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Determination of Stresses on Laminated Aircraft Transparencies by the Strain Gage-Hole Drilling and Sectioning Method

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Final rept. Jan 1989-Sep 1990

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Reducing the incidence of transparency changeouts due to craze of the outer acrylic ply requires knowledge of stress levels in in-service aircraft transparencies. Laboratory and field craze data may then be correlated to predict craze onset. Experiments conducted during the first phase of this study verified that the strain gage-hole drilling method, with calibration modification, can measure biaxial stress states in laminated transparencies with an error of less than 10 for stresses over 500 psi. A device to apply this method to full-scale transparencies was then designed and fabricated. In the second phase, stresses due to three mechanisms residual stress, installation, and cabin pressurizing in full-scale transparencies were measured. For comparison purposes and to identify the contribution of particular mechanisms to the total stress state, the sectioning method was also used. The measurements made on full-scale transparencies removed from service due to craze indicate residual stresses are too low to cause crazing in a uniform environment one free from wide temperature variation and extended contact with crazing agents. Stresses due to transparency installation and cabin pressurizing were also low relative to the maximum principal stress criteria used in this study. Recommendations include examination of other craze criteria, examination of other stress inducing mechanisms such as moisture desorption from the surface, and examination of cyclic loading and chemical exposure on craze.

Subject Categories:

  • Attack and Fighter Aircraft
  • Laminates and Composite Materials
  • Machinery and Tools

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