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Analyzing failure modes using RBDs in Weibull++

ReliaSoft Weibull++ provides the ability to use a reliability block diagram (RBD) to model series, parallel and k-out-of-n configurations. In this article, we will give an example of how to use an RBD to conduct failure modes analysis.



An electronic device can fail due to six independent primary failure modes: A, B, C, D, E and F. The component fails if mode A, mode B or mode F occurs. If mode C, mode D or mode E occurs alone, the component does not fail; however, the component will fail if any two (or more) of these modes occur (i.e., C and D; D and E; E and C; or C, D and E). The objective is to analyze each data set using the 2-parameter Weibull distribution with MLE and to determine the lower 1-sided 90% confidence interval on the reliability of this component at 100 hours.

Experiment and data


The following tables present the time-to-failure data for these modes (in hours).

Mode A   Mode B   Mode C
Number State Time   Number State Time   Number State Time
in Group in Group in Group
1 F 1144   1 F 2221   1 F 19
1 F 1719   1 F 2257   1 F 140
1 F 2129   1 F 2569   1 F 292
1 F 2803   1 F 3029   1 F 432
1 F 3020   1 F 3805   1 F 528
1 F 3082   10 S 5000   1 F 552
1 F 3589            1  F 605
1 F 3973            1  F 734
1 F 4337            1  F 779
1 F 5011            1  F 874
1 F 5029                
18 S 5500                


Mode D   Mode E   Mode F
Number State Time   Number State Time   Number State Time
in Group in Group in Group
1 F 1063   1 F 1290   1 F 605
1 F 1085   1 F 2261   1 F 760
1 F 1399   1 F 2355   1 F 773
1 F 1445   1 F 3209   1 F 854
1 F 1550   1 F 3284   1 F 890
1 F 2056   1 F 3394   1 F 1165
1 F 4384   1 F 3596   1 F 1220
1 F 4863   1 F 4203   1 F 1320
8 S 5000   1 F 4254   1 F 1967
        1 F 4294   1 F 2606
        1 F 4420   1 F 2834
        10 S 5000   15 S 3000



Step 1: Using Weibull++, the first step is to create a new data sheet for grouped times-to-failure data with suspensions.

Step 2: Rename the folio to "Component" and the data sheet to "Mode A." Enter the data given for Mode A and use the 2-parameter Weibull distribution with MLE to calculate the parameters, as shown next.

Step 3: Insert another data sheet of the same type named "Mode B," enter the data given for Mode B and calculate the parameters. Repeat for the remaining failure modes.

Step 4: Choose Home > Insert > Diagram.

Step 5: Build the RBD that describes the reliability-wise configuration of the failure modes, as shown next.

Note that the node in the diagram requires 2 of the 3 paths to succeed.


Step 6: Click Calculate to analyze the diagram.

Calculate Icon


Then use the Quick Calculation Pad (QCP) to calculate the reliability at 100 hours with the lower 1-sided 90% confidence bound, as shown next.

The lower bound is estimated to be 96.71%.