# What is the fatigue limit for copper pipe with a factor of safety of 5?

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• MysticDream
MysticDream
TL;DR Summary
Trying to estimate the fatigue limit for very low stress.
I’m wanting to estimate the fatigue limit for copper piping that has a factor of safety of about 5 (yield FOS). All I can find online is graphs of curves that go to about 100 million cycles, but I need more. Obviously for these materials that don’t have a distinct fatigue limit like steel, the slope of the curve goes more horizontal as stress levels decrease, but I want to know what the slope is up to a billion cycles if possible. This piping will be subjected to external gas pressure cycles of 300 cycles per min, 24/7. That means after one year, it will have gone through about 157,680,000 cycles. I need to have a reasonable estimate of how long these would be in service before they fail from fatigue. If anyone has any insight into this, I’d greatly appreciate any assistance.

I would think electrical conductors see 50 Hz continuously (in UK at least) for many decades. Perhaps there is some data about any mechanical degradation evidenced from this. How big is the pressure and the pipe?

MysticDream
hutchphd said:
I would think electrical conductors see 50 Hz continuously (in UK at least) for many decades. Perhaps there is some data about any mechanical degradation evidenced from this. How big is the pressure and the pipe?
Yeah, that’s a good point, but that max stress is probably way lower than the max stress in my design with a FOS of 5. Electrical conductors probably have a FOS of 100 or more given that the stress it’s subjected to is so small.

The pipe is .625” OD with a wall of .028”. The min and max external pressure in the cycle is from 300 to 350 psi. An FEA showed a yield FOS around 5 for that max pressure of 350.

## 1. What is the fatigue limit for copper pipe?

The fatigue limit for copper pipe, or the stress level below which it can endure an essentially infinite number of cycles without failing, is generally not well-defined as it is for ferrous metals. For non-ferrous metals like copper, fatigue strength is often provided for a specific number of cycles, typically 10^7 cycles.

## 2. How is the fatigue limit determined for copper pipe?

The fatigue limit for copper pipe is determined through fatigue testing, where samples are subjected to cyclic loading until failure. The results are used to create an S-N curve (stress vs. number of cycles) that helps estimate the fatigue strength for a given number of cycles.

## 3. What is a factor of safety, and how is it applied to fatigue limits?

A factor of safety is a design criterion that provides a margin of safety for uncertainties in material properties, loading conditions, and potential flaws. When applied to fatigue limits, it reduces the allowable stress to ensure a higher reliability of the component. For example, with a factor of safety of 5, the allowable stress would be one-fifth of the fatigue limit determined from testing.

## 4. How do you calculate the allowable stress for copper pipe with a factor of safety of 5?

To calculate the allowable stress for copper pipe with a factor of safety of 5, you first need the fatigue limit or fatigue strength for the desired number of cycles. Then, divide this value by 5. For example, if the fatigue strength at 10^7 cycles is 100 MPa, the allowable stress would be 100 MPa / 5 = 20 MPa.

## 5. Are there any standard values or references for the fatigue limit of copper pipe?

Standard values for the fatigue limit of copper pipe can be found in engineering handbooks or material databases. However, these values can vary based on the specific alloy, processing, and conditions of use. It is often recommended to refer to manufacturer data or conduct specific testing for the most accurate information.

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