Brief introduction into some of the changes and updates to both the ISO 6892-1 and ASTM E8/8M tensile testing standards for metals and ambient temperature, importantly strain control

1. UPDATES TO METALS
STANDARDS 2017

2. 22
Questions We Want to Answer
What is important?
• Which metals testing standards have changed recently?
• Why and how have the standards changed?
• How does this affect you?
• How can you ensure your testing machine is compliant with these latest
standards?

3. 33
Evolution of Testing Standards
How did they evolve?
BS18
ISO 6892-1:2009DIN 50145
EN10002
ISO6892
1904 1970 20091990 2016
ISO 6892-1:2016
ASTM E8-24T ASTM E8
ASTM E8M
ASTM E8/E8M

4. 44
Notable Changes
What are the key changes?
• In 2009: ISO 6892-1
• Added a strain rate method – “Method A”
• Separated Method A, strain rate, and Method B, stress rate
• ASTM E8/E8M-11
• The two separate standards have been combined into one standard for imperial and metric
• Clarified testing speeds and added tolerances for strain rate
Prior to this, defined strain rates mostly found in product standards such as
aerospace (Nadcap/EN 2002)

5. 55
EN vs ISO?
What are the differences between EN & ISO?
EN 10002 ISO 6892-1
Method A: Strain Rate Control
Method B: Stress Rate Control
Stress Rate
Control
Stress Rate
Control

6. 66
“Method B” Rates
How is Method B applied?
• Method B – Stress Control
• Stress control during elastic region, then “sets” constant crosshead speed
• Stress = Force/Area – Accurate Load Measurement and Specimen Dimensions Required
• Two Different Ranges
• Aluminum samples: 2 – 20 MPa/Second
• Steel samples: 6 – 60 MPa/Second (Huge variation!)

7. 77
Changes in Method B?
Added a new note into Method B
"It is not the intent of Method B to maintain constant stress rate or to control stress rate
with closed loop force control while determining yield properties, but only to set the
crosshead speed to achieve the target stress rate in the elastic region. When a specimen
being tested begins to yield, the stressing rate decreases and may even become negative
in the case of a specimen with discontinuous yielding. The attempt to maintain a constant
stressing rate through the yielding process requires the testing machine to operate at
extremely high speeds and, in most cases, this is neither practical nor desirable."
Stress Control
Stress
Strain
Crosshead Speed Control

8. 88
“Method A” Rates
How is Method A applied?
• Method A – allowed strain rates
• Range 1 – 0.00007 s−1 ±20%
• Range 2 – 0.00025 s−1 ±20% (recommended)
• Range 3 – 0.002 s−1 ±20%
• Range 4 – 0.0067 s−1 ±20% (recommended)
• Range(s) chosen are determined by material
properties being sought
• Either in strain control or ‘estimated rate’
• ISO 6892-1 Method A
• MOST restrictive of metals standards

9. 99
Rates on Logarithmic Scale
Simpler way of displaying rates and calculations

10. 1010
Strain Rate?
What is Strain Rate?
• Change in specimen length/original gauge length/time
• Change in strain per unit of time (in/in/min, mm/mm/s,
%/s, s-1)
• Typically measured with an extensometer on metals
specimens
∆𝑙
𝑙
𝑙
Typically measured with
an extensometer for
metals specimens

11. 1111
Strain Rate?
Why is strain rate important?
• Some metals mechanical properties are strain-rate dependent
• Extract from ISO 6892-1:
“Method A is intended to minimize the variation of the test rates during
the moment when strain rate sensitive parameters are determined and
to minimize the measurement uncertainty of the test results.”

12. 1212
Strain Rate?
Why is strain rate important?
• From ASTM E8/E8M-11:
• In strain control, “the reproducibility of yield strength test results from machine to
machine and laboratory to laboratory is good.”
• “In order to reproduce yield test results, for strain-rate sensitive materials, it is
important that strain rates during the determination of yield are similar.”
• “The yield strength of some materials can change by more than 10% when tested
with the slowest and then the highest speeds permitted by E8/E8M.”

