Glass is a brittle material that can break easily if handled carelessly, but skilled workers can cut entire panes into the sizes people need. First, they use a tool called a glass cutter to draw a line on the glass surface, then snap the glass open without it breaking into pieces. Why does this happen?
Cutting Glass After the Industrial Revolution in the 18th and 19th centuries, the understanding of object destruction processes developed into a "strength theory." According to this theory, when an object is subjected to an external load that reaches a certain "threshold value" of strength, the object will be destroyed. However, when cutting glass, the strength of the external load is far below its "threshold value," yet the glass can still be cut cleanly. Why is this? In 1921, British scientist Griffith became interested in this question and conducted meticulous research, thereby founding a new discipline called fracture mechanics. According to his theory, there is stress concentration at the front of a "notch" (or "crack"), and as long as the external load reaches the "threshold value" for fracture, the crack will expand. Moreover, the smaller the crack is, the lower the fracture "threshold value" will be. Therefore, once the crack starts to expand, it becomes difficult to stop it, which is known as "low stress brittle fracture." Therefore, glass with a notch, combined with a not-too-large load, will quickly break along the notch, while in areas without notches, the classic "strength theory" still applies, as these parts do not reach the strength "threshold value" and will not break.
Packaging Tear Openings In daily life, fracture mechanics also has many applications. For example, some food plastic packaging bags have a pre-made "V"-shaped small notch on their edges, which makes it very convenient to tear open the resilient plastic bags. However, if the manufacturer accidentally leaves a rounded notch, it will be more troublesome to use.