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

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.

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

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

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

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

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)

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

ELIMINATING HANDLING EFFECTS FULL AUTOMATION COST SPECIMEN FINGERS GLOVES TIME EFFECTIVENESS

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

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

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

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

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