Fatigue calculations for stainless steel seamless tubing

[tquigley]

Looking to understand Fatigue calculations for stainless steel seamless tubing

Problem:
Material: 304 Stainless Steel tubing (.375 OD, .072 wall) per ASTM specifications used at 158F degrees maximum
Ambient UTS 515 MPa, YTS 205 MPA, temperature de-rated per MIL-HDBK-5J to UTS 473.8 MPa, YTS 192.7 MPa.
Burst Pressure: 710 bar
Proof Pressure: 568 bar
Operating Pressure: 420 bar

Looking to find the fatigue life of this tubing when cycled from ambient to 420 bar at max temperature of 158F.
Looking to generate a chart reflecting the stress-life and just understanding the fatigue of this application.

Attached are my current fatigue calculations.

I will be setting up practical tests to verify this application, however I'm looking to predict that this tubing will have a acceptable safety factor.

Any help that is offered is greatly appreciated.

 

[Navin A S]

Solution:In order to calculate the fatigue life of the stainless steel tubing, you will need to first determine the fatigue strength of the material. This can be done using a stress-life (S-N) diagram or curve. The S-N diagram plots the number of stress cycles that the material can endure before failure as a function of the applied stress level. Generally, the higher the stress level the fewer cycles the material can endure before failure. For the specific application, you will then need to determine the total number of stress cycles that the tubing will endure under your operating conditions. This is done by multiplying the number of stress cycles per second (1/second) by the total number of seconds in the cycle. For example, if the cycle time is 1 minute, then the total number of stress cycles will be 60. Once you have determined the total number of stress cycles, you can then compare this number to the fatigue strength of the material. If the total number of stress cycles is below the fatigue strength of the material, then the material should be acceptable for your application. If the total number of stress cycles is above the fatigue strength of the material, then you may need to consider using a different material or modifying the design of the part to reduce the stress levels.In addition to calculating the fatigue life of the material, you should also consider performing practical tests to verify your results. This can help ensure that the material is suitable for the application and can provide valuable feedback on the performance of the material.