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Research Papers

Direct Testing of Gradual Postpeak Softening of Fracture Specimens of Fiber Composites Stabilized by Enhanced Grip Stiffness and Mass

[+] Author and Article Information
Marco Salviato

Research Assistant Professor
Civil and Environmental Engineering,
Northwestern University,
Evanston, IL 60208

Viet T. Chau, Weixin Li

Civil and Environmental Engineering,
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

Gianluca Cusatis

Associate Professor
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 83(11), 111003 (Aug 24, 2016) (11 pages) Paper No: JAM-16-1360; doi: 10.1115/1.4034312 History: Received July 18, 2016; Revised July 22, 2016

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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Figures

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Fig. 1

Simplified schematic of a universal servo-hydraulic testing machine

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Fig. 2

Dimensions of 2D woven composite specimens

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Fig. 3

(a) Experimental setup considered in the analysis and (b) new massive grips designed to achieve stable postpeak

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Fig. 4

Load-point control stability analysis for various values of grip mass, mg

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Fig. 5

Schematic of machine and testing setup including the PID control

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Fig. 6

Effect of grip mass on (a) open system, (b) proportional control, (c) differential control, and (d) integrative control

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Fig. 7

Effect of grip stiffness on (a) open system, (b) proportional control, (c) differential control, and (d) integrative control

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Fig. 8

(a) Example of MTS standard grip and (b) newly designed grips of increased stiffness and mass

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Fig. 9

Examples of typical stable load-displacement curves obtained with the newly designed grips for carbon fiber woven composites (a) and (b) and for glass fiber textile composites (c)

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Fig. 10

Geometry of single edge notched tension (SENT) specimens used (units: mm)

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Fig. 11

(a) Typical load-displacement curves of [0deg]8 geometrically scaled SENT specimens of various sizes, showing decreasing nonlinearity at increasing specimen size. Typical failure patterns of SENT specimens for width (b) D = 20 mm, (c) D = 40 mm, and (d) D = 80 mm. (e) Magnification of fracture surface for the large size specimen showing extensive tow failure and pull out (the postpeak is here dynamic; note that, for size effect testing, a stable postpeak control is not needed).

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