How do cracks typically begin to form in brittle materials like glass?

Brittle materials, such as glass, are particularly susceptible to cracking due to their inherent structural characteristics. At the atomic level, imperfections are often present in the crystalline structure of these materials. These imperfections can manifest as defects, such as vacancies (missing atoms), dislocations, or other irregularities that weaken the material's overall integrity.

When stress is applied to brittle materials, these microscopic flaws can act as stress concentrators. Because brittle materials do not have significant ductility, they cannot deform plastically to absorb and distribute applied stresses. Thus, when the load exceeds the material's strength at these imperfections, cracks initiate at these weak points and propagate rapidly, leading to fracture.

While the other options could contribute to damage or failure under certain circumstances, they do not adequately explain the primary cause of crack initiation in brittle materials. For example, excessive heat can cause thermal expansion or stress, but it is not a common primary mechanism for initiating cracks in glass. Similarly, while external forces are involved in the process of fracture, it is specifically the imperfections that typically serve as the starting point for cracks to form. Chemical reactions can lead to material degradation, but they do not fundamentally explain the process of crack initiation in the same way that atomic-level imperfections do.