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Strain Measurement Techniques for Composites Testing
The document provides an overview of strain measurement techniques used in the testing of composite materials, specifically focusing on continuous fiber polymer matrix composites. It details various strain measurement methods, including strain gauges and extensometers, and discusses their applications in different testing scenarios such as tensile, compression, and shear tests. Additionally, it highlights the importance of testing conditions and calibration for accurate strain measurements.
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Strain Measurement Techniques for Composites Testing
- 1. Ian McEnteggart Composites MarketManager Strain Measurement Techniques for Composites Testing
- 2. 2 Topics • Introduction • CompositeMaterials • Testing of Composite Materials • Strain Measurement Techniques for Composite Coupon Testing • Strain Gauges • Contacting Extensometers • Non-contacting Extensometers • Full-field Strain Measurement • Examples of Tests • Tensile • Compression • In-Plane Shear (IPS)
- 3. Introduction Composite Materials andProperties
- 4. 4 Composite Materials • Twoor more distinct phases • Matrix and Reinforcement • Common Matrix materials • Polymer Matrix Composites (PMC) • Ceramic Matrix Composites (CMC) • Metal Matrix Composites (MMC) • Many others e.g. wood • Common Reinforcements • Continuous fibers (carbon, glass, basalt) • Discontinuous fibers / Particles Today’s focus is on testing of Continuous Fiber Polymer Matrix Composites (CFRP, GFRP or simply Carbon/Glass Fiber) Coupons
- 5. 5 • 3 Poisson’sratios υ12, υ23, υ13 3 1 2 • 3 Tensile moduli/strengths E1t , E2t , E3t , S11t , S22t , S33t • 3 Shear moduli/strengths G12, G23, G13 ,,S12, S23, S13 • 3 Compressive moduli/strengths E1c, E2c , E3c , S11t , S22t , S33t Composite Material Properties • Most metals and plastics are isotropic - properties independent of direction • Composites are anisotropic - properties depend on direction • Composite Intralaminar properties • Composite Interlaminar fracture properties are important, too!
- 6. 6 Tension: Fiber dominantproperty. 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 fibres 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 Composites Require Many Different Tests to Characterise Also a range of “structural tests” on Coupons e.g. open hole tension & compression, bearing load, Compression after Impact (CAI)
- 7. Strain Measurement Techniquesfor Coupon Testing
- 8. 8 Strain Measurement forCoupon Testing • Require high accuracy/resolution • Extensometers: ASTM E-83 class B-1/2 • Strain Gauges: Resolution < 50µε (my estimate) • Non-contacting Video / DIC great for research but not generally used for standard tests • Most strain measurements on coupons made using either strain gauges or clip-on extensometers • Use of averaging Axial strain measurement in order to correct for and/or monitor specimen bending is common • Use of Biaxial (Axial + Transverse) strain measurement to determine Poisson’s ratio and Shear strain • Majority of composites testing is done at temperature, mostly between -80 to 200 °C (-112 to +392 °F) but the upper temperature limit is increasing as resins improve • Composite coupons are often “conditioned” in hot/wet environments or liquids e.g. water, hydraulic fluids, fuel prior to testing - this can make bonding of gauges difficult • Explosive failures are common, extensometers often removed prior to failure
- 9. 9 Strain Measurements forComposites Testing Non-contacting DIC Video Strain gauges Contacting Extensometers Bi-axial clip-on Axial clip-on Automatic
- 10. 10 Strain Gauges • Useof strain gauges is very common in composites testing. • In some cases e.g. measurement of local shear strain, there is no extensometer solution available • In other cases the test specifications mandate the use of strain gauges • Use of multiple strain gauges is usual, strain readings are typically combined to generate • Average axial / width strain • Difference in axial strain (bending) • Shear strain (axial - transverse strain)
- 11. 11 Electrical Interface toSingle Strain Gauge • Strain gauges used for strain measurement are usually used singly and have an absolute calibration (i.e. a Gauge Factor relating change in resistance to change in strain). • Using a single strain gauge with an strain channel requires: • Bridge completion (Quarter Bridge – see above) • A method of providing an absolute calibration. • NOTES: • A single active gauge in a voltage driven bridge has an inherent non- linearity which is acceptable (<1%) at low strain levels (<1% Strain). • A 3-wire connection to the strain gauge (see above) provides compensation for changes in wire resistance due to temperature changes
