The Hendrix Group Reporter^©

In Mechanical Engineering, ASME has published an informative article on recent changes to the material design factor (MDF) used in calculating allowable stress values under the Code's Section I and Section VIII, Division I Standards. The new figure is contained in the 1999 Addenda to the 1998 edition of the Code and relaxes the material design factor from 4.0 to 3.5. Relaxation of the MDF has the effect of increasing material allowable stresses used in pressure vessel design. No change was made to maximum allowable stress based on the existing 1.5 yield strength MDF figure. Nor were changes made to allowable stress values in the time-dependent regime, where creep and stress rupture properties are dominant.

The MDF reduction decision was based on advances in steelmaking quality, fracture toughness technology, welding and non destructive inspection techniques over the last half century. The article continues with a discussion of the history of the MDF and revisions to it since the Code was first published in 1914. For more information on this subject, see Mechanical Engineering, Vol. 122, No. 2, February 2000.

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