COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES Table 1.2 Approximate Actual(US$/kg)Values of Saving One Unit of Weight: Costing Based on Some Late 1980s Estimates ●small civil$80 advanced fighter $500 .civil helicopter $80-$200 ·VTOL$800 military helicopter $400 ·SST$1500 ·large transport$300 Space Shuttle $45,000 large commercial $500 Chapter 2 describes the basic principles(micromechanics)of fiber composite materials.As an example,to a good first approximation,the stiffness under loading in the fiber direction(unidirectional fibers)may be determined by the simple law of mixtures.This is simply a sum of the volume (or area)fraction of the fibers and the matrix multiplied by the elastic modulus.The strength estimation is similar(for a reasonably high fiber-volume fraction)but with each elastic modulus multiplied by the breaking strain of the first-failing component. In the case of carbon fiber/epoxy composites,this is generally the fiber-breaking strain.If,however,the lowest failure strain is that of the matrix,the first failure event may be the development of extensive matrix cracking,rather than total fracture.This damage may or may not be defined as failure of the composite. However,toughness is usually much more than the sum of the toughness of each of the components because it depends also on the properties of the fiber/ matrix interface.Therefore,brittle materials such as glass fibers and polyester resin,when combined,produce a tough,strong composite,most familiarly known as fiberglass,used in a wide range of structural applications. Control of the strength of the fiber/matrix interface is of paramount importance for toughness,particularly when both the fiber and the matrix are brittle.If the interface is too strong,a crack in the matrix can propagate directly through fibers in its path.Thus it is important that the interface is able to disbond Table 1.3 Summary of the Approach for Development of a High-Performance Fiber Composite ·Fibers ·Polymer Matrix 。Composite stiff/strong/brittle/low -low stiffness and strength toughness through density ductile or brittle synergistic action -high temperature -can be polymer,metal, (woodlike) capability or ceramic -high strength and able to carry major load -transmits load to and stiffness in fiber as reinforcement from fiber direction,weak at -usually continuous -forms shape and protects angles to fiber axis -oriented for principal fiber tailor fiber directions to stresses optimize propertiesCOMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES Table 1.2 Approximate Actual (US$/kg) Values of Saving One Unit of Weight: Costing Based on Some Late 1980s Estimates • small civil $80 • civil helicopter $80-$200 • military helicopter $400 • large transport $300 • large commercial $500 • advanced fighter $500 • