Date of Award

Spring 1999

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Computational Analysis and Modeling

First Advisor

Raja Nassar

Abstract

The problem of long-term structural behavior of CIPP liners under external hydrostatic pressure has been characterized as a structural instability problem which is induced by time-dependent material deformation or creep. It is also a contact problem since the liner deflection is externally constrained by the host pipe. The intrinsically nonlinear problem is investigated by means of finite element simulation, with emphases on (a) the essential structural behaviors and mechanisms of buckling, and (b) the influences of inelastic material properties (i.e. yield strengths and creep rates) and geometrical parameters on liner's buckling resistance.

A liner first deforms as a free pipe into a two-lobe pattern because of the existence of an initial gap. It will transition to a one-lobe mode when one of the two competing lobes becomes dominant. The finite element results show excellent agreement with experimental observations. Because mode transition and hence the critical pressure depend greatly on geometric factors which are usually not controllable in pipe rehabilitation, predictions based on the conservative one-lobe model should be used in liner design.

The relationship between critical time and external pressure derived from finite element simulation results show that critical time can be expressed as a monotonic function of the ratio of applied pressure to the critical (short-term) pressure. This relationship shows as expected that critical time at the two extreme pressure levels (zero and critical pressure) is infinity and zero, respectively. The model gives excellent agreement with the finite element results, and is better than other models used in the literature to correlate experimentally observed buckling times with pressure levels.

Finite element simulations are carried out to investigate the effects of three essential geometric parameters (i.e. the dimension ratio of the liner, the gap between the liner and its host pipe, and the ovality of the host pipe) and two geometric imperfections (i.e., variation in liner thickness and initial local imperfection in liner shape).

Several issues which are important in design, including the discussion on failure states, and an appropriate way to choose a safety factor, are discussed. A methodology is presented by which finite element simulation results can be used to CIPP liner design. Design curves are given for designing CIPP liners made of a specific CIPP resin. An empirical design equation is presented which can determine a safe and cost-effective thickness for a given design pressure and given host-pipe configuration. (Abstract shortened by UMI.)

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