Axial loading and the change of wall thickness in a cylinder

[Appa]

I'm having trouble with understanding how to calculate a wall thickness change in a hollow cylinder unde axial loading. The length, outside diameter and initial wall thickness are given as well as the size of the load, Young's modulus and Poisson's ratio.
I guess this isn't really a hard question but I just can't get my head around it... I computed the shortening of the pipe as well as the change in diameter, but I don't understand how you should go about working out the change in wall thickness, as I guess you need to think about the solid cylinder that's "missing" from inside the hollow one but do I just calculate the amount that the solid one would extend and then subtract it from how much the hollow one has extended? Sounds wrong...

 

[Chestermiller]

Yes, this is correct. It is the same thing you get if you just say that the strain in wall thickness is equal to minus the axial compression strain times the Poisson ratio.