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Composites Testing: Challenges & Solutions

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The following presentation discusses challenges and solutions of composites testing, with a focus on standards, alignment, and strain.

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Composites Testing: Challenges & Solutions

  1. 1. Ian McEnteggart Composites Testing: Challenges & Solutions JEC Europe - March 2015
  2. 2. 2 Challenges • Testing Productivity given the large number of test setups and standards • Flexible testing equipment • Easy change over • Standard test methods • Achieving and maintaining accurate Alignment • Need for accurate alignment • Nadcap alignment criteria • Measuring Strain • Standardized testing • Non-uniform strain fields
  3. 3. 3 Composites Require Various Tests to Characterize Tension: Fiber-dominant property. Dependant on the tensile stiffness and strength of the fiber. Compression: Matrix-dominant property. Dependant on the stiffness and adhesion qualities of the resin being able to maintain the fibers as straight columns and not buckle. Shear: Matrix-dominant property, transferring stresses across the composite. Flexure: Combination of above three: upper =compression; lower = tension; middle = shear Also a range of “structural tests” on coupons; e.g. open-hole tension & compression, bearing load, compression after impact (CAI)
  4. 4. Challenge 1: Managing a Wide Range of Test Types and Standards
  5. 5. 5 • Unsupported Gauge length (Short) • ISO 14126 • ASTM D3410 (ITTRI) • Celanese • ASTM D6641 (CLC) • AITM 1-0008 (Hydraulic Grips) • Supported Gauge Length (Anti-Buckling) • ISO 14126 • Modified ASTM D695 • ASTM D6484 Compression Testing Configurations Unsupported Gauge Section Supported Gauge Section (Anti-Buckling)
  6. 6. 6 Flexible Modular System for Range of Tests Over a Range of Temperatures Load Cell with 1000:1 Range Precise Grip Alignment Temperature Chamber CAI Compression to ASTM D695, etc. “Piggy back” Compression Platens with Spherical seats Alignment Fixture
  7. 7. 7 Bluehill® 3 Composites Test Methods • Range of Testing Modes • Tension • Compression • Shear • Flex • Fracture toughness • Special, e.g. bearing • Range of Standards • ASTM, ISO, EN, Pr-EN… • AITM, BSS….. • Methods include example data and report • Flexible software easy to create new tests and/or modify existing tests.
  8. 8. Challenge 2: Alignment
  9. 9. 9 What is Alignment? IV
  10. 10. 10 Why is Alignment so Important for Composites Testing? Ductile Metal Test Piece • Misalignment introduces uneven stress distribution • Metal yields in high stress region, but continue to carry load • Stress redistributes reducing the effect of misalignment on test results Fiber Composite Test Piece • Misalignment introduces uneven stress distribution • Fibers in high-stress region fail • Stress in remaining fibers increases causing rapid failure • Misalignment has a significant effect on test results
  11. 11. 11 Grip Design for Repeatable Alignment • Moving body provides repeatable jaw engagement • Side-to-Side symmetrical wedge “pocket” ensures accurate alignment • Front-Back symmetrical body maintains accurate alignment under load • Specimen stops ensure accurate specimen location NOTE: Design principles apply to both Manual and Hydraulic grips
  12. 12. 12 Measuring Alignment - Typical Alignment Specimens & Electronics
  13. 13. 13  Allow small adjustments of angularity and concentricity position to optimize alignment between upper and lower grip on a testing machine  Fixtures designed to allow adjustment under load are much easier to adjust as the effects of adjustments can be seen immediately Alignment Fixtures
  14. 14. 14 Compression Alignment • Hydraulic Wedge Grips • Shear loading • High lateral stiffness to maintain alignment under load • Spherical Seated Compression Platens • Quick and easy mounting on to fixed grips • Center of rotation located in the center of the platen surface • Lockable • Meet Nadcap compression alignment requirements
  15. 15. 15 Nadcap - Tensile Alignment Requirements • Alignment measured under load using strain gauged specimens* that are representative of the specimens being tested • For AC7101 (Metals) • The alignment specimen shall produce ~1,000 µstrain at the lowest yield strength of the weakest material being tested. • The acceptable bending (PBS) is as follows: • Static tests: 10% • Cyclic tests: 5% • For AC7122 (Composites) • Typical alignment specimen designs are shown in the Nadcap document. • The acceptable bending (PBS) is as follows. • Static tests: 8% • Cyclic tests: 5% • ASTM E1012 standard is cross-referenced by Nadcap and other standards. • Contains detailed procedures • Latest version (E1012-12) includes classifications at 5%, 8% and 10% PBS. *NOTE: Nadcap procedures for verifying alignment are more demanding than those described in E1012 and they do not allow “alignment cell compensation”; this means that very accurate alignment cells are needed to perform these verifications. IV
