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Materials Fatigue Data – from Fatigue Test Machine to Usable Design Data

Robin Anderson, M.A., M.Met., Ph.D.Technologist - Materials

HBM - nCode Products

EIS Day at Instron

High Wycombe

25th November 2015

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General View of Typical Test Laboratory

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Schematic of Servo-hydraulic Test Machine

PCPC

PC Displays

PC – based Controller & Datalogger

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1. Static

2. Fatigue

3. Creep and elevated temperature fatigue

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Extensometer on Tensile Test Specimen

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Specimen Nomenclature

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Cylindrical Tensile Specimens

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Sheet Tensile Specimen

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• Static or what is commonly called “Tensile Testing”

• Applicable Standards• BS EN 2002-02, Metallic Materials – test method, Part 2: Tensile

Testing at elevated temperatures.

• BS EN ISO 6892-01 Metallic Materials – test method, Part 1 Method of test at ambient temperatures.

• BS EN ISO 6892-02 Metallic Materials – test method, Part 2 Tensile Testing at elevated temperatures.

• BS EN ISO 527 – 01 Plastics – Determination of Tensile Properties. General Principles.

• BS EN ISO 527 – 02 Plastics – Determination of Tensile Properties. Test Conditions for Moulding and Extrusion Plastics.

• …..

Note: ASTM E8M, which is the US standard for this, is not very different to ISO 6892….but there are detail differences such as the measure of elongation is expressed over 4 times the gauge diameter c.f. ISO practice of using 5 times the gauge diameter.

Static Testing

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Reminder….

KeyA – limit of proportionalityB – yield pointC – proof strength at certain value of strain (such as 0.2%) since A & B may not be clearly discernableD – ultimate tensile strength – point at which necking occursE – final failureF – fracture stress – only obtainable from true plot

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• Most of our tests are on M12 or M18 tensiles

• Strain rate is usually ~ 2000µε per second (or 0.002 per second) unless elevated temperature tests are also done.

• In the latter case two rates are used:• Slow – 0.0005 per second

• Fast(er) – 0.01 per second

• The latter pairing of tests shows quite clearly when creep effects start becoming apparent (see following slide).

• The usual parameters provided are proof stress, UTS, reduction in area, elongation (noting different national standards), n, K and an estimate of Young’s Modulus.

• Three at each rate would be the minimum number to be tested.

In Practice…

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Two Strain Rates at Different Temperatures

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1. Static

2. Fatigue

3. Creep and elevated temperature fatigue

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Stress-life (S-N )Testing

( )bffa NS ⋅⋅= 2`σ

• Constant amplitude load is applied to specimen – usually at R=0.1

• Plot stress vs. number of cycles to failure on log-log plot and fit a curve

• Curves can be derived for smooth specimens, individual components, sub-assemblies or complete structures

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Strain-life (E-N) Testing

• Constant amplitude strain is applied to specimen – usually at R=-1

• EN test controls plastic strain, the parameter that governs fatigue, inherently a better test!

• Plot strain vs. number of cycles to failure on log-log plot and fit a curve

( ) ( )cff

bf

fa NN

E22 `

` ⋅+⋅= εσ

ε

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Strain-life Test in Progress

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Sheet Specimen inside Anti-buckling Guides

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Polymer Testing Rig

Specimen

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Typical Cylindrical Fatigue Specimens

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Typical Sheet Fatigue Specimens

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• Applicable Standards• BS7270: 2006 Constant Amplitude Strain Controlled Fatigue Testing.

• ASTM E606 – 04e1 Standard Practice for Strain-Controlled Fatigue Testing.

• ISO 12106 Metallic Materials – Fatigue Testing, Axial Strain Controlled Method.

• BS ISO 12107 Metallic Materials – Fatigue Testing, Statistical Planning and Analysis of Data

• BS EN ISO 527 – 01 Plastics – Determination of Tensile Properties. General Principles.

• BS EN ISO 527 – 02 Plastics – Determination of Tensile Properties. Test Conditions for Moulding and Extrusion Plastics

• ….

Fatigue Testing

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• Source material for which the need arises carefully• ideally the specimens should be extracted from components in the

region that analysis identifies as critical (see picture below).

