During the operation of the transformer, these sheets get deformed temporarily due to the magnetic flux. This phenomenon is called magnetostriction. Over long periods of time, due to repeated movement, gaps form between the surface of the sheets.
These gaps affect the magnetic circuit of the transformer circuit. This causes a reduction in the flux of mutual inductance. The transformer current increases by about 10 percent.
The gaps in the core sheets also distort the flux lines and cause an increase in the leakage flux. This leakage flux causes eddy current heating in metallic components of the transformer such as the tank and other fixtures. This causes the current to increase by another 5 percent.
This causes an overall reduction in transformer capacity.