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VABS Recovery: Failure Index Question

  1. Dana Frye


    I am running VABS via preVABS and I had a question regarding the reported values for "Failure Index" when the tsai-hill failure criteria is specified. I've gotten tripped up in the past due to Abaqus and Nastran reporting different values for the tsai-hill failure index. I was hoping to get confirmation, in preVABS/VABS, is the reported tsai-hill failure index calculated using the first formula or the second formula?


    FI #1: σ112 / X2 - σ11*σ22 / X2 + σ222 / Y2 + σ122 / S2


    FI #2: sqrt(σ112 / X2 - σ11*σ22 / X2 + σ222 / Y2 + σ122 / S2)

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    Replying to Dana Frye

    1. Wenbin Yu

      What you listed only works for plane-stress. What uses in VABS is for 3D stresses. Please see attached. 

      For 3D stresses, we output failure index the same as your FI#1 for plane-stress. The strength ratio is the 1/FI#2 for plane-stress. 

      Please let us know you other questions. Please also try iVABS (, many scripts you need to write yourself are also available in that package. 

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      Replying to Wenbin Yu