More D When testing lower strength materials such that untabbed CLC specimens can be used Procedure A , the benefits of combined loading become particularly prominent. It may not be possible to successfully test untabbed specimens of these same materials using either of the other two methods. Factors that influence the compressive response include: type of material, methods of material preparation and lay-up, specimen stacking sequence, specimen preparation, specimen conditioning, environment of testing, speed of testing, time at temperature, void content, and volume percent reinforcement. Composite properties in the test direction that may be obtained from this test method include: 5.
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The CLC fixture consists of two pairs of steel blocks, each pair being clamped together with four bolts, as indicated in Fig. When installed, the ends of the 5. The 1.
These gripping surfaces are coated with tungsten carbide particles, as shown in Fig. And since these surfaces are relatively smooth grit particles , they do not damage the surface of the specimen. This permits the use of untabbed specimens in many applications, a major advantage in terms of specimen fabrication cost.
By adjusting the bolt torque, the ratio of end- to shear-loading of the specimen can be controlled, i. This permits the successful testing of stronger materials than if the specimen was purely end-loaded by avoiding end crushing , and less clamping force than if the specimen was purely shear-loaded lower clamping-induced stress concentrations.
The fixture blocks also prevent gross buckling of the specimen. Two 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 a 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 gage length the distance between the restraining end blocks , at 0.
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, Fig. Three different size CLC fixtures are shown in Fig. We have designed and fabricated even larger and smaller CLC fixtures. A special CLC fixture, with four alignment rods and linear bearings, is shown partially disassembled at the left of Fig.
A partially disassembled standard fixture is shown at the right. With increasing interest in the use of Digital Image Correlation DIC techinques to obtain full-field displacements of the surface of the test specimen, it is necessary for the instrumentation to be able to view the face of the test specimen.
The two alignment rods of the standard fixture block this view. By using four alignment rods, a line of sight is available between the pairs of rods, while maintaining the fixture symmetry. Another view of the special fixture with four alignment rods of Fig. A completely assembled fixture is shown in the background along with a standard fixture. This special fixture weighs 20 lb, approximately twice that of the standard fixture, and can accommodate specimens up to 2. The clear space between alignment rods is slightly more than 1.
The tungsten carbide particle grip surfaces of any of the above fixtures will eventually show wear after long term use typically many hundreds of specimen tests , and may have to be recoated. As wear progresses, higher and higher bolt torques will be required to achieve equivalent results. An extreme example of surface wear is shown in Fig. Normally use of the fixture is not continued to this point. The original coating can be ground off and the fixture recoated, restoring the fixture to its original condition.
Sources of Additional Information: 1 D. Adams and J. Wegner and D. Welsh and D. Adams and G. Coguill and D.
ASTM D6641 Composite Compression Testing