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

1. Ian McEnteggart
Composites Testing:
Challenges & Solutions
JEC Europe - March 2015

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
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. Challenge 1:
Managing a Wide Range of Test Types
and Standards

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
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
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. Challenge 2:
Alignment

9. 9
What is Alignment?
IV

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
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
Measuring Alignment - Typical Alignment
Specimens & Electronics

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
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
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
… 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. Challenge 3:
Strain Measurement

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
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
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
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
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. Advanced Video Extensometer 2

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
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
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
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
DIC Example 2 – Open-Hole Tension

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. ~Thank you for your attention~
Any Questions?