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    The Failure of Aluminum Compact Shear Specimens Under Mixed-Mode Loading

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    The failure behavior of 7075-T6 aluminum under mixed-mode loading was investigated using the compact shear specimen. This specimen is capable of providing a range of Mode I and Mode II loading conditions at the crack tip, depending upon the boundary conditions provided by the loading fixtures.

    Finite-element analyses of the specimen were made which included the effects of loading fixture constraints and the material's elastic-plastic properties. The symmetric and antisymmetric components of the stress and deformation fields were used to calculate the applied J-integral values, JI and JII, corresponding to the Mode I and Mode II loading at the crack tip. Methods to obtain the critical values of the J-integral at crack initiation from the load displacement curves of the specimen are discussed.

    The failure behavior of the material varied greatly depending upon the relative amounts of Mode I and Mode II loading at the crack tip. For near Mode I loading, the specimen behavior at failure loads was nearly elastic, and the failure processes appeared to be typical of those for Mode I plane-strain fracture toughness testing. A fairly significant amount of shear loading seemed to have little effect on the failure load. For near Mode II loading, the amount of plastic deformation is much more significant, with the ligament ahead of the crack tip nearly fully plastic at the failure load. The failure processes in the material, as evidenced by the fracture surface morphology, are different as compared to Mode I type failure. There is evidence of slow stable crack growth occurring at loads near the departure from linearity of the specimen load-displacement curve.

    For a mixed-mode case in which Mode II dominates with a smaller amount of positive Mode I loading, the critical J-integral value is substantially larger than the value of the critical J value for Mode I loading. When it is transformed into the corresponding critical stress intensity factor values, KIIc and KIc, it is found that KIIc is approximately twice KIc. However, there is some question as to whether the specimen failure is controlled by a critical stress intensity value or by the applied shear stress.

    The use of the J-integral as a failure criterion and as a means to predict the angle of crack growth for elastic-plastic failure is discussed.


    mixed-mode loading, Mode II failure, Mode II fracture toughness of 7075 T-6 aluminum, J-integral, elastic-plastic fracture mechanics, fracture test methods

    Author Information:

    Riddle, RA
    Engineer and section leader, Lawrence Livermore National Laboratory, Livermore, CA

    Streit, RD
    Engineer and section leader, Lawrence Livermore National Laboratory, Livermore, CA

    Finnie, I
    Professor, University of California at Berkeley, Berkeley, CA

    Committee/Subcommittee: E08.08

    DOI: 10.1520/STP23237S