Composites Testing: Challenges & Solutions

Ian McEnteggart

Composites Testing:

Challenges & Solutions

JEC Europe - March 2015

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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

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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)

Challenge 1:

Managing a Wide Range of Test Types

and Standards

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• 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)

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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

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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.

Challenge 2:

Alignment

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What is Alignment?

IV

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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

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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

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Measuring Alignment - Typical Alignment

Specimens & Electronics

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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

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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

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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

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… 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

Challenge 3:

Strain Measurement

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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

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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

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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

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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

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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

Advanced Video Extensometer 2

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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

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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

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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.

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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

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DIC Example 2 – Open-Hole Tension

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Why Instron® DIC?

Integrated and synchronous collection of all data from testing machine, e.g. Force, and camera.

Only 1 PC Integrated 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.

~Thank you for your attention~

Any Questions?