Page 112 - Failure Analysis Case Studies II

Failure Analysis Case Studies II
D.RH. Jones (Editor)
0 2001 Elsevier Science Ltd. All rights reserved 97

Catastrophic failure of a polypropylene tank Part XI:

comparison of the DVS 2205 code of practice and the design of
the failed tank

G.W. Weidmann*, P.R. Lewis
Department of Materials Engineering, Faculty of Technology, The Open University, Milton Keynes MK7 6AA, U.K.

Received 19 October 1998; accepted 4 November 1998

Abstract

The design of a failed, large (20 m3) polypropylene storage tank is compared with the recommendations
of the German Code of Practice, DVS 2205, to which it allegedly conformed. It is shown that the tank was
seriously under-designed, and that the situation was exacerbated by the introduction of residual tensile
stresses in its walls during its manufacture. 0 1999 Elsevier Science Ltd. All rights reserved.

Keywords: Code of practice; Design; Failure; Polypropylene; Standard; Tank; Weld

1. Introduction

The problem of designing load-bearing structures in plastics differs from that of designing
comparable structures in metals such as steels in several important ways, particularly if the design
life of the structure is intended to be a long one (20 or 30 years, say). These differences arise because
the behaviour of plastics under load is not only time-dependent but also non-linear, because their
range of recoverable strains is typically some ten times larger than in metals, because plastics can
often be more sensitive to stress concentrations than metals, and because plastics react in a different
way to environmental agents than metals. Failure to appreciate these differences has led (and
unfortunately still does lead) to premature failure of plastics products, and to their acquiring an
early reputation for being ‘cheap and nasty’.
The basis of much rational design with plastics is the so-called ‘pseudo-elastic design method’
proposed initially by Baer et al. [l]. In this, the appropriate time- and temperature-dependent
values of modulus and Poisson’s ratio are substituted for the elastic ones in the standard stress-
strain solutions for a given loading configuration and part geometry. Initially, before sufficiently

* Corresponding author. Tel.: 01908-653271; fax: 01908-653858.
Reprinted from Engineering Failure Analysis 6 (4), 215-232 (1999)