The model of thin shell theory shows that the maximum stress value is located at intrados of the bended pipe

but the practical bending process makes the tube-wall thickness on the extrados to be thinned

and the existence of non-circularity enhances the stress concentration on extrados.Under joint effect of the internal pressure and the external bending moment

the non-circularity of elbow has three change trends

namely to reduce

to increase

or to keep unchanged.The "plural circles" can't be seen as the only development trend of creep damage on elbows.The data of previous tests show that the material of elbows isn't aging faster than that of straight pipes

thus

it can't be considered that the aging process of pipe metal at elbows has a typical significance for the whole pipeline

therefore

the metallograpphic examination of traight pipes can't be neglected.