CompositesWorld
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17 CompositesWorld.com Te Mixed Mode Bend (MMB) test method, standardized as ASTM D 6671 3 in 2001, combines the loading of the Mode I DCB and Mode II ENF tests using the confguration shown in Fig. 2c. Loading hinges are bonded to the delaminated end of the 150-mm long specimen and attached to the fxture base and the upper loading lever. Varying ratios of Mode I to Mode II loading are applied by adjusting the position of the translating loading saddle. Pure Mode II loading is produced when the loading saddle is positioned directly over the central loading roller (c = 0, Fig. 2c), resulting in three-point fexure loading. Increasing ratios of Mode I to Mode II loading are obtained by moving the loading saddle away from the specimen's delaminated end, producing an increasing lever arm that opens the delaminated portions of the specimen, as in the DCB test. ASTM D 6671 provides equations for calculating the Mode I and Mode II components (G I and G II ) and total fracture toughness, G c . For most composite materials, G c increases as the percentage of Mode II loading increases, and delami- nation growth transitions from stable to unstable. Although equations have been proposed to predict mixed-mode fracture toughness values based solely on measured values of G IC and G IIC , MMB testing is often performed at varying mode mixities and a curve ft of the data is used to arrive at a mixed-mode fracture crite- rion for delamination growth in composites. Fracture Mechanics Testing REFERENCES 1 ASTM D 5528-13, "Mode I Interlaminar Fracture Toughness of Unidirectional Fiber-Reinforced Polymer Matrix Composites," ASTM International (W. Conshohocken, PA, US), 2013 (frst issued in 1994). 2 ASTM D 7905-14, "Determination of the Mode II Interlaminar Fracture Toughness of Unidirectional Fiber-Reinforced Polymer Matrix Composites," ASTM International (W. Conshohocken, PA, US), 2014. 3 ASTM D 6671-13, "Mixed Mode I-Mode II Interlaminar Fracture Toughness of Unidirectional Fiber Reinforced Polymer Matrix Composites," ASTM International (W. Conshohocken, PA, US), 2013 (frst issued in 2001). ABOUT THE AUTHOR Dr. Daniel O. Adams is a professor of mechanical engineering and director of the Composite Mechanics Laboratory at the University of Utah and is vice president of Wyoming Test Fixtures Inc. (Salt Lake City, UT, US). He holds a BS in mechanical engineering and an MS and Ph.D in engineering mechanics. He has published more than 120 technical papers. He chairs the Research and Mechanics Divisions of ASTM Committee D30 on Composite Materials and the Testing Committee of the Composite Materials Handbook (CMH-17). a: Double Cantilever Beam test (ASTM D 5528) b: End Notched Flexure test (ASTM D 7905) c: Mixed Mode Bend test (ASTM D 6671) FIG. 2 Test methods used for fracture mechanics testing of composites.