equivalent mass and effective mass

[richard98]

Hello,
I know that the natural frequency of a structure is the square root of the ratio of equivalent stiffness (in the form of elastic modulus) and equivalent mass (in the form of density).
I have two questions about this equation.
In my model, an epoxy block was sandwiched by two Kovar blocks (structure like 'ABA'), and the bottom of these three blocks were all fixed.

Question 1:
When the elastic modulus of epoxy changes, the equivalent stiffness of the structure will change, but does the equivalent mass change also?
I am not very sure about it. I think the equivalent mass remains the same because the densities of both Kovar and epoxy remain the same. But I found in ANSYS finite element analysis that the modal effective mass varied with the elastic modulus of epoxy.

So, question 2 is:
What is the difference between equivalent mass and effective mass?

Thanks in advance.

Best regards,

Richard

 

[hacksaw]

you appear to be confusing "modal mass" with the actual mass.

the modal parameters are the equivalent mass and stiffness for each mode as if each were a simple spring mass system. they are used in constructing the overall response.

the departure of the modal mass from the actual mass gives you some idea of the relationship of the mode shape with the actual mass distrbution.

semantics, yes; confusing, it certainly can be.