Z is the 1947 Batdorf parameter:

Z = L*2(1 - nu*2)*(1/2)/(rt)

The scattered points are from experiments.

FROM:

George Gerard and Herbert Becker, “Handbook of Structural Stability, Part III - Buckling of Curved Plates and Shells”, NACA Technical Note 3783, August 1957

SUMMARY: Available theories and test data on buckling of curved plates and shells are reviewed. For torsion and external pressure loadings, the test data are correlated in terms of linear buckling theories for both the elastic and inelastic ranges. The cases which exhibit a marked disagreement between linear theory and test data include those of curved plates and cylinders under axial compression, cylinders under bending, and spherical plates under external pressure. These cases have been analyzed by a unified semiempirical approach for both the elastic and inelastic ranges which is satisfacotry for analysis and design purposes. The effects of internal pressure on buckling of elements under uniaxial loads are discussed and data on various combined loadings are presented in interaction form.

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