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Understanding Changes to Key Plastics Testing Standards


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Understanding Changes to Key Plastics Testing Standards: ISO 527- 1, 2, ASTM D638, and ISO 178

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Understanding Changes to Key Plastics Testing Standards

  1. 1. Understanding Changes to Key Plastics Testing Standards ISO 527- 1, 2; ASTM D638; and ISO 178 STANDARDS COMPLIANCE
  2. 2. The following slides are provided as a brief summary of changes in the most common plastics testing standards, and are designed to enlighten users on a selection of the key changes that have taken place in the past several years that they may not be aware of. Users should obtain a copy of the international testing standards prior to testing for a full understanding of specifications and requirements.
  3. 3. 1993 (1st Edition) 1993/COR 1:1994 1993/AMD 1:2005 2012 MODULUS TENSILE STRENGTH DEFINITION NOMINAL STRAIN SPEED GAUGE LENGTH (multipurpose) Specific Young’s Modulus Max Tensile Stress During Test Based off Grip Separation One speed allowed 50 mm Newly defined Modulus Newly defined Tensile Strength New alternative preferred method Two speeds allowed Now 75 mm (50 mm okay for QC) EVOLUTION OF ISO 527 – 1, 2 = read on!
  5. 5. NEW TENSILE STRENGTH DEFINITION ɛ σ Tensile strength 1993 versionTensile strength 2012 version
  6. 6. A REVIEW OF STRAIN TERMS ɛ σ (0) (-) or ASTM D638Uniform deformation Non-uniform deformation (necking) ISO 527-1 Inhomogeneous strain distribution (yielding or necking) Homogeneous strain distribution ɛ σ
  7. 7. STRAIN TERMS Elongation and Strain Measured from an extensometer Nominal Strain Calculated from grip separation (With the exception of ISO 527-1, Method B) ε = ΔL/L0 L0 L0
  8. 8. WHY DOES NOMINAL STRAIN EXIST? When specimen necking occurs outside of the gauge length, strain captured by the extensometer is useless Nominal strain is based off the entire specimen straining
  9. 9. EXTENSOMETER REMOVAL POINT Some labs incorrectly calculate Percent Elongation at Break with their extensometers, particularly when automatic extensometers are used. Extensometers are not an acceptable strain source for materials that neck or have a yield point Using an extensometer for strain at break is OK only if strain is homogeneous within the GL (no necking or yield) ɛ σ ɛ σ Remove extensometer at yield Leave extensometer on through failure
  10. 10. WHAT TO REPORT? Both ASTM D638 and ISO 527 specify Nominal Strain for materials that exhibit inhomogeneous strain It is important to understand what Type of Strain to report because they all produce different results Example: PP ɛ σ ɛ σ 100% 66.3% 63.6% PERCENT STRAIN Extensometer Only NOMINAL STRAIN D638 & 527-1, Method A NOMINAL STRAIN 527-1, Method B
  11. 11. KEY TAKEAWAYS NOMINAL STRAIN • Used only for materials that neck or have zero-slope yield points • D638: measured entirely based off crosshead displacement • ISO 527 two methods: • Method A: ~D638 • Method B: combination of extensometer and crosshead displacement • Nominal Strain ≠ Percent Elongation … so make sure you are comparing apples to apples when investigating differences in results!
  12. 12. 1993 1997 2001 2004 (AMD) 2003 2010 2012 + A1:2013 DEFLECTION ACCURACY MICROMETER/ CALIPER ANVILS SPEED Error should not exceed 1% of value No specification 1%/min (2 mm/min) ISO 178 Extensometer required (ISO 9513 Class 1) Width: 1 mm diameter (flat) or 1 mm rectangular Thickness: 4 mm diameter (flat) and 50 mm radius (spherical) Method B (New): Allows 2 speeds in 1 test Width: 4-6.4 mm rectangular faces Thickness: 1.5 to 6.4 mm diameter (flat) or ≥ 50 mm radius (spherical)
  14. 14. ASTM D638 vs. ISO 527 – 1, 2 Key differences related to producing different results include: • Different specimen sizes • Different test speeds • Multiple speeds vs. single speed • Different Tensile Strength calculations • Different Nominal Strain calculations • Micrometer anvil shapes vary – difference directly linked to how much plastic “sink” exists Expect different results even when testing the same material
  15. 15. 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
  16. 16. THANK YOU FOR YOUR TIME! Please contact your local Instron® Sales Representative with any questions.