As I would cut fabric, with a pair of scissors, I’d soon get to a point where the fabric just rips apart. This happens with a certain amount of “pressure” on the scissor, a certain point on the scissor blade etc etc.
Is there any physics to the conditions necessary for the fabric to start ripping rather than getting cut by shear forces from the blade ?
The essential physics in cutting is just the inclined plane (in the case of scissors, a compound intersection of planes); essentially, at some acute angle, the force needed to sever fibers of the textile by pushing the scissors forward is lower than the effort to close the scissors, so you can just zip the scissors forward in a straight cut.
The actual mechanics of cutting flexible structures like textiles is, on the other hand, really complex, involving a bunch of different mechanics of materials phenomena such as friction, torsion, viscoelasticity, strain and fracture limits, et cetera. Even something “simple” like paper is actually really complicated when you get down to the microscopic level. When you start getting into fiber-reinforced composite materials, things start to get seriously weird. Don’t ask unless you really want to ramp up your anxiety next time you get on a modern airliner.
Stranger
Assume you are restraining the fabric by holding it down on or near the edge that you are going to cut, but not right at the point you are about to cut.
When you first start cutting, there is nothing or not much that is restraining the fabric from moving in the direction in which you are cutting. Your hold is basically perpendicular to the direction of cut, leaving the fabric able to bunch up, or move away, in the direction you are cutting. So you have to close the scissors down on it to apply the force required to cut it.
The further in you go, the more your direction of cut is away from the point of restraint, meaning your restraint can prevent the fabric moving away from the cutting point, meaning that the movement of the scissors can itself bring sufficient force to bear to cut the fabric.
Easier to show with a diagram
I get what you are saying, but wanted to see if there was a repeatable pattern to it, i.e. given a scissor’s sharpness, a certain pull from the point of restraint, maybe certain characteristics (say yield strength, thickness, etc, etc) of the fabric, can you predict how much “you have to go away from the point of restraint” that the cloth will start ripping.
I think @Stranger_On_A_Train has provided good reasons to investigate further or not.