Strength and Damage Tolerance Evaluation of Sandwich Structure Manufactured Utilizing the Automated Tow-Placement Process

This report presents information regarding research conducted at the Helicopters Division of Boeing Defense & Space Group concerning tow-placed Kevlar-based material systems in sandwich structure designed for rotorcraft primary airframe structure. The purpose of this work was threefold: 1) to evaluate the tow-placement manufacturing process for sandwich, 2) to evaluate the impact resistance and damage tolerance of tow-placed sandwich, and 3) to evaluate water permeability of thin tow-placed laminates. Successful fabrication of shear panels demonstrated the ability of the tow-placement equipment to place minimum thickness material into the core recess of the tool, and verified production concepts for fabricating core stiffened structures using the automated tow-placement process. Undamaged compression specimens failed due to face wrinkling at stress levels comparable to conventional fabric sandwich specimens. Residual strength after damage was similar to previous fabric results. One shear panel was tested and failed due to a delamination originating in the doubler region near the test section. Shear strain levels in the center of the test section were greater than for fabric sandwich shear panels previously tested. Some facesheets consisting of 4 plies of .005” tow permeated a small amount of water due to porosity, as determined by a test using a water-filled vaporometer in a vacuum chamber.