Page 113 - Failure Analysis Case Studies II

98

comprehensive data on the creep and creep rupture behaviour of specific plastics became available,
this approach was limited to strains small enough that an assumption of linear viscoelastic
behaviour was a good approximation. Nowadays, this restriction does not apply since copious
data are available on all the commoner thermoplastics largely generated from investigations into
the long-term behaviour of materials for pressurised pipes.
One of the few, perhaps the only, report of a combined theoretical and experimental investigation
into the design against failure of large plastics tanks is that of Forbes et al. [2]. They applied the
pseudo-elastic design method to polypropylene tanks with capacities up to 9100 gallons (41 m3).
The design was based on a stress analysis solution of a fourth order linear differential equation as
given by Timoshenko and Woinowsky-Kreiger [3] which takes into account the effects of the
transition from horizontal base to vertical wall and of transitions in wall thickness. These effects
are manifested as increases in the radial expansion of the tank walls just above the transition
points, but they can also be thought of as kinds of stress concentrating features. Using a limiting
hoop strain of I%, the results of this analysis produced a design chart for the wall thickness of
tanks of increasing capacity up to 10,000 gallons (45 m3). Their results were vaIidated by full-scale
tests on two large tanks.
The failure of a 20 m3 polypropylene storage tank and the ensuing investigation were described
in Part I of this work [4]. The tank was constructed to a design which was verified by the calculations
of a consultant engineer and allegedly conformed to the design code DVS 2205 [5], the German
Code of Practice for the design of free-standing thermoplastics containers (there is no cor-
responding British Standard, although there is one for GRP tanks, BS4994: 1987). This code of
practice provides a guide to the determination of the maximum permissible stresses that will avoid
different modes of failure in thermoplastics containers over specified lifetimes. It takes into account,
interalia, the type of thermoplastic, its chemical interaction, if any, with the contents of the
container, the operating temperature, and effects arising from changes of wall section and method
of manufacture. This paper reviews the design methodology of DVS 2205, and compares the design
of the failed tank with the detailed recommendations that result from DVS 2205. Figure 1 shows
the dimensions of the tank as designed (taken from the design sketch), together with the wall
thicknesses, in mm, at different heights.

2. Design methodology of DVS 2205

The following translated extract from DVS 2205, Part 1 [5] outlines the essentials of the design
methodology.

3. Strength parameters
3.1. General
The fundamental bases of the design calculations are the long-term values of materials
parameters. In general, depending on the type of loading, three limiting criteria are possible:

(1) stress or strain
(2) deformation (e.g. excessive bending)
(3) stability (e.g. kinking or buckling)