Static and dynamic analysis of the fracture tests of fiber composites in hydraulically servo-controlled testing machines currently in use shows that their grips are much too soft and light for observing the postpeak softening. Based on static analysis based on the second law of thermodynamics, confirmed by dynamic analysis of the test setup as an open system, far stiffer and heavier grips are proposed. Tests of compact-tension fracture specimens of woven carbon-epoxy laminates prove this theoretical conclusion. Sufficiently, stiff grips allow observation of a stable postpeak softening, even under load-point displacement control. Dynamic analysis of the test setup as a closed system with proportional-integrative-differential (PID)-controlled input further indicates that the controllability of postpeak softening under crack-mouth opening displacement (CMOD) control is improved not only by increasing the grip stiffness but also by increasing the grip mass. The fracture energy deduced from the area under the measured complete load-deflection curve with stable postpeak is shown to agree with the fracture energy deduced from the size effect tests of the same composite, but the size effect tests also provide the material characteristic length of quasibrittle (or cohesive) fracture mechanics. Previous suspicions of dynamic snapback in the testing of stiff specimens of composites are dispelled. Finally, the results show the stress- or strain-based failure criteria for fiber composites to be incorrect, and fracture mechanics, of the quasibrittle type, to be perfectly applicable.
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November 2016
Research-Article
Direct Testing of Gradual Postpeak Softening of Fracture Specimens of Fiber Composites Stabilized by Enhanced Grip Stiffness and Mass
Marco Salviato,
Marco Salviato
Research Assistant Professor
Civil and Environmental Engineering,
Northwestern University,
Evanston, IL 60208
Civil and Environmental Engineering,
Northwestern University,
Evanston, IL 60208
Search for other works by this author on:
Viet T. Chau,
Viet T. Chau
Civil and Environmental Engineering,
Northwestern University,
2145 Sheridan Road,
Evanston, IL 60208
Northwestern University,
2145 Sheridan Road,
Evanston, IL 60208
Search for other works by this author on:
Weixin Li,
Weixin Li
Civil and Environmental Engineering,
Northwestern University,
2145 Sheridan Road,
Evanston, IL 60208
Northwestern University,
2145 Sheridan Road,
Evanston, IL 60208
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Zdeněk P. Bažant,
Zdeněk P. Bažant
McCormick Institute Professor and
W.P. Murphy Professor
Civil and Mechanical Engineering and
Materials Science,
Department of Civil and
Environmental Engineering,
Northwestern University,
2145 Sheridan Road, CEE/A135,
Evanston, IL 60208
e-mail: z-bazant@northwestern.edu
W.P. Murphy Professor
Civil and Mechanical Engineering and
Materials Science,
Department of Civil and
Environmental Engineering,
Northwestern University,
2145 Sheridan Road, CEE/A135,
Evanston, IL 60208
e-mail: z-bazant@northwestern.edu
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Gianluca Cusatis
Gianluca Cusatis
Associate Professor
Civil and Environmental Engineering,
Northwestern University,
2145 Sheridan Road,
Evanston, IL 60208
e-mail: g-cusatis@northwestern.edu
Civil and Environmental Engineering,
Northwestern University,
2145 Sheridan Road,
Evanston, IL 60208
e-mail: g-cusatis@northwestern.edu
Search for other works by this author on:
Marco Salviato
Research Assistant Professor
Civil and Environmental Engineering,
Northwestern University,
Evanston, IL 60208
Civil and Environmental Engineering,
Northwestern University,
Evanston, IL 60208
Viet T. Chau
Civil and Environmental Engineering,
Northwestern University,
2145 Sheridan Road,
Evanston, IL 60208
Northwestern University,
2145 Sheridan Road,
Evanston, IL 60208
Weixin Li
Civil and Environmental Engineering,
Northwestern University,
2145 Sheridan Road,
Evanston, IL 60208
Northwestern University,
2145 Sheridan Road,
Evanston, IL 60208
Zdeněk P. Bažant
McCormick Institute Professor and
W.P. Murphy Professor
Civil and Mechanical Engineering and
Materials Science,
Department of Civil and
Environmental Engineering,
Northwestern University,
2145 Sheridan Road, CEE/A135,
Evanston, IL 60208
e-mail: z-bazant@northwestern.edu
W.P. Murphy Professor
Civil and Mechanical Engineering and
Materials Science,
Department of Civil and
Environmental Engineering,
Northwestern University,
2145 Sheridan Road, CEE/A135,
Evanston, IL 60208
e-mail: z-bazant@northwestern.edu
Gianluca Cusatis
Associate Professor
Civil and Environmental Engineering,
Northwestern University,
2145 Sheridan Road,
Evanston, IL 60208
e-mail: g-cusatis@northwestern.edu
Civil and Environmental Engineering,
Northwestern University,
2145 Sheridan Road,
Evanston, IL 60208
e-mail: g-cusatis@northwestern.edu
1Present address: Assistant Professor, William E. Boeing Department of Aeronautics and Astronautics, University of Washington, Seattle, WA 98195.
2Corresponding author.
Contributed by the Applied Mechanics Division of ASME for publication in the JOURNAL OF APPLIED MECHANICS. Manuscript received July 18, 2016; final manuscript received July 22, 2016; published online August 24, 2016. Editor: Yonggang Huang.
J. Appl. Mech. Nov 2016, 83(11): 111003 (11 pages)
Published Online: August 24, 2016
Article history
Received:
July 18, 2016
Revised:
July 22, 2016
Citation
Salviato, M., Chau, V. T., Li, W., Bažant, Z. P., and Cusatis, G. (August 24, 2016). "Direct Testing of Gradual Postpeak Softening of Fracture Specimens of Fiber Composites Stabilized by Enhanced Grip Stiffness and Mass." ASME. J. Appl. Mech. November 2016; 83(11): 111003. https://doi.org/10.1115/1.4034312
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