05 June 2009
05 June 2009
Reichhold has proposed a simple, quick and inexpensive test method to estimate the long-term HDB of pipes, which is quite distinct from the complex, time-consuming and expensive classical method described in ASTM D 2992.
The HDB, or Hydrostatic Design Basis, is the stress or strain that fails the composite pipes on the long-term. It is estimated by extrapolating the stresses/strains that fail water filled pipe specimens subjected to different pressure levels. Since the pipe specimens are tested filled with water, their mode of failure is by weeping, not by burst. The estimation of the HDB is a difficult and expensive process. As a result, to avoid this cost and expense, the industry has shown interest in replacing it with simpler short-term tests.
The new method recognizes that weep occurs when small cracks that initiate and grow in the critical ply eventually coalesce to form a pathway for the water to pass through. If the strains on the critical ply are small enough, below a certain threshold value, these cracks do not form and the pipe does not weep. The first acoustical emissions measured on strained isolated critical plies serve as an indicator of this threshold strain. Below the threshold limit, the regression line changes its slope and becomes horizontal.
While the classical ASTM D 2992 derives a short-term regression line by extrapolating weep failures, the new method obtains the long-term HDB by measuring the threshold acoustical emissions of the isolated critical ply.
The method was presented in the paper ‘Weep failure and the HDB’, by Antonio Carvalho Fº from Reichhold at the Construction, Corrosion and Infrastructure Conference in Las Vegas. The paper highlighted the dominance of the resin system on the HDB, with a minor influence from the sizing on the glass fibres. As a consequence, the HDB is a property of the resin, quite independent of the pipe’s manufacturing process and wall thickness.
There are three independent long-term failure modes for composite pipes. The first mode, burst, is controlled by the continuous glass fibres. The second mode, weeping, is controlled by the resin. And the third mode, known as strain-corrosion, is controlled by both the glass and the resin. Over the last few years Reichhold has developed a series of three papers dealing with each of these modes of failure. The first paper was presented in 2007 and discussed burst failure. This year’s paper is a sequel to the first and addresses the HDB and weep failure. The third and last paper in the series, dealing with the topic of strain corrosion and the long-term performance of composite pipes in aggressive environments, will be presented in the future.