ASTM D2241 Standard, stipulated the test method of the flattening test on Clause 8.6:

8.6 Flattening Test—Flatten three specimens of the pipe, 2 in. (50 mm) long, between parallel plates in a suitable press until the distance between the plates is 40 % of the outside diameter of the pipe. The rate of loading shall be uniform and such that the compression is completed within 2 to 5 min.

On removal of the load, examine the specimens for evidence of splitting, cracking, or breaking.

– Minimum load required (no deflection limit indicated)
– Flattening test to 40% with no visible cracking

ASTM D2241 - 15
Standard Specification for Poly(Vinyl Chloride) (PVC) Pressure-Rated Pipe (SDR Series)