How to determine variable amplitude in a fatigue analysis

[lodhss]

I am in the process of setting up some mechanical fatigue test of a medical device. I’d like to load the device ___ times with a daily activity load, and then load it once with a high impact load, and then go back to ___ times of a daily activity load. I've seen engineers in other industries (auto, aero, etc) use variable amplitude fatigue loading cycles, but was looking for specific examples. If you have references/publications, I'd love to read them. Would be interested to know how adding bouts of high impact loads affect fatigue life of a product/part. Thanks for your help.

 

[lodhss]

In addition, are there any iso or astm standards that the industry uses for variable amplitude loading fatigue tests?

 

[rb1957]

occasional high amplitude loads usually have the effect of slowing crack growth ... google "fatigue crack retardation".

google "fatigue analysis" to see how to set up a load spectrum ... which is basically figuring out a typical load magnitude (a max and a min if you need it, most often min = 0 is good enough) and then how often this happens.

another day in paradise, or is paradise one day closer ?

 

[Dougt115]

What kind of medical device?

 

[tbuelna]

While I appreciate the fact that you wish to learn more about the subject of fatigue, you also seem to be missing the reason such tests are conducted. Your "mechanical fatigue test" should be used to validate fatigue life analysis work that has been performed on the design. You should be using a conforming test article, and the test conditions should match what was used for the related analysis work. If you haven't established what the fatigue life load/cycle conditions are for your device and performed some analysis based on these variables, then you are not ready for testing, and it would largely be a waste of time and money.

 

[Bicky22]

I agree with Tbuelna. "Your "mechanical fatigue test" should be used to validate fatigue life analysis work that has been performed on the design."

To help validate your fatigue life analysis based on different load levels and number of times loaded, I would read up on Palmgren-Miner's rule.

http://aerospacengineering.net/?p=166

Again, meaningless and a waste of time and money if analysis work has not been preformed.

 

[lodhss]

A medical device like a joint replacement would be loaded in vivo with loads that can vary tremendously over the course of a day. Current iso standards of a knee replacement, for example, are just to load the knee replacement with loads comparable to walking which have loads that vary in amplitude and direction in a very complex way. In patient use, however, the loads and directions can be even more extreme when people climb stairs, jog, bike, etc. I'm looking for ways other industries (e.g. shocks of a car) have explored ways to incorporate variable amplitude loads to test fatigue life of a product to better replicate its use in the field, and see if it can be translated to a medical device like a joint replacement by adding more than just walking loads.

 

[Bicky22]

Halt/Hass testing.

 

[Dougt115]

Items like knees have a standard that they have to meet, ISO 10328. This standard tells you the loading, cycles and test setup for each load rating.

You can do more testing if you want but it will not help your sales. Industry knows what an ISO rating of 275lbs means and what testing it has to meet. Additional unique testing will mean nothing to anyone but you and your company.

Unless you get brought to court and then you will lose no matter what. Juries always favor the handicapped, insurance companies will settle out of court and your company will end up paying.

 

[lodhss]

I am more interested in improving industry standards, and right now it seems that products are getting onto the market without being fully vetted. Again, if other industries have introduced random high loads (e.g. load applied after hitting a pothole), i'd be interested to know how a loading profile in this scenario was devised.

 

[TVP]

Automotive industry definitely does work in this area. Here are some references that include details on the subject. See if you can't review them at a nearby University engineering library:

SAE Fatigue Design Handbook

http://books.sae.org/ae-22/

Fatigue Testing and Analysis, 1st Edition
Theory and Practice

http://store.elsevier.com/Fatigue-Testing-and-Analysis/YungLi-Lee/isbn-9780080477695/

Fatigue of Structures and Materials

http://www.springer.com/us/book/9781402068072