SubscribeStandard Extrapolation Method and Impact Performance of Pipes from Impact Laboratories.
Pipe SEM: Standard Extrapolation Method
IMPACT Laboratories offers pipe pressure testing over a range of pressures and temperatures to generate data of the form shown above in accordance with ISO/TR 9080. SEM-calculations then define the classification of pipe, which indicates the minimum required strength at 20°C for a lifetime of 50 years.
Impact Performance of Pipes
Impact failure in pipes is potentially catastrophic; the nightmare scenario is one of rapid crack propagation (RCP) axially along a pipeline. This potential situation is simulated using the small scale S4 test (ISO 13477) and on a larger scale via the British Gas full scale test (ISO 13478).
Specifically selected charpy impact assessment offered by IMPACT laboratories can significantly reduce pipe and material development costs.
In pipe RCP tests, a crack is initiated and propagated into a section of pressurised pipe. The propagation of this crack along the full test length is governed by the temperature, pressure and material RCP resistance of the pipe test section. Observation of the fracture surfaces from RCP tests shows that surface yielding is the mechanism for promoting RCP resistance.
For pipes of a given dimension and material, a critical temperature can be defined (Tc ) as the temperature above which crack propagation will not occur in a pipe, irrespective of pipe pressure.
In laboratory Charpy impact tests, the fracture surface of the impact specimen shows identical features to those observed in pipe failures - crazing at the notch which controls fracture initiation and surface yielding which controls crack propagation.
The energy of fracture can be split into initiation and propagation contributions, allowing us to measure the propagation energy.
This test can be performed on compression-moulded sheet to assess material suitability or directly on specimens cut from extruded pipe to indicate final performance.
A one-to one correspondence exists between the critical pipe temperature and the ductilebrittle temperature as shown.
The corresponding measure of surface yielding behaviour is the ductile-brittle transition temperature (Tdb ).
This is determined by performing instrumented Charpy tests at a range of temperatures to determine the temperature (Tdb ) below which there is zero propagation energy and thus no resistance to crack growth.
The important consequence for pipe performance is that assessment of RCP resistance can be made from Charpy tests on pipe and hence minimise the need for expensive S4 and full scale pipe evaluation.