A study is described that evaluated the effect of the mode mix used to create a precrack on the perceived delamination toughness of laminated composites. Mode I double cantilever beam, mode II end-notched flexure, and mixed-mode symmetrically delaminated single leg bending tests were first performed on unidirectional graphite/epoxy specimens with mode I precracks. The tests were then repeated on new unidirectional specimens with mode II precracks. Through an examination of the mechanisms involved, it is shown that toughness values for materials that do not exhibit fiber bridging will be most accurate, at all mode mixities, if test specimens are precracked at the same mode mix as the test itself.
1. Davidson, B. D., "Prediction of Delamination Growth in Laminated Structures,"Failure Mechanics in Advanced Polymeric Composites, AMD-Vol. 196, G. A. Kardomateas and Y D. S. Rajapakse, Eds., American Society of Mechanical Engineers, 1994, pp. 43-65.
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2. O'Brien, T. K., "Generic Aspects of Delamination in Fatigue of Composite Materials,"Journal of the American Helicopter Society, Vol. 32, No. 1, 1987, pp. 13-18.
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3. O'Brien, T. K., "Towards a Damage Tolerance Philosophy for Composite Materials and Structures,"Composite Materials: Testing and Design (Ninth Volume), ASTM STP 1059, S. P. Garbo, Ed., American Society for Testing and Materials, 1990, pp. 7-33.
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4. O'Brien, T. K.., Murri, G. B. and Salpekar, S. A., "Interlaminar Shear Fracture Toughness and Fatigue Thresholds for Composite Materials,"Composite Materials: Fatigue and Fracture, Second Volume, ASTM STP 1012, P. A. Lagace, Ed., American Society for Testing and Materials, 1989, pp. 222-250.
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5. Murri, G. B. and Martin, R. H., "Effect of Initial Delamination on Mode I and Mode II Interlaminar Fracture Toughness and Fatigue Fracture Threshold,"Composite Materials: Fatigue and Fracture (Fourth Volume), ASTM STP 1156, American Society for Testing and Materials, 1991, pp. 239-256.
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6. Polaha, J. J., Davidson, B. D., Hudson, R. C. and Pieracci, A., "Effects of Mode Ratio, Ply Orientation and Precracking on the Delamination Toughness of a Laminated Composite,"Journal of Reinforced Plastics and Composites, Vol. 15, No. 2, 1996, pp. 141-173.
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7. Davidson, B. D., Fariello, P. L., Hudson, R. C. and Sundararaman, V., "Accuracy Assessment of the Singular Field-Based Mode Mix Decomposition Procedure for the Prediction of Delamination,"Composite Materials: Testing and Design (Thirteenth Volume), ASTM STP 1242, S. J. Hooper, Ed., 1997, pp. 109-128.
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8. Johnson, W. S. and Mangalgiri, P. D., "Investigation of Fiber Bridging in Double Cantilever Beam Specimens,"Composite Technology Review, 1987, pp. 10-13.
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9. Russell, A. J., "Micromechanisms of Interlaminar Fracture and Fatigue,"Polymer Composites, Vol. 8, No. 5, 1987, pp. 342-351.
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10. Koudela, K. L., Strauch, E. C. and Strait, L. H., "Effects of Mode I and Mode II Precracking on Critical Strain Energy Release Rates,"Recent Advances in Composite Materials Research, MD-Vol. 56, S. R. White, H. T. Hahn and W. F. Jones, Eds., American Society of Mechanical Engineers, 1995, pp. 97-118.
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11. Davidson, B. D., "An Analytical Investigation of Delamination Front Curvature in Double Cantilever Beam Specimens,"Journal of Composite Materials, Vol. 24, 1990, pp. 1124-1137.
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12. Davidson, B. D., Kr Uger, R. and Konig, M., "Three Dimensional Analysis and Resulting Design Recommendations for Unidirectional and Multidirectional End-Notched Flexure Tests,"Journal of Composite Materials, Vol. 29, No. 16, 1995, pp. 2108-2133.
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13. Davidson, B. D., Altonen, C. S. and Polaha, J. J., "Effect of Stacking Sequence on Delamination Toughness and Delamination Growth Behavior in Composite End-Notched Flexure Specimens,"Composite Materials: Testing and Design (Twelfth Volume), ASTM STP 1274, C. R. Saff and R. B. Deo, Eds., American Society for Testing and Materials, 1996, pp. 393-413.
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14. Krfiger, R., Rinderknecht, S. and Konig, M., "Twoand Three-Dimensional Finite Element Analyses of Crack Fronts in a Multidirectional Composite ENF Specimen," ISD Report No. 97/1, Institut fur Statik und Dynamic der Luftund Raumfahrtkonstruktionen, Universitlit Stuttgart, May 1997.
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15. Bhashyam, S. and Davidson, B. D., "Evaluation of Data Reduction Procedures for the Mixed Mode Bending Test,"AIAA Journal, Vol. 35, No. 3, 1997, pp. 546-552.
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16. Martin, R. H. and Hansen, P. L., "Experimental Compliance Calibration for the Mixed-Mode Bending (MMB) Specimen,"Composite Materials: Fatigue and Fracture (Sixth Volume), ASTM STP 1285, E. A. Armanios, Ed., American Society for Testing and Materials, 1997, pp. 305-323.
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17. Military Handbook 17B, "Polymeric Matrix Composites," Vol. 1, Guidelines, Department of Defense, 1988.
