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(OP)
Is there a non-graphical method (Brock and Franklin) method of converting Is to Is50? (e.g. formulae)

Reading from the correction graphs is a little clumsy and can give errors.

In my opinion, the point load not a very accurate test but tend to give an approximation of strength. In this case errors according to graph ranges are not a big deal.  Otherwise you have to work on the equation of the curve - if mentioned.

Formula is

Is(50) = F x Is

where F = (De/50)** 0.45    (** means to the power of 0.45)
De is the "equivalent core diameter"

De**2 = D**2 for diametrical test,
De**2 = 4A/phi for axial. block and lump tests
(A = WD= minimum cross sectional area of a plane through platen contact point)

Reference :
1.ISRM - "Suggested method for determining Point Load Strength"
2. N. Brook , "The equivalent core diameter method of size and shape correction in Point Load testing", Intl Journal of Rock Mech., Mineral Sci. and Geomechanics  vol 22 no. 2 pp 61-70, 1985

Regards

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