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Composites, Tsai Wu Failure Index

[ Edited ]

Hello,

 

one question to the Tsai Wu failure index in NX, which appears in the graphical laminate report. Everything above 1 means the laminate will fail and everything below 1 means "everything" is ok, is this right?

 

Thanks

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Accepted by topic author tsai_wu
‎05-20-2016 01:35 PM

Re: Composites, Tsai Wu Failure Index

[ Edited ]

Hei

 

Yes, a failure index above 1 reports over usage.

From NX advanced simulation help:

"When you use the shell composites (PCOMP/PCOMPG) or solid composites (PCOMPS), you can optionally output a failure index and strength ratio for individual laminae and for the laminate bonding material.

Failure indices assume a value of one on the periphery of a failure surface in stress space.

  • If the failure index is < 1, the lamina stress is interior to the periphery of the failure surface and the lamina is predicted to be safe.

  • If the failure index is > 1, the lamina stress is exterior to the periphery of the failure surface and the lamina is predicted to have failed.

The failure indices represent a phenomenological failure criterion in that only an occurrence of a failure is indicated and not the mode of failure.

Strength ratio is a more direct indicator of failure than the failure index since it demonstrates the percentage of applied load to the failure criteria. Strength ratio is defined as:

Strength Ratio (SR) = Allowable Stress / Calculated Stress"

 

Personally I prefer to use the Strength Ratio (SR). This is similar to the traditional Safety Factor, meaning the factor you can increase the load with before having failure. Note that consequently the SR is a "linearized" factor, the traditional Tsai-Wu factor is not. Look in the litterature and NX help for more help.

 

Best regards,

Geir Olav Guddal