V-notched shear testing of composites

 Many test methods are available for measuring the shear properties of composite materials. The V-notched methods are currently among the most popular, in part because they can produce a relatively pure and uniform shear stress state in the test specimen, and they can measure the shear modulus and shear strength for the in-plane and interlaminar material orientations. The original V-notched shear test for composites, commonly referred to as the Iosipescu shear test, was introduced in the early 1980s¹ and became an ASTM standard (ASTM D 5379²) in 1993. In more recent years, the V-notched rail shear test was developed and standardized (ASTM D 7078³); a V-notched, combined loading shear test has been developed as well. 

The test fixtures and specimens used for the three methods are compared in Fig. 1, at left. All three apply an asymmetrical four-point bend loading to the specimen. As illustrated in Fig. 2, this type of loading can be visualized (and experienced) by the reader using a pencil or pen as the specimen, and applying the load, using the index finger and thumb on both hands. For starters, if the index fingers are placed on the outer bottom surfaces of the specimen and the thumbs placed at the inner top surfaces (Fig. 2a), significant bending stresses are produced in the central section between the reader’s thumbs. This is the loading method used in conventional four-point flexure testing. However, if the reader simply rotates one hand by 180°, such that the thumb loads the bottom and the index finger loads the top of the specimen (Fig. 2b), the resulting asymmetric four-point loading changes from bending to shear in the central section.

The 90° V-notches machined into the specimens are used to produce a uniform state of shear stress in the central test section between the notch tips. Without notches, the shear stress distribution varies parabolically across the width of the specimen, ranging in magnitude from zero at the top and bottom specimen edges to a maximum at the horizontal midplane. The V-notches effectively remove the outermost regions of the specimen under relatively low shear stress, leaving the central section between the notches with a surprisingly uniform state of shear stress. Using V-notches to produce a uniform stress state may seem counterintuitive for readers who are unfamiliar with this type of shear test, because notches are known to produce large stress concentrations and nonuniform stress distributions in axially loaded specimens. However, the 90° V-notches in the shear specimens, with depths of 20-23% of the total specimen width, produce minimal stress concentrations.

Although the same asymmetrical bend loading (Fig. 2b) is used in all three V-notched shear tests, the specimen surfaces where the load is applied and the size and shape of the test specimens differ between them. In the Iosipescu shear test method (Fig. 1a), the fixture loads the top and bottom edges of the 76-mm long by 19-mm wide specimen. Although limited load can be applied through the specimen edges without causing crushing, it is adequate to produce failure in unidirectional or cross-ply composite specimens, such that the shear strength and shear modulus of the composite material can be measured.

Although it is still common practice to test 0° unidirectional composite specimens with fibers oriented lengthwise, there are significant advantages to using a [0/90]_ns cross-ply laminate for in-plane shear testing. Cross-ply specimens are not only less fragile, but also produce a more uniform state of shear stress and strain in the notched test section of the specimen. The latter has important implications for accurate shear modulus determinations. To calculate the shear modulus, the average shear stress τ_ave (calculated as the applied force divided by the specimen cross-sectional area between the notches) is divided by the measured shear strain γ, or 

G = τ_ave/γ.

Because the average shear stress is used in the shear modulus calculation, the average shear strain between the notch tips, γ_ave, also must be used to obtain the correct shear modulus. When relatively small strain gages are centered between the notches, the measured shear strain can be 5-10% lower than γ_ave when testing 0° unidirectional specimens. Therefore, the measured value of shear modulus will be 5-10% too high. The use of cross-ply specimens results in greater shear strain uniformity and more accurate measurement of shear modulus. Note that if 0° unidirectional specimens are to be used, specialized shear strain gages⁴ that extend the entire distance between the notch tips may be used to record accurate values for the average shear strain, γ_ave, and thus the shear modulus, G. 

The V-notched rail shear test fixture (Fig. 1b) loads the specimen through its faces, allowing for greater load transfer and the testing of higher shear strength composite laminates. Face-loading of the 76-mm long by 56-mm wide V-notched specimens is accomplished by gripping the outer 25 mm of each specimen end, using clamping bolts. Up to 500 MPa shear strengths have been obtained from [0/±45/90]_ns quasi-isotropic and [±45]_ns laminates⁵, but specimens must be kept relatively thin to avoid slipping in the grips. The V-notched rail shear specimen also has a central test section that is three times larger than that of the Iosipescu specimen, allowing proper shear testing of relatively coarse textile composites with large unit cell sizes. The test method also is suitable for unidirectional and cross-ply composite laminates. As with the Iosipescu shear test, the use of cross-ply laminates is recommended to produce a robust specimen and a more uniform state of shear strain between the notches.

Although the V-notched rail shear test fixture is capable of higher specimen loadings than the Iosipescu shear test fixture, there is increasing interest in testing the thicker, high-shear-strength laminates used in structural applications. For such cases, the combined loading shear test fixture provides even higher shear load capacity. Shown in Fig. 1c, this method combines the edge-loading method of the Iosipescu test and the face-loading method of the V-notched rail shear test. Although the central V-notched region of the specimen is the same as the V-notched rail shear specimen, the gripping region on either end is elongated by 25 mm to provide twice the length for edge and face loading. The resulting 127-mm long by 56-mm wide specimen has been shown to produce acceptable gage-section failures of relatively thick laminates that require failure loads above 100 kN⁶. Note that the fixture body is much heavier and contains larger-diameter bolts on each side, compared to the V-notched rail shear test fixture. 

In summary, the choice of V-notched shear test method is best made with the composite to be tested firmly in mind. To obtain the in-plane or interlaminar shear properties of composite materials, all three test methods can be used. However, the Iosipescu shear test, which uses a smaller specimen and a smaller, less expensive fixture, is the preferred method for most users. The V-notched rail shear test provides a significantly larger test section required for testing relatively coarse woven or braided textile composites, as well as an increased loading capacity to enable the testing of multidirectional composite laminates. Finally, the combined loading shear test offers the greatest shear load capacity, permitting the shear testing of relatively thick specimens from high-shear-strength laminates used in structural applications.  

Wyoming Test Fixtures Inc.

References

¹ D. E. Walrath and D. F. Adams, “The Iosipescu Shear Test As Applied to Composite Materials,” Experimental Mechanics, Vol. 23, No. 1, 1983, pp. 105-110.

² ASTM D 5379/D 537M-12, “Standard Test Method for Shear Properties of Composite Materials by the V-Notched Beam Method,” ASTM International (W. Conshohocken, PA, US), 2012 (first issued in 1993).

³ ASTM D 7078/D 7078M-12, “Standard Test Method for Shear Properties of Composite Materials by the V-Notched Rail Shear Method,” ASTM International (W. Conshohocken, PA, US), 2012 (first issued in 2005).

⁴ “Plane-Shear Measurement with Strain Gages,” Vishay Precision Group Tech Note TN-512-1, Document No. 11062, September 2010, www.vishaypg.com/docs/11062/tn5121tn.pdf

⁵ D.O. Adams, J.M. Moriarty, A.M. Gallegos, and D.F. Adams, “The V-Notched Rail Shear Test,” Journal of Composite Materials, Vol. 41, No. 3, 2007, pp. 281-297.

⁶ D.J. Litz, “Development of the Combined Loading Shear Test Method and Shear Strain Measurement in the V-Notched Rail Shear Test,” MS Thesis, Department of Mechanical Engineering, University of Utah, 2012.