Accelerated Durability Testing Test.pdf · Accelerated Durability Testing ... Introduction to...

© nCode 2005 Fatigue Design of Vibrating Components Slide 1

Accelerated DurabilityAccelerated DurabilityTestingTesting

Dr. Andrew Halfpenny&

Frederic Kihm

© nCode 2005 Fatigue Design of Vibrating Components Slide 2

Introduction toIntroduction toAccelerated TestingAccelerated Testing

Mission ProfilingAnd

Test Synthesis

Fatigue Design of Vibrating Components Slide 3

What Do We Want From A Durability Test?What Do We Want From A Durability Test?

• Durability test that’s suitablefor the item in question:– a component,– sub-assembly– or a whole vehicle

• Test must replicate the same failuremechanisms as seen in the real world

• Test should be representative of the realloading environment

• Test should be accelerated where possible to reduce project timescales and costs

• Test specification can be used in FE based virtual test or realphysical test


What Steps Are Involved?What Steps Are Involved?

1. Duty or Mission Profiling– Find out what’s expected of the vehicle / component– How long should it last?– Determine the ordinary loads that it’s likely to see every day– Determine the extraordinary loads it might see and be expected

to survive

2. Test Synthesis– Synthesise a test that exhibits the same damage as the Mission

Profile


How Do I Obtain Real Load Data?How Do I Obtain Real Load Data?

In-service Measurement - usingwheel force transducers, load cells,strain measurements, etc. frominstrumented prototypes & customervehicles inrealconditions

Proving Ground / Test Flights /Engine Test Cells –using measurements taken over knownevents with well establishedcorrelation to the real world

Pros.• Good source of data• Much less data to analyse than

aboveCons.• Requires a prototype• Tends to be biased towards extreme

events

Pros.• Good source of data• Much less data to analyse than

aboveCons.• Requires a prototype• Tends to be biased towards extreme

events

Pros.• The best source of data possibleCons.• Requires a prototype• Long record length required• Ideally should include many

statistically representative samples

Pros.• The best source of data possibleCons.• Requires a prototype• Long record length required• Ideally should include many

statistically representative samples


How Do I Obtain Real Load Data?How Do I Obtain Real Load Data?

Analytical modeling - usingMulti-Body Simulationsoftware.

Pros.• Doesn’t require a prototype, fully

analytical simulation• Carried out early in design cycle• Ideal for the new ‘Breed’ product

where no legacy data is availableCons.• Variable quality of data depends a lot

on experience• Needs a prototype test to support it

Pros.• Doesn’t require a prototype, fully

analytical simulation• Carried out early in design cycle• Ideal for the new ‘Breed’ product

where no legacy data is availableCons.• Variable quality of data depends a lot

on experience• Needs a prototype test to support it

Engineering Judgment – Estimate loadsusing experience of what worked in thepast

Pros.• Ideal where long standing experience availableCons.• Many tests have poorly recorded heritage• Often have unknown safety factor• Often intolerant to changes in material or

changes in intended use

Pros.• Ideal where long standing experience availableCons.• Many tests have poorly recorded heritage• Often have unknown safety factor• Often intolerant to changes in material or

changes in intended use


What is Mission Profiling?What is Mission Profiling?

ProvingGround

• A Mission Profile is a set of loading events thatconstitute the expected loads seen by a vehicleover its life.

• Events might be time series records of discreteevents like curb strikes or potholes or extra-ordinary emergency cases (deterministic)

• or PSD records of continuous vibratory loadingsuch as standard road surface, engine inducedvibration, etc. (stochastic)

Mission Profile

x 200 rpts

x 100 rpts

x 100 hrs

+

+

etc…


What is Mission Profiling?What is Mission Profiling?

• The damage on most Automotive structuralcomponents is dominated by deterministic timeevents

• Aerospace components and automotive bracketsare usually dominated by continuous stochasticprocesses

Take off

Cruise

Air combat

Intercept

Cruise

Descent

Land

-10

0

10

20

30

55.8 56 56.2 56.4Time (s)

Acc

eler

atio

n (g

)

-0.2

0

0.2

0.4

100 200 300Time (sec)

Acc

eler

atio

n (g

)

Deterministic

Stochastic


Test Synthesis – Type of TestTest Synthesis – Type of Test

Vibration Controlled

• Test rig driven by an accelerationor displacement input

• Component is fixed only to test rigand is vibrated against its owninertia, ∴ frequency and amplitudemost important

• Force in component:F =a.m

m = massa = acceleration



Rig Controlled(Cyclic or “Closed Load Path”)

• Test rig driven by a displacementinput

• Component is fixed to test rig andan external rigid restraint,amplitude important, frequencynot important

• Typical of many structuralcomponent tests

• Force in component:F = k.x or F = c.v

k = stiffness c = dampingx = displacement v = velocity



Remote Parameter Control (RPC)(Simulation)

• Enter a number of measuredchannels

• Let rig controller determine inputlevels that recreate measured data

• Run test and monitor responses tomake sure they match originalmeasured channels

• Tests are usually inertia reactedbased on whole vehicle or subassemblies, ∴ frequency andamplitude important


Test Synthesis – Route MapTest Synthesis – Route Map

DeterministicStochastic

Quasi-staticDynamic

UniaxialMultiaxial

Test Synthesis Frequency DomainTime DomainPeak-Valley Domain

Dynamic Quasi-static

UniaxialUniaxial

Load ScalingLoad Scaling



• Scaling up the load will reduce the test durationexponentially.

