Whereas the outer barrier failure distribution is applied to a specified number of packages which are present in the Source, the inner barrier failure distribution is applied to each individual package. That is, it represents the failure distribution of the inner barrier(s) within a given package. One very important implication is that unlike the outer barrier, failure of the inner barrier is not discretized. That is, you do not specify the number of inner containers within each package. Hence, if your conceptual model is that a single inner container exists in each package (e.g., a liner within a drum), the failure distribution represents the fraction of the single inner container which has failed as a function of time. If your conceptual model is that multiple inner containers exist in each package (e.g., multiple drums within a large concrete container), the failure distribution represents the fraction of the inner containers which have failed as a function of time. In either case, the key point is that the fraction does not necessarily represent an integer number of inner containers.
If you specify the Number of Barriers for your Source to be "double", the Inner Barrier… button will be available on the Source dialog. The Inner Barrier… button accesses the following dialog for defining the inner barrier failure modes:
Failure modes can be added and deleted using the Add Failure Mode and Delete Failure Mode buttons. You can move between existing failure modes using the Previous Mode and Next Mode buttons at the top of the dialog.
The behavior of the failure distribution is controlled by the following inputs:
Start when outer barrier fails?: If this box is checked, the failure distribution will not begin until the outer barrier fails. Otherwise, the failure distribution begins at the specified Start Time.
Start Time: Unlike the outer barrier failure distributions, which are defined relative to the Effective Time for the failure mode, inner barrier failure distributions are defined relative to a specific start time. That is, you directly specify the time at which the failure distribution begins (i.e., whether or not it is delayed). If Start when outer barrier fails? is checked, the beginning of the inner barrier failure distribution is controlled by the time of the failure of the outer barrier (and the Start Time is ignored). Otherwise, the failure distribution begins at the elapsed time specified in this field.
Probability: This is the probability that the mode is active for a given portion of an inner container. The default probability for a failure mode is 1 (all parts of the inner container are susceptible). Note that the probability should not be specified as a function of time (if it is, its initial value will be used).
Distribution: This is a list box with six options: Uniform, Weibull, Exponential, User-Defined, Immediate and Table. These define the shape of the failure distribution. Each type of failure distribution requires some number of parameters (e.g., a Uniform requires one, a Weibull requires two). These additional inputs appear in the box directly below the Distribution field labeled "Failure Mode Definition".
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