• These specimens should be as large as is physically possible and preferably cylindrical • this is frequently not easy, hence the variety of specimen sizes and

types in the tables given above.

• Surface finish is critical.

• Our “standard” (conventional) curve is based on 15 good fatigue results.• good is defined as showing neither anomalous behaviour nor defects in

the fracture surface.

• Back this up with tensiles –• either 3 as-received or those plus 3 cyclically-stabilised ones – used in

the past to get an extra point in the plastic region…..

• Strain levels –• usually start in the mid range of strain amplitudes and work both up

and down from here (this is where experience comes into play)

Specifying a fatigue test set (1)

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Specimen extraction – potential locations

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• Test frequencies • from 0.1 Hz at strain amplitudes of ≧ ±10000µε to 20Hz at strain

amplitudes around ±2000µε at RT

• Waveform • can be either sinusoidal or triangular. The latter is most used at

elevated temperatures to enable creep effects to be detected.

• Range of lives• usually from a few hundred reversals up to several million

• Set discrete target maximum life (say 3E6 cycles) and then set a stop test value of 5E6.

• Definition of “failure” or failure criterion:• specimen separation if stress-life, life at a particular load drop from the

stable value if strain-life (see plots that demonstrate this).

• If a value for “endurance” is required then this would be done via a staircase method in load control.

• The latter can be used to generate knock-down factors for differing surface finishes – and this can be done in bending

Specifying a fatigue test set (2)

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• Dimensional accuracy• Avoid tolerance issues that lead to non-uniform strain

• Consistency among specimens and across sets

• Minimize impact due to machining• Grinding burn

• Plasticity

• Residual stresses

• Final specimen• Surface finish ( which needs to be specified )

• No machine marks

This is an area where you should not cut corners (p un intended!).

Your fatigue properties will only be as good as you r test specimens

Test Specimen Preparation

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Surface Finish – in General

After Juvinall……

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Surface Finish - Machined

Our in house findings on a high strength steel (towards the right of this picture)are that lives varied from 4500 to 2,500,000 cycles for CLA roughnesses from about 32 down to 4 (Ra 0.9 to Ra 0.1 microns) at a single Test Level…..

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Profilometry – how that finish is “Measured”

Source: Mahr Federal Inc.

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But be careful how you interpret it!

Source: Mahr Federal Inc.

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Specimen Finishing Device

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Strain-life Test in Progress

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Typical Strain-life Test Record from One Test

Control parameter - constant strain amplitude

Initial higher stress falls away as material cyclically-stabilises

Material response

Loop width at zero stress defines plastic strain; wider heremore plasticity, nearlynone, predominantlyelastic behaviour

Crack starts to form – tensile load direction only

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Tensile Load Carrying Capacity Drops as Crack Grows

Green hysteresis loop is near final failure, notice how it leans over

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Showing how a Variety of Load Drops can be Used

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“All Elastic” Test

Loop width negligible- this is an elastic test

Slight load rise at end shows failure outside extensometer knife edges

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But what if it is Formed Sheet?

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Deep Drawing Steel – Three Thicknesses: SAME Batch

Three mm

Two mm

One mm

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Traditionally fatigue data would often look like this

4 tests at 3 levels

3 of the 4 tests at this level were runouts; the most expensive tests to run are the least useful

Find the mean and distribution for each level, then fit the elastic and plastic line, effectively using only 4 points!

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BS 3518 -1 : 1993

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Foreword to BS 3518-1

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ASTM E739 – 10 Recommendations w.r.t. Specimen Nos.

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Repeat or individual test levels?

• The traditional approach of several tests at fixed levels was appropriate when the calculations and curve fitting were done manually, but with modern computers the need falls away now…

• We tend to advise testing at individual levels for each specimen, producing a more defined and representative curve with improved statistics:-

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Effect of Test Set Size

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Why the ‘Design Curve’?

• The standard regression fit through the centre of the data means that by definition, 50% of the specimens will have failed, with only 50% confidence in this assumption– Do you want to design to that?

