Actually, it is very likely that cracks form at right angles because mathematically, that is the most efficient way of intercepting the stress raiser in an advancing crack. (at least by Dugdale, Griffith, and Rice’s crack theories.)
Whenever a homogeneous and brittle material cracks, there is a zone of increased stress preceding the tip of the crack. (That’s consistent with why things crack continually in one direction along a shear plane.) When the tip of an advancing crack intercepts another crack at less than 90 degrees, there is some tendency for the crack to continue to travel. When it hits a 90 degree intercepting crack, there is a chance the advancing crack will halt and no longer present a dynamic cracking mechanism.
The other way is for the crack to intercept a surface such as a hole or circular crack. That is one reason Navy sailors drill holes in steel and ram cones into them to stop a sheet metal crack in a bulkhead.
Whenever a homogeneous and brittle material cracks, there is a zone of increased stress preceding the tip of the crack. (That’s consistent with why things crack continually in one direction along a shear plane.) When the tip of an advancing crack intercepts another crack at less than 90 degrees, there is some tendency for the crack to continue to travel. When it hits a 90 degree intercepting crack, there is a chance the advancing crack will halt and no longer present a dynamic cracking mechanism.
The other way is for the crack to intercept a surface such as a hole or circular crack. That is one reason Navy sailors drill holes in steel and ram cones into them to stop a sheet metal crack in a bulkhead.
Thanks, Cvengr for the explanation and your source. Appreciate you sharing your expertise. Best wishes.