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When certain materials are subjected to the combined effects of sustained tensile stress and corrosive environment, over a period of time, structural failure may occur at a stress lower than the material would normally withstand. This phenomenon is known as stress corrosion cracking, and it may occur in an environment which is not severely corrosive in the normal sense. To avoid failure, tensile stresses must not exceed a threshold value which is a function of the material and the application.
The boss in which a Lee Plug is installed will generally not be susceptible to stress corrosion cracking if a highly resistant boss material is used, and the D/d ratio is 2.5 or greater. The Lee Plug is not subject to stress corrosion since it is in compression throughout. Other values of D/d may be required if the mechanical properties of the boss differ significantly from those of the plug.
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| ALLOY | CONDITION | YIELD STRESS (MIN.) |
% ELONG. (MIN.) |
|---|---|---|---|
| 2024 Alum | T851 | 58 ksi | 5 |
| 6061 Alum (a) | T6 | 35 ksi | 10 |
| 7075 Alum | T73 | 56 ksi | 8 |
| 355, 356 Cast Alum | T6 | 17 ksi | 3 |
| 15-5PH Cres (b) | 1075 | 125 ksi | 13 |
| 300 Series Cres (a) | Cold Drawn | 75 ksi typ. | 35 typ. |
| 4130 Cast Steel | Quenched & Tempered |
85 ksi | 16 |
| 6AI-4V Titanium (a) | Annealed | 120 ksi | 10 |
| Monel Alloys (a) | Cold Drawn | 85 ksi typ. | 20 typ. |
| Inconel Alloy 600 | Annealed | 35 ksi | 30 |
(a) All conditions are highly resistant to SCC.
(b) Condition H1000 and above is highly resistant to SCC.
*The following alloys have low resistance to SCC, and are generally not recommended for Lee plug bosses (partial list):
Alum: 2024-T3 & T4, 7075-T6, Cres: 440c (all cond.)