longtimelurker
Member
So I've been re-working a design to work with 6061-T6 instead of 2024-T3. In my original design, I selected the limit stress for airframe life using the SN diagram for riveted lap joints from Bruhn (Fig C13.14). This same diagram is reproduced in Hiscock's "Design of Light Aircraft" (Fig 15.4). So now I need to find an appropriate SN diagram for 6061-T6.
6061-T6 doesn't appear to be in Boyer's "Atlas of Fatigue Curves", however I found some curves in Kaufman's "Properties of Aluminum Alloys: Fatigue Data and the Effect of Temperature".
The thing is - the curves for notched 6061 (Kt = 3.0, R = +0.1) I found in Kaufman seem pretty much to correspond to the upper band of "riveted lap joints" data as shown in Bruhn;
If I use Kaufman's curves directly - I'm getting 10x the cycle lifetimes I used before with 2024. And I've always thought that 6061 was worse in fatigue than 2024.
So then I went to look for the original source for the C13.14 figure in Bruhn - only to find that the reference is listed as [16. Christensen, R.H., "The Fatigue of Materials and Structures", Collection of Notes for In-Plant Course on Fatigue at Douglas Aircraft Co;, Inc., Santa Monica, Calif. 1954.] Which I don't believe I can locate to see where the original data came from.
So right now I'm using the 6061-T6 curve with a 20 ksi limit (4.4g) design stress and a safety factor of 40 (which still gives me a 2500 hour airframe life). But I can't believe that there's not a better source of fatigue data for 6061-T6 somewhere..
What have other designers referenced when using SN Curves for 6061-T6 ???
6061-T6 doesn't appear to be in Boyer's "Atlas of Fatigue Curves", however I found some curves in Kaufman's "Properties of Aluminum Alloys: Fatigue Data and the Effect of Temperature".
The thing is - the curves for notched 6061 (Kt = 3.0, R = +0.1) I found in Kaufman seem pretty much to correspond to the upper band of "riveted lap joints" data as shown in Bruhn;
If I use Kaufman's curves directly - I'm getting 10x the cycle lifetimes I used before with 2024. And I've always thought that 6061 was worse in fatigue than 2024.
So then I went to look for the original source for the C13.14 figure in Bruhn - only to find that the reference is listed as [16. Christensen, R.H., "The Fatigue of Materials and Structures", Collection of Notes for In-Plant Course on Fatigue at Douglas Aircraft Co;, Inc., Santa Monica, Calif. 1954.] Which I don't believe I can locate to see where the original data came from.
So right now I'm using the 6061-T6 curve with a 20 ksi limit (4.4g) design stress and a safety factor of 40 (which still gives me a 2500 hour airframe life). But I can't believe that there's not a better source of fatigue data for 6061-T6 somewhere..
What have other designers referenced when using SN Curves for 6061-T6 ???
