For each design load case, at both limit and ultimate conditions, HyperX will calculate two different load types: Strength and Buckling. The FEA Loads are processed separately for each load type. During sizing and analysis, Strength loads are run for failure criteria that are more localized in nature, such as material strength and local buckling. Whereas buckling loads are evaluated for failure criteria that are more zone-level, such as panel buckling.
The plot below, taken from HyperLoads, shows an example of the different design-to load envelopes - strength vs. buckling - that are calculated for a single zone.
Note
These envelopes are also affected by the limit and ultimate safety factors of each design load case. So, it is not always expected that the limit vs. ultimate load envelopes be mathematical scalars of one another, as limit and ultimate factors are not always the same for each design case.
Strength Loads are used for material strength, crippling, sandwich, and local buckling failure categories.
Tip
Strength loads are calculated based on the FEA Loads Processing technique prescribed to the zone via a load property.
By default, tensile or compressive stiffness properties are used when computing margins of safety with strength loads. Tension/compression depends on the sign of the dominant load direction (\(N_x\) or \(N_y\)).
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If \(|N_x| >= |N_y|\), the sign of \(N_x\) is taken to define tension vs. compression load state.
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If \(|N_y| >= |N_x|\), the sign of \(N_y\) is taken to define tension vs. compression load state.
Alternatively, average stiffness properties can be used with strength loads by enabling the "Use Average Modulus" option in the Advanced Settings.
Buckling Loads are used for panel buckling and stiffener buckling failure categories. Compressive stiffness properties are always used when computing margins of safety for buckling loads. When extracting buckling loads from FEA, the average compressive element force are used for Design-To Buckling Loads.
Tip
Buckling loads are unaffected by the load property assigned to the zone, as buckling loads are always calculated as an element area weighted average over the zone. (see Average Loads Processing).