Parts stress modelling
Parts stress modelling is a method in
engineeringand especially electronicsto find an expected value for the rate of failureof the mechanical and electronic components of a system. It is based upon the idea that the more components that there are in the system, and the greater stress that they undergo in operation, the more often they will fail.
Parts count modelling is a simpler variant of the method, with component stress not taken into account.
Various organisations have published standards specifying how parts stress modelling should be carried out. Some from electronics are:
* MIL-HDBK-217 (US Department of Defense)
* SR-322 (
Telcordia, previously Bellcore)
* HRD-4 (
* and many others
These "standards" produce different results, often by a factor of more than two, for the same modelled system. The differences illustrate the fact that this modelling is not an exact science. System designers often have to do the modelling using a standard specified by a customer, so that the customer can compare the results with other systems modelled in the same way.
All of these standards compute an expected overall failure rate for all the components in the system, which is not necessarily the rate at which the system as a whole fails. Systems often incorporate redundancy or
fault toleranceso that they do not fail when an individual component fails.
Several companies provide programs for performing parts stress modelling calculations. It's also possible to do the modelling with a
All these models implicitly assume the idea of "random failure". Individual components fail at random times but at a predictable rate, analogous to the process of
nuclear decay. One justification for this idea is that components fail by a process of wearout, a predictable decay after manufacture, but that the wearout life of individual components is scattered widely about some very long mean. The observed "random" failures are then just the extreme outliers at the early edge of this distribution. However, this may not be the whole picture.
All the models use basically the same process, with detailed variations.
* Identify the components in the system
** Such as R123, 10kOhm carbon film resistor
* For each component, determine the component model to use from the standard
** Such as "resistor, film, < 1 Megohm" or "Connector, multi-pin"
* From the standard's component model, discover what, if any, complexity parameter is needed, and find the value of that parameter for this component
** Such as pin count for a connector or gate count for a chip
* From the standard's component model, discover what thermal stress curve applies, and find the value of the temperature in operation for this component
** The failure rate of connectors may change little with temperature, while that of capacitors may change greatly
* From the standard's component model, discover what, if any, part stress parameter is needed, what part stress curve applies, and find the value of that part stress parameter for this component in this application
** A part stress might be the applied power as a fraction of the component's rated power, or the applied voltage as a fraction of the rated voltage
* From the standard's component model, find the base failure rate for this component, and modify that according to the complexity parameter, the operating temperature and thermal stress curve, the part stress parameter and part stress curve, with arithmetic specified by the standard. This now is the expected failure rate for this component in this application
* Add up all the results for every component in the system to find the overall failure rate for all components in this system.
Other global modification parameters can be employed, which are assumed to have the same effect on every component failure rate. The most usual are the environment, such as ground benign or airborne, commercial, and the purchasing quality assurance process. The standards specify overall multiplier factors for these various choices.
Wikimedia Foundation. 2010.
Look at other dictionaries:
Stress (physics) — Stress is a measure of the average amount of force exerted per unit area. It is a measure of the intensity of the total internal forces acting within a body across imaginary internal surfaces, as a reaction to external applied forces and body… … Wikipedia
Reliability engineering — is an engineering field, that deals with the study of reliability: the ability of a system or component to perform its required functions under stated conditions for a specified period of time. [ Definition by IEEE] It is often reported in terms… … Wikipedia
South Asian arts — Literary, performing, and visual arts of India, Pakistan, Bangladesh, and Sri Lanka. Myths of the popular gods, Vishnu and Shiva, in the Puranas (ancient tales) and the Mahabharata and Ramayana epics, supply material for representational and… … Universalium
Structural engineering — is a field of engineering dealing with the analysis and design of structures that support or resist loads. Structural engineering is usually considered a speciality within civil engineering, but it can also be studied in its own right. [cite… … Wikipedia
Plant defense against herbivory — Poison ivy produces urushiol to protect the plant from herbivores. In humans this chemical produces an allergic skin rash, known as urushiol induced contact dermatitis … Wikipedia
Conservation of slow lorises — Slow lorises, such as this Bengal slow loris (Nycticebus bengalensis) were once considered common, but are now recognized as threatened species. Slow lorises are nocturnal strepsirrhine primates in th … Wikipedia
Hydrothermal vent — Marine habitats White smokers emitting liquid carbon dioxide at the Champagne vent, Northwest Eifuku volcano, Marianas Trench Marine National Monument Littoral zone … Wikipedia
Western sculpture — ▪ art Introduction three dimensional artistic forms produced in what is now Europe and later in non European areas dominated by European culture (such as North America) from the Metal Ages (Europe, history of) to the present. Like… … Universalium
Continuum mechanics — Continuum mechanics … Wikipedia
drawing — /draw ing/, n. 1. the act of a person or thing that draws. 2. a graphic representation by lines of an object or idea, as with a pencil; a delineation of form without reference to color. 3. a sketch, plan, or design, esp. one made with pen, pencil … Universalium