When cracks occur in a chip multilayer ceramic capacitor due to mechanical or thermal stress being applied, and cracks reach the active area of the internal electrodes (figure 1), leakage may occur between the internal electrodes of that portion, causing a deterioration (short) of the insulation resistance.
The "discharge under a high electric field of the cracked portion" can be the main mechanism of insulation resistance deterioration.
For example, as shown in figure 2, when cracks occur between the internal electrodes, it can be assumed that a slight air gap was created in the dielectrics between the electrodes.
In this model, when voltage V is applied, assuming that the field intensity in the dielectric layer is "E," the field intensity of the air gap can be expressed as "εE."
Therefore, most of the voltage applied to the capacitor covers the air gap, and if the dielectric breakdown voltage of the air is exceeded, a discharge will occur; the insulation resistance will deteriorate with this repetition.
Figure 1
Figure 2