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Study on Viscoelastic Behavior of Plastic Pipe Reinforced by Cross-Winding Steel Wire

Author: LiXiang
Tutor: ZhengJinYang
School: Zhejiang University
Course: Chemical Process Equipment
Keywords: Composite materials Composite pipe Stress Analysis Experimental study Orthotropic Creep buckling Viscoelastic Drawn steel wire
CLC: TB333
Type: PhD thesis
Year: 2008
Downloads: 299
Quote: 2
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Abstract

Plastic pipe reinforced by cross helically wound steel wires (PSP) is a new plastic-matrix steel composite pipe developed in China recently. PSP is composed of two layers of thermoplastic high density polyethylene (HDPE) matrix and a high strength steel wires mesh skeleton, which is sandwiched in the middle of the inner and outer HDPE layers. The skeleton is formed by cross helically wound steel wires with high strength, and tightly integrated between the inner and outer HDPE layers using high quality cohesive resin. As the skeleton is inlaid in the pipe wall, the inner and outer HDPE layers of PSP join through the gaps among steel wires, which makes the HDPE and the steel wires interlace with each other and integrate together. The contact interface between HDPE and one steel wire is discontinued with the interface between HDPE and another steel wire. Owing to the unique structure, PSP not only integrates the advantages of plastic and metal, but also overcomes drawbacks of steel pipe lined with plastic, such as delaminating easily under long-time alternating stress. With the excellent performance of PSP, there has been a growing interest in using it.However, up to now, the manufacture of PSP is mainly by experience and experimental results in the factory. The factual problems happened in the producing can not be solved in a scientific way, which seriously constrains the using of the PSP. Supported by New Century Excellent Talents in University of China and Specialized Research Fund for the Doctoral Program of Higher Education, this paper researched on short-term and long-term mechanical response, long-term satiability under external pressure, and the failure mechanism of the PSP system. Some conclusions are drawn form this paper:(1) In order to understand the mechanical properties of PSP under internal pressure, a four-layer analytical model, which considers the torsion caused by the differences between the winding angles of the inner and outer steel wires layers, has been proposed using structural mechanics. The model includes an inner high density polyethylene (HDPE) layer, an inner steel wires layer, an outer steel wires layer, and an outer HDPE layer. To investigate the mechanical properties in inner and outer steel wires layers, the elastic parameters of the composite monolayer plate were deduced from rectangle outside and circle inside model. During the elastic response of PSP subjected to internal pressure, the stresses and strains in four layers and pressures between the interfaces were obtained. The three-dimensional numerical simulation method was proposed and FEA model is constructed. For better showing stresses and strains, and compared to the HDPE pipe with the same geometric parameters, A three-dimensional numerical simulation method was presented by considering the torsion caused by the differences between the winding angles of the inner and outer steel wires layers. Good agreement among theoretical results, FEA results and experimental data was observed, which shows the mechanical model and FEA model are correct. The failure modes of the PSP were researched by experiment. The failure modes and torque values between PSPs manufactured by two different methods were compared, and the influence of the change of the winding angle on the strain in exterior surface was also discussed. It is better to choose the full winding method to manufacture PSP.(2) The analysis method of the long-term stress based on the structural characteristic of PSP is established. Tensile creep experiments are carried out on the HDPE splines to obtain viscoelastic constitutive equation of matrix material. A linear visco-elastic mechanical model of PSP is put forward based on the structure characteristic and viscoelasticity of matrix material and bonding resin, the laws of stress and strain vs. time of each layer are archived, and the mechanical model is verified with experiments. The strain of PSP and average stress of steel wire vs. time are predicted on a 50 years level, and the impaction of volume percentage of steel wires and winding angle on viscoelasticity of PSP is obtained as well.(3) In order to further understand the stability of the buckling of the PSP, a four-layer analytical model has been proposed using structural mechanics. And the instantaneous critical buckling pressure of the PSP subjected to external pressure was deduced from Donnell flat shell theory. Good agreement between theoretical results and experimental data was observed, which shows that the present model can predict the instantaneous critical buckling pressure of the PSP. The relations between the instantaneous critical buckling load and design parameters were analyzed, and some methods were presented to increase the stability of the PSP. (4)The mechanical model to analyze the stress of the end part of PSP system is proposed, and a calculation method of critical length of PSP system joints is established as well. Based on thorough analysis of failure mechanism of PSP system and factors influencing failure of joints, the mechanical model to analyze the stress of the end part of PSP system is proposed, and the law of stress distribution on the interface of steel wire and bonding resin of the end of PSP system. The effect of various drawing speed on the results of pull-out is analyzed in detail, the corresponding failure criteria is put forward, and actual failure behavior of the steel wire drawing process is also discussed. Experimental studies on the drawing behavior of the steel wire and the interface of steel wire and bonding resin is carried out, and a calculation method of critical length preventing failure of PSP system joints is established as well. And the calculation method is verified with a hydraulic testing on extended PSP joints.

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