INSTRON 5565A Flexural Testing

[smcen1]

Hi guys, I have a question regarding using INSTRON 5565A for flexural testing of poly methyl methacrylate (PMMA) resin.
During testing, after 100 seconds the force seems to begin to drop despite the sample not breaking. Does anyone know why this is happening? Is it to do with the settings on the INSTRON Bluehill system?
Any one with ideas is greatly appreciated.

 

[Compositepro]

All plastics creep.

 

[btrueblood]

It shouldn't...unless it's programmed to do that. Did you RTFM?

http://bcodata.whoi.edu/Carrington/Instron_5565_Materials_Testing_Frame_M10-14190-EN(RevA).pdf

That machine supposedly has a limited cycle testing capability, are you sure you are not operating in that mode?

Alternatively, the material is yielding/relaxing/creeping under load and the actual load required for a given crosshead speed really does drop (i.e. it may be a material characteristic). Is the drop repeatable for a given material composition?

Last, yes, you might be exceeding the force output of the machine, or it may be "old and tired" and you are seeing a thermal reset fuse operate - I would expect the machine to give error codes if this happened, though.