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The S4 test evaluates the resistance of a pipe specimen against the steady-state RCP (rapid crack propagation) at a given temperature (e.g. at 0 deg C). The crack is initiated by an impact of a striker-blade that penetrates into the pipe wall at a velocity of (15 +/- 5) m/s. The fast-running longitudinal crack thus initiated is driven by the potential energy of pressurised gas in the pipe. The test is carried out according to the ISO/FDIS/13477 standard 'Thermoplastic pipes for the conveyance of fluids - Determination of resistance to RCP (rapid crack propagation) - Small-scale steady-state test (S4 test)'. The standard defines the following conditions for the crack initiation: the shape and velocity of the crack-initiation equipment, conditioning of the pipe and the maximum time interval (3 minutes) allowed for the manipulation prior to the test. It is shown that certain parameters of the equipment are defined insufficiently, namely the shape of crack-initiation equipment and a maximum allowed difference of the striker velocities. These latter may, however, become a source of very great differences of the rate and the extent of deformation. It is also shown that the temperature gradient across the pipe wall is significantly changed during the allowed manipulation time interval - a factor that can seriously influence the crack-initiation. This is demonstrated by the temperature dependence of the length of the impact-initiated crack on a PE 80 grade polyethylene pipe. In addition, the reliability of determination of critical pressure is discussed. This is defined as the highest crack-arrest pressure below the lowest crack-propagation pressure. This situation can be described as a limiting state of the pressurised pipe before the transition state.. This limiting state has to be looked upon as a statistical variable. Analogies can be found in the temperature dependence of the Charpy impact strength (ISO 179-1) or falling-weight dart tests (ISO 6603-1, ASTM D 3029). To formulate the statement about the reliability of the determined critical pressure as a statistical variable is very difficult. It can be shown, however, that the probability level of the critical crack assessment depends substantially on the number of tests performed immediately below the lowest pressure (in the range of one experimental pressure step) at which the RCP was observed.