PSI - Issue 23

Jan Poduška et al. / Procedia Structural Integrity 23 (2019) 293–298 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction Recycling of plastic solid waste is an important issue worldwide, as post-consumer plastic waste is being produced in massive quantities (according to Singh (2017) the production crosses 150 million tons per year globally). The low degradability of plastics in landfills has caused an increased demand for the recycling of plastic waste. The recycling of polymer material used for pipes has become an important issue also for the industry branch dealing with plastic pipes. The TEPPFA (The European Plastic Pipes and Fitting Association) has made a commitment to use a quarter million tons of recycled material by the year 2020 (according to Calton (2016)). In the future, the EU initiatives will probably force even larger amounts of virgin material to be replaced by recycled material in the production of plastic pipes. The mechanical recycling is the most common type of recycling for the used plastic pipe materials, usually the HDPE (high density polyethylene) – plastics are ground down and simply reprocessed by mixing the recycled material with the virgin material or by creating a new product solely out of the recycled material. However, the recycled material does not have the properties of the virgin material. The change in mechanical and other properties of various types of PE caused by recycling has been a subject to numerous studies, e.g. Jin (2012), Reis (2013), Cruz (2013), Navratil (2015). Due to the worse mechanical performance of the recycled polyethylene for piping applications, the use of the recycled material is limited to non-pipe applications or to non-pressure pipes only. According to current regulations, adding recycled content is not allowed at all for pressure piping systems. However, it can be assumed that the use of the recycled materials for pressure pipes will be allowed eventually. In this paper, some ways of incorporating the recycled material into a wall of a pressure pipe are investigated. It is assumed that the recycled content would form a layer in a multi-layer pipe. Combining layers of recycled HDPE with the virgin HDPE layers could grant a performance similar to the performance of single-layer pipes made entirely of the virgin material. The most important property of any HDPE pressure pipe is its lifetime. The lifetime of these pipes is closely related to their resistance against a mechanism called the slow crack growth (SCG). This resistance is significantly reduced by the recycling. To compare different types of multi-layer pipes, FEM simulations of the crack growth in the pipes were carried out and lifetime estimations were calculated. The linear elastic fracture mechanics approach was used to calculate the lifetimes. The material parameters necessary for the lifetime calculations were obtained by measuring the crack growth rate in the recycled HDPE by CRB (cracked round bar) test. The possibility of using recycled material for pressure pipe applications as a part of a multilayer pipe system is discussed here, based on the obtained results.

Nomenclature A

material parameter characterizing the crack growth rate ((mm /s)/(MPa×m 1/2 ) m )

a

crack length (mm)

a ini a fin

initial crack length (mm) final crack length (mm)

d

nominal outer diameter of the pipe (mm) crack growth rate at static loading (mm/s)

da/dt

Young’s modulus (MPa)

E

stress intensity factor (MPa×m 1/2 )

K I m

material parameter characterizing the crack growth rate (-)

R

load cycle asymmetry

s

total thickness of the pipe wall (mm) thickness of the inner layer of the pipe (mm) thickness of the outer layer of the pipe (mm) thickness of the middle layer of the pipe (mm) time of slow crack growth in a pipe, lifetime estimation

s i

s m s o

t SCG

µ

Poisson’s ratio (-)

σ hoop

hoop stress in the pipe wall caused by internal pressure