Maarten Kruijer
Papers # 2016 Berlin
To ensure the structural integrity of long term load bearing thermoplastic (like PVC or HDPE/PP) components, the time-dependent behavior of the raw (thermoplastic) material needs to been assessed and integrated in the design procedure. This paper describes how the time-dependent material behavior can be integrated in the design process. As an example, the design of a PVC sewage inspection chamber is elaborated more in detail.
Over the past decades many non-plastic sewage products have been replaced by thermoplastic equivalents. Thermoplastics are more easy to process to a particular complex shape, easier to handle (lighter in weight) and have a lower carbon footprint. On the contrary, thermoplastics in general show time-dependent behavior which is more complex to deal with compared to for instance concrete. Especially, in load-bearing applications, the plastic component is subjected to external loads during its complete lifetime. This leads to the fact that deformations will continue in time and can ultimately lead to premature failure. It is therefore a key issue to have a clear understanding of the time dependent constitutive behavior of the used thermoplastic. In addition, it has to be incorporated in the design methodology for thermoplastic pipes and sewage products.
Having such a design methodology, premature failure can be avoided and effectiveness /competitiveness of thermoplastics over other raw materials is increased as heavy over dimensioning is no longer needed. It aids to reduce the costly trial–and–error time and direct towards a ‘first–time–right’ design. Especially for injection moulded products the (experimental) trial and error approach is costly, as for each design, mould adjustments may be required.
In this paper the design methodology incorporating the time-dependent behavior is presented and applied in the development and optimization of a PVC inspection chamber. An inspection chamber can be installed up to 6 meters deep, and as such, being subjected to a 6m water column (0.6 bar) during its entire lifetime. As a consequence, continuing deformations of the chamber can lead to failure or unfit for usage after a period of time.
To this end, firstly the base material is tested for its time dependent constitutive behavior by creep experiments. Subsequently, a viscoelastic material model is set-up for the PVC and is implemented in a finite element program. Finally, the mechanical (long-term) requirements for the chamber are outlined and the design procedure is used to engineer the PVC chamber to be optimized for its usage.
