimate strength. The fatigue ratio versus hardness data is shown in Figure 2. The fatigue strength does not appear to be a fixed percentage of the ultimate tensile strength throughout the entire hardness range. At 200 Brinell the fatigue ratio is approximately 0.45, while at 700 Brinell it is 0.35. As the hardness and strength increases the fatigue strength/ultimate strength becomes a smaller ratio.

figure 2Figure 2

 

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2 Responses to Hardness versus Fatigue Strength

  1. Peggy Jones says:

    What fractographic and metallographic work was performed on the lowest fatigue ratio specimens? The data plots or descriptions need to include test details: load type (bending, torsion, axial), load or strain controlled test, load or strain ratio, frequency, test temperature, and test atmosphere (e.g. lab air). Perhaps these details could be included as a link?

    • Unless otherwise stated, all testing in this program is fully reversed strain controlled axial tension compression testing per ASTM E606, done at room temperature. The details for each test can be found at http://www.autosteel.org. On the lower right of the page is an icon for the bar steel fatigue database where you can log in. The test information can be found by clicking on, “master index by iteration”. The test details can be found by clicking on, “documents”. Most iterations have representative microstructure photos. Fractography has not typically been done in the past, but some work has been done recently with carburized case samples. The work has shown surface initiation at lower cycle life, and subsurface inclusion initiation above 250,000 reversals.

      Response from Greg Fett, SMDI Program Manager Bar Steel Fatigue Sub-Committee

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