Monday, October 26, 2009

Inspecting High Strength Materials

The Aardvark Syndrom - built strong but easy to break

Drop a glass onto the floor and it shatters, drop a block of wood onto the floor and it doesn't - yet both have approximately equal tensile strengths. A high tensile strength steel bolt might be twice as strong as a mild steel bolt yet it fractures in two whereas the mild steel bolt bends but still holds the structure together.

We learn to handle glass objects differently than wooden or metal objects because "they break easily"and "are brittle." For the same reason if we use brittle materials in structures we need to 'be careful" and provide additional protection to avoid breakage. A high tensile strength steel bolt might be 3 times stronger than a mild steel bolt but it takes 10 to 100 times less energy to break!1.

A mild steel bolt can handle small nicks, or a little bit of corrosion pitting not because it is strong but because it is hard to fracture (crack). Even if it does crack, the crack growth is so slow that we use various NDT methods to detect cracks and replace the bolt before it breaks.
Not our high-tensile steel bolt. The energy required to break it is 10 to 100 times less. A bit of corrosion that creates a pit that concentrates stress might be all it takes to start a crack. Because it doesn't take much energy to grow the crack, the part may fracture as fast as a broken glass.
There are various methods of using "high-strength-low fracture energy materials, such as better envirnomental protection, non-critical applications, redundent load paths, crack arresting structures. But using a high tensile steel bolt ("Grade 8") as a single point of attachment on a trailer hitch that is bathed in road salt and submerged in lakes where corrosion occurs hidden under the head or shank is not one of them. A better idea would be to match strength with fracture energy!

More inspections are required for high-strength low fracture energy materials:
  • More frequent inspections for corrosion.
  • Protection from scratches and marks.
  • Protected tooling that won't mar the surface.
  • More frequent application of corrosion inhibitors.
  • More adequate and detailed inspection and rejection instructions.
  • Increased education of mechanics and their employeer on why this is so.

For procurement, better quality audits of the manufacturing process, as these parts require more precise materials , process, and heat-treat.

Note 1. Approximate work of fracture J/(m squared) for mild steel is 100,000 to 1,100,000. For high tensile strength steel it is approximately 10,000.

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