EXPERIMENTAL EXAMINATION OF FATIGUE PROPERTIES USING SELF TAPPING SCREWS FOR MECHANICAL ATTACHMENT TO SANDWICH COMPOSITE MATERIALS

摘要

An increase in the use of advanced composite materials to construct structures has highlighted the need for effective outfitting attachment methods. One such method uses mechanical fasteners, which provide an efficient means of mounting various types of equipment onto the composite structure. Data concerning the static and hygrothermal effects of a self-tapping screw/sandwich composite attachment method have been presented; however, fatigue data for this attachment method has not been discussed. Test specimens examined in this report consisted of a sandwich composite of two vinyl ester-glass laminate facesheets with a balsa core. Representative outfitting brackets were attached to the upper facesheet with four self-tapping screws. These brackets were then connected to a test fixture, and a hydraulically operated testing machine was used to subject the samples to fatigue loading conditions. The fatigue samples were subjected to a minimum of 1,000,000 cycles at various loads in an attempt to cause failure. If the sample did not exhibit a fatigue failure prior to completion of at least 1,000,000 cycles, the fatigue test was stopped and a load-displacement plot was generated using a uniaxial static test to determine the effects of fatigue on the sample's properties. Results demonstrated a general decrease in panel stiffness throughout the test and that panels subjected to greater than 60 percent of the baseline uniaxial tension screw pullout load were more likely to fail than those at lower mean load levels.