WYOMING COMBINED
LOADING
COMPRESSION TEST FIXTURE
(ASTM D 6641)
Model No. CU-EL (low carbon steel)
Model No. WTF-EL (17-4PH stainless)
Assembled Fixture, with Specimen
Installed
The Wyoming Combined Loading
Compression (CLC) Test Fixture, originally developed at the
University of Wyoming (References 1 through 3), became ASTM Standard
D 6641 in 2001 (Reference 4). It is a modification of the Wyoming
End-Loaded, Side-Supported (ELSS) Compression Test Fixture developed
almost 20 years ago (Reference 5).
The CLC
fixture, like its predecessor the ELSS, consists of two pairs of
steel blocks, each pair being clamped together with four bolts, as
indicated in the photos. When installed, the ends of the specimen
are flush with the ends of the blocks. The 1.2" wide gripping
surfaces of the blocks are thermal-sprayed with tungsten carbide
particles (see view of partially disassembled fixture), intended for
testing specimens of any width up to 1". Indexing pins and spacer
bars are provided to center 1/2" and 1" wide specimens in the
fixture. Specimen thickness can also be varied as desired.
The Wyoming
Combined Loading Compression (CLC) Test Fixture is designed to
accommodate untabbed specimens 5.5" long. This is the same length as
commonly used with the IITRI and Celanese tabbed specimen test
methods. The gage length (the distance between the restraining end
blocks), at 0.50" is also the same. The CLC fixture can be used to
test nonstandard gage length specimens by simply altering the
specimen total length.
Non-standard
fixtures can also be designed and fabricated. For example, the
following photograph compares the standard fixture (left) with a
fixture designed to accommodate specimens of the same 5.5” length
but up to 2.5” wide.
ASTM D 6641
Standard Fixture (left) and Special 2.5” Specimen Width Fixture
(right)
The next
photograph shows a reduced-size fixture, designed to test specimens
only 2.5” long and up to 0.5” wide, with a standard fixture in the
background for comparison.
Special Sub-Scale Fixture shown
(disassembled) in the Foreground,
with a Standard Fixture in the
Background
The tungsten
carbide particle thermal-sprayed gripping surfaces, shown in the
following photograph, are relatively smooth (approximately 100 grit)
and do not damage the surface of the specimen. This permits the use
of untabbed specimens, a major specimen fabrication cost advantage.
Partially Disassembled Fixture
By adjusting
the bolt torque, the ratio of end- to shear-loading of the specimen
can be controlled, i.e., combined loading can be achieved. This
permits the successful testing of stronger materials than if purely
end-loaded (by avoiding end crushing), and less clamping force than
if purely shear-loaded (lower clamping-induced stress
concentrations).
The fixture
blocks also prevent gross buckling of the specimen. Alignment rods
in linear bearings are used to keep the upper and lower pairs of
blocks aligned, the linear bearings eliminating frictional binding
of the alignment rods. The circular recess shown in one face
provides clearance for an edge-mounted extensometer, if used.
Alternatively, bonded strain gages can be used on one or both faces
of the specimen.
The assembled
fixture, with a specimen installed, is placed unconstrained on the
flat base of the testing machine, and loaded directly on its top
face by a flat platen mounted in the crosshead of the testing
machine.
The Wyoming
Combined Loading Compression (CLC) Test Fixture is designed to
accommodate untabbed specimens 5.5" long. This is the same length as
commonly used with the IITRI and Celanese tabbed specimen test
methods. The gage length (the distance between the restraining end
blocks), at 0.50" is also the same. The CLC fixture can be used to
test nonstandard gage length specimens by simply altering the
specimen total length.
The CLC test
method has been shown to be totally suitable for the compression
testing of materials of moderate strengths, e.g., composites
exhibiting compressive strengths up to about 150 ksi, which includes
cross-ply and quasi-isotropic lay-up laminates, fabric composites,
random chopped-fiber composites, and other materials of similar or
lower strength. In such cases the CLC test fixture has been shown to
produce compressive strength and modulus values fully equivalent to
those obtained using more complex and expensive test fixtures such
as the IITRI, Wyoming Modified IITRI, Celanese, and Wyoming Modified
Celanese configurations (References 1 through 3).
For very high
compressive strength, highly orthotropic materials such as
unidirectional carbon- or glass-reinforced polymer-matrix
composites, a tabbed specimen can be used, just as with the Modified
ASTM D 695 Compression Test Method, a pure end-loading method.
However, this does negate the simplification and cost savings
associated with the use of an untabbed specimen. A viable option is
to use an untabbed, thickness-tapered specimen when testing these
highly orthotropic composites (References 6 and 7).
Sources of
Additional Information:
1)
1) D.F. Adams and J.S. Welsh, "The Wyoming Combined Loading
Compression (CLC) Test Method," Journal of Composites Technology
and Research, Vol. 19, No. 3, 1997, pp. 123-133.
2)
2) P.M. Wegner and D.F. Adams, "Verification of the Combined
Load Compression (CLC) Test Method," Report No. DOT/FAA/AR-00/26,
Federal Aviation Administration Technical Center, Atlantic City, New
Jersey, August 2000.
3)
3) J.S. Welsh and D.F. Adams, “Current Status of Compression
Test Methods for Composite Materials,” SAMPE Journal, Vol.
33, No. 1, January 1997, pp. 35-43.
4)
4) ASTM Standard D 6641-01 (2001), “Test Method for Determining
the Compressive Properties of Polymer Matrix Composite Laminates
Using a Combined Loading Compression (CLC) Test Fixture,” American
Society for Testing and Materials, West Conshohocken, Pennsylvania
(first issued in 2001).
5)
5) M.N. Irion and D.F. Adams, "Compression Creep Testing of
Unidirectional Composite Materials," Composites, Vol. 2, No.
2, April 1981, pp. 117-123.
6)
6) D.F. Adams and G.A. Finley, “Experimental Study of
Thickness-Tapered Unidirectional Composite Compression Specimens,”
Experimental Mechanics, Vol 36, No. 4, December 1996, pp.
348-355.
7)
7) S.L. Coguill and D.F. Adams, “Use of the Wyoming Combined
Loading Compression (CLC) Fixture to Test Unidirectional
Composites,” Proceedings of the 44th International SAMPE
Symposium, Long Beach, California, May 1999, pp. 2322-2331.
