S-Basis in MMPDS-03 1

[feajob]

Hi,

I would like to understand something about S-Basis in MMPDS-03 (because, my knowledge is limited in M & P engineering field).

In order to be complaint with the FAR regulation (item 23.613), one economical method is to select materials in MMPDS that contain both A- & B- Basis allowables that are recognized in the industry. Unfortunately, I need to use Aluminum 7175-T74 (AMS 4149) with 3"-4" thickness from Table 3.7.11.0(b). The design mechanical properties are provided only for S-Basis.

I understood that S-Basis design values do not have reliable statistical significance. But, on page 9-10 of MMPDS-03, it is written that "Traditionally, the statistical assurance of S-Basis values has not been known. However, the statistical assurance with S-Basis values established since 1975 is known within the limitations of the qualification sample and the analysis method used to evaluate the data. Within those constraints S-Basis values established since 1975 may be viewed as estimated A-Basis values."

How can I make sure the S-Basis values for this specific Alloy is established before or after 1975?

I appreciate to know your opinion.

Thank you,
A.A.Y.

 

[rb1957]

are you going to test the part ?

if yes, then i'd use S-basis.

if no, i'd use the data available, and either aim for a high MS, or include a "fitting factor". The true A-basis is almost certainly going to be very close to the published S-basis, maybe off by a few psi.

in any case, i expect will (taylor) will have an opinion ! probably contray

also 7175 is a pretty recent alloy, quite probably post 1975.

 

[ESPcomposites]

As you mentioned, I believe the FAR requires the use of A or B basis.

I have run into the awkward situation where only S basis was available and we could simply could not use the data. Making matters worse, the part had a very high static margin (fatigue critical), but there was no way to show acceptance.

So while I don't think the data would be that much different, I don't know that you can get FAA approval. I would be interested to hear otherwise.

Brian

http://www.espcomposites.com

 

[Sparweb]

When purchasing the material, you should receive a test report with the traceability data. This should confirm that:
a) composition of the ordered material is identical to that tested in MMPDS
b) heat treatment is the same as the MMPDS samples.
c) FTU+FTY are greater or equal to the S-basis value in MMPDS

Some of these test reports give additional information such as sampling rate. I'm not suggesting you carry out your own chapter 9 analysis - it's just good background about the mill's production.

Other sources of information available to you are hardness tests, NDI inspections of incoming materials that shows that the raw materials are consistent.

When the static margin of safety is very high, load testing 100% of the parts and the drawing-to-part conformity inspection reduces uncertainty to zero.

If the ultimate load test would destroy the part then you could do your own coupon tests. Again, not so much to do all the statistics, just to validate the data you are given from the mill.

None of these options are as easy or as cheap as just having an A-basis value to go on. Often you find materials that have an A-basis FTU given in the L and LT directions, but don't in the ST direction. When complicated parts with complex loads are analyzed and stresses are found across the ST plane, one can get stuck in this situation, which is similar to the one you are in.

Looking back at your posting, I see FAR 23 being the reference, not FAR 25. This prompts me to check the exact requirement to which you want to comply. Paragraph *.613 is very different between the two. It will also vary with the age of the aircraft. The regulation has changed over the years. You ought to be using the same basis as it was at original certification. There are reasons why one is compelled to use the latest regulation, at times, but often the orgininal basis is good enough.

Steven Fahey, CET

 

[feajob]

Hi,

Thank you for your responses. I am agree with you (rb1957) and I also believe that the true A-basis mechanical properties should be very close to the published S-basis. But as Brian mentioned that, we may face real challenges to obtain FAA approval.

Steven, you are right we receive a test report when purchasing the material and we perform the Ultimate strength test and hopefully our parts should withstand the Ultimate load conditions. But, I don't know the answer of this question:

Q1) For the purpose of certification, if we pass successfully the Ultimate load tests with S-basis alloy then can authority still ask for A-basis alloy in our stress analysis report?

FYI, we are dealing with FAR 23, because it is related to a VLJ (MTOW is less than 5,700kg and above 750 kg). You may read more details with regard to FAR 23 versus FAR 25 in the following link:

http://www.pprune.org/tech-log/136001-far23-vs-far-25-aircraft-5700kgs.html

My second question is related to this sentence in FAR 23.613 "Within those constraints S-Basis values established since 1975 may be viewed as estimated A-Basis values."

Q2) They wrote that "may be viewed", I think that this may is used to indicate possibility or probability of considering S-Basis as A-Basis for any alloy fabricated after 1975. Now, how can we take advantage of this possibility?

