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.

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).

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

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 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 Carbon+Glass/Epoxy Composite specific modulus fatigue property

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%.