Reduction in Fbru for 7075-T6 Sheet as of MMPDS-05

[SuperStress]

In MMPDS-04 (and previous) the allowable for 7075-T6 bare sheet per QQ-A-250/12 is Fbru=156ksi (0.040"<t<0.125", e/D=2.0, A-basis, Table 3.7.7.0(b1)).

In MMPDS-05 (and on) the allowable is Fbru=146ksi (same parameters, Table 3.7.8.0(b1)). The table says that it was last revised in Apr 2010 per MMPDS-05 Item 09-29.

We are looking at a repair to an older airframe that was designed/substantiated around the MIL-HDBK-5B days, and the margins are low enough that simply replacing the vintage material with new would indicate a negative margin.

I'm trying to find out the reason for this change. My initial impression was that it might be a "wet pin"/"dry pin" reduction, but I can't find anything to back that up, and the reduction seems a bit small for that anyway. I know that there have been reductions on thicker plate materials identified in the tables at the beginning of Ch. 3 (Bearing Property Reductions for Thick Plate of 2000 and 7000 Series Alloys), but I've not seen anything similar for thin sheet.

Does anyone know the history behind this reduction in allowables?

SuperStress

 

[Aerodesign]

Hi Superstress,

I'm afraid l don't know the reason for the reduction, l suspect it may be due to a reappraisal of the test data submitted which derived the MIL-HDBK-5xx values.
Look at the presentation here:

https://www.mmpds.org/wp-content/uploads/2015/03/mmpds_2015_2007aeromat_presentation.pdf

which may be useful.

I can contribute to the dry/wet pin allowance which in my organisation requires a KDF of 0.85 to be applied. Note d in Table 3.7.9.0(b1) MMPDS-08 states dry-pin values for [all] inc 146ksi (e/D=2.0) A-Basis.

When l was Chief Stress at Pilatus it was clear that legacy data vs later data 'deltas' could cause some problems (being an organisation that dealt with repairs, mods and new aircraft). The way around it now is to establish a guidelines document per project (mandatory adherence) which specifies values directly or gives references to MMPDS or other published data.

This doesn't help you much - it strikes me that if you are that close to the limit a rethink on the repair methodology may be needed.

l hope you resolve the problem.

Aerodesign

 

[SuperStress]

Thanks Aerodesign.

I contacted Battelle and your suspicion was spot-on. The data were examined to see if the legacy (1950s) data reduction methods and production lot release data met current standards, and they did not. The updated allowables reflect the change to current methods. The MMPDS community is aware of the issues that this is creating and they are working to find acceptable solutions.

SuperStress

 

[Sparweb]

In case you're in the mood to frighten the people in your purchasing dep't...

Some exotic alloys from MMPDS-06 (sorry, I don't have any more recent):

Table 3.7.5.0(f) Design Mechanical and Physical Properties of 7055-T762 Aluminum Alloy Sheet
AMS 4267
Fbru = 155 ksi (L direction, e/D=2)
Fbru = 158 ksi (LT direction, e/D=2)

Table 3.7.17.0(b). Design Mechanical and Physical Properties of 7475-T61 Aluminum Alloy Sheet and Plate
AMS 4084
Fbru = 154 ksi (e/D=2)


STF

 

[SuperStress]

Thanks SparWeb.... lol. I suspect the chances of getting the Air Force to order anything that isn't currently in the depot would be slim, but I'll pass those suggestions on to the responsible engineer.

SuperStress