  16. 16. 16 … Not Just about Alignment … • Adherence to Nadcap procedures mean: • Alignment in accordance with Nadcap • Alignment specimens are representative of materials being tested • Calibration frequency is adhered to and calibrated to ISO/ASTM standards for load cells, extensometers, displacement, crosshead/speed, etc…. • Preventive maintenance plan is followed • Training is recorded … operators shall be trained to recognize proper operation of equipment • … Services provided by Instron • Alignment, Gauged Alignment Cells, Calibration, PPM, Training, Advice
  17. 17. Challenge 3: Strain Measurement
  18. 18. 18 Strain Products for Composites Testing Non-Contact AutoX750 Automatic Clip-On & Strain Gauges Biaxial Clip-Ons Static Clip-Ons Dynamic Clip-Ons DIC Replay SVE/AVE 2 Strain Gauges
  19. 19. 19 Clip-On Biaxial Extensometer  Main applications in composites testing  Tensile (including Poisson's ratio)  In Plane Shear (IPS)  Key Features:  Covers a wide range of test standards  Wide temperature range (-200 to +200 °C /-328 to +392 °F)  Single-handed attachment  Versions with independent axial output options (allows simultaneous monitoring of total average strain and PBS*)  Compatible with all current /existing Instron® Systems *PBS (Percentage Bending Strain)* 𝑃𝐵𝑆 = ∈ 𝑓−∈ 𝑏 ∈ 𝑓+∈ 𝑏 × 100 Where ∈ 𝑓 and ∈ 𝑏 are the strains on either side of the specimen
  20. 20. 20 Biaxial Extensometer • Averaging Axial • Corrects for specimen bending • Versions with independent axial outputs allow for measurement of average and PBS (Percentage Bending Strain)* 𝑃𝐵𝑆 = ∈ 𝑓−∈ 𝑏 ∈ 𝑓+∈ 𝑏 × 100 Where ∈ 𝑓 and ∈ 𝑏 are the strains on either side of the specimen. • Versions with Transverse Strain measurement allow for determination of Poisson’s ratio • 𝑃𝑜𝑖𝑠𝑠𝑜𝑛𝑠 𝑅𝑎𝑡𝑖𝑜 𝜈 = ∈ 𝑇 ∈ 𝐴 Where ∈ 𝐴 is the Axial strain and ∈ 𝑇 is the Transverse strain
  21. 21. 21 Automatic Extensometer • Automatic contacting extensometer • 1 micron accuracy • Capable of testing multiple gauge lengths • Suitable for tension and compression • Measures strain through failure • Automatically closes on specimen to test
  22. 22. 22 AutoX750 for Composites Testing • 1µm accuracy. • Robust - arms can be left on until failure • Repeatable positioning and attachment ensures consistent results • 1µm accuracy • Low clamping force does not damage specimen • Low drag force minimizes specimen bending • 1 um accuracy • Less expensive than strain gauges • Robust - arms can be left on until failure • Easy to use Tow – Tensile ASTM D4018 Laminate – Tensile ASTM D3039 & ISO527-4/5 Laminate Compression ASTM D695 Laminate Flexure ASTM D790/7264 EN2562/2746 ISO178/14125 • 1 um accuracy • Robust - arms can be left on until failure • Easy to use
  23. 23. Advanced Video Extensometer 2
  24. 24. 24 Accurate and Consistent • High-accuracy strain measurement meets most composites standards • The 1 micron accuracy allows measuring modulus to ISO 527-4/5 • 490 Hz frame rate prevents missing fast events such as break • Patented LED lighting and fan system prevents environmental influences • Doesn’t require operator to attach extensometer, reducing operator influence and increasing consistency
  25. 25. 25 Versatile and Capable • Can be used to measure both tensile and compressive strain • Can be used on chambers for cold and hot tensile tests • Can be used with any testing machine with a +/- 10V input • Can be used for full field strain measurement using Digital Image Correlation software
  26. 26. 26 What is Digital Image Correlation? Images Displacement Strain Analysis of image surface over time Use of cross correlation to determine displacement Strain calculated from displacement An optical method to measure deformation on an object surface.
  27. 27. 27 DIC Example 1 - Vee-Notch Shear • Test to determine shear properties • Vee–notched specimen • Approximately uniform shear stress distribution in notch • Traditional approach is to use strain gauges mounted at +/- 45º required to measure shear strain (see below) • DIC allows determination of actual strain distribution and can be used to validate measurements from gauges ASTM D5379 ASTM D7078
  28. 28. 28 DIC Example 2 – Open-Hole Tension
  29. 29. 29 Why Instron® DIC? Integrated and synchronous collection of all data from testing machine, e.g. Force, and camera. Only 1 PCIntegrated camera and lighting unit sits on the frame, out of the way of testing area. Polarized light is used so ambient light doesn’t matter. Users can focus on analyzing and understanding their results rather than assembling test rigs.
  30. 30. ~Thank you for your attention~ Any Questions?