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Case Study #25 — Fastener Failure Workshop

Wood Processing
 

Industry:

Wood Processing
Flake Board Manufacturing

Problem:

The fasteners holding knives on a log flaker were breaking. When the fasteners were broken, the sharp knives were then thrown through the air.

Injuries:

None.

Damages:

One of the knives released with such force that it was thrown through a wall made of concrete block. Fortunately no one was in its path. Naturally, when a blade comes loose, this stops production and subsequent income, but more importantly this created an extremely dangerous situation.

Observations:

The fasteners breaking were 5/8" - 11 x 6-1/2" (165mm) socket cap screws. All failures were at the head-to-body juncture. Three screws were used on each knife to secure them to the carrier arm disk. There are a total of 24 knife pockets with each holding 2 knives 18-1/2" (470mm) long. They are mounted end-to-end to make a cutting width of 37" (940mm). These machines cut tree lengths or shorter logs into flakes for the production of flakeboard.

Investigation:

We learned that the carrier disk revolves at about 200RPM and that each knife weighed 177lbs (80kg). This clearly explained the inertia and force that allowed the knife to penetrate the concrete wall. We also learned that the knife's edges are finished to very sharp conditions, as the normal cutting thickness of the wood flakes is only 0.025" (0.6mm). The screws had a plain finish, and corrosion products "red rust" were very obvious, particularly under the heads. There was also a rather strong odor coming from the screws. Laboratory tests of the fastener's metal showed a proper core hardness and a well-tempered martensitic structure with an ideal amount of slight decarburization. The fasteners also had been carefully installed in a proper manner with calibrated torque wrenches. A lab evaluation with a scanning microscope (SEM) showed that the fracture surface had a rock candy intergranular appearance. An EDAX microprobe scan of that surface showed a high concentration of chlorine with a fair amount of potassium. The chlorine was obviously the source of the odd smell on the parts. The fracture was 360° around the fillet radius area and the cracking initiation was directly attributed to the corrosion products.

Conclusion:

This was a clear case of stress corrosion cracking (SCC). The plain finished products were susceptible to cracking just by virtue of normal rusting. This generates atomic hydrogen and will lead to the cracking. However, the element of chlorine magnifies this opportunity as chlorine develops hydrogen at a very fast rate. The source of the chlorine and potassium is believed to have come from the fertilizers used to spark the tree's growth.

Recommendations:

With a high concentration of chlorine, plating or coatings would prove ineffective. Therefore, the best solution would be to make the screws from Inconel 718.

 

As appeared in Fastener Technology International magazine.

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