As you probably noticed, English is not my first language. I believe that the way FAR items are written, we need to be a lawyer with perfect English skill.

Thanks,
A.A.Y.

 

[rb1957]

it sounds like you're involved in an initial design, so that the plane will be completely tested, and this is the basis of certification. "if we pass successfully the Ultimate load tests" then you're good-to-go.

your analysis is (mainly) for risk reduction and to support MRB activities.

you could ask for batch testing, but the supplier will probably not like this.

your alloy (7175) is also probably post 1975, so S-basis is GTG.

clear as mud ?

btw, having perfect english is a handicap when reading the FARs.

 

[WKTaylor]

feajob... another approach to consider...

S basis [specification basis] values indicate that the users will have to validate all allowables for the material in a specific application, a technique often called "point design".

7175-T74 Die Forgings [[AMS4149, MMPDS-03 Table 3.7.11.0(b). Design Mechanical and Physical Properties of 7175 Aluminum Alloy Die Forging]] are giving way to 7085-T7452 DFs [[AMS4403, MMPDS-03 Table 3.7.8.0(c). Design allowables for 7085-T7452 Aluminum Alloy Die Forgings]] for a host of reasons... not the least of which are exceptional mechanical, SCC and EXCO properties plus excellent toughness & durability [high KIc and dA/dN] up to 12-inches section thickness at HT. See attachment for a taste of what is Alcoa is doing with 7085-T7452 DFs: some huge forgings!!!

The 7085-T7452 [AMS4403] A & B-basis allowables have been generated for 1.0--12.0 section thickness. Pretty impressive... but then the A-380 and F-35 use massive numbers of these forgings in huge sizes/thicknesses.

But...Yeah... it's ALCOA proprietary material.

NOTE. MMPDS-04 added A & B basis allowables for 7085-T7452 AMS 4414 hand forgings [forged block, etc] 2-to-12.0-inch section thickness. This material has allowables, SCC, EXCO and durability properties ALMOST identical to the DF material... but allow latitude for short runs of 100% machined parts without the cost and lead-times of a DF lot order. Conventional shot-peening or laser shock-peening of finished parts helps retard crack initiation. Etc... etc.

If this were my decision, I would consider changing to 7085-T7453 DF or FB option, with mandatory ultrasonic inspection of the stock [class A], and mandate that the DF or FB material certification [tests] meet/exceed "B-basis" allowables in MMPDS-04 [table 3.7.8.0.(c) or (d)].

Regards, Wil Taylor

 

[rb1957]

will,

what's the price difference ? in any case, i imagine that a VLJ is pretty much limited by manufacturing minimums rather than material strength, so a "better" material is not necessarily justified.

 

[WKTaylor]

rb1957...

Often a "better material" runs-rough-shod over "good enough" material because production quantities-of, and experience with, the "better material" sky-rockets... while the "good enough" material stagnates in both production and usage.

A classic example is 4340 steel. Most is NOW made per the higher quality standards of 4340M [+ Vacuum-Arc Remelt, etc]. If a lesser-grade [air melt] steel is required, most vendors just "cross-qualify the 'superior material' backwards" to meet/exceed the lower grade air-melt steel spec. Otherwise, the inferior air-melt steel would be virtually impossible to obtain except in special production runs... probably at equivalent, or higher cost, relative to the superior "M" VAR steel.

Never forget that obsolescence is a problem with materials that can present massive substitution challenges in the future.

Regards, Wil Taylor

 

[lbell]

I have to weight in here to say do NOT use any of these new forging materials without running your own tests to verify the material properties with actual parts. Size, shape, and many other factors will contribute to how strong and durable the actual parts will be. The real fatigue properites of these forgings will have to be determined over time.

 

[WKTaylor]

IBell...

Concur...

Any time critical monolithic parts are part fabricated, the entire process from billet stock to packaging and shipping of the finished part must be verified inspected and tested; and then all aspects/processes be "written in stone".

In the case of parts machined from die or hand forgings, the raw stock chemistry and metallurgy, the initial forgings and the initial machined parts should undergo full dimensional, metallurgical and destructive testing [static and fatigue]. Subsequent production lots should under-go the same rigorous inspections destructive testing on >randomly< selected pieces... and full NDI on all 100% of the remaining pieces.

OH... and by-the-way... never forget to permanently serialize each billet, each forging and each finish machined part. That was a hard lesson one of my vendors learned when documents revealed that a second HT process was not performed on "2-parts": played hell trying to establish which of the fully finished parts were the "2-parts". Duhhhhhhh.

Regards, Wil Taylor