plez tel me ,wat is the importance and uses of failure criterias(theory)in design of machine elements?how to select and use the suitable theory for a particular design?plez tel with some examples(situations)...
In very basic terms, when designing any machine, the reliability of its individual components influence the reliability of the end product. Let's say you are building something with three components each at .99 reliability, for instance:
.99 * 365 = 361.4 of good operational days per year for each component
However .99*.99*.99= 0.97 reliability for the unit or 354 days of good operation per year. (Hours or minutes of operation can be used instead of days.)
Therefore, when designing complex pieces of equipment, it is very important to have good reliable components with extremely high reliability scores, e.g. 0.9999973. The end result being a system with little or no failure.
Other factors to consider, based on actual data are MTBF, Mean Time Between Failure, and MTTR, Mean Time To Repair. If a failure does occur, then it would be in your best interests to repair it as soon as possible. Going back to my original example, imagine that it was your local utility, and being without water or electricity for 11 days out of the year. In a manufacturing environment, it could be extremely costly or catastrophic. The reliability component minimizes this risk.
With electronic components, quite often the failure occurs most readily in the infancy stage. Then there is a comparatively long period of time where the component reliability will be fairly constant, until finally it reaches its useful life limit and the failure rate increases again. If you were to plot this out on a time scale, it would resemble a bathtub, hence the nomenclature "bathtub curve" for this pattern.
thanks a lot sir.but i've some doubts.In all the theories they said if a particular stress exceeds a particular value of some stress while conducting particular test,ex."failure will occur when the maximum principal stress in a system
reaches the value of the maximum stress at elastic limit in simple tension".how they come to those types of conclusions.plez tell in detail.
The conclusions are drawn based on material testing, whether in compression, tension, or flexure. While each specimen tested exhibits a maximum value, one can infer a particular behavior based on the average or minimum results obtained. This is the reason for the safety factors incorporated into the calculations. Components will fail at the weakest link when more than one stress is being exerted on it.
Suppose you have a component with a maximum elongation of 10% on the average. If you stretch the component to 10%, failure is likely to occur around this number, perhaps 9%, 11%, but failure becomes more likely as you approach to the average maximum strength, whether expressed in terms of elongation, tensile strength, compressive or flexural strength.
I understand your zeal in questioning established practices, as this has often led to revision of long held theories, but you must do additional research to either confirm your acceptance of them or lead the way in providing a mathematically plausible alternative.
failure theories....
Re: failure theories....
