To: Barry Lotz & other interested parties Hey, I hadn't expected to get this deeply involved in the details, but since you asked about "relaxation vs. time under stress" in the second paragraph of your September 10th message: 1. Basically, material creep is a primary reason that my initial message included the statement "Do NOT be tempted by the ease of plastics"--- since even the best engineering plastic will exhibit a creep rate which is typically a couple orders of magnitude worse than metals. 2. As you have discovered, however, even metals--- under continuous loads producing relatively high stress--- will exhibit some creep. This is not unique to flexures; you already pointed out that it can happen to music wire, and most of us have seen cases where coil springs have "relaxed" when under load for long periods. Even knife-edge pivots have been shown to have some (small) amount of "cold flow". 3. Therefore, there should be little, if any, concern about creep in flexures for "horizontal-boom" seismic devices, where the stress level is low. I am, however, a fan of using comparatively thicker spring-metal flexures, say in the .004 to .008 inch range for spring steels, for your typical size and weight instruments--- but this is primarily because of the ruggedness which is provided. For those, like Sean-Thomas, who want to maintain gravity as the primary centering force, you can readily compensate for flexure thickness by increasing the free-length (i.e., distance between clamps) per my "Calculations" paragraph of previous message. 4. But, back to your case of the "VBB sensor using an ~.018" strip bent in ~180 degrees". The stress level is obviously rather high, so what can be done?--- 5. As you have noted, creep is time dependent--- gradually decreasing over a period of months. Therefore, one helpful "trick" used by many instrument manufacturers is to speed up the "aging". A typical method of doing this is the cycle the metal (a minimum of 100 times, with 1000 being better) to a stress level which is approximately 50% higher than the load which will be seen in service. Obviously, you don't want to exceed the yield strength or proportional limit! Application of heat--- perhaps 250 degrees F, but again well below any temperature used for heat treatment (or normalizing) of that particular metal--- during the cycling will also assist in the stress aging. 6. Yes, the selection of the spring metal is also important in controlling creep. However, the metallurgists and other experts tend to use a lot of caveats--- items such as differing times and temperatures during tempering, amount of cold-working due to rolling or drawing to final size, etc. Therefore, I haven't seen any good comparisons in tabulated data. (Anyone else?) One general rule of thumb seems to be to avoid those spring metals which are hardened entirely, or primarily, by strain hardening ("cold working") rather than heat ("precipitation hardening" and "transformation hardening"). This would indicate the avoidance of phosphor and silicon bronzes, 18-8 stainless, and the 300 series of stainless steels if your application demands minimum creep. ---Dean E. Gladow--- _____________________________________________________________________ Public Seismic Network Mailing List (PSN-L)
Larry Cochrane <cochrane@..............>