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Some results of the supersonic transport materials research conducted by Douglas-McDonnell in the areas of fracture toughness, and by Douglas in the area of crack propagation, are presented. The results of tests on centrally notched fracture toughness specimens of steels, titaniums, and superalloys are described. The crack propagation studies show results of testing cracked specimens under uniaxial and biaxial states of stress at room and elevated temperatures, and the extent of material deterioration in stiffened panels. Methods of analyses for crack propagation and residual crack strength are presented and comparisons are made between analysis and test.

The newest addition to the Douglas family of airplanes is the short to medium haul DC-9. This paper includes a brief description of the DC-9 hydraulic systems highlighting special design criteria. Also included is a discussion of some of the new built-in design features which have been incorporated to assist the airlines in trouble shooting and maintenance. The attendant reduction in unnecessary component removals will not only reduce operating costs and overhaul time but will also result in a marked improvement in reliability.