Boeing Company Undifferentiated or Other Model
Other Stress Analyst; Ph.D. Materials Science
Other / Unknown
Deviation / Discrepancy - Procedural Published Material / Policy
Environment - Non Weather Related; Procedure
A nonconservative margin of safety calculation has been used on components for the [fleet of aircraft] at the ZZZ location of the [Company]. The calculation of margin of safety; MS; compares ultimate tensile strength Ftu to stress s:MS = Ftu/s - 1A positive MS is wanted to prevent overload failure. The maximum principal stress s1 was sometimes used at [Company] in the MS calculation; i.e.; s = s1. This calculation is non-conservative; specifically when the stress state includes a compressive principal stress; s2. In the two-dimensional case; the Tresca failure criterion stress is:sTresca = s1 - s2Because s2 < 0; i.e.; compressive; sTresca is larger than the maximum principal stress; s1.Using sTresca in the MS formula therefore predicts failure at lower load levels than using s1.For ductile metals such as aerospace aluminum alloys; the Tresca criterion matches observed failure behavior; as described in Dowling's text Mechanical Behavior of Materials. The first principal stress criterion applies only to brittle materials such as gray cast iron.An example calculation shows the possibility of generating a specious positive margin of safety. The MS1 calculated with first principal stress indicates positive margin (predicts no overload failure):s1 = 70 ksis2 = -50 ksiFtu = 100 ksiMS1 = 100/70 - 1 = 0.42but the margin MSTresca calculated with sTresca predicts failure:MSTresca = Ftu/(s1 - s2) - 1 = 100/[70 - (-50)] - 1 = 100/120 - 1 = -0.17      An explanation of the stress tensor and its expression in terms of principal stresses can also be found in Dowling's text.
Reporter has concerns about calculating structural safety margins. Reporter was involved with calculating structural safety margins for fuel tanks and other components. Reporter insinuated that proper analysis was done in the past. Reporter stated that reporter did not know for sure; but thinks that for these fuel system components; a negative structural margin of safety is possible on overload.
Design Engineer reported that nonconservative structural margin analysis/calculations were used on aircraft fuel tanks and fuel system components.