1. 1 Chap. 9 Design of Composite Materials 9-1. Advantages of Composite Materials in Structural Design The main advantages of using composites in structural design are as follows. Flexibility  Ply lay-up allows for variations in the local detail design.  Ply orientation can be varied to carry combinations of axial and shear loads. Simplicity  Large one-piece structures can be made with attendant reductions in the number of components.  Selective reinforcement can be used. Efficiency  High specific properties, i.e., properties on a per-unit-weight basis.  Savings in materials and energy. Longevity  Generally, properly designed composites show better fatigue and creep behavior than their monolithic counterparts.

2. 2 9-2. Fundamental Characteristics of Composite Materials Composite materials come with some fundamental characteristics that are quite different from conventional materials.  Heterogeneity : Composite materials, by definition, are heterogeneous. There is large area of interface and the in situ properties of the components are different from those determined in isolation.  Anisotropy : Composites in general, and fiber reinforced composites in particular, are anisotropic. The modulus and strength are very sensitive functions of fiber orientation.  Coupling Phenomena : Coupling between different loading modes, such as tension-shear, is not observed in conventional isotropic materials. These coupling phenomena make designing with composites more complex.  Fracture Behavior : Monolithic, conventional isotropic materials show what is called a self similar crack propagation. This means that the damage mode involves the propagation of a single dominant crack; one can then measure the damage in terms of the crack length. In composites, one has a multiplicity of fracture modes. A fiber reinforced composite, especially in the laminated form, can sustain a variety of subcritical damage (cracking of matrix, fiber/matrix decohesion, fiber fracture, ply cracking, delamination).

3. 3 9-3. Designing with Composite Materials Characteristics of Composite Materials 1) Physical properties of composites depends on - Physical properties of components - Volume fraction, shape & geometric arrangement of components - Interface characteristics 2) Anisotropic Modulus and Strength of Fiber Reinforced Composite (65 vol.%) Carbon Fiber/Epoxy Laminated Composite - 3 layer stacking sequence - 3 layers Young's modulus, tensile strength vs volume fraction of plies  "Performance Charts" or "Carpet Plots" can tailor the properties of composites

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5. 5 ex) Specification : Longitudinal tensile strength higher than steel. Transverse modulus similar to Al. Point A : If we change the volume fraction of 0 o ply and 90 o ply, Point B : If we decrease the volume fraction of 0 o ply and increase the volume fraction of 90 o ply, Point C : Reversing the x and y coordinates, satisfy specification

6. 6 Thermal Expansion Coefficient of Fiber Reinforced Composite B/Al, B/Epoxy, C/Epoxy Carbon, Kevlar Fiber - negative CTE parallel to fiber direction.

7. 7 Control of thermal expansion coefficient → important for aerospace application (-70°C ~ 200 °C) electronic packaging application (R.T. ~ 150 °C) ex) Leadless chip carrier (LCC) Si + Alumina Chip Carrier + Carbon/Epoxy Composite By controlling carbon fiber orientation, → Thermal expansion mismatch = 0 → No thermal fatigue problem Si + SiCp/Al

8. 8 Hybrid Composite Composite using more than 1 type of fiber Selection of fibers highest strength in highly Control of fiber alignment stressed location and direction

9. 9 Carbon+Glass/Epoxy Composite specific modulus fatigue property

10. 10 ARALL (Aramid Aluminum Laminates) Alcoa, 3M Company (1985) Arrange aramid fibers between Al sheets. High strength, excellent fatigue & fracture resistance. Good formability & machinability - similar to metal. Applications : Aircraft - fuselage, lower wing, tail skin. Increase the fatigue and fracture resistance. Weight saving ~ 15-30%.