I'm not familiar with COMSOL. But I think you may give a input displacement along that direction and let finite element software to compute the required force. Then you can use the obtained force and the input displacement to calculate the stiffness.

Thank you for the quick reply. Is this (unit load/unit displacement) method a well established procedure to find stiffness of flexure mechanisms. Can i use this result directly in my research without any analytical or experimental validation?

It's a well established procedure for linear stiffness structure. You need to make sure that the deflection is less than 5% of the length of the flexure. To my understanding, unit load method is an energy-based method and is different from the aforementioned procedure.

Hello Rahul, 

This question is elegantly answered in a two part blog from COMSOL itself. Have a look:

https://www.comsol.com/blogs/computing-stiffness-linear-elastic-structures-part-1/

https://www.comsol.com/blogs/computing-stiffness-linear-elastic-structures-part-2/

Hope this helps.

Rahul Chauhan ....Good question… Please check reference below.. hope it will be helpful for you…

Regards…

http://www.mecheng.iisc.ernet.in/~suresh/Theses/LimayeMastersReport.pdf