Stress

Páginas: 8 (1985 palabras) Publicado: 23 de mayo de 2012
STRESS - STRENGTH INTERFERENCE
PROBABILITY OF FAILURE
John Toksoy Cummins Inc.
November 7, 2002

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QUIZ 1!
DESIGN = MARGIN Strength Design Stress
50 45 Fatigue Strength (ksi) 40 35 30 25 20 15 10 5 0 A B C D Material Types

§ Which one of the
materials would you choose for a higher fatigue strength?

§ Would you pay a

premium for material “D” over material “A”?

Stress &Strength Interference

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VARIATION IS THE ENEMY… § Would the additional information like the variation in
strength change your answers?
50 45 Fatigue Strength (ksi) 40 35 30 25 20 15 10 5 0 A B C D Material Types
Stress & Strength Interference

§ Is material “C”

better then “B”? quantify the difference between these materials?

$2.6 / kg $2.2 / kg
VARIATION

§ How would you

$1.5/ kg

$1.7 / kg

§ Is design margin

alone a good criteria to select a material?

AVE

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Early Practice

- Design Margin

§ Early practice in stress-strength relationship
dealt almost entirely along the lines of design margin.
Factor of safety!

§ Design margin approach use the mean value
of stress & strength ignoring the natural scatter that each may possess.
n n

§Utilization of design margin is justified when
It is based on considerable experience Component design changes are not too different than the existing design. Ø Geometry, processing, function
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Stress & Strength Interference

Recent Practice

– Probability of Failure

§ The variation in stress and strength results
in a statistical distribution and a natural scatter in these variables.§ When these two distributions interfere,

that is when stress becomes higher than strength, failure results. a practical engineering sense and means of calculating the resulting interference (probability of failure) is the heart of this seminar.
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§ Means of expressing these distributions in

Stress & Strength Interference

Outline

§ Definition of failure - Unreliability §Reliability in simple terms § Part Strength & Stress § Normal Distribution § Probability of failure § Reliability quantified § Example
Stress & Strength Interference

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Definition of Failure - Unreliability

§ Failure
n

The inability to meet customer required function The manner in which the item fails, not the display of the failure Ø It is very important to identify the root cause andseparate the failure modes When mission is interrupted such that the item cannot or should not be operated until repair occurs
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§ Failure Mode
n

§ Mission Disabling Failure
n

Stress & Strength Interference

How Do Customers Talk About Reliability

§ “ A system that does what I want

(function), when and where I want to use it (conditions), for as long as I want to use it (time) ”

§“ No surprises - no unscheduled downtime” § “ Get me up and running quickly when
failures occur ”
n

This is as important as not having a failure in the first place.

Stress & Strength Interference

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Definition of Reliability

§ “The ability of an item to perform a

required function under stated conditions for a stated period of time”

§ “Quality over time” § “It is alsodefined and/or measured as the
probability that an item performs…”
n

It is with this definition that we can quantify Reliability.

Stress & Strength Interference

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Reliability Measures

§ Cumulative failure rate at a stated time
n

Repairs Per Hundred (RPH) within warranty period Failure rate per hour, month, mile in service Mean Time Between Failures (MTBF) B10 life – time at which10% of the items have failed

§ Instantaneous failure rate or hazard rate
n

§ Time it takes to fail
n n

§ Probability of failure
Stress & Strength Interference

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Causes of Different Failure Types

§ Infant Mortality
n n n

Manufacturing & assembly issues Quality control issues Supplier Issues
Titanic

§ Random Failures
n n

Interference of inherent strength and...
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