Chapter 3: Reliabilty Allocation and Software Reliability Flashcards
(77 cards)
What is reliability allocation
It is a process of assigning reliability requirements to individual components for achieving specified system reliability. It is a process of assigning reliability values to subsections of a system and these add up to become the reliability of the entire system.
What are the benefits of reliability allocation
It forces people involved in design to understand and establish appropriate relationships between reliabilities of components and parts. It forces engineers to consider reliability equally with other design parameters such as cost, performance, weight, etc.
What are the most common methods of reliability allocation
Hybrid, Similar Familiar Systems Method, Factor of Influence Method
What is the similar familiar system Method of reliability allocation
This method makes use of failure data collected on a similar system and subsystems.
What is the major disadvantage of the similar familiar method of reliability allocation
It assumes that the life cycle, cost, and reliability of past similar designs were satisfactory.
What is the Factors of Influence method for Reliability Allocation
It is based on the assumption that factors such as failure criticality, environment, complexity & time, and state of the art affect the system’s reliability.
How does the factor of influence method work?
Each item is evaluated with respect to each of the four factors on a scale of 1-10. 10 means the item is most affected, 1 means least affected.
What is failure criticality
This factor considers the criticality of failure in the item on the system.
What is the environment factor
This accounts for the item’s exposure or susceptibility to environmental conditions such as vibration, temperature, and humidity.
What is the Complexity and time factor
Relates to the number of subsystem parts and the relative operating time of the item during the functional period of the system.
What is the state of the art factor
Takes into account the advancement in the state of the art at a given time.
How does the Hybrid reliability allocation work
Combines the similar familiar systems and the factor of influence methods by assigning weights to both.
What are the major classifications of failure in electronic systems?
Load exceeding rated or derated values under transient, steady-state or test conditions, and non-load related factors.
What is the MIL-HDBK-217 standard
A standard for reliability prediction in electronic systems providing failure rate models for electronic components.
What are the main features and assumptions of MIL-HDBK-217
The system is repairable, component failures contribute to system failure rate, and failure times are independent and exponentially distributed.
What is the general failure rate model given by MIL-HDBK-217
λp = λb * πQ * πE * πA (base failure rate, quality factor, environment factor, application factor)
What is the base failure rate of an electrical component
Related to temperature and based on diffusion reaction rates governed by the Arrhenius equation.
What is the formula for the Arrhenius equation
R(s) = e^{-lamda * t}
What’s the difference between the failure in mechanical and electrical systems
Mechanical systems wear out; electrical systems do not wear out in the same sense.
What are the stress and non stress factors that can cause failure in electrical components
Parameter drift, transient, electromigration, electromagnetic interference.
What are Transient electrical stresses
Short duration high voltages or currents affecting delicate electrical components.
What are the causes of transient electrical stresses
Capacitive/inductive effects, ESD, lightning, switching transients.
What type of transients are lower power diodes vulnerable to
Transients
What are metal oxides vulnerable to
ESD
