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Plastics Testing: Reducing Variablity in Test Results

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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.

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Plastics Testing: Reducing Variablity in Test Results

  1. 1. ISO 527-2, ASTM D638 REDUCING VARIABILITY IN TEST RESULTS
  2. 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. 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. 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
  5. 5. SPECIMEN ALIGNMENT Proper Alignment leads to • Increased strain • Better repeatability • Higher yield stress • Increased tensile strength Affects proven on: PP (30% glass), ABS, variousTPOs Misaligned Specimen Aligned Specimen
  6. 6. CORRECTING SPECIMEN ALIGNMENT • Ensure specimen is vertical and centered in grips • Available for most side-acting and wedge grips Specimen Centering Aids
  7. 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. 8. THERMAL EFFECTS Heat from fingers may: • Causes necking • Lower strain • Lower yield stress Affects proven on: PP (neat and 30% glass) & PPSU
  9. 9. ELIMINATING HANDLING EFFECTS GLOVES SPECIMEN FINGERS FULL AUTOMATION TIME COST EFFECTIVENESS
  10. 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. 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. 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. 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. 14. THANK YOU FOR YOUR TIME! Please contact your local Instron® Sales Representative with any questions.

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