Failure analysis of notched graphite-epoxy tubes

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Virginia Tech

Notched unidirectional graphite/epoxy tubes with fiber orientations of 2.5°, 15°, 45°, and 87.5° were failed in tension, compression, torsion, and combined compression-torsion loading. The stress field around the slot-like notches, aligned with the tube axis, was determined using an infinite flat plate elasticity solution with an elliptical hole. The normal stress ratio theory was used to predict crack location, crack direction, and failure stress. The experimental failure modes of the tubes were determined using scanning electron microscopy and related to the stress field in the vicinity of the notch.

The results showed that independent of loading the cracks usually initiated at the discontinuity of the notch where the semi-circular end intersects the straight sides and then grew along the fiber direction either at the fiber/matrix interface or within the matrix. The normal stress ratio theory correctly predicted the direction of crack growth but not the location of crack initiation since the model did not account for the notch discontinuity. The prediction of far-field failure stresses exhibited only limited agreement with the experimental results. When there was agreement, the predicted far-field failure stresses were dependent on the elliptical semi-axis ratio used to model the notch. The material principal stresses at the location of maximum normal stress ratio were correlated with the failure mode of the unidirectional tubes. Matrix failure tended to occur when the material principal shear stress and transverse stress (perpendicular to the fibers) were nearly equal in magnitude, while fiber/matrix interface failure was predominant when the stresses differed by a factor of 2.0 or more.

In addition, several notched angle-ply ± 87.5° tubes were failed in torsion. The normal stress ratio theory was applied to these tubes and correctly predicted that fiber breakage would occur. The predicted crack initiation stress agreed poorly with experimental results and the location of the crack was influenced by the notch discontinuity not included in the model. The direction of crack growth and the failure morphology of the angle-ply tubes were dependent on the direction of torsion applied to the tubes.