A strength of materials based model was developed to determine the interlaminar tensile stress at the corner radii of curved composite structures. The model assumes that the stress state in the local area near the corner is under pure bending. The force in each layer (ply) of the laminate is determined by classical lamination theory and the curved beam analysis method. The interlaminar tensile stress is then obtained by enforcing the force and moment equilibrium in each layer.
To verify the accuracy of this analytical model, a series of corner specimens were fabricated and tested. These specimens were fabricated under controlled conditions and no porosity was detected in the specimens. The corner specimens were tested by pulling apart the sides of the corner to introduce bending moment at the corner. The maximum interlaminar tensile stress computed using the analysis model was compared to the flatwise tensile strength of the material in order to predict the failure moment of the test specimen. Excellent correlations between the measured and predicted failure moment were achieved.
A series of tests were conducted on flange-web corner elements obtained from graphite composite frames that had porosity in their corners. The objectives of these tests were to determine: (1) level of porosity in the corners by using ultrasonic inspections and photomicrographic examinations, and (2) the effect of porosity in the corner on the bending strength and the interlaminar tensile strength. Ultrasonic inspections revealed the locations along the corners where porosity was present. Photomicrographic examinations showed that the porosity occurred in clusters between plies and appeared similar to delaminations. Qualitative levels of porosity, based on photomicrographic observations, were established; these were (1) no porosity, (2) low porosity, and (3) high porosity. Attempts to use acid digestion methods to establish quantitative measures of porosity were not successful because the porosity was localized between only a few plies.
A relationship was established between ultrasonic signal attenuation loss due to porosity and interlaminar tensile strength of porous structures. Strength testing was conducted on the flange-web corner specimens. Post-failure examination of the test specimens showed that interlaminar tension dominated the failure with little evidence of shearing effects. The failure strength data were used with the analytical model to calculate the interlaminar tensile failure stress for each specimen.