Interpretation of Material Stress-Strain curves from MMPDS

[Burner2k]

I've been having trouble interpreting material stress-strain curves provided in MMPDS.

I'm posting a curve of 2024-T3 sheet from MMPDS

If the tabular properties from Table 3.2.3.0 (b1) Pg 3-71, ref MMPDS-01 is to be believed, for a bare Al 2024-T3 sheet thickness <0.25", the Ftu is around 65ksi. But in the above figure, the curve for L-Tension plateau's around 52-54 ksi.

Figure 3.2.3.1.6(u) seems like represents a stress-strain curve which matches the tabular Ftu values, but the curve is for clad sheet and I could not find a similar curve for bare in MMPDS.

Would appreciate help in getting a better understanding.

 

[testing]

Given the scales on the x-axis, those look like they're fairly consistent with each other. The first plot does not appear to be going to failure, so I wouldn't expect Ftu to show up on there. Instead, the yield stresses of 47 ksi (L tension) and 42 ksi (LT tension) are consistent with the table (as are the 39 ksi (L) and 45 ksi (LT) in compression). The second plot shows failure at a strain that is almost 16x higher than the x-axis maximum of the first plot.

 

[ESPcomposites]

Right. The first curve is used to closely observe what is happening in the elastic and yield strength regions (and not near Ftu). The second curve is used to observe the total plastic response and Ftu.

Brian

http://www.espcomposites.com

 

[rb1957]

wrong ... at 10% strain, the lower graph has 60ksi, the upper <54ksi.

and I don't think thickness explains it ... table properties are very consistent ...
T351 clad gets to 55ksi at about 3" thick,

personally I'd use the clad curve for unclad material (conservative)

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

 

[ESPcomposites]

You may want to reread the previous posts. The upper curve is simply not appropriate for Ftu because it doesn't even get close to the strain to fracture. That curve is used to examine the initial yielding response and Fty (and perhaps elastic modulus). The bottom curve can not do that very well because the strain range is too large. Each curve has a different intended purpose.

 

[testing]

@rb1957: I agree that the lower graph is 60 ksi at 10% strain. The top graph only goes to 1% strain though. The bottom graph is roughly at 52 ksi at 1% strain which matches reasonably with the top curve. As ESPcomposites pointed out, the top curve doesn't give you enough information to determine Ftu while the bottom does.

 

[rb1957]

oops

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

 

[Burner2k]

Thanks guys for the clarification.

Somehow I did miss the bottom scale 'Strain' resolution between the two curves.

Is there any reliable & acceptable open source (or proprietary as well) for material mechanical properties information other than MMPDS?

 

[tbuelna]

Depends on what you plan to use the data for. MMPDS properties are accepted by most of the aerospace industry for analysis, but you may find them to be more conservative than you would like. Many aerospace OEMs also develop their own proprietary material property standards that may differ from MMPDS. These proprietary OEM material properties are not usually made available to the public.

 

[jotunn]

The other thing to remember is that the tables usually publish A, B, or S basis values, as identified within the tables. I have always interpreted the plots, labeled as "typical", as average/nominal/not statistically reduced, stress-strain curves.

 

[GrandpaDave]

ANC-5 then MIL-HDBK-5 and today MMPDS are and were the stress engineers bible for lots of stuff.
E. Bruhn covered ANC-5 in his text books. Now, I was in Aeronautics at Purdue years ago and Bruhn was head of the Aeronautics school.
Now the guys across the street in Civil and Mechanical engineering schools might have used other sources for material properties.
But, even in ANC-5, etc. the properties are referenced to specific specification (QQ-As, MIL- etc...) at least back in the olden days.
I always found it interesting to trace the origin and history of all this "stuff".
G-pa