Time-to-Failure Data

Time-to-failure (continuous) data is the most commonly observed type of reliability growth data. It involves recording the times-to-failure for the unit(s) under test. Time-to-failure data can be applied to a single unit or system or to multiple units or systems. There are multiple data entry schemes for this data type and each is presented next.

Failure Times Data

This data type is used for tests where the failure times are recorded. The data can be entered in a cumulative or non-cumulative format, as shown next.

Figure 3.2: Failure Times data for a single system in cumulative format

Figure 3.3: Failure Times data for a single system in non-cumulative format

Grouped Failure Times

This data type is used for tests where the exact failure times are unknown and only the number of failures within a time interval are recorded (e.g. inspection data). For a single system, multiple failures can occur before the operator stops the test. In this case, X number of failures are found after Y hours of test time. Failures X1, X2, X3, etc. could have occurred at any time period up to the termination time, thus exact times for each failure are not available. This is commonly called interval or grouped data. When multiple units are tested, the units are inspected at predetermined time intervals and the number of failed units is recorded. The failed units are then subsequently repaired and put back into the test or removed. When entering the time at which the number of failures occurred for grouped data, the time is equal to the total accumulated test time for all of the units being tested. The number of failed units is simply equal to the number of failures that occurred during the current interval. Figure 3.4 shows an example of data entry for grouped data.

Figure 3.4: Grouped Failure Times data

Multiple Systems (Known Equivalent Times)

This data type is used for tests where a number of systems are tested. If a failure occurs in any system, a corrective action is taken on the failed unit and any design changes are incorporated into all test systems. Once the corrective actions have been implemented, the test is resumed. The time-to-failure for the failed system, along with the current operating times of all other systems, are recorded. The data can be cumulative or non-cumulative. Consider the following table where the Failed Unit ID column indicates which unit failed. For example, if you enter 2 into the Failed Unit ID column, this indicates that the unit in the Time Unit 2 column is the one that failed. For the units that did not fail, you must enter the appropriate operating times for each unit at the time of the failed unit into the corresponding Time Unit columns.

Figure 3.5: Multiple Systems (Known Equivalent Times) data

In this table, two units are undergoing testing and the units do not accumulate age at the same rate. At 10 hours into the test, unit 1 fails and corrective action is taken on both units 1 and 2. By this time, both units have accumulated 10 hours of operation. At 17 hours, unit 2 fails and corrective action is again implemented on both units; however, unit 1 has accumulated 5 hours and unit 2 has accumulated 7 hours since the last event, and so forth.

Multiple Systems (Unknown Equivalent Times)

This data type is used for tests where a number of systems are tested and the event (start, failure or end) and the time of the event are recorded. This data type assumes uniform time accumulation and that the systems are tested simultaneously. As an example, consider the data of two systems shown in Figure 3.6. System 1 begins testing at time equals 0 (with a start event, S) and failures are encountered and corrected at 281, 312 and 776. Testing stops at 1000 hours (an end event, E). System 2 begins testing at time equals 0 (with a start event, S) and failures are encountered and corrected at 40, 222 and 436. Testing stops at 500 hours (an end event, E). RGA has two options to view this type of data. Figure 3.6 shows the normal view, while Figure 3.7 shows the advanced view. Both figures represent the same data.

Figure 3.6: Normal view for Multiple Systems (Unknown Equivalent Times) data

Figure 3.7: Advanced view for Multiple Systems (Unknown Equivalent Times) data

When entering data within the normal view each system must be initiated with a start event. The format displayed in Figure 3.6 is simply a suggestion.

Multiple Systems (Unknown Equivalent Times) - Using Dates

This is similar to the previous data type except that dates are utilized along with operating hours. This assumes noncontinuous usage and the software computes equivalent (average) usage rates. Figure 3.8 shows an example of this in RGA.

Figure 3.8: Normal view for Unknown Equivalent Times for Multiple Systems data with dates

Models for Time-to-Failure (Continuous) Data

The following models can be used to analyze time-to-failure (continuous) data sets. Models and examples using the different data types are discussed in later chapters of this on-line reference.