Rational creep mechanics and engineering applications to high temperature pipework design

The thesis is divided into three sections: The first section is aimed at providing a rational basis for the behaviour of complex structures deforming in time due to creep under the action of arbitrary loading. Two constitutive models were chosen to be studied - the "internal variable theory" and the well known "time hardening theory". The main content of the approach is the derivation of an equation of evolution for the growth of inelasticity; this equation may be rigorously defined. Resulting from this description are coupled evolution equations for the stress, strain and displacement field. It is the author’s contention that these equations are fundamental to understanding the process of creep in a structure and bring out the essential nature of the temporal response. Such an approach allows the basic properties of solution to a creep problem under variable loading to be studied. The second section applies the ideas from the first section to the study of flexible piping components - in particular the curved pipe; however to begin with, in order to fix ideas the straight pipe is discussed. A novel nonlinear shell model of a curved pipe is derived (to cope with internal pressure). The elastic behaviour of the in-plane bending of this model is studied numerically. Two distinct cases of the creep of a curved pipe, with no internal pressure, are investigated in detail; these analyses are realized from the equation of evolution of stress and allow comparison with simple approximate methods - which turn out to be quite successful. The third section examines the more complex problem of a high temperature piping system. On a basis of results of the second section a simple approximate method is suggested in the spirit of well known ’’flexibility" methods for elasticity. A simple example is worked out to demonstrate the method. The thesis concludes with a view into a possible future for inelastic pipework analysis and creep mechanics in general.

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