Stress Corrosion Cracking - Design Allowables 3

[HANDCALC]

Forgive the length of this post,

Stress Corrosion Cracking (SCC) is a type of material failure that results from the combined action of environmental conditions and sustained surface tensile stresses. Certain aluminum and steel alloys are prone to SCC failure, which can occur at much lower stresses than would reasonably be expected. I am aware that when dealing with metallic (aircraft) materials, careful selection of alloys, heat-treatment, grain direction and protective finishes are the standard design measures to avoid SCC failures. I have also reviewed the following guidance material:

DEF-STAN 00-970 Part 1 Section 4, Leaflet 7, “Protection of Structure, Stress Corrosion Cracking”
MSFC-SPEC-522B “Design Criteria for Controlling Stress Corrosion Cracking”
MSFC-STD-3029 “Guidelines for the Selection of Metallic Materials for Stress Corrosion Cracking Resistance in Sodium Chloride Environments”
NASA PD-ED-1227 “Controlling Stress Corrosion Cracking in Aerospace Applications”
NASA-STD-6004 (DRAFT) “Selection of Metallic Materials for Stress Corrosion Cracking Resistance in Sodium Chloride Environments”
NASA SP-8082 “Stress Corrosion Cracking in Metals”
SAE ARP 823 “Minimizing Stress Corrosion Cracking in Wrought Heat-treatable Aluminum Alloy Products”
SAE ARP 982 “Minimizing Stress Corrosion Cracking in Wrought Titanium Alloy Products”
SAE ARP 1110 “Minimizing Stress Corrosion Cracking in wrought Forms of Steels and Corrosion Resistant Steels and Alloys”

MMPDS-01 doesn’t always list design allowables in the short-traverse (ST) direction for all alloys and thicknesses, for example 2024-T351 per Table 3.2.3.0 (e1). I recall reading (somewhere) that for 2024-T351, design stresses in the ST direction should be kept below 7 to 10 ksi. Most aircraft companies have design manuals, which unfortunately I do not have access to at my place of employment.

My specific question is do others in this forum have access to specific design allowables, like “stress corrosion threshold stresses” for the different aluminum (and steel) alloys where sustained tensile stresses are unlikely to result in SCC failure?

 

[der8110]

Your research is very thorough, I had the same issue 10+ years ago and you show some new references. The problem is that SCC also depends on the concentration of the corrosive solution. It also takes time, lots of time, for SCC to show up and is therefore an unnatractive topic for university research projects. Similar for industry.

I doubt you are going to find anything handy like MMPDS allowables or s-N curves simply because there are additional variables. And the effects of concentration as mentioned.

Another factor in preventing SCC is to remove the water which couples the metal and corrosive salt.

In all practicality, SCC is best controlled by design details and protective treatments. Reducing stresses has the associated weight penalty.

Are you stuck with 2024-T351 (or was that just an example?), or could you use CRES or Ti?

 

[TVP]

I can't remember whether or not the ASMH or Damage Tolerant Design Handbook has data for design against SCC. Anybody know for sure?

https://www.continuinged.purdue.edu/media/mateng/#aero

 

[HANDCALC]

Thanks der8110,

My position requires me to review (and internally approve) structural design modifications to our aircraft, which operate under experimental flight permits. We are a small organization with limited aircraft design resources, but having worked in the aerospace industry for 20 years, I've learned to pay attention to details. Hence the question about SCC.

Yes, we have an application where 2024-T351 has already been specified for some hogged-out components (in work), and where it will see moderate loading in the ST direction. I realize that supplemental heat-treatment to a different end-temper condition is one possibility, as is shot-peening the surface to induce a compressive residual zone. Boeing used to have excellent design manuals, but I was hoping that one of the forum members could point me to a more comprehensive public-domain document with specific SCC design allowables, preferably in tabular form.

Thanks TVP,

If my memory serves me correctly, the Damage Tolerant Design handbook may have KIscc values, not exactly what I am looking for...

 

[rb1957]

i think you'll be hard pressed to find real, usable numbers. prevention for scc is mainly by (as der8110 posts) surface treatment (water displacing compounds). i'd include a saturation shot peen after machining, to colose up the exposed grain structure. as an experimental flight test plane, could you add an annual inspection of the detail?

