ASTM D6641 Compression Testing for Polymer Matrix Composite Laminates Test Fixtures
Standard Test Method for Compressive Properties of Polymer Matrix Composite Materials Using a Combined Loading Compression (CLC) Test Fixture
Using a combined loading compression (CLC) or equivalent test fixture, ASTM D6641 specifies a process for determining the compressive strength and stiffness parameters of polymer matrix composite materials. Combined Loading Compression (CLC) uses a mixture of end and shear loading, in this method the shear stresses are reduced and it is possible to generate the clamping forces with using bolted clamps.
Two procedures are included in ASTM D6641; Procedure A: to be used with untabbed specimens such as fabrics, composites made of chopped fiber, laminates with no more than 50% 0 plies, and to be used with tabbed specimens having a higher orthotropy, such as unidirectional composites, is procedure B.
To increase the load-bearing area at the specimen ends, tabs must be used. It is highly essential to read the entire specification in the relevant ASTM publication before performing ASTM D6641 test procedure.
The main benefit of the test method is the combination of end loading and shear loading. Inducing just enough shear loading will prevent the specimen ends from being crushed by the end loading. In reality, the required clamping force can be achieved with as little as 2.5 to 3.0 N-m of torque per screw.
When compared to other compression test fixtures for composite materials, CLC test method has some important advantages.
Because of the test fixture is small size and easy to use CLC test fixture simplifies the testing proses. (Especially high temperature testing)
Untabbed straight-sided specimens can be tested using combined load without experiencing high levels of stress.
ASTM D6641 Specimen Dimensions
Test specimen can be untabbed (procedure A) or tabbed (Procedure B).
A typical ASTM D 6641 test specimen (Drawings that is below shows typical specimen dimensions in the ASTM D 6641 Standard) has a 140 mm length and 12 mm width, and unsupported gauge length of 12 mm when inserted in the fixture. Gauge lengths between 12 mm to 25 mm are acceptable dimensions according to ASTM D 6641 testing standard.
Key Notes from ASTM D 6641 Standard:
For both Procedures A and B, the specimen ends must be perpendicular to the specimen's longitudinal axis and parallel to one another within 0.03 mm.
The nominal specimen and tabbing thickness can differ, but they both need to be consistent. Thickness irregularities (such as thickness taper or surface flaws) must be less than 0.06 mm [0.002 in.] along the specimen grip length or tab length and 0.03 mm [0.002 in.] across the specimen or tab width.
Tabs typically have square ends and are about 1.6 mm [0.06 in.] thick, though the thickness can be changed as requirement.
The faces of the specimen could be slightly lapped to smooth out any small surface flaws and irregularities, creating flatter surfaces for a more uniform fixture grip.
13 mm gauge length is an enough space for inserting strain gauge device for opposite surface.
Combined Loading Compression Test Fixture Specimen Preparation
Guide pins are inserted the specimen preparation apparatus.
The CLC test fixture is placed on the specimen preparation apparatus.
Alignment bar should be inserted to gauge length of the fixture to adjust gauge length.
Insert the specimen through the on top of the fixture so that it sits on top of the guide pins.
Insert the alignment bar to the gauge length of the CLC test fixture.
Tighten the fixture with an allen wrench. (According to the testing standard ASTM D 6641; torques of all the eight of the 6mm diameter screws should be 2,5 to 3 Nm)
Alignment bars should be removed from the test fixture.
ASTM D6641 Combined Loading Compression Testing
Cutting tools should be used to give the rectangular shape to the material. The long side of the sample should be parallel to the direction to be tested.
Once the sample has been cut, insert the sample into the ASTM D 6641 test fixture with the specimen preparation apparatus.
Place the fixture between 2 compression plate (well aligned, fixed) in the testing machine. (Compression plates surfaces should be surfaces parallel within 0.03 mm [0.001 in]).
If strain gages or other transducers are being used, attach the lead wires to the data acquisition device.
Load the specimen in compression to failure at a nominal rate of 1.3 mm/min [0.05 in./min], while recording strain data, force and displacement.
Inspect the tested specimen and note the type and location of the failure. For valid test results, final failure of the test specimen must be occurred within the gauge section.
At least 5 sample must be tested.
Laminate Compressive Strength
Laminate Compressive Modulus (Chord Modulus)
Compressive Poisson’s Ratio by Chord Method are the compressive properties that can be calculated.