Failure of elevated temperature Al6061 fatigue specimens 1

[will123123]

I am currently fatigue testing aluminium 6061 at 250°C. Frequency 25Hz and stress ratio 0.1. The specimens are being tested in the elastic range. The failure mechanism of the samples is not cracking and braking in two, but by yielding and necking in the gauge length area.

Initially, I assumed that there was initiation of a crack that reduced the section such to cause localised stresses to exceed yield, but crack testing on tested specimens reveal that no cracks are present.

I suspected localized overheating, but the latest two samples necked directly below the temperature control thermocouple. There is no evidence of a change in temperature from the data.
I did wonder whether overaging is causing the strength of the material to fall below the fatigue loads, but I have run a couple of tests at slower frequency - these failed at similar number of cycles to samples run at the same load at the standard 25Hz,despite being expose at temperature for much longer. I am going to carry out hardness tests to confirm, but this does not appear to cause.

I have not observed failure like this before and your thoughts would be appreciated.

 

[winstonsk]

250C is high for Al in my experience.
Is it creeping?

 

[WKTaylor]

Winstonsk... I concur ... 250C is way beyond the normal operating temperature range for 6061-'any temper'.

wil123123... what is the configuration of Your test specimens, IE: classic round bar necked in the middle, other?

Are You bringing the test specimen slowly or rapidly up-to temperature before beginning the test?

Sounds like You're failing fairly quickly in this highly ductile state. Any discoloring or grainy surface appearance? what do post failure hardness and conductivity readings reveal [AMS2658 Hardness and Conductivity Inspection of Wrought Aluminum Alloy Parts]?



Regards, Wil Taylor

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[will123123]

Thank you for your replies. Further testing with more appropriate data logging has confirmed creep. I appreciate the temperature is very high, but the service life of the parts are only of the order of 10 hours.
The test specimens are typical uniform gauge length, round fatigue specimens with 5mm diameter and 20mm parallel length. The samples reach temperature in approximately 20minutes - the load frame is controlled to apply zero load prior to test. The samples fail by necking, the test end criteria is extension over 3mm.
There is no discolouration or other change in appearance of the samples. I have not yet had a chance to carry out post failure hardness testing.