 

[HANDCALC]

Thanks rb1957,

It's a good idea for me to go back and read my copy of "Shot Peening Applications" by the Metal Improvements Company, and review what the proper process specification would be for Alumen intensity, coverage, etc.

For our application, you're absolutely correct that we can imposed inspection requirements, and have our Maintenance department include these in the associated Instructions for Continuing Airworthiness.

If I keep looking, one day I may find what I'm looking for.

Thanks to all...

 

[crackman]

Handcalc

Only real reference I know of off hand which includes "Max Allowable Sustained Tensile Stresses" for a several 2000 and 7000 series aluminums (as well as steels) in plate, bar, extrusion and forging forms is the old Lockheed book published back in the 1970's by T. Lunde entitled "Fatigue and Stress Corrosion Guidelines" which I thought was copyrighted but not restrited and I think it was published for general consumption back then. Tough to find copies these days though. Anyways, I remember 2024-T3/-T4 had allowables published in it for L, LT and ST in all product forms including plate, rod, bar, extrusion and forging. The low end I recall being 5 ksi and the high being 15 for rod and bar. Anyways, this probably doesnt help much other than adding one more reference to your list. However, these days all SCC effects are evaluated using KISCC and not using the old max allow tensile stress method.

 

[HANDCALC]

Crackman,

Thanks for the reference to "Fatigue and Stress Corrosion Guidelines" since I actually have access to this material. The document was prepared by Tjerand Lunde, and the one I've seen is copyrighted 1976, by the Lockheed-California Company. I'll check this out later today.

You earned a well deserved ***star*** for pointing me in the right direction...

I also appreciate the valuable suggestions provided by der8110, rb1957 and TVP...

 

[prost]

A common mistake when dealing with corrosion and fatigue in metallics is to confuse some of the terms. Based on what you have written, it doesn't appear as if you are, but you never know! Is this a true SSC, in which due to constant stress (either applied through a gravity induced load or residual due to manufacturing) on a structure which is in a degrading environment, or is this a cycling structure, with residual surface stresses, that is being damaged by corrosion, say pitting, exfoliation, etc.? I have seen some very intelligent people, who don't know much about corrosion/fatigue it turned out, that see the phrase "stress corrosion cracking" or "sustained stress corrosion cracking" and assume they can use the SSC curves to predict structural lives for structures in corrosive environments.

Just making sure...

 

[HANDCALC]

Prost,

The components are part of a pressurized equipment pod, carried underneath the wing of one of our test aircraft. The environment the pods could be exposed to would include maritime (salt-laden) atmosphere and aircraft engine emissions.

My question about SCC was not to predict the structural life of the components using a fatigue or damage tolerance analysis, but to ensure that the component was statically stressed as conservatively as possible. Weight penalties are negligible. Initially I was given a stress substantiation that compared Von-Mises stresses against L and LT allowables, for an alloy (2024-T351) that exhibits poor SCC properties in the ST direction. This makes it difficult for me to sleep at night!

CRACKMAN nailed the reference to Lockheed's "Fatigue and Stress Corrosion Guidelines", Table C2.a(2) with 2024-T3 (ST - 5 ksi) and I found another reference in some old Boeing course notes showing 2024-T3 (ST - 7 ksi). Our design staff repeated their analysis and demonstrated that the principal stress (in the ST direction) could be kept below about 5 ksi with some minor refinements.

Whew!

Thanks to all, and to you PROST for the additional input...

 

[Sparweb]

There is a chart in Niu's Airframe Structural Design (the green book) with sustained stress allowables, including the ST direction.
Page 116.

Steven Fahey, CET

 

[WKTaylor]

handcalc...

MIL-HDBK-5B Table 3.1.2.3.1 contains the data You are looking for. REPEAT... MIL-HDBK-5B >>1973<< [not C, D, J or MMPDS, etc...].

NOTE: as far as the data is concerned, -T3, T351, etc are all lumped together.

It is available from IHS.

Regards, Wil Taylor

 

[HANDCALC]

SparWeb and wktaylor,

My thanks, and a **STAR** to both of you for adding Mike Niu's "Airframe Structural Design", Figure 4.7.7, page 116 (Ref 4.1 Mil-HDBK-5D) and MIL-HDBK-5B.