Modulus of elasticity
We have shown that the modulus of elasticity, E, is an important design consideration. In
axial tests (compression or tensile), the stressstrain curves of PVC are non-linear, so one needs
to consider both the tangent modulus and the secant modulus (Fig. 7). The tangent modulus is
larger than the secant modulus. In high-load applications the use of the secant modulus is more
appropriate for precise deflection calculations. Because sheet piles are manufactured by an
extrusion process, one must determine whether any directionalities of the properties are induced
in the material. A recent study of a manufactured vinyl sheet pile by Tom and Tom (2002) of
ERDC-GSL has shown that there is no significant anisotropy in the material. Temperature has a
significant effect on the elasticity modulus. The modulus increases at lower temperatures and
decreases at higher temperatures. Tests performed on a commercial sheet piling PVC (Fig. 8)
have shown that above 140F the reduction of modulus is significant, and above 180F the
modulus reduces drastically.
Figure 7. Stressstrain curve of a visco-elastic material.
The other most important property is strength. Both tensile and compressive yield strengths
are important for determining at what loads or moments the sheet pile will fail. Of course the
PVC sheet pile, given its high elongation property, can hardly fail under service load but may
become very unstable from accidentally applied extreme overload. Flexural strength properties of
the PVC sheet pile materials are also considered useful, especially to assess the deflection
parameters directly under the flexural loads and their modes of failure.
All visco-elastic materials suffer creep, in which the material continues to deform under a
sustained constant load until it fails (Fig. 9). At low loads, creep is hardly a problem, because it
takes an extremely long time to deform; however, at a higher applied load, PVC may creep, and a
higher temperature may accelerate the creep deformation.