1. REDUCING VARIABILITY IN TEST RESULTS
ISO 527-2, ASTM D638

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
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…
Specimen Alignment
Thermal Effects
Grip Type
Extensometer Type
Grip Faces
Extensometer Attachment
Weight of Extensometer
Speed of Test
Method Setup
Technique
Specimen Measuring Anvils
Procedural Errors
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)
Accuracy of Equipment
Data Rate
Algorithm
Rigid v. Flexible Load String
= most common

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)
“Real” Curve
Load
20lbf
Curve from test
Strain

5. SPECIMEN ALIGNMENT Proper Alignment leads to Aligned Specimen
Increased strain
Better repeatability
Higher yield stress
Increased tensile strength
Aligned Specimen
MAIN POINTS 1. 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
Misaligned Specimen
Affects proven on: PP (30% glass), ABS, various TPOs

6. CORRECTING SPECIMEN ALIGNMENT
Specimen Centering Aids
Ensure specimen is vertical and centered in grips
Available for most side-acting and wedge grips
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

7. EXTENSOMETER PLACEMENT/ATTACHMENT
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).
Aligned Extensometer
Automatic Extensometers
Modulus
Better repeatability
Higher mean
Strain
Extensometer placement/attachment – things to mention
Could be angle/high/low
Potential damage to specimen when attached (if not careful)
Misaligned Extensometer
Affects proven on: PP (neat), PEEK, ABS, Data based off PP (30% glass)

8. THERMAL EFFECTS Heat from fingers may: Causes necking Lower strain
Lower yield stress
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
Affects proven on: PP (neat and 30% glass) & PPSU

9. ELIMINATING HANDLING EFFECTS
FULL AUTOMATION
COST
SPECIMEN FINGERS
GLOVES
TIME
EFFECTIVENESS

10. DECREASING VARIABILITY IN MODULUS
CAUSE
SOLUTION
Inconsistent gripping force
Pneumatic grips; Apply a preload
Insufficient data capture rate
Increase data rate; modify calculation (overlap/region)
VARIABILITY
Inconsistent specimen alignment
Alignment aids; different grips
Inconsistent extensometer alignment
More care attaching extensometer; Automatic extensometer

11. DECREASING VARIABILITY IN STRAIN AT BREAK
CAUSE
SOLUTION
Inconsistent gripping force
Pneumatic grips; Apply a preload
Insufficient data capture rate
Increase data rate; modify calculation (overlap/region)
VARIABILITY
Inconsistent specimen alignment
Alignment aids; different grips
Inconsistent extensometer alignment
More care attaching extensometer; Automatic extensometer
Avoid excessive contact with specimen: wear gloves, wait
Heat from fingers

12. Expect different results even when testing the same material
ASTM D638 vs. ISO 527 – 2
Expect different results even when testing the same material
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

13. NOW WHAT? CONFIDENCE IN RESULTS
Based on this information, it is recommended
that you check your lab for the following
Procedure reflects the latest standard
Equipment meets the latest requirements
Requirements are all interpreted correctly
CONFIDENCE IN RESULTS

14. THANK YOU FOR YOUR TIME! Please contact your local
Instron® Sales Representative
with any questions.