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Crack Propogation



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Crack Propogation

Initiation and Propogation of Cracks in Carbide Material 

 From the book Building Superior Brazed Tools  Buy the Book 

 Tungsten carbide and wood both have a grain structure. 

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Wood’s structure looks something like this.   It is individual cells arranged in tubes.   How well wood splits depends on where you hit it. 

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If you try to cut wood against the grain it is a lot of work.  When you cut wood down the grain it is much easier to separate it in smaller pieces than it is cutting across the grain.  

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Crack initiation

Deals with the fact that if you hit something hard enough you can put a crack in it.   If you hit anything hard enough you will make an impression in it.  

Crack propagation 

Crack propagation deals with how far the crack runs.   In many ways this is much more important.  Quite often a crack in carbide runs through the carbide and destroys it. 

Preventing crack propagation

In traditional carbide grades they made the carbide tougher by making it with a much higher percentage of Cobalt which, as a metal, is softer than carbide grains. You can also make carbide tougher by using larger grains or even mixing larger and smaller grains. The piece of clear, straight grained wood on the left split much further with a single blow than the knotty piece on the right. 

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A cutting tool tip has forces on it from many directions.  Even a saw tip that theoretically cuts straight still has a great deal of stress on in from the sides as the density of the wood changes and the blade wanders. 

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If you take a log and make plywood out of it then you get a much tougher piece of material.  With repeated impacts you can split a piece of wood along the grain.  When you hit piece of plywood you can initiate a crack but it doesn’t go anywhere.  You have to keep cutting and cutting in the same place until you gnaw your way through it.

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Ordinary carbide has tight grains like wood. You can make carbide tougher with multi grains like knotty wood. We make carbide really tough with a sophisticated grain structure and we use a special metalloid binder like the glue in plywood to get truly phenomenal strength.

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In ordinary carbide it is pretty easy for fault lines to pass straight through 

We add a definite structure much  like rebars in concrete

Then we tie the whole thing together so it acts like a singe piece rather than many pieces glued together