13. 1313
Strain Rate?
Testing on different systems
When testing on different systems at the same speed, you can get different results
• Upper Plot controlled at
constant crosshead speed on
‘stiff’ system
• Lower Plot controlled at same
constant crosshead speed but
on weaker system

14. 1414
Strain Rate?
Testing on the same system at different rates
• Upper Plot controlled at
Maximum Stress Rate per
Method B
• Lower Plot controlled at
Minimum Stress Rate per
Method B
When testing on the same system at different speeds, you can get different results

15. 1515
So What’s New?
What changed in 2016?
• Method A was commonly confused and has been clarified
• It was assumed that Method A required ‘closed loop strain
control’ to be able to conform. This is NOT true.
• You can achieve the strain rate with a constant crosshead
speed
• Calculated by multiplying the required strain rate by the
parallel length

16. 1616
ISO 6892:2016
What has changed?
Strain Control (Extensometer)
Stress Control (Load/Force)
Extension Control (Crosshead/Position)

17. 1717
ISO6892-1:2016
Method A1 – Closed
Loop Strain Rate
Method A2 -
Estimated Strain Rate
Method B – Stress
Rate
How Does This Compare with ASTM E8?
What are the differences?
ASTME8M-15
Method A – Rate of
Stressing
Method B - Rate of
Straining
Method C – Crosshead
Speed Control
Strain Control
(Extensometer)
Stress Control
(Load)
Extension Control
(Crosshead)

18. 1818
ASTM E8/E8M-2015 Testing Rates
What are ASTM E8 rates?
• The following control methods can be used “in the absence of any specified
limitations”:
• Method A – Rate of Stressing 1.15–11.5 Mpa/s (10-100 ksi/min)
• More restrictive than ISO 6892-1 Method B (6-60 MPa/S)
• Method B – Rate of Straining 0.00025 s−1 ± 40%
• Less restrictive than ISO 6892-1 Method A (± 20%)
• Method C – Crosshead Speed 0.015 ±.003 (+/-20%) mm/mm/min of
specimen gauge length
• Similar to ISO 6892-1 Method A ‘estimated rate’
• Similar recommendations and control modes as ISO 6892-1:2009, but less
restrictive tolerance for strain rate and more restrictive for stress rate

19. 1919
ISO 6892-1:2016 Method A1 & A2
It is important to remember the required rates and the +/-20% Tolerance is the
SAME for BOTH methods.
What is important?
So, how can you conform?

20. 2020
Method A2
Estimated Strain Rate

21. 2121
Method A2 “Estimated Strain Rate”
Did it exist before?
Extract from ISO 6892-1:2009:
“Two different types of strain rate control are described in this section. The first is
the control of the strain rate itself, that is based on the feedback obtained from
an extensometer. The second is the control of the estimated strain rate over the
parallel length, which is achieved by controlling the crosshead separation rate at
a velocity equal to the desired strain rate multiplied by the parallel length.”
Method A1 and A2 were already there!

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Method A2 “Estimated Strain Rate”
How do you do it?
Wording within ISO 6892-1:2016 Method A2 notes the ‘estimated strain rate over
the parallel length’
• This is achieved by controlling the crosshead separation rate at a velocity equal to the desired
strain rate multiplied by the parallel length

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Method A2 “Estimated Strain Rate”
How do you do it?
Example:
For a specimen with 80 mm Lc (parallel length), the required crosshead
separation rate would be:
• Range 2: (0.00025 mm/mm/s)
0.00025 mm/mm/s X 80 mm = 0.02 mm/s (or 1.2 mm/min)
• Range 4: (0.0067 mm/mm/s)
0.0067 mm/mm/s X 80 mm = 0.536 mm/s (or 32.16 mm/min)
This would suggest that the speeds above will mean you are compliant.
However, NOT NECESSARILY!
The strain rate still needs to be within the +/-20% tolerance. Annex F?

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Method A2 “Estimated Strain Rate”
Why would it be different?
Experiment: Same testing speed, two different machines. Results the same?

25. 2525
ISO 6892-1 Annex F
How can you get the correct estimated strain rate?
Estimation of the crosshead separation rate in consideration of the stiffness (or
compliance) of the testing equipment.
This equation it takes into account the full machine compliance and will give a
more accurate constant crosshead speed to achieve the target strain rate. This is
particularly helpful for systems that are not able to perform closed loop strain
control.

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ISO 6892-1 Annex F
How can you get the correct estimated strain rate?
Using powerful Bluehill® Universal Software, utilizing Expression Builder, there are now
default methods built for Annex F of ISO 6892-1:2016.

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Method A2 “Estimated Strain Rate”
Test performed in extension control
What does this look like?
±20% Error
bounds
Constant crosshead
displacement
0.2% Offset yield
(strain rate within
±20% at this
point)
Strain rate moves into ±20% error band during
yield. Correct rate during Rp0.2 calculation
point

28. 2828
Method A2 “Estimated Strain Rate”
Why would it be different?
Positives to using ‘estimated rate’:
• Simpler control method – can be used on
most testing machines
• Still get comparable results
Negatives to using ‘estimated rate’:
• Test times are increased
• Further calculation for machine
compliance/stiffness
• May need to break specimens to tune test
• Different results from machine to machine
if not tuned
• Common misunderstanding of ‘Method A’ in
ISO 6892-1

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Method A1
Closed Loop Strain Rate Control

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Method A1 “Closed Loop Strain Control”
System Requirements
• Load Frame
• Precise and stable drive system
• Grips
• Able to securely grip the specimen during the test without slippage
• Extensometer
• High-precision device with stable feedback
• Good gauge length to parallel length ratio
• Software/Controller
• Responsive control loop able to maintain +/-20% or +/-40% limits
• Automatic tuning or manual tuning for yield region
• Lab Environment
• Free of vibration and shock
*Not achievable
on every system

31. 3131
Method A1 “Closed Loop Strain Control”
What is the feedback “loop”?
Extensometer
Feedback
Control Error
(Extension)
Machine Controller
e.g. 5900
Error Signal
GAIN Adjustments
Set Point from
Software

32. 3232
Method A1 “Closed Loop Strain Control”
Gain Adjustment
• Gain adjustment is what will dictate the performance of the machine control
• “PID” Controller is most common. Proportional gain requires greatest
adjustment, and will change throughout the test

33. 3333
Method A1 “Closed Loop Strain Control”
The effect of stiffness or “compliance”
Zero Load Under Load
Deflection
L
o
Zero Load
Lo+ΔL
Under Load
Compressed Lead Screw and Drive
System Deflection
Load Cell Deflection
Crosshead and Base
Deflection
Grip, Jaw Face and Adaptor
Deflection
Amplified deflection!

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Method A1 “Closed Loop Strain Control”
What does this look like?
±20% Error
bounds
0.2% Offset yield
Strain control slows the
crosshead down to
compensate
Significant and rapid change in
system stiffness during the onset of
yield
Test performed in closed loop strain control

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Method A1 “Closed Loop Strain Control”
Why would you want to do this?
0
100
200
300
400
500
600
700
0 5 10 15 20 25 30 35 40 45 50 55 60 65
TensileStress(MPa)
Time (s)
Closed Loop Estimated Rate
40% Time Saving
when compared to estimated rate

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Method A1 “Closed Loop Strain Control”
Summary of why you should care
• Repeatable and comparable results
• As some metals are strain-rate sensitive, this reduces variance between labs and systems
• Improved efficiency
• Shorter testing times with a range of specimens
• 5900 Automatic tuning reduces setup time
• No need to use a specimen for tuning
35% Time Saving
when compared to estimated rate
ISO 6892-1 Method B – 10 Mpa/s
ISO 6892-1 Method A – Estimated Rate 0.00025/s
ISO 6892-1 Method A – Closed Loop Rate 0.00025/s
Seconds
50 100
Time required to run an ISO 6892-1:2016 test on aluminium:
0 25 75

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ISO 6892-1:2016 Control Modes Summary
Simple Comparison between the methods

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Which Instron Systems Meet What?
Testing Systems 5900 Series vs 3300 Series

39. 3939
Conclusions
When looking into new testing machines or accessories, ask:
• Can this achieve requirements for ISO 6892-1:2016 Method A1?
This will enable you to:
1. Future proof your laboratory
2. Increase repeatability
3. Ensure accurate results
4. Decrease your testing times!
Using advanced 5900 controller enables:
1. Less operator training
2. Always with +/-20% tolerance
3. No setup time

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