The natural frequencies need to meet |[K] - w^2[M]| = 0, for isometric material if the modulus reduces 4% from 100 C to 400 C, the [K] will reduce 4% too, the natural frequencies will reduce 1 - ( 1 - 4% ) ^ 0.5 = 2%. That means if you have one mode with a frequency at 10 Hz at 100 C, the natural frquency at 400 C for that mode will be 9.98 Hz.

The dynamic analysis is different from static one, high temperature causes the system more flewxible, which may reduce the nozzle loads due to the temperature. But, it also reduces natural frequencies which makes system easier to get exicted by dynamic loads.

Regards,

Shawn Zhong
COADE, Inc.