### 2.2.4. Linearization of Inverse Reserve Factors

Failure criteria are functions that describe a failure envelope and the output of the function is the inverse reserve factor (IRF). IRF is a measure of where the load point is in relation to the failure envelope. IRFs calculated in ACP-Post can differ from those determined in Mechanical APDL. This is because IRFs in ACP-Post are linearized.

For failure criteria without quadratic terms, such as maximum strain or maximum stress, the failure output from Mechanical APDL should match closely with IRFs from ACP-Post. For those involving quadratic terms, however, IRFs output from ACP are normalized and will thus not match those output from Mechanical APDL. Because of this normalization of IRFs, an increase of twice the force does not result in four times the IRF in ACP-Post.

The determination of failure criteria being exceeded does not change as a result of the linearization (because ). However, the numerical values for failure criteria involving nonlinear differs between ACP-Post and Mechanical APDL.

The implementation of some failure criteria (Puck, LaRC, etc.) differs between ACP-Post and Mechanical APDL.

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