Hello!
I have got a question that I find difficult to assign to a specific forum. I am using Ansys, so for the best of my knowledge I will try it here but if a moderator thinks it should be asked in a more general engineering thread, feel free to move the post.
I am simulating a ceramic cylinder (piezo element) which is loaded with pressure. The piezo element is integrated in to a bigger part which deforms while loading. As the force transmission on the piezo is not clearly axial; I had a look on the shear stresses – those should be minimal or non-existent fpr piezo ceramics. Unfortunately there is a lack in literature on material values but a manufacturer told us that the shear stress should be below 1000 psi (~6.9 MPa) for monolithic elements and below 1 MPa for multilayer Elements.
As even for my different bearing conditions (e.g. semi spherical end pieces) of the piezo element the shear stresses where above the mentioned levels I did simulations of a simplified setting:
- Cylinder
- Statically defined bearing see picture
- (A: base area uz=0
- (B: 1 point/node fully fixed
- (and C: 1 point uy=0 to inhibit the remaining possible rotational movement (avoiding rigid body movement))
- Force on top area (so pressure) in an axial direction
I wonder why for the small loading (von Mises stress within the material ~6.36 MPa) already a constant shear stress is present (Max shear ~3.18 MPa, see picture)
I was stunned but think that may be explained by Mohr’s circle: The minimal principle stress has the greatest absolute value (S3=-6.36 MPa) and amounts up to the value of the von Mises stress (seen that S1 is nearly zero). Therefore the shear stress must be S3/2 (radius of Mohr’s circle), which agrees with the shear stress values.
Note: The loading is for the specific piezo element definitely realistic or even small and best case (axial) - so should be withstood (for monolithic and multilayer elements!).
My question now is: How can I evaluate the presence of shear stress as characteristic for a non-axial loading and as a value that has to be minimized if those stresses are “per se” present. Or: How do I evaluate the additional part that is caused by a non-axial loading.
And further: How does that relates to the Values that I got from the manufacturer.
In principle I want to move from that simple approach to my more realistic model with a loading that may be non-axial: to evaluate my design.
I appreciate any help even if you can just reveal my errors in reasoning.
I hope that I could present my question(s) in a comprehensible manner, if not please ask for clarification!
Thanks in advance
Anatol
I have got a question that I find difficult to assign to a specific forum. I am using Ansys, so for the best of my knowledge I will try it here but if a moderator thinks it should be asked in a more general engineering thread, feel free to move the post.
I am simulating a ceramic cylinder (piezo element) which is loaded with pressure. The piezo element is integrated in to a bigger part which deforms while loading. As the force transmission on the piezo is not clearly axial; I had a look on the shear stresses – those should be minimal or non-existent fpr piezo ceramics. Unfortunately there is a lack in literature on material values but a manufacturer told us that the shear stress should be below 1000 psi (~6.9 MPa) for monolithic elements and below 1 MPa for multilayer Elements.
As even for my different bearing conditions (e.g. semi spherical end pieces) of the piezo element the shear stresses where above the mentioned levels I did simulations of a simplified setting:
- Cylinder
- Statically defined bearing see picture
- (A: base area uz=0
- (B: 1 point/node fully fixed
- (and C: 1 point uy=0 to inhibit the remaining possible rotational movement (avoiding rigid body movement))
- Force on top area (so pressure) in an axial direction
I wonder why for the small loading (von Mises stress within the material ~6.36 MPa) already a constant shear stress is present (Max shear ~3.18 MPa, see picture)
I was stunned but think that may be explained by Mohr’s circle: The minimal principle stress has the greatest absolute value (S3=-6.36 MPa) and amounts up to the value of the von Mises stress (seen that S1 is nearly zero). Therefore the shear stress must be S3/2 (radius of Mohr’s circle), which agrees with the shear stress values.
Note: The loading is for the specific piezo element definitely realistic or even small and best case (axial) - so should be withstood (for monolithic and multilayer elements!).
My question now is: How can I evaluate the presence of shear stress as characteristic for a non-axial loading and as a value that has to be minimized if those stresses are “per se” present. Or: How do I evaluate the additional part that is caused by a non-axial loading.
And further: How does that relates to the Values that I got from the manufacturer.
In principle I want to move from that simple approach to my more realistic model with a loading that may be non-axial: to evaluate my design.
I appreciate any help even if you can just reveal my errors in reasoning.
I hope that I could present my question(s) in a comprehensible manner, if not please ask for clarification!
Thanks in advance
Anatol
