Strong is not necessary better. High strength bolts (usually ones of 150,000 psi or greater, including "grade 8" bolts and most NAS bolts) are subject to hydrogen embrittlement and stress corrosion cracking.

Both of these problems attack the fastener at the grain boundaries of the metal and initiate a crack. To avoid this type of failure you can:

• Avoid use in applications subject to periodic condensation or weather.
• Use a lower clamp load (torque) as lower stress lowers the likelihood of either type of failure. Or better yet, if you don't need the clamp load then why use such a high strength bolt.
• Avoid plating as this diffuses atomic hydrogen into the steel.
• Protect the fastener from corrosion, wetting, and hydrogen.
• Avoid cadmium, zinc, or cadmium plating.

Good practices to avoid conditions that might promote hydrogen embrittlement and stress corrosion cracking should be implemented any time you are using high-strength fasteners.


To quote from the Navy Ships Technical Manual, Chapter 075, Fasteners, Page 75-60:

"Sacrificial metal coatings (cadmium, zinc, metallic-ceramic of high strength steel fasteners (over 150 ksi tensile strength) for corrosion protection increases their susceptibility to hydrogen embrittlement and the potential for failure...Therefore, zinc or aluminum coated fasteners of a tensile strength greater than 150,000 psi shall not be used in applications in the weather, or where subjected to periodic wetting or heavy condensation. Cadmium plated fasteners of this strength shall not be used in the weather or subject to periodic wetting."

An interesting example of hydrogen embrittlement is the fatal accident of a Bell 206 helicopter in British Columbia in June of 2000. The screws in  the fuel control unit broke due to hydrogen embrittlement. The repair facility replaced the screws during overhaul with standard AN503 screws. Ordinarily hydrogen embrittlement is not a problem with these screws because the rated tensile strength is 125,000 psi, well under the 145,000 psi where hydrogen embrittlement becomes a problem.

However, the screws tested much stronger than they should have been due to improper heat treatment. The cadmium plating applied to the screws then introduced hydrogen into the steel.  If the screws had been  manufactured to the proper tensile stress, they would not have failed and the fatal accident would not have occured. This accident was caused by screws that were stronger than they should have been.  In fact, the entire lot of screws were non-conforming.

ref. Transportation Safety Board of Canada, Aviation Investigative Report, AW00W0105

This 160,000 psi aircraft NAS bolt can fail at much lower stress if damaged by corrosion and hydrogen embrittlement.