I'm currently doing a small project in college for structures being damped. We're adding weights to a structure and measuring the damping affect. We've calculated the damping ratio and coeffecients, however, we currently have damping ratio increasing with mass, which I do not believe is correct following the below equation:

The above equation, assuming k and c are constant, suggests that zeta decreases as mass increases.

However, the results we are getting are entirely different, and we do not have a constant value for the damping coefficient:

We got the initial value of damping ratio by interpreting the graphs by using the following equation and process:

Where the above was rearrange from:

where 'n' = the number of full wave cycles between x_0 and x_n

Ultimately, I'm curious if the damping ratio should decrease with increase in mass, and should the damping coefficient stay constant?