Recently, we find many kinds of flexible devices in trade shows and markets. Such devices and materials will break by various factors. Researchers are doing endurance tests to confirm its durability. It is one purpose to find when and where a crack happens. However it is difficult because they are too small, or nobody knows when and where it happens, or a specimen had broken down when a researcher does not observe it. It may be possible to find cracks with a microscope if a specimen is good enough small, but if a specimen is larger than visible area of a microscope, a crack may happen out of observed area. Mechanoluminescence (ML) can show a point where huge strain energy is affected. If a crack happens where huge strain energy is affected, it may be a solution to observe the points indicated by the bright ML with a microscope. And also, since ML can show not only energy of deforming but also energy of destruction, ML can help to find a crack from a wide area. It is important to study where stress is concentrated and when and where a crack begins to improve durability of flexible devices.