# Example of Random Interaction for Constant Amplitude Fatigue Events

When you select Random interaction, the software mixes peak stresses from different events to evaluate the alternating stress.

Different loading events can interact in such a way that the maximum positive peak stress from one event combined with the minimum peak stress from another event produces a larger alternating stress than either one of the fatigue events.

The Random interaction option predicts a higher fatigue damage factor than applying the different fatigue events sequentially (No Interaction). The Random interaction method is based on the ASME Boiler and Pressure Vessel Code.

For example, consider these three events:
 Event 1 Event 2 Event 3 Number of cycles 80,000 230,000 120,000 Min Stress 0 -300 MPa 0 Max Stress 500 MPa 0 200 Mpa

If Random interaction is considered, the sequence of events that results in the largest stress variation is when one cycle of Event 2 proceeds each cycle of Event 1. In this case the stress fluctuates from 500 MPa (max stress of Event 1) to -300 MPa (min stress of Event 2), resulting in a 800 MPa stress variation (alternating stress is 400 MPa and average stress is 100 MPa).

Since there are only 80,000 cycles of Event 1, this sequence of events can happen only 80,000 times. After the first sequence of events, there are only 150,000 (= 230,000 - 80,000) cycles remaining of Event 2.

For the next event sequence, let's assume that one cycle of Event 3 proceeds each of the remaining cycles of Event 2, resulting in a stress variation of 500 MPa (alternating stress of 250 MPa and an average stress of -50 MPa).

Since there are only 120,000 cycles of Event 3, the second sequence of events can happen 120,000 times.

Finally, 30,000 cycles of Event 2 are left. This corresponds to an alternating stress of 150 MPa, with an average stress of -150 MPa.

 Random Interaction Event 1 followed by Event 2 80,000 cycles of a 400 MPa alternating stress with an average stress of 100 MPa Event 2 followed by Event 3 120,000 cycles of a 250 MPa alternating stress with an average stress of -50 MPa Event 2 30,000 cycles of a 150 MPa alternating stress with an average stress of -150 MPa No Interaction Event 1 80,000 cycles of a 250 MPa alternating stress with an average stress of 250 MPa Event 2 230,000 cycles of a 150 MPa alternating stress with an average stress of -150 MPa Event 3 120,000 cycles of a 100 MPa alternating stress with an average stress of 100 MPa
• For studies with fatigue events referencing multiple studies, the program calculates peak stresses based on the Random Interaction option even if you select the No interaction option.
• The ASME Boiler and Pressure Vessel code recommends using the Random Interaction option. It is generally more conservative (predicts higher damage) than the No interaction option.