Aluminum Alloy Fatigue Information

[minger]

Hey guys, I'm working on a project and am looking for some fatigue data for aluminum alloys, A356-T6 in particular. I've looked through the MIL-5 and Aerospace (Cindas) handbooks and haven't found any decent information.

The textbook I have "Mechanical Engineering Design" by Shigley and Mischke only lists data for steel and other steel alloys. Does anyone have any decent references for stress vs cycles or anything decent?

Thanks,

 

[Mech_Engineer]

The data is available out there, it's just a matter of whether you're willing to pay for it or not. SAE has a fatigue design handbook that might have some data in it. Otherwise, a quick search on Google turned up this ScienceDirect.com journal article:

http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6W7V-42DRCFD-8&_user=7150423&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_version=1&_urlVersion=0&_userid=7150423&md5=0b0a18db6a2b5b8f86b8bb101b0a3dc6

 

[Q_Goest]

Hi Minger. Good fatigue data on aluminum is hard to come by, especially charts showing test results.

MatWeb has some data depending on whether it's a http://www.matweb.com/search/DataSheet.aspx?MatGUID=595aeac7a5af4446bf4d912226644759" cast. Note the ASTM specs for these are B26 (sand) and B108 (perm mold).

I've designed a number of sand cast parts are highly stressed and under cyclic conditions and the only data I've been able to find is the matweb stuff which I've had to depend on. Seems to be ok, though I've always put lots of extra margin in.

 

[minger]

Well, the only thing I could find off of there was the Fatigue Strength for 5e8 cycles, which can be useful, but I really wish there was some more data, an Ultimate Fatigue Strength would be nice; as then I could apply my own Marin endurance modifying factors.

Either way I appreciate the help, I guess I'll go through the library and see if there's any good fatigue books.

Thanks again,

 

[Q_Goest]

Well, the only thing I could find off of there was the Fatigue Strength for 5e8 cycles, which can be useful, but I really wish there was some more data, an Ultimate Fatigue Strength would be nice; as then I could apply my own Marin endurance modifying factors.

The problem with aluminum is it doesn't have a typical 'fatigue limit'. The more stress cycles that are imposed on aluminum, the lower the stress cycles need to be to eventually result in failure. This is different than steel which has some distinct endurance limit.

If you were to plot peak alternating bending stress (y axis) against a log scale of life cycle (x axis) you'd find a knee in the curve at around 10^7 cycles. So that by 10^8 cycles, the graph is almost flat. It isn't flat though, it continues to decline, meaning that as you continue to impose more cycles on your test specimen, the peak alternating bending stress needed to result in failure continues to drop.

The most important things to understand about fatigue calculations isn't the specific limit of the material anyway. With the data MatWeb gives, one can be pretty sure where your stresses need to be to create a safe design. The more important issues are always things like stress concentrations, surface finish and absolute magnitude of loads and in what direction. These are the factors that I've found are typically not properly accounted for. In addition, realizing how far off some of these factors can be, in addition to how far off your real life loading conditions may be, the best thing to do is to be conservative in your design and ensure the safety factors you select reflect this lack of accuracy.

 

[minger]

Awesome, well I appreciate the help. Turns out my still-active ohiolink.edu account let's me pull some of that experimental data up.

I should be good to go now for what I need. Thanks again!

 

[fatiguepen]

Some of the FD&E data is on-line at
http://fde.uwaterloo.ca/Fde/Materials/dindex.html

Including a couple of A356 files sets.
maybe more in future if I have time.