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Unformatted text preview: Stiffness T il i for Improved C S iff Tailoring f I d Compressive i Strength of Composite Plates with Holes
Raphael T. Haftka, James H. Starnes AIAA Journal Anirban Chaudhuri Aerospace Structural Composites PLATE WITH HOLES
Two ways to deal with minimizing stress concentration around y g the hole using optimization methods without incurring large mass penalties. Modify shape of the hole. hole Reinforce the region near the hole (more practical). For composite material the problem of stress concentration due to holes is more severe than metals as: More brittle than metals and provide little or no relief due to plastic load redistribution. Load transferred from plate to hole reinforcement through soft and weak resin layer which causes shear lag and shear failure in g reinforced region. COMPOSITE PLATE WITH HOLES
Softer material can be used near the hole as it can withstand higher strains. strains. This paper looks at increasing the failure load of compression loaded composite plate with a hole by using softer material near the hole. ea t e o e. This method is also called cross-sectional stiffness tailoring. crosstailoring. ANALYSIS & OPTIMIZATION
Due to symmetry only q y y y quarter of the plate is modeled with each ply orientation being modeled by a separate finite element. element The plate is divided into 2 sections: Outer Laminate and Inner Laminate. Laminate. Results of finite-element analysis are finiteused to predict failure by using the pointpoint-strain failure criterion. ANALYSIS & OPTIMIZATION contd contd.
Optimization of plates is done to achieve minimum mass such p p that the strain in any element is less than the critical value. value. Experiments were conducted to verify the results on different laminates obtained by the optimization. DESIGNS OF LAMINATES
TailoredTailored-stiffness designs: Plates with two different designs: laminates with stiffness discontinuity between outer and inner laminate. Th l i t The outer laminate b i stiffer th the inner t l i t being tiff than th i (softer) laminate. UniformUniform-stiffness designs: Plates with only one laminate. designs: Hybrid Graphite/Glass-Epoxy Designs Graphite/GlassGlass Graphite-reinforced glass-epoxy laminate: Glass-epoxy Graphiteglasslaminate with 0o glass fibers replaced by 0o graphite fibers in the outer laminate (H1). (H1). Glass-softened graphite-epoxy laminate: Graphite-epoxy Glassg p graphite- p y Graphite- p y p
laminate with 0o graphite fibers near the hole (inner laminate) replaced by 0o glass fibers (H2). (H2).
Specimen H1 H2 Outer laminate (024Gr/4520Gl/904Gl) (028/4528/906)Gr Inner laminate (020/4520/904)Gl (08Gl/4528Gr/906Gr) RESULTS & CONCLUSION
Amongst all designs, tailored-stiffness designs exhibited g g , tailoredg higher strength-mass ratio as compared to the heavier strengthuniformuniform-stiffness designs. The i Th improvement in strength -mass ratio was highest for ti t th ti hi h t f GraphiteGraphite-reinforced glass-epoxy laminates. glassSubsta t a Substantial improvements were obtained for all-graphitep ove e ts we e obta ed o all-g ap te a graphiteepoxy plates by removing the 0o plies from inner laminate. Example of all-graphite-epoxy plate: all-graphiteSpecimen Tailored-stiffness Uniform stiffness Outer laminate (036/4536/906)Gr (036/4536/906)Gr Inner laminate (4536/906)Gr --Failure Load, kN 523 491 Mass, kg 0.44 0.55 THANK YOU A O Example of softer material
Replacing high modulus 0o graphite fibers with low modulus p g g g p high failure strain 0o glass fibers increases failure load of tension loaded composite plate. 45o fiber-dominated graphite-epoxy laminates are softer and fibergraphitehave higher failure strains than 0o fiber-dominated graphiteave g e a u e st a s t a fiber-do be ated graphiteg ap te epoxy laminates. ...
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- Spring '11