CompositesWorld

JUL 2018

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JULY 2018 10 CompositesWorld DESIGN & TESTING » In my October 2015 column, I compared three V-notched shear test configurations used to measure the shear stiffness and shear strength of composite materials: e V-notched beam shear test (ASTM D5379 1 ), the V-notched rail shear test (ASTM D7078 2 ) and the V-notched combined loading shear test. All three use specimens with 90° V-notches machined into the central test section to produce a region of uniform shear stress. e primary differences between the three shear test configurations are the size and shape of the test specimen as well as the methods of load application: through the top and bottom specimen edges (ASTM D5379), through the specimen faces (ASTM D7078) or both (combined loading shear test). Here, we'll focus on shear testing of thick, high-shear-strength laminates for which the combined loading shear test method is best suited. e combined loading shear test fixture (Fig. 1a) is similar to that used in the V-notched rail shear test, but is larger and has bolt-adjustable specimen edge loaders. e specimen is loaded into one fixture half, using an alignment jig to properly position it. e edge loader bolt is tightened to ensure specimen edges are in contact with the fixture. Next, the bolts for the face loaders are adjusted to align the specimen's centerline with the centerline of the fixture half before tightening. e second fixture half is mounted in the same way onto the specimen's other end. Finally, the edge loader bolts for both fixture halves are loosened, then retightened to ensure that no preload is applied to the specimen edges. e assembled fixture is mounted into a universal testing machine via pinned adapters and loaded in tension. Although the central V-notched region of the combined loading shear specimen has the same geometry as in the V-notched rail shear test, the gripping region's length is increased from 25 to 51 mm, providing twice the area for both edge and face loading. e resulting 127-mm-long by 56-mm-wide specimen (Fig. 1b) has been shown to produce acceptable gage-section failures in relatively thick, high-shear strength laminates that require applied shear loads up to 100 kN 3 . High-shear strength composite laminates are of interest for many struc- tural applications, including the central web region of composite beams, which carry the majority of the shear stress under transverse loading. Unlike tension and compression stresses, which are carried most effi- ciently by reinforcing fibers oriented in the direction of the stress (Fig. 2a), shear stresses are carried most efficiently when fibers are oriented at ±45° angles (Fig 2b). is concept can be visualized by considering the shear stress element rotated 45°, showing that the ±45° fibers are actually carrying the stresses directly in tension and compression. For this reason, high-shear-strength composite laminates typically have a relatively high percentage of ±45° plies. Note that a quasi-isotropic [0/±45/90] s laminate is expected to have relatively high shear strength, having 50% of its plies in ±45° orientations. e ability to mechanically test high-shear-strength laminates is espe- cially important because of the challenges associated with predicting their shear strength. Using laminated plate theory analyses with progressive ply failure 4 , shear-loaded multidirectional laminates typically are predicted to experience matrix-dominated ply-level damage prior to reaching their Shear testing of high-shear strength composite laminates FIG. 1a Combined loading shear test fix- ture. Source (all images) | Dan Adams FIG. 1b Combined load- ing shear test specimen. FIG. 3 Failure mode in quasi- isotropic shear specimen. FIG. 4 Failure along 45° planes in epoxy shear specimen. Edge Loading Bolt Face Loading Bolts Edge Loader Face Loader Gripping Regions 127 mm 56 mm a: Normal stresses b: Shear stresses σ 90 σ- 45 T± 45 FIG. 2 Efficient fiber orientations for stresses σ 0 90° -45° 0° 45° σ45

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