×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Contact US

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!

*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

Effect of speed on fatigue life?

Effect of speed on fatigue life?

Effect of speed on fatigue life?

(OP)
Spring: about 5 in dia x 9 in tall barrel style coil spring using about .5 in wire.  Fully compresed height is roughly 1.5 in.
Fatigue test: cycle spring from almost compressed height to about 4 in height at some rate until it breaks.
Question:  How does that rate affect the life of the spring?

RE: Effect of speed on fatigue life?

Can you give more info? Are all the coils have the same diameter? If so then if the springs is squared and ground there is only one active coil?

RE: Effect of speed on fatigue life?

Speed is only significant when the spring is near its resonant frequency.  Engine valve toss is one example.  Have you calculated the resonant frequency of your design?

RE: Effect of speed on fatigue life?

(OP)
The 3" OD end coils are squared off so the thing stands up straight on the bench, then the coils expand out to ~5" OD middle coils.
I know that it doesn't actually operate near the resonant frequency, but I was thinking about cycle testing as fast as 5 or 6 Hz.  What does resonating actually do to the spring?  I'm guessing inertial forces would add to the spring force pushing on whatever the spring is hooked up to.  But how does that affect fatigue life of the spring?
OK, are there some numbers to crunch to figure out the resonant frequency?  Or should I hit it with a hammer and time the wobbles?

RE: Effect of speed on fatigue life?

(OP)
These springs have gone through some type of stress-relief exercise like shot peening during manufacture. (No, I don't understand all this stuff - just enough to be ;-D dangerous.)  I also know that brand new springs demonstrate a force output decline over the first X hours of their life.  The rate of decline decreases to almost nil after several days.  Since I'm talking about testing brand new springs, how does this new-spring force decay affect the life cycle testing?  I wondered if the two phenomena (force decay & cycle fatigue) are related.  OK, not if, but HOW?

RE: Effect of speed on fatigue life?

(OP)
OK, I ran the numbers on frequency & came up with a resonance of around 90 Hz.  I'm nowhere near that, so the only weird thing I can think of will be the total time of compression during the life cycle test.
Let me put it another way.
If four identical springs are compared before & after a life cycle test:
# action                 result
1 sits relaxed           no change
2 sits compressed        loses force
3 cycles quickly         loses some force, fatigues
4 cycles slowy           loses more force, fatigues differently?

RE: Effect of speed on fatigue life?

The new-spring force decay does affect the life cycle test but it will actually prolong the life because the force reduce cause a stress reduce. Therefore the spring will have longer life. However, can you tolerate the force reduce?

The force reduce can be a result result of a relaxation or simply because the stress was (higher than the yield shear stress where the spring actually gone into the plastic zone and yielded. The result is a minor preset (scrag). If this is the case you do not have a sufficient safety factor.

You should calculate the stress at the desired force and compare it to the material yield shear stress. It should not be more than 35% to 50% of the ultimate tensile stress depends on the spring material.

You described the spring as barreled spring, is the gap (pitch) between the coils constant? or it is variable? Is the 5" coil bottoms first or all the coils bottom (touch) each other at the same time when spring is pressed to solid?

What is the type of material the spring is made off?

RE: Effect of speed on fatigue life?

(OP)
Wire is chromium silicon SAE 9254.  
There are 4½ coils total, including the squared-off ends of about ½ coil at the small diameter.
I can live with losing some force.
My goal is to do cycle testing faster without compromising the results.  Slower cycling will help the spring last longer?  OK.  But that means faster cycling will kill it sooner, invalidating my test?

RE: Effect of speed on fatigue life?

As I mentioned previously, the speed at which the spring is cycled during testing is irrelevant below the natural frequency.  israelkk is talking about force loss due to compression set, which is a totally different phenomenon.

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! Already a Member? Login


Resources

Low-Volume Rapid Injection Molding With 3D Printed Molds
Learn methods and guidelines for using stereolithography (SLA) 3D printed molds in the injection molding process to lower costs and lead time. Discover how this hybrid manufacturing process enables on-demand mold fabrication to quickly produce small batches of thermoplastic parts. Download Now
Design for Additive Manufacturing (DfAM)
Examine how the principles of DfAM upend many of the long-standing rules around manufacturability - allowing engineers and designers to place a part’s function at the center of their design considerations. Download Now
Taking Control of Engineering Documents
This ebook covers tips for creating and managing workflows, security best practices and protection of intellectual property, Cloud vs. on-premise software solutions, CAD file management, compliance, and more. Download Now

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