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The public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Department of Defense, Washington Headquarters Services, Directorate for Information ABSTRACTZ-pins are small-diameter carbon rods that are ultrasonically inserted through composite laminates in the z-direction to provide extra resistance to crack growth and delamination. They are normally inserted through co-cured composites, but this report presents an experimental study on a new z-pin stubble manufacturing technique that is compatible with the co-bonding process. Early researchers were limited to an exposed stubble height of 0.030 inch, which would not provide all of the benefits of a full-depth co-cured z-pin reinforced joint. This report describes a study in which the stubble lengths were increased to 0.250 inch. Double cantilever beam specimens were used to characterize the effect of stubble diameter and height on the Mode I crack resistance. It was found that increasing the stubble height from 0.040 inch to 0.125 inch resulted in a significant improvement in Mode I properties. Z-pins are small diameter carbon rods that are ultrasonically inserted through composite laminates in the zdirection to provide extra resistance to crack growth and delamination. They are normally inserted through cocured composites, but this paper presents an experimental study on a new z-pin stubble manufacturing technique that is compatible with the co-bonding process. Z-pin stubble is created by inserting pins through the thickness of an uncured laminate and trimming the pins to a certain height above the surface. After initial cure, another uncured composite part is cured on the exposed stubble. Early researchers were limited to an exposed stubble height of 0.030 inch, which would not provide all of the benefits of a full-depth co-cured z-pin reinforced joint. This paper describes a study in which the stubble lengths were increased to 0.250 inch. Double cantilever beam (DCB) specimens were used to characterize the effect of stubble diameter and height on the Mode I crack resistance. It was determined that the new approach using a second ultrasonic step is indeed necessary for stubble heights on the order of 0.125 inch. It was also found that increasing the stubble height from 0.040 inch to 0.125 inch resulted in a much more significant improvement in Mode I properties than increasing the aerial density from 2% to 4%. Finally, taking that final step of increasing the stubble height from 0.125 to 0.250 inch did not improve the properties as much as would be expected from previous studies.iii

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