- 12. 12 Verification of StrainGauge Measurements • No accepted primary standard for strain (primary standard for extensometers is displacement) • Gauge manufacturer checks gauges on a sample basis using bending beam rig or similar and supplies a certificate stating the Gauge Factor • Strain gauge Gauge Factor is influenced by the Poisson's ratio of the material—this can be important when making measurements on composites • Verification of strain gauge data acquisition channels is performed using a traceable strain gauge simulator
- 13. 13 Contacting Extensometers –Clip-on • General • Manually attached • Strain gauged types - temperature range -70 to 200 ºC • Capacitive types – temperatures up to 600 ºC • 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 • Biaxial • Versions with Transverse Strain measurement allow for determination of Poisson’s ratio (∈ 𝑇 ∈ 𝐴 )
- 14. 14 Automatic Extensometer • Automaticcontacting 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
- 15. 15 AutoX750 for CompositesTesting • 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
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- 17. 17 Instron AVE 2Non-contacting Video Extensometers • 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
- 18. 18 Applications of Non-ContactingVideo Extensometers • Can be used to measure tensile and compressive strain • Can be used on chambers for cold and hot tensile tests • Can be used with any test machine with +/- 10V input • Can be used for Full-field strain measurement using Digital Image Correlation software
- 19. 19 What is DigitalImage 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.
- 20. 20 How Does itWork?
- 21. 21 Calculating Full-field Displacement •Repeated for each subset over the entire surface • The result is a regular map of displacements over the entire specimen surface Specimen Surface image Split into small subsets Pattern recognised for each subset As the specimen deforms, axial (x) and transverse (y) displacements for each subset are calculated
- 22. 22 Calculating Strain • Strainat each location is calculated using central differencing • Strain calculated in the x and y directions separately • For the x direction: ∆𝐿 = 𝐿 𝑡 − 𝐿0 𝜀 = ∆𝐿 𝐿0
- 23. 23 Analysis of VariousStrain and Displacement Data Axial Strain Transverse Strain Shear Strain Poisson’s Ratio Minimum Normal Strain Maximum Normal Strain Axial Displacement Transverse Displacement
- 24. 24 Extracting 1D Plots •Use virtual extensometer for calculating strain/displacement between two points • Use virtual strain gauge for calculating average strain over a defined area
- 25. 25 DIC Example 1- Vee-notch Shear • Test to determine shear properties • V – 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 left) • DIC allows determination of strain distribution (see below right) ASTM D 5379 ASTM D 7078
- 26. 26 DIC Example 2– Open Hole Tension Test • Composite Laminate – Open Hole Tension • Complex 2D strain distribution • Measure all components of 2D Strain Tensor (Axial, Transverse, Shear) along with Maximum and Minimum Principle Strains Shear Strain Axial Strain
- 27. Examples of StrainMeasurement in Composite Tests
- 28. 28 Strain Measurement forIn-Plane Tensile • In-plane (laminate) tensile • Specimens may have different orientations (e.g. 0/90º) • Biaxial extensometer or axial + transverse strain gauges required for determination of Poisson's ratio ASTM D 3039 ISO 527-4/5 EN 2597 n = - etrans / elongitudinal
- 29. 29 Strain Measurement forCompression Testing • Use of strain gauges is common as short unsupported gauge sections and support fixtures provides little room for extensometers • Some specialized extensometer solutions are available • Independent measurement of strain on both sides of the specimen is required (to allow measurement of bending) Unsupported Gauge Section Supported Gauge Section
- 30. 30 In-Plane Shear Propertiesby +/- 45 Degree Tension Test • Test specimen has fiber directions of +/- 45 degrees • Test set up similar to tensile test • Axial and Transverse Strain measured using biaxial extensometer or axial + transverse strain gauges • Simple test but not a pure shear stress state (shear + axial tension) • Shear Strain = Axial Strain – Transverse Strain ASTM D 3518 ISO 14129 AITM 1-1002 prEN 6031
- 31. 31 Thank you foryour time! Please contact Instron® with any questions. Visit www.instron.com for more information.