E unidirectional ber reinforced composites if the

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: nion fairings and wing landing gear doors (graphite/Kevlar) • Brakes (structural carbon) | v v For any class of fiber-reinforced composites, the ones with the highest specific strength and modulus values generally have all their fibers aligned in one direction (i.e., unidirectional fiber-reinforced composites). If the loading direction is known (and is always the same), then the composite can be designed and fabricated so that the strong and stiff (fiber) direction coincides with the loading direction. In this case the “weakness” in the other directions is not a problem. If, however, the loading direction is not known, or varies | e-Text Main Menu | Textbook Table of Contents pg581 [R] G1 7-27060 / IRWIN / Schaffer iq Chapter 14 12.01.98 plm QC2 rps MP Composite Materials 581 FIGURE 14.2–4 Glass/epoxy 4 A plot of specific strength versus specific elastic modulus for several monolithic structural materials and polymer-based composite materials. The data in the figure demonstrate the superiority of composites as structural materials. (Source: Adapted from C. Zweben.) Specific tensile strength = Tensile strength/density (Arbitrary units) Kevlar/epoxy 3 Graphite/epoxy (T-300) Boron/epoxy 2 Graphite/epoxy (HM-S) 1 Titanium Steel Aluminum Beryllium Magnesium Graphite/epoxy (GY-70) 0 0 1 2 3 4 5 6 7 Specific tensile modulus = Elastic modulus/density (Arbitrary units) 8 with time, then a nearly isotropic composite is required. The fibers must be arranged so that a portion of them is oriented in each of several directions within the material. This type of fiber architecture yields properties that are between those of the “strong” and “weak” directions in aligned fiber composites. That data at the lower left end of the ranges in Figure 14.2–4 correspond to nearly isotropic composites. ....................................................................................................................................... EXAMPLE 14.2–1 A robotic arm with a circular cross section used in a space vehicle is subjected to a constant tensile force along its axis. The length of the arm is fixed by other design considerations. Derive an expression for use in selecting a material that will minimize the weight of the arm. Assume that the applied stress cannot exceed 50% of the ultimate strength. How would the material choice change if the design requirement called for a fixed stiffness value? [Hint: From mechanics, the bending stiffness S of the arm is given by S EI , where E is the elastic modulus and I is the bending moment of inertia (I for a circular rod is d 4 64).] Solution Since this is unidirectional loading, we can use the properties of unidirectional composites when comparing materials. If the applied force is F, the stress is F A F d 2 4 where d is the arm diameter. We can thus write, 4F d2 0.5 u where u ultimate tensile strength. The mass M of an arm with length l and density is M d 2 4 l. If we solve the stress equation for d 2 and substitute this expression into the mass equation, we find: 2 Fl...
View Full Document

This note was uploaded on 02/25/2013 for the course PHYS 2202 taught by Professor Sowell during the Spring '10 term at Georgia Institute of Technology.

Ask a homework question - tutors are online