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    Height-Tapered Double Cantilever Beam Specimen for Study of Rate Effects on Fracture Toughness of Composites

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    A height-tapered double cantilever beam specimen was introduced for the study of loading rate effects on Mode I delamination fracture toughness of graphite/epoxy composite. The material investigated was AS4/3501-6 graphite/epoxy. The height contour of the specimen was designed to provide a slightly decreasing compliance with increasing crack length in order to produce a stable and smooth crack propagation at high rates of loading. Furthermore, the specimen geometry selected allowed the attainment of much higher crack propagation velocities than was possible with uniform or width-tapered double cantilever beam specimens. The specimens were machined from 24-ply and 48-ply unidirectional plates. A 5.08-cm-long (2-in.) initial crack was machined at the narrow end of the specimen. The specimens were loaded in an Instron electrohydraulic machine at constant crosshead (opening deflection) rates ranging from 7.5 × 10−3 mm/s (1.8 × 10−2 in./min) to 460 mm/s (1.07 × 103 in./min) corresponding to crack extension rates of up to 26 m/s (61 500 in./min). This represents a 500-fold increase in crack velocity over that studied in previous work. Crack extension was monitored by means of a conductive-paint circuit applied along the path of the propagating crack. Continuous records were obtained of the load, deflection, and crack extension for determination of the strain energy release rate. The latter was calculated by the beam analysis method extended to include the effects of transverse shear and kinetic energy. It was found that the strain energy release rate increase with crack velocity up to a value of approximately 1 m/s (2360 in./min); thereafter, it decreases with increasing crack velocity. The maximum value of the strain energy release rate was approximately 46% higher than the quasi-static value.


    graphite/epoxy, delamination, fracture toughness, rate effects, test methods, double-cantilever beam (DCB), crack propagation, strain energy release rate, stability of crack propagation

    Author Information:

    Yaniv, G
    Visiting assistant professor and professor, Northwestern University, Evanston, IL

    Daniel, IM
    Visiting assistant professor and professor, Northwestern University, Evanston, IL

    Committee/Subcommittee: D30.02

    DOI: 10.1520/STP26139S