Why are my XRD residual stress measurements so inconsistent for burnished Ti-6Al-4V?

[zenite]

I am doing a research project on measurement of residual stresses for Ti-6Al-4V that has been burnished. I did the first scan to get my peaks, and selected the last peak (the highest 2 theta value) for the stress scan (sin^2 PSI method).

The problem is that the value I got for 3 samples (that are theoretically identical) is 99MPa, -300MPa, -360MPa. Also, the standard deviation for all 3 samples are in the region of +/- 300MPa (!). That is highly inaccurate.

Now, I have read many texts on XRD and the theories. But I have no idea why the accuracy for my sample is so bad. Can anyone advise me? My samples have some scratches are really small pitted holes, so will that affect the result?

I did another scan on the highest peak (instead of the one with the largest 2theta value), I ended up with -3000MPa. Really lost now.


I am using Philips Analytical X-Ray B.V. with Cu tube anode. Let me know if I have to provide any other details.

 

[yilmazerh]

I also have same problem. I have been used Bruker D8 XRD machine. Some results are higher than 0.2% proof stress and tensile strength. I am confuse it can or can not. I need another accurate way to measure residual stress. but my sample is small (20mm diameter disk sample)