Learning Forum

[kashfia.mahin]

[peteroznewman]

nIn Static Structural model, under Analysis Settings, turn on Large Deflection. For a drum head, there is very little stiffness to the initial or linear displacement of the drum head normal to its surface. It is only through nonlinear iteration that the tension in the drumhead can be developed, which will greatly increase the stiffness of the drum head.nA 1 N force is a very small force, maybe you need a larger force. Please show how that force is applied and how the drum is supported. How is the drum meshed? Is it shell elements on a surface? That is what you want.n

[kashfia.mahin]

thanks for your reply.nFor the Mesh on the water tank, Shell elements are not used. nForce is applied on the one fave of the tanknFixed support is given in the lower edges of the drumnI changed the Force from 1N to 100N on the face. I didn't see much change in the eigenvalue.nTotal Deformation for 1N forcennn

[ErKo]

It is membrane actions generating stress stiffening effects, and thus increase in the bending stiffness.nnThe best example is to put an axial load/stress on a beam, and then see how the 1st bending mode frequency changes as we increase our axial load. This is the same thing that happens when increasing the tension in a guitar string, where the frequency goes up with increased tension.nnThis is the best example to start with in order to understand this effect (stress stiffening).nnAll the bestEriknn

[peteroznewman]

nErik's example is very good to learn from.nFor your model, you should definitely replace the solid body with a surface body and get shell elements.nWhich mode vibrates that surface? You should see that mode move when you have significant force, but you don't have a rigid structure so you can't build up much tension in the shell elements from that force.nn

[kashfia.mahin]

Thanks nI have tried first the way Mr. Erik said. For the free boundary condition of that tank, the frequencies are very smaller than when I applied the support on the edges. nBut when I applied the force on the outer face, it doesn't change much in the eigenfrequency. nthe above picture is the 1st Mode of Deformation of the tank while considering only the Modal analysis.nThe 2nd picture is the 1st mode of deformation of the tank after Static Analysis.nAs I am a new learner in structural dynamics, sometimes I miss the parameters to consider. n