• Target life is influenced by endurance limit andonset of local plasticity as well as dynamicresponse of component

• Scaling should be used with extreme care toavoid local yielding and changing the load paths

• Not suitable for most inertia reacted tests

• Destroys Amplitude• Maintains Sequence• Maintains Phase between multiple

channels• Maintains Frequency Content

1 10 100 1 .103 1 .104 1 .105 1 .106 1 .107 1 .108 1 .109 1 .1010100

1 .103

1 .104

Number of Cycles to Failure

Stre

ss R

ange 2UTS

1000 NC1

Scaled Range

Original Range

Real Duration

Test Duration

1/ b

Where b is the Basquin Exponent (gradient of SN curve)This is only approximate!




Quasi-staticDynamic

UniaxialMultiaxial

• Peak Valley Extraction• Block load sequence• Statistical Exceedence• Equivalent Sinus



UniaxialUniaxial



Peak Valley Extraction (PVX)Peak Valley Extraction (PVX)

• Typical 90% reduction in signallength

• ‘Gate’ small cycles on range,rainflow or fatigue contribution

• Take care with slew rates, etc.

• Maintains Amplitude• Maintains Sequence• Destroys Phase between

multiple channels• Destroys Frequency content

360 Points

36 Points


Block Load Sequence (with mean)Block Load Sequence (with mean)

• Very simple test specification• No sequence information ∴

cannot distinguish overloadeffects in EN analysis norcrack closure effects in LEFM,etc.

• Maintains Amplitude• Destroys Sequence• Destroys Phase between


Data available in lists of:• Max, Min, Cycle count• Range, Mean, Cycle count• Range, R, Cycle count• Etc…


Block Load Sequence (no mean)Block Load Sequence (no mean)

• If mean stress effects are notimportant then you can outputin simple load blocks.I.e. 306 cycles at 200MPafollowed by 56 cycles at11MPa, etc…




Statistical ExceedenceStatistical Exceedence

• Very simple test specification• G-Exceedence test data is

widely used in the Aerospaceindustry




Damage Equivalent Constant AmplitudesignalDamage Equivalent Constant Amplitudesignal

• Much easier signal to reproduce ontest rigs

• Damage Equivalent and not CyclesEquivalent like Block Load Sequence

• Makes the comparison of the damagepotential of different signals easy

• Destroys Amplitude• Destroys Sequence• Destroys Phase between


Both sets of signalscreate same damage! … 100 000 or more

cycles




Quasi-staticDynamic

UniaxialMultiaxial



• Is it proportional i.e.dominant plane?

• Multiaxial PeakValley Extraction


• Increase Load Frequency

UniaxialUniaxial



Resultant / Critical Plane AnalysisResultant / Critical Plane Analysis

• Proportional multi-axial, or caseswith a dominant fatigue plane

• Establish critical plane• Eliminate non-damaging channels• Determine a single drive channel

with fixed proportions betweeninputs or align component on theuniaxial test rig at a given angle Resultant Load Plane


Multi-axial PVXMulti-axial PVX

• Maintains phase relationshipbetween multiple channels bykeeping points that correspondwith a peak or valley in adifferent channel

• Ordinary peak valley wouldapply all peaks / valleyssimultaneously thereforechanging the load paths

• ‘Gate’ small cycles

• Maintains Amplitude*• Maintains Sequence• Maintains Phase between



Increase Loading FrequencyIncrease Loading Frequency

• Doubling the frequency will halfthe test time

• Limit acceleration to max 1/3 firstmode natural frequency

• Not suitable for inertia reactedtests

• Maintains Amplitude• Maintains Sequence• Maintains Phase between multiple

channels• Destroys Frequency content

1/3 * natural frq




Quasi-staticDynamic

UniaxialMultiaxial





• Buffered FatigueAnalysis:Damage Editing


• Increase Load Frequency

UniaxialUniaxial



Buffered Fatigue AnalysisBuffered Fatigue Analysis

Rainflow cycle count and calculate fatiguedamage for each cycle proportioning damage tothe start and end times of the cycle

Divide the time signal into buffers and Removenon or low damaging buffers of the original drivesignals and splice remaining data together using awindowing envelope to maintain frequencycontent and prevent high slew rates and impactringing

Take a time signal at the critical location(s)


Buffered Fatigue AnalysisBuffered Fatigue Analysis

• Maintains all key attributes• Typical acceleration 50-80%

depending on amount of damageto be retained and number offailure locations assessed

• Can be used with uniaxial ormultiaxial fatigue solvers

• Can be used with a potentialdamage solver

• Maintains Amplitude• Maintains Sequence• Maintains Phase between

multiple channels• Maintains Frequency content

• You should always compare thedamage before and afterreduction to make sure it’s stillequivalent




Quasi-staticDynamic

UniaxialMultiaxial

• Peak Valley Extraction• Block load sequence• Statistical Exceedence• Equivalent Sine




• Buffered FatigueAnalysis:Damage Editing

Optional Load ScalingOptional Load Scaling


Later

• Increase frequency of Time Series

UniaxialUniaxial


The Final Test SpecificationThe Final Test Specification

• If Deterministic factors are dominant the test can bedefined wholly in the time domain

• If Stochastic factors are dominant the test can bedefined wholly in the frequency domain

• Where components have both Stochastic andDeterministic events (or Extraordinary events) thenTime and Frequency tests may be required

Test Profile

x 2 hrs

x 4 rpts

x 2 hrs

+

+

etc…

x 4 rpts

+


SummarySummary

• Loading environment can be splitinto Deterministic and Stochastic

• Deterministic loads should berepresented in time domain byblock loads or edited time signals(Chassis, Steering, Suspension &Drivetrain)

• Stochastic loads should berepresented in frequency domainby PSDs (Engine Components,Brackets, Electronics & Ancillary)

• Load inputs can be Uniaxial orMultiaxial

• Components can behaveStatically or Dynamically

• Tests can be Vibration Controlled,Rig controlled or RPC.

• Loading can be Inertial, Viscose orDisplacement induced.

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