• We therefore derive a design curve, for example where 97.7% of the specimens have survived, and we want 95% confidence in that fatigue curve

• The safety margin from a 97.7% CoS : 95% CI curve combines with those on loads and geometry to give a satisfactory performance, equivalent to 6-sigma

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Experimental Scatter

Distribution of lives at individual strain level

Regression line – plot through mean points

Design line – plot through statistically deduced points

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• The more samples you test the more confident you are about the scatter

• Increase the scatter band, using statistics, to account for the actual number of samples tested

• e.g. 97.7% Certainty of Survival with a 95% Confidence

1 10 100 1 103× 1 10

4× 1 105× 1 10

6× 1 107× 1 10

8×1 10

3−×

0.01

0.1

1

10

Reversals to Failure

Str

ain

Am

plit

ud

e (

uE

)

4− 2− 0 2 4

Standard Deviations

97.7:95 curve

50:50 Regression Curve

Deriving the Design Curve

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In Practice…

• Most of our tests are on • M12_LCF_321_RT,

• M12_LCF_321_HT,

• M18_LCF_321 or

• JDW-09-040_Iss1 specimens

• Waveform is sinusoidal or triangular – the latter is used particularly for elevated temperature testing when the rate is held constant – and at that corresponding with the tensile tests

• Frequencies vary between 0.1 and 20Hz depending on the strain level at which/to which the test is run

• Most of our testing is strain-life although materials and component stress-life are carried out too

• Bend testing (in load control) is carried out for developing knock-down factors for various finishes – usually involving a staircase method

• Failure criteria are load drop or rupture based as appropriate

• σf’, b, c, εf’, n’, K’ or SRI1 and b are derived noting that the S-N parameters are readily derived at the same r-ratio as any E-N test results

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How long will tests last?

• Typical range of fatigue curve• Time required to test specimens for these lives at10 Hz

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• Select appropriate material

• Make sufficient specimens – in other words SPARES

• Machine them carefully

• Decide on life range required• Minimum life

• Maximum life and then “stop-test”

• Ensure enough results in each region

• Repeats at fixed control parameter levels or spread out?

• Confirm waveform

• State failure criterion or criteria

• Analyse carefully

Summary

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1. Static

2. Fatigue

3. Creep and elevated temperature fatigue

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Creep Testing – not done in-house

See: http://www.smart-swansea.com/index.htm

• Testing under constant load conditions

• 3 repeats at five stress levels at each temperature

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• Isothermal E-N tests

• Isothermal S-N tests, 2 frequencies, one slow enough to allow creep effects to show…

• Hold Tests with various dwell times

• Supporting tensile tests at two strain rates.

Elevated Temperature Fatigue

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Resistance-heated Furnace with Extensometer in Place

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SiC Igniter-based Furnace

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Creep

• BS EN ISO 204:2009 Metallic Materials – Uniaxial creep testing in tension – Method of test.

Fatigue

• BS ISO 12111:2011 Metallic Materials – Fatigue testing – Strain-controlled thermomechanical fatigue testing method.

• ASTM E2714-09 Standard test method for Creep-Fatigue Testing.

• ASTM E2368-10 Standard Practice for Strain Controlled Thermomechanical Fatigue Testing

• …….

Elevated Temperature Testing

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Creep Results for Cast Iron at 600°C

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TMF – in strict sense…

Varying both mechanical stress and temperature…..

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• No one test covers all bases• you can’t simply do the RT static tests and deduce (or guess as I would

put it!) all the other parameters

• Statistics is all• testing too few specimens is false economy!

• Don’t assume that because a material “meets a standard” then all equivalent materials will behave or be the same in, for example, fatigue• material standards focus more on the purchase and consistency of

supply aspects not the parameters Engineers need

Summing Up

“If you stick around long enough Robin, then it will break!”

- Ken Morton, one of founders of nCode

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• Engineering Materials 1 & 2, 4th Editions 2012.

Mike F. Ashby & David R.H. Jones; Publ. Butterworth Heinemann.

• Mechanical Metallurgy

George E. Dieter

• The Practical Use of Fracture Mechanics

David Broek

• Fatigue of Structures and Materials

Jaap Schijve

• Fundamentals of Fracture Mechnics

John F. Knott

References

HBM nCode: Public measure and predict with confidence

www.hbm.com/ncode

Robin Anderson, M.A., M.Met., Ph.D.Technologist - MaterialsHBM - nCode Products

Tel: +44 1629 735 821Email: robin.anderson@hbmncode.com