Sunday, December 16, 2012

Why Things Break - Getting to know Fracture Energy

 Metallic structures break not because they are weak (low tensile strength), but because they aren't tough enough. The "high-strength Grade 8" bolt fails not by forces and stress, but by way of energy--and often at a very low force! As mechanic's repairing structures, strength tells us little about structural integrity. The engineer designs the structure to be strong enough to support the loads- the mechanic makes the structure endure.

Even the smallest scratch or crack can locally intensify stress beyond the materials ultimate strength (Inglis equation for stress intensity). There is no advantage in using a bolt that is stronger than it needs to be to support the load if a scratch or corrosion pit can easily concentrate stress beyond even its ultimate strength -- in fact it is often disadvantageous. Since every structure has cracks and scratches, there is something else besides strength that holds the structure together.  Griffith showed us what happens when the local stress from a stress riser (corrosion pit) exceeds the material's strength; and it has nothing to do with strength but something called fracture energy.

What is easier to break, a piece of glass or a piece of plywood? Drop the glass and it shatters, drop the plywood and it doesn't. Plywood is tough; glass is not. Glass cutters work on glass by making a scratch across the glass surface, why doesn't this work on plywood? Because it takes a lot more fracture energy to break plywood than it does to break glass.
  • Glass work of fracture 1-10 joule per square meter
  • Plywood work of fracture 10,000 joules per square meter
  • Mild Steel work of fracture 100,000 to 1,000,000 joules per square meter
  • High Tensile Steel work of fracture 10,000 joules per square meter
Glass and plywood need to be strong (and stiff) enough to function in a structure (with a large safety factor), but maintaining the structure to assure that it doesn't fail often has little to do with strength and everything to do with controlling energy.

For metallic structures:
  • as strength and hardness increases, energy required to break them decreases,
  • as stress increases, stress corrosion cracking potential increases,
  • as strength increases, hydrogen embrittlement potential increases,
  • as strength increases, the ability to detect cracks before complete failure decreases.

How does this knowledge help the mechanic? In three ways:1
  1. Choose parts that have both adequate strength to support the load and also that require a high energy to break them.
  2. Carefully inspect for scratches cracks, corrosion pits structures that require low energy to break them (high strength steels and aluminum alloys).
  3. As long as that high-strength metallic part remains in service it must be protected from hydrogen embrittlement. The higher the strength, the more protection. Protection is by preventing corrosion and preventing chemical (particular acid) exposure.
Examples of applying our energy knowledge:

A high-tensile strength bolt is used on a trailer hitch (exposed to road salt). Since stress corrosion cracking (SCC) increases with tensile load, we might want to use a lower tightening torque than what standard bolt torque charts show. Tighten the bolt to satisfy the needs of the joint, add in a safety factor, but no tighter.This lowers the tensile stress and reduces the susceptibility to SCC. You might be better served by using a lower strength but higher work of fracture bolt if the requirements of the joint allow for it.

Cleaning the inside of your Chinook Helicopter (or underside of your new Jeep) with a household chemical cleaner exposes high-strength aluminum and steel parts to hydrogen embrittlement. Better to use a cleaner that has been tested for hydrogen embrittlement and approved by a major aircraft manufacturer.2 The army lost on of their Chinook Helicopters and had to decontaminate the rest of the fleet by cleaning the insides of their Chinook Helicopters with Simple Green cleaner.3

Scratches and corrosion is more serious on high-strength metallic parts. Principle structural components carrying heavy loads and made of light-weight aluminum or steel alloys are fragile structures. Protect from scratches and corrosion. Strength in the presence of corrosion is only temporary.

  1. Another consideration often overlooked is that high tensile strength alloys are more difficult to manufacturer. The possibility of quality and process errors increase with strength. What is labeled as "high-strength" might not be. Hydrogen embrittlement becomes increasingly more difficult to control during the manufacturing process.
  2. FAA Advisory-Circular-AC43-205
  3. Extreme Simple-Green is the aircraft version which meets Boeing-specification-D6-17487P and is designed to be safe on aircraft aluminum.

No comments:

Post a Comment