2.1. Failure Mode Measures

Failure mode measures are used to represent the loading of materials by means of a value. Three failure mode measures are available:

  • IRF = Inverse Reserve Factor (IRF) defines the inverse margin to failure. Load divided with IRF is equal to the failure load. IRF > 1 discloses failure.

  • MoS = Margin of Safety (MoS) defines the margin to failure. MoS is defined as (1/IRF - 1). MoS < 0 discloses failure.

  • RF = Reserve Factor (RF) defines the margin to failure. Load multiplied with RF is equal to the failure load. RF < 1 discloses failure.

 

2.1.1. Detailed Descriptions

The reserve factor indicates margin to failure. The applied load multiplied by the reserve factor gives the failure load:

(2–1)

Reserve factor values greater than one indicate positive margin to failure and values less than one indicate negative margin. The values of reserve factors are always greater than zero.

The critical values of reserve factors lie between zero and one, whereas the non-critical values range from one to infinity. Whether the results are shown in numeric form or as contour plots, the non-critical values tend to be emphasized in comparison to critical values. Therefore, the inverse reserve factor is often preferred in practical use:

(2–2)

The non-critical values of range from zero to one and the critical values from that on.

The margin of safety is an alternative for the reserve factor in indicating margin to failure. The margin of safety is obtained from the corresponding reserve factor with the relation

(2–3)

A positive margin of safety indicates the relative amount that the applied load can be increased before reaching failure load. Correspondingly, a negative margin of safety indicates how much the applied load should be decreased. Margins of safety are typically expressed as percentages.

 

2.1.2. 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 can differ from those determined in Mechanical APDL. This is because IRFs in ACP 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. For those involving quadratic terms, however, IRFs output from ACP are normalized and will therefore 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.

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

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