The F111 swept wing fighter was made with the most exotic ultra high-strength (UHS) D6ac steel available at the time. D6ac is heat treated to 220 to 240 ksi strength range. As a comparison, NAS high strength fasteners have a strength of 160 ksi. During ground testing of prototype aircraft the D6ac steel broke well below its rated strength. New aircraft experienced several in-flight break-ups with catastrophic results. In each case the steel was of adequate strength and the loads were well below D6ac's rated strength. The F111 failures (and the Titanic and C5a Galaxy) created several troubling questions when using high-strength materials:
How can a part rated and tested to withstand a force of 220-240 ksi break at a much lower force?
Why will high strength fasteners break at loads below their rated strength?
What other properties need to be considered besides strength?
What environmental conditions need to be considered before we use high strength steel?
How important is it to control and protect the surfaces of high strength steel?
There is a fundamental difference between strength and toughness. Strength gets all the attention, but toughness can be just as important. So what is the difference? Strength is the force required to break. Toughness is the ability to absorb energy without breaking. To better understand the difference we will look at why the D6ac steel in the F111 was strong but not tough.
Quality Control needs to test strength and toughness in heat treated parts
Testing on sections of D6ac steel from the F111 showed that although the strength was within specifications, the toughness varied by a factor of three times. The reason for this was in the way the steel was heat treated. Steel of low toughness was used in the F111 because quality control did not test for toughness until several catastrophic failures.
The steel used in the Titanic hull had adequate strength but low toughness in cold temperatures. When the Titanic hit the iceberg, instead of the steel causing small cracks and bending, a crack grew very fast and very large. As an analogy, a glass bottle will shatter when hit with a hammer whereas a bottle will absorb the energy and deform but not break. The glass is harder and stronger but the plastic is tougher. In this example, the plastic has a greater fracture toughness.
A sword is a good example of combining strength and toughness. The back of the sword is made tough so that it can withstand repeated blows without breaking. The cutting surface is made strong and hard to retain sharpness in order to cut and penetrate. Saw blades are also made with hard, strong cutting faces and softer, tougher backs to prevent breakage.
Importance of Surface Protection from nicks and scratches
In the F111 the D6ac steel was strong but not tough. Early F111 aircraft had an operational fatigue life of a few hundred hours. Because the steel was not tough, any small scratches or machine marks caused cracks. Extremely small cracks would quickly grow to complete fractures. As a result, great care was used during manufacturing to prevent and inspect for any machining marks in critical areas. Critical parts were inspected and any machining marks were polished out. Maintenance person all were trained to take great care to not scratch, nick critical parts.
Corrosion Protection is critical to prevent failure
Later, another problem developed in the F111 that caused more failures. Small corrosion pits quickly grew into cracks and then fractures. Stress corrosion cracking (SCC) quickly resulted in failure because of the low fracture toughness. Corrosion pits are similar to small scratches and nicks in that they concentrate stress into a very small area. The difference is that during corrosion, hydrogen is forced into the metal and reduces its strength. The crack then grows into this area of weakness and the process is repeated. This action is like cancer - no amount of corrosion is acceptable as it grows. Normally, when we see a small amount of corrosion we stop it and clean it up and don't worry about it as the the metal thickness has not been reduced by very much so it should be just as strong as before. This doesn't work with high-strength steel that lacks toughness. Small, microscopic pits created by the corrosion create cracks. Because of the metal's lack of toughness the cracks quickly grow. Cracking in the C-5 Galaxy is a good example of this process.
Flight hours cannot be used to establish inspection intervals
The C-5 Galaxy upper fuselage lobe skin is made from 7079-T6 aluminum. It was discovered that small corrosion pits caused SCC that, in one instance grew to a length of 4 inches, while the aircraft were parked. Crack growth was independent of flight hours and stress! Stress Corrosion Cracking occurs as a result of corrosion and material properties whether the aircraft is flying or parked.
Emphasis on strength-to-weigh ratio created F111 failures
D6ac steel is little used today. The aluminum alloy used in the F111 had similar low toughness problems and is not used and has even been removed from most material references.
High Strength in the lab does not mean high strength in use
Often I have been asked what is stronger; a Grade 8 bolt or a common AN aircraft bolt. The Grade 8 is stronger; but the Grade 8 may break sooner than the AN bolt.
Summary for Mechanics
Be careful not to scratch or nick high-strength parts. Do not clean high-strength parts with wire brushes or other tools that may scratch the surface.
Corrosion protection is critical
Pitting is cause for rejection
NDT inspections need to be more sensitive to detect cracks early.
Inspection intervals are calendar based.
Undocumented and untraceable high-strength fasteners should viewed with suspicion.
