Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here


No of cycles in a sine test

No of cycles in a sine test

No of cycles in a sine test

I wonder: How many cycles are accumulated in a sine test? If we are sweeping through frequencies and we are hitting some natural frequencies in the process, will the number of (fatigue) cycles be the fn*60 cycles pr minute? Since the time at peak stress will be rather short, will that be real fatigue cycles, or should we count cycles within the rms stress range. sqrt(2)/fn-sqrt(2)*fn. The latmter will be more cycles at lower stress level. Anyone with any experience here? We ar sweeping at 2 Oct/min.

Appreciate any input from experienced test engineers.

RE: No of cycles in a sine test

You could calculate the actual number of cycles at each frequency fairly easily from the sweep time, but for fatigue calculations we conservatively use the highest frequency and assume the entire test was at this frequency. It's quicker too.


Jim Kinney
Kennedy Space Center, FL

RE: No of cycles in a sine test

Yes, but do you use the frequency for the peak response and count the cycles at the peak only, or do you count cycles for å broader band of frequencies like the rms value. Theoreticaly it will only dwell (or not dwell at all) for a very short time at the stress peak. The structure will surely accumulate cycles on each side of the peak. I suppose it also must be connected to the Factor of Safety used for the Fatigue life calculations?

RE: No of cycles in a sine test

Typically we test 20-2000Hz at 1-4 octaves per minute. So 100 to 400 sec * 2000 Hz = 200000 to 800000 cycles at peak stress.

Usually our loads are low enough that we don't worry about fatigue, but we have to check the box that we looked at it.

If it were critical, then you are correct about counting around peak responses. Search rainflow counting for another method.


Jim Kinney
Kennedy Space Center, FL

RE: No of cycles in a sine test

Thanks for your input.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close