Hybrid materials design to control creep in pipes.
Thesis DisciplineMechanical Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
A hybrid material design has been developed to improve creep performance in pressurized metallic pipes subjected to high temperatures. Metallic pipes were reinforced with various arrangements of external wires which have substantially greater creep resistance than the pipe material. This research was conducted to explore the field of reinforcement of piping for creep reduction, exploit the creep strength of refractory metals, and investigate structure-property relationships in architectured materials. Two basic wire reinforcement architectures were tested, simple helical windings and braided sleeves. By adjusting the architecture of the reinforcement, apparent tangential (hoop) and longitudinal stresses on the pipe are altered, thereby allowing multiaxial creep strains to be controlled. The utilization of a reinforcement layer in a hybrid layup, where it is not bonded or embedded in a matrix is a relatively unexplored field. Hybridization allows the most desirable properties to be extracted from each component and have them work together in parallel. The use of braided refractory reinforcement is also a particularly novel concept, with refractory materials for reinforcement purposes traditionally being utilized in particle, whisker and discontinuous fibre form. Rather than testing in a uniaxial stress state, the present approach to creep testing pressurized pipes at high temperature remains largely underutilized, and is especially relevant to industry applications where creep takes place in the complex, multiaxial stress state of a pressurized pipe. In a low-temperature reinforcement architecture optimization study of a brass-stainless steel system, designed for ease of fabrication and to negate oxidation issues, pipes were pressurized and creep rupture tested at 400°C. Even in an unoptimized state, braided reinforcement was observed to out-perform a simple iv helical wrap by at least 22%, giving a 10-times life extension without rupture, and a reduction in creep rate in excess of 45-times for reinforcement oriented at a 50°. A simple analytical model from reinforced pressure vessel theory predicts a neutral angle (θN) of 54.7°, at which point the reinforcement is oriented to act proportionally to the applied pressure stresses. An empirical model of effective creep rate with varying reinforcement angle was derived in the present study, and used to find that a braid angle of approximately 54.7±1.5° is optimal to minimize the effective multiaxial creep rate of a hybrid pipe under internal pressure, reducing it to the point of being negligible. The braided reinforcement was observed to be constantly shifting towards the equilibrium point of θN, but only for initial angles below θN. This concept of braid reorientation is generally associated with rapid elastic deformation or static reinforcement of systems at room temperature, and the gradual shift towards θN facilitated by creep deformation has not been reported previously. A relationship for -θ (i.e. creep rate for a given reinforcement angle) was derived, including the reduction in as θ tends to θN. Findings of this optimization study were applied to a high temperature system which served as an acceleration of reformer furnace operating conditions: 253MA pipes were reinforced with tungsten wire and creep rupture tested at 1030-1040°C. Using braided reinforcement oriented at 52.6±1.4° a life extension in excess of 700x was observed, with no signs of bulk deformation after a 309x life extension. These high temperature results were considered in light of the intended industry application, with a balance of life extension, weight reduction and increased operating temperature preferred over outright life extension for the reformer furnace application.