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[[Image:rga2.1.png|thumb|center|400px|Growth potential and management strategy.]]
[[Image:rga2.1.png|center|400px|Growth potential and management strategy.]]

Revision as of 23:36, 18 April 2012

Why Reliability Growth?

It is typical in the development of a new technology or complex system to have reliability goals. Each goal will generally be associated with a failure definition. The attainment of the various reliability goals usually involves implementing a reliability program and performing reliability tasks. These tasks will vary from program to program. A reference of common reliability tasks is MIL-STD-785B. It is widely used and readily available. Table 2.1 presents the tasks included in MIL-STD 785B.

Table 2.1 - MIL-STD-785B reliability tasks
Design and Evaluation Program Surveillance and Control Development and Production Testing
101 Reliability Program Plan 201 Reliability modeling 301 Environmental Stress Screening (ESS)
102 Monitor/Control of Subcontractors and Suppliers 202 Reliability Allocations 302 Reliability Development/Growth Test(RDGT) Program
103 Program Reviews 203 Reliability Predictions 303 Reliability Qualification Test (RGT) Program
104 Failure Reporting, Analysis and Corrective Action System (FRACAS) 204 Failure Modes, Effects and Criticality Analysis(FMECA) 304 Production Reliability Acceptance Test (PRAT) Program
105 Failure Review Board(FRB) 205 Sneak Circuit Analysis(SCA)
206 Electronic Parts/Circuit Tolerance Analysis
207 Parts Program
208 Reliability Critical Items
209 Effects of Functional Testing, Storage, Handling, Packaging, Transportation and Maintenance


The Program Surveillance and Control tasks (101-105) and Design and Evaluation tasks (201-209) can be combined into a group called basic reliability tasks. These are basic tasks in the sense that many of these tasks are included in a comprehensive reliability program. Of the MIL-STD-785B Development & Production Testing tasks (301-304) only the RDGT reliability growth testing task is specifically directed toward finding and correcting reliability deficiencies. For discussion purposes, consider the reliability metric mean time between failures (MTBF). This term is used for continuous systems, as well as one shot systems. For one shot systems this metric is the mean trial or shot between failures and is equal to [math]\displaystyle{ \tfrac{1}{failure\text{ }probability} }[/math] .
The MTBF of the prototypes immediately after the basic reliability tasks are completed is called the initial MTBF. This is a key basic reliability task output parameter. If the system is tested after the completion of the basic reliability tasks then the initial MTBF is the mean time between failures as demonstrated from actual data. The initial MTBF is the reliability achieved by the basic reliability tasks and would be the system MTBF if the reliability program were stopped after the basic reliability tasks had been completed. The initial MTBF after the completion of the basic reliability tasks will generally be lower than the goal. If this is the case then a reliability growth program is appropriate. Formal reliability growth testing is usually conducted after the basic reliability tasks have been completed. For a system subjected to RDGT, the initial MTBF is the system reliability at the beginning of the test. The objective of the testing is to find problems, implement corrective actions and increase the initial reliability. During RDGT, failures are observed and an underlying failure mode is associated with each failure. A failure mode is defined by a problem and a cause. When a new failure mode is observed during testing, management makes a decision not to correct or to correct the failure mode in accordance with the management strategy. Failure modes that are not corrected are called A modes and failure modes that receive a corrective action are called B modes. If the corrective action is effective for a B mode, then the failure intensity for the failure mode will decrease. The effectiveness of the corrective actions is part of the overall management strategy. If the RDGT testing and corrective action process are conducted long enough, the system MTBF will grow to a mature MTBF value in which further corrective actions are very infrequent. This mature MTBF value is called the growth potential. It is a direct function of the design and management strategy. The system growth potential MTBF is the MTBF that would be attained at the end of the basic reliability tasks if all the problem failure modes were uncovered in early design and corrected in accordance with the management strategy.
In summary, the initial MTBF is the value actually achieved by the basic reliability tasks. The growth potential is the MTBF that can be attained if the test is conducted long enough with the current management strategy. See Figure GP.
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Growth potential and management strategy.