Eli5 What is the underlying physical mechanism by which a diamond-tipped tool, when used to mark glass, facilitates a precise and controlled fracture along the intended cutting line?
Eli5 What is the underlying physical mechanism by which a diamond-tipped tool, when used to mark glass, facilitates a precise and controlled fracture along the intended cutting line?
Comments
Eli5:
You cut a small slith in the glass making it the weakest point on the structure, thus paving the way for guaranteeing it will snap at that point.
A defect in the material will decrease its resistance a lot, when you make that small sharp edged incision you make sure that will be the weakest surface by an order of magnitude. Therefore you can apply a force that will break just that part and nothing else.
When you cut into the glass you introduce stress into the glass that goes much deeper than the scored line itself.
When you then apply pressure on the glass it will then concentrate to these stress points, create stress fractures and break along the line.
You can see the effect yourself if you construct a Polariscope (a polariscope is a tool for studying how light refracts inside glass or crystal, so it’s used to, among other things, to see stress inside glass).
P.S: And of course someone has made a youtube to explain it https://www.youtube.com/shorts/6qWt1CK7v2c
Try to rip a piece of construction paper in half.
Now, fold a piece of construction paper back and forth in half. Try to rip this piece of paper in half again along that line.
You’ll notice when we fold the paper that you can see it is damaged a bit. When we drag the hard rock(diamond) against the glass, we are damaging the glass in a similar way. When we pull on the outer edges, the damaged area rips first. By deciding what part to damage first, we can decide what part rips first.
Any sharp change in the thickness of a material creates a point where the material is significantly weaker (called a stress riser). https://en.wikipedia.org/wiki/Stress_concentration This can be true even at a shoulder where the remainder of the part is thinner. This is why parts often have a radius in corners.
When you score a piece of glass (or any other material), even very shallowly, it creates a stress riser along that line. So when force is applied, that’s the weak point, and it will tend to break there. And while the depth of cut isn’t very important, that it be continuous is, since if there’s any point where the glass is at full strength, it can easily deviate from that line.
Note that this same principle is why chipped glass containers are far more likely to fracture, and why metal parts that have a scratch are likely to fail over time.
Stress concentration. Every material has a stress limit but sudden changes in shape increase the stress that the material experiences.
As a fun fact, the first pressurized planes had square windows with sharp corners. This sharp corners were stress concentrators and the planes fuselage fractured, starting at this corners. Now planes have smooth oval windows. This windows also concentrate stress, but not as much.
You know how when you open some plastic bags, they put a little nick in it as a place for you to start ripping? It’s the same idea as that.
Basically, glass, plastic, in simple terms are big networks of atoms all “holding hands” with their neighbors. They provide support for each other, literally energy in their bonds holding them together.
When you introduce a failure point, like a nick in the plastic or a scratch on the surface, you break some of those bonds in a very specific location. This leaves the atoms next door slightly weakened. They have fewer neighbors they are holding hands with.
Then, when a big force comes in to snap the glass or tear the plastic, it’s highly likely that those weakened points in the structure will break first. Then it’s a chain reaction: your tearing breaks more bonds on the weakest parts, leaving the next atoms over weakened, and it continues and continues. Always breaking where it’s weakest