Probably, he was getting all values as "Infinity".

@chemr2 :
The problem is because of the 1/12 in the denominator.

Both 1 and 12 are integers. Change at least one of them to float to get proper results.
This should be sufficient :
Code: CPP
Sigma = (1500.0 * .020)/ ((1.0/12)*(.060)*pow(.040-2.0*a,3));

BTW, this is NOT what we call an infinite loop.
Your loop is finite, but the output values are infinite