Safety Factors

Except for gauged elements (previously discussed), safety factors of 4.0 on ultimate and 3.0 on yield must be maintained on parts and hardware. Plastic bending analysis is not accepted.

Refer to the Ames fastener guidelines later in this section. Also, if desired, contact the Test Manager for a current copy of the NASA Ames Research Center Fastener Supplier list.

Shear Stresses

If the shear ultimate strength of the material is unknown, calculate it as 60 percent of the tensile ultimate strength.

Thermal Stresses

Any thermal stresses that could occur must be algebraically subtracted from ultimate tensile and tensile yield strength of materials before the factors for allowable stresses are applied.

Material Properties

Material stress properties should reflect the expected minimums that will occur within the expected temperature range.

Buckling Stress

The allowable compressive stress in columns and skins must be equal to or less than one-third of the critical buckling stress.

Oscillating Stresses

Allowable oscillating stresses caused by oscillating loads with or without accompanying steady-state loads must be computed as follows:

• The mean stress, if any, must be applied to the proper Modified Goodman Diagram to which a safety factor of four (4.0) has been applied.

• The gross allowable oscillating stress must then be obtained from this diagram.

• The allowable oscillating stress must be obtained by dividing the gross oscillating stress by the appropriate stress concentration factor, if any.

Impact Strength

All material must have a minimum Charpy V impact strength of 15 foot-pounds at test conditions.