Thickness-tapered unidirectional composite specimens

Sometimes a testing concept is known for a long time but is never developed into an established test method. It isn’t discarded, but the concept languishes for lack of a champion; that is, a strong proponent who promotes its use and, eventually, guides it through a standardization process.

The use of thickness-tapered specimens is in this category (Fig. 1b). It had no champion because in the early days of composite materials use, removal of material from the specimen edges was accepted, but it was generally considered unacceptable to disturb the as-cured surfaces of a composite through material removal by any means. The concern was that the inherent composite material properties, particularly strength and stiffness, would be degraded. Today there is much less resistance to machining the faces of a composite material. For that reason, thickness-tapered specimens deserve another look.

To understand why, it is necessary to step back and review why a tapered specimen is a necessary tool. Unlike isotropic materials, such as most metals and plastics, most composite materials are orthotropic. But unidirectional composites (those with all of the reinforcement oriented in the same direction) are the extreme case. The axial tensile and compressive strengths of unidirectional composites are typically much higher than the transverse normal and shear strengths. To determine the axial tensile and/or compressive strengths of a unidirectional composite, therefore, the applied loading during the test must be transferred into, and uniformly redistributed within, the test section by shear stresses. Thus, there is the risk that the test specimen might fail in shear before the ultimate axial tensile or compressive stress is reached.

This risk is present when a traditional dog-bone shaped specimen is used, that is, a specimen with a narrower width in the center section than at the ends (Fig. 1a). This could be referred to as a width-tapered specimen, as opposed to the thickness-tapered specimen that is the topic of the current discussion. The reason for the dog bone shape is to increase the cross-sectional area of the specimen at its ends and thus reduce the average stress in these end regions where stress concentrations are induced by the applied loading. This typically nonuniform applied loading is more uniformly redistributed throughout the specimen by shear stresses. But if a width-tapered specimen is used to test a unidirectional composite, these induced shear stresses will cause the enlarged ends to shear off the specimen, as indicated in Fig. 1a, effectively converting the specimen into a straight-sided specimen that will fail in tension or compression prematurely due to the grip-induced stress concentrations at the specimen ends. If the specimen is thickness-tapered rather than width-tapered, there is a much larger area where shear failure must occur to shear off the ends and leave a specimen of uniform thickness (Fig. 1b). Thus, by properly designing the thickness-tapered specimen geometry, these premature shear failures can be prevented.

Thickness-tapering offers benefits similar to those achieved by bonding end tabs to a straight-sided specimen of uniform thickness (Fig. 1c). The tabs increase the cross-sectional area at the specimen ends. But thickness tapering avoids the induced stress concentrations that result due to differences between material properties of the specimen, the tabs and the adhesive used to bond the tabs to the specimen. Also, stress concentrations due to discontinuities in geometry at the ends of the gage section can be reduced. Further, less labor is required to thickness-taper the specimens than to prepare and bond tabs.

Although thickness-tapering can be used for tension and compression specimens, tensile testing of a unidirectional composite tends to be easier because a relatively thin specimen can be used. This reduces the applied load that is required to induce failure. On the other hand, a compression specimen must be thick enough relative to its unsupported length (gage length) to avoid buckling. This greater required thickness makes specimen gripping and uniform load introduction much more difficult. As a result, most of the work to date using thickness-tapered specimens has been related to compression testing.

Some of the earliest compression specimen thickness-tapering studies were performed at the Royal Aircraft Establishment (RAE) in the U.K., during the early 1970s¹. A thickness-tapered specimen was used, with its ends adhesively bonded into slots machined in aluminum blocks. The concept was to introduce a significant portion of the applied compressive load into the specimen by shear transfer through the adhesive, as opposed to total end loading. It was estimated that approximately one-half of the applied load was thus introduced by shear. Although specimen preparation was more involved, and it was not practical to reuse the aluminum blocks, the experimental results were quite promising.

Although thickness tapering was introduced more than 40 years ago, its use, unfortunately, was not pursued, and it soon faded away. About 20 years later, the concept was revisited at the University of Wyoming (Laramie, Wyo.)^2,3. An extensive study was conducted — both experimental and analytical — to determine favorable specimen geometries. One configuration in particular was identified as the most desirable.

A few years later, this favorable geometry was revisited experimentally⁴ and shown to produce compressive-strength results consistent with those obtained by testing a cross-ply laminate and then backing out the 0° ply strength using classical lamination theory. This result was particularly significant because it had been demonstrated in the interim that the compressive strength of the 0° plies in composite laminates of various layup configurations was consistent with that obtained by testing a cross-ply laminate5. It also should be noted that these unidirectional composite compressive strengths were significantly higher than those measured using conventional tabbed specimens (Fig. 1c).

Thickness-tapered uni composite spe-cimens have been shown to directly produce compressive strengths comparable to those obtained by testing actual laminates, including cross-ply laminates, and then backing out the uni ply strength using an analysis. Perhaps it is time for a champion to come forward, refine the thickness-tapered specimen compression test method and specimen geometry and propose a new standard.

Wyoming Test Fixtures Inc.

References
¹Purslow, D., and Collings, T.A., “A Test Specimen for the Compressive Strength and Modulus of Unidirectional Carbon Fibre Reinforced Plastic Laminates,” RAE Technical Report No. 72096, Royal Aircraft Establishment (Farnborough, U.K.), 1975.
²Finley, G.A., and Adams, D.F., “An Analytical and Experimental Study of Unidirectional Composite Thickness-Tapered Compression Specimens,” Report No. UW-CMRG-R-95-101, Interim Report: Federal Aviation Administration, Composite Materials Research Group, University of Wyoming (Laramie, Wyo.), January 1995.
³Adams, D.F., and Finley, G.A., “Experimental Study of Thickness-Tapered Unidirectional Composite Compression Specimens,” Experimental Mechanics, Vol. 36, No. 4, December 1996.
⁴Coguill, S.L., and Adams, D.F., “Use of the Wyoming Combined Loading Compression (CLC) Test Fixture to Test Unidirectional Composites,” Proceedings of the 44^th International SAMPE Symposium (Long Beach, Calif.), May 1999.
⁵Adams, D.F., and Welsh, J.S., “The Wyoming Combined Loading Compression (CLC) Test Method,” Journal of Composites Technology & Research, Vol. 19, No. 3, July 1997, pp. 123-133.