Rajagopal,

Thermal stresses are not always secondary, then can be peak also. Read Appendix 4 of sec VIII Div 2 for detail.

Also there are situations where thermal stress can actually act as a primary stress.As an example,if a thin tube is subjected to a length-wise temperature profile that has a sharp jump discontinuity at some point along the tube and this discontinuity moves repeatedly along a length larger than, typically, a couple of radii of the tube, then the shakedown limit is near equal to the elastic limit, i.e. 2 is replaced by 1.
For details on this subject , you can type " A.R.S.Ponter" in google, Professor A.R.S.Ponter is an international expert in this field and you will get more information.

Regards