The mechanical properties (stiffness and strength) of the fiberglass are time dependant, which is the characteristic that most of the plastic materials have.
The TOPFIBRA fiberglass pipe (as all the GRP-BRP pipes) shows both the creep and the relaxation. Several performed tests ascertain and measure these phenomena according to the relevant international standards.
Creep was studied through the long term pressure tests according to ASTM D2992. Several specimens were loaded with the constant internal pressure (i.e. the constant stress) at different levels. For the creep phenomenon, the strain in the pipe wall increases continuously over the time, leading to the pipe failure (leaking) after several hours. The failure points were recorded and analysed according to the standard.
The result of the test is a decay of approximately 35% in the failure stress after 10,000 hours, which is extrapolated to 45% after 50 years.
The design stress for the internal pressure is generally taken at the ½ of the failure stress at 50 years and thus ¼ of the short term failure stress. This is also reflected by most of the standards that require a short time strength of the pipe wall in the hoop direction, which is exactly 4 times the hoop stress for the pressure class. For an example of this, see the Table 8 of the ASTM D3517 Standard Specification or the equivalent Table 10 of the AWWA C950 Standard.
The long-term failure stress due to the internal pressure is called the “Hydrostatic Design Basis” (HDB), and it is often expressed as a strain rather than a stress. The long or short time failure stress varies considerably in the fiberglass pipes, due to the variability of the wall structures that can be realized, with different reinforcement content and arrangement. On the contrary, the failure strain is more constant. The excess strain causes most of the failures of the pipe, resulting in the micro-cracks in the resin; debonding of the reinforcement; and weeping of the pipe, without catastrophic ruptures of the reinforcement.
The design strain, applying the Safety Factor of 1.8÷2 as required by the Specifications, generally ranges between 0.18% and 0.35%. The starting value conservatively suggested by TOPFIBRA is 0.25÷0.30%, even if the results of the tests are often quite higher.
Another creep test is conducted according to the ASTM D5365 “Standard Test Method for Long-Term Ring-Bending Strain of "Fiberglass" (Glass-Fiber-Reinforced Thermosetting- Resin) Pipe”. Several pipe ring specimens are subject to the constant loads of different values, up to a catastrophic failure or to a significant reduction in the mechanical properties. The parameter resulting from this test is called the “Strain basis” ( ) and is the long term failure strain of the pipe in a deflected condition. This value is used, for example, to calculate the allowable vertical deflection for a buried pipe (AWWA M45, Section 5.7.2). b S
Relaxation is measured with another long-term test, similar to ASTM D5365, but with the pipe ring specimens subject to the constant deformations of different extent during the test. The guiding standard is ASTM D3681 “Standard Test Method for Chemical Resistance of "Fiberglass" (Glass-Fiber-Reinforced Thermosetting-Resin) Pipe in a Deflected Condition”, also called the “strain corrosion test”, i.e. failure of the pipe wall caused by the exposure of the inside surface, while in strained condition, to a corrosive environment for a period of time. Generally, the used testing solution contains 5% of the sulphuric acid.
The strain corrosion test is especially required for the sewer pipes, mainly in the warm climatic conditions. The design parameter derived from the test is the “Strain Basis”, which is used alternatively to the one derived from the creep test to ASTM D5365.
The elastic modulus would also require a similar analysis.