For ISO 14692 2017, specifies the long-term axial stress at 0:1 stress ratio. Typically, the axial stress (hoop stress is 0 at this point) is lower than the axial stress al(2:1) (hoop stress is double the axial stress at this point).
Typical units (depending on your unit system):
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lb/in2
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KPa
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N/mm²
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For information on failure envelope points, see Figure B.1 of ISO 14692-2:2017.
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The ratio of these stresses, called bi-axial stress ratio, can range between 0.5 and 0.75 for plain pipe depending on the winding angle and specific pipe type.
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If you do not define al(0:1), then you must define a_R and h_R. The software then calculates al(0:1) from a_R, h_R, al(2:1), and hl(2:1) according to section B.1 in ISO 14692-2:2017.