Hello,

 I am not specialist in mechanics, so I would like to consult with you on
the follow subject.  It concerns straight and very long beams (i.e.
lenght/height ratio is around 100) with rectangular cross-section, where the
height of a beam is larger than its width.

What I have found about Single Cristal Silicon I would like to ask you to
comment the points mentioned below. I would be very grateful for any
comment.


1. It is a brittle material. For the brittle materials the characteristics
is linear in the elastic region until the yield point and the SI is
fractured, so linear stress-strain curve.

2. From the point 1 and the fact that the straight beam during the bending
doesn't change its shape I assume it is a linear material.

3. Accroding to Gere and Timoshenko "Mechanics of Materials", 3rd SI
Edition, 1991, pp.34-35,  the allowable stress is obtained from the relation
ultimate stress divided par safety factor.

4. Petersen in his article " Silicon as a mechanical material" pointed only
yield strength of Silicon, however some group of researcher pointed the
fracture strength e.g., *"Due to its brittleness at room temperature, SCS
fractures before it deforms plastically. Fracture strength, instead of yield
stress which is used for most metals, becomes the design parameter that
limits the maximum load a silicon structural member can bear"
* (Yi T. et al, Sensors and Actuators 83 (2000), pp. 172-188)

And here are some questions:

a)  For brittle materials in the room temp., what strength then should be
considered concerning MEMS components like beams - fracture strength or
yield strength to calculate teh allowable stress - I mean tension and
compresion stresses?

 b) Have you ever seen in the literature what the safety factor is for the Si?

Best regards.

Karolina