The following slides are provided as a summary of the most common explanations for test data variability. Users should have a full understanding of any international testing standards specifications and requirements prior to testing.
2. The following slides are provided as a summary
of the most common explanations for
test data variability. Users should have
a full understanding of any international testing
standards specifications and requirements prior
to testing.
3. FACTORS THAT INFLUENCE RESULTS
Specimen
Alignment
Thermal Effects GripType
Extensometer
Type
Method Setup Technique
Specimen
Measuring Anvils
Procedural
Errors
Grip Faces
Extensometer
Attachment
Weight of
Extensometer
Speed ofTest
= most common
Accuracy of
Equipment
Data Rate Algorithm
Rigid v. Flexible
Load String
So you’re performing the tests to the standard and you’re still seeing variability?
There are many factors that can contribute to variability in results…
4. BALANCING LOAD BEFORE TEST
Zeroing out or auto balancing load after
specimen insertion will result in incorrect
load/stress data, which impacts all
stress-related calculations (like modulus)
Load
Strain
“Real” Curve
Curve from test
20lbf
7. When extensometers are not attached properly,
it can negatively effect the results. Some
extensometers (Automatic) are much more
consistent and easily-aligned.
Some extensometers are more susceptible
to misuse and misalignment (Clip-Ons).
EXTENSOMETER PLACEMENT/ATTACHMENT
Automatic Extensometers
• Modulus
• Better repeatability
• Higher mean
• Strain
• Better repeatability
Affects proven on: PP (neat), PEEK, ABS, Data based off PP (30% glass)
Aligned
Extensometer
Misaligned
Extensometer
8. THERMAL EFFECTS
Heat from fingers may:
• Causes necking
• Lower strain
• Lower yield stress
Affects proven on: PP (neat and 30% glass) & PPSU
10. DECREASING VARIABILITY
IN MODULUS
VARIABILITY
Inconsistent
gripping force
Inconsistent
specimen
alignment
Insufficient data
capture rate
Inconsistent
extensometer
alignment
Pneumatic grips; Apply
a preload
Alignment aids;
different grips
Increase data rate;
modify calculation
(overlap/region)
More care attaching
extensometer;
Automatic
extensometer
SOLUTIONCAUSE
11. DECREASING VARIABILITY
IN STRAIN AT BREAK
VARIABILITY
Inconsistent
gripping force
Inconsistent
specimen
alignment
Insufficient data
capture rate
Inconsistent
extensometer
alignment
Pneumatic grips; Apply
a preload
Alignment aids;
different grips
Increase data rate; modify
calculation
(overlap/region)
More care attaching
extensometer; Automatic
extensometer
CAUSE SOLUTION
Heat from
fingers
Avoid excessive contact
with specimen: wear
gloves, wait
12. ASTM D638 vs. ISO 527 – 2
Key differences related to producing different results include:
• Different specimen sizes
• Different test speeds
• Multiple speeds vs. single speed
• Tensile strength calculation differs
• Nominal strain calculation differs
• Micrometer anvil shapes vary – difference directly
linked to how much plastic “sink” exists
Expect different results even when testing the same material
13. CONFIDENCE IN RESULTS
Procedure reflects the latest standard
Equipment meets the latest requirements
Requirements are all interpreted correctly
NOW WHAT?
Based on this information, it is recommended
that you check your lab for the following
14. THANK YOU FOR YOUR TIME!
Please contact your local
Instron® Sales Representative
with any questions.
Editor's Notes
Algorithm – end of test criteria, least square fit differences
Data rate – particular for quick events, max strain for a brittle material
Others not listed:
Damage or wear (ex knife edges)
MAIN POINTS1. First, ask people if they know what causes misalignment
Examples: Fatigue, rushing, no alignment device, large/long jaw faces, ergonomics (low grips/tall person)
Cover impact – mention that study is ongoing and we’d like materials to conduct more testing.
Lead into solutions
Most newer grips have alignment accessories.
Many people have them and never used them, hence forgot they existed, didn’t know what they were and they are now gone
Homemade solutions – tape or glue a stopper onto grips, one that is adjustable if varying specimen widths exist
Extensometer placement/attachment – things to mention
Could be angle/high/low
Potential damage to specimen when attached (if not careful)
Affects proven on: Polypropylene (30% glass and neat), polyphenylsulfone (PPSU
Yield stress and tensile strain at yield were significantly lower when Leo tested these specimens with hot hands. This heat from hands was only tested with the AutoX
There is a significant difference in average Modulus and standard deviation of results when using the AutoX and Clip-on extensometer. This may be due to the specimen’s susceptibility to the clip-on weight or clip-on knife edges weakening the specimen
For materials with Rubber, elongation could reduce from heat because of rubber, it is inherit to rubber