I did a series of hot compression tests on TNM-B1 alloy at 1150/1175/1200C with strain rates of 0.001 and 0.01 s-1, and noticed a non-uniform barreling in some specimens. Besides, some flow curves showed anomaly: the flow curve at 1175C/0.0013s-1 locates above the curve at 1150C/0.0013s-1. The alloy was produced by spark plasma sintering at 1237C and contains a duplex microstructure of lamellar colonies plus globular Gamma-phase grains and beta-phase as well, and has fine grain structure. Has anybody experienced the similar behavior? Might the behavior be related to any microstructural aspects? or it is just a technical problem?
Thanks a lot.
Barreling is a well-known phenomenon in compression tests when friction is developed between the sample's end surfaces and the test machine's anvils. This friction force is directed from a periphery towards the center of the sample end surface. It is necessary and normal practice to use solid lubricants such as molybdenum disulfide of graphite in compression tests. The non-uniform barreling may develop due to high-temperature test and improper height/diameter ratio. On the other hand, friction and compression forces form a triaxial cone stress state close to the end surfaces and only small central part might be under uniaxial compression. Therefore , the sample shape after lubricated compression test looks curved and bent without any barrelling. At high temperatures there may be some intermediate states when friction is unstable and threfore sample shape is changed
What is non-uniform barelling? Usually the ends of the specimen bulge the least and the middle bulges the most. This is due to friction as explained by Sergei.
Are you observing something else?
This type of anomaly in flow curves is usually attributed to dynamic strain ageing, but in your material there is the additional possibility of anomalous hardening due to ordering at these temperatures.
It seems I can imagine what is meant by the non-uniform barreling. We did compression testing on copper parallelepiped singlecrystals using lubrication and obtained shapes as shown in Figure. The sample was segmented into at least three parts and looked as heavily distorted one with some barreling
For more details please see attached below PDF file of paper Article Macrosegmentation and strain hardening stages in copper sing...
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