shotpeening and residual stress relaxation 2

[jozef155]

Hello,

I have shotpeened specimen (AISI 316 Ti), I performed XRD residual stress measurements after 1000, 10 000, 50 000, 100 000, 5 mil ... cycles (load 430MPa).

I noticed not very smooth relaxation curve, but after 10 000 cycles the residual stress rapidly decrease and then after 100 000 cycles increase.

Could someone say why this is happening?

Thanks

 

[TVP]

Measurement error, or variation in location of measurement, or change in loading condition, specimen alignment, etc.

 

[hydtools]

You have taken the material into plastic deformation. You have begun work hardening. You have exceeded the compressive stress induced by the shot peening.

Ted

 

[unclesyd]

I agree with hydtools on the probable cause of the reversal. You can only get some much penetration, after which you begin to move metal. Many years ago I worked with shotpeening 304L SS in hopes of mitigating corrosion at the grain boundaries. Huey testing of the samples revealed very little was gained from blasting using Wheelabrator beyond 5 cycles. In those days there no method to check stresses in a part, especially SS.
Link to Metal Improvement Company discuss the maximum stress that can be achieved.

If I recall correctly one of Dr David Kirk's papers discusses results similar to what you are seeing.you are seeing

http://www.shotpeener.com/library/kirk_articles.php

http://www.metalimprovement.com/shot_peening.php