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Validation of Reduced Composite Allowable Variation Via Prestressing
Final rept. Jul 1973-Jun 1974
NORTHROP CORP HAWTHORNE CA AIRCRAFT DIV
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Three types of boronepoxy prepreg tape were prestressed to fracture weak sites along the fiber by winding over 0.3- to 0.6-inch diameter rollers prior to lamination. The prestressed prepreg was then laminated, and design allowable testing was conducted to determine if mechanical strength properties are increased and data scatter is reduced by prestressing. The types of prepreg studied were standard Rigidite 55054 prepreg, carbon substrate boron fiber prepreg, and a prepreg made from defect tungsten substrate boron that was manufactured in a high-speed, low-cost, production process. The strength of angleply composites of both Rigidite 55054 and carbon substrate boron composites were unaffected by prestressing. A study was made to determine if prepreg costs could be reduced by manufacturing low-cost defect boron fiber and prestressing it to improve its properties. The results of this study were inconclusive. The test results show prestressing marginally improved some composite properties while others were reduced. On Rigidite 55054 unidirectional composites, fatigue strength was significantly improved by prestressing, while longitudinal tensile strength was reduced at room temperature and 350 F. On unidirectional carbon substrate boron composites, the longitudinal tensile strength at room temperature and 350F was increased with attendant variability, while fatigue strength at high stress levels was reduced but not affected at low stress levels.
APPROVED FOR PUBLIC RELEASE