Figure 2: Modified Goodman Diagram used to determine acceptable stress levels.


In the example used for Figure 2, the endurance limit was calculated as a percentage of the ultimate tensile strength (typically 42.5% of the ultimate strength for this particular material). Other factors which might further decrease the endurance limit should be considered when determining safe limits; including: surface condition, operating temperature, size, load, and criticality. Testing of a material specimen is the most accurate way to determine the endurance limit, and should be performed for critical components and those which are design to shear at a prescribed level in the event of an overload (shear pins, shear grooves, etc.)


A limitation of the Modified Goodman approach is that it assumes the alternating stresses occur at consistent amplitude. In most cases the amplitude of the torsional vibration (and hence alternating stress) will vary. Additionally, reversing torques as shown in Figure 3, are better analyzed with other methods. The Palmgren-Miner rule is one method for determining fatigue life in cases of variable amplitude loading.


Figure 3: Variable amplitude and reversing torque. THE REPORT | SEP 2024 | ISSUE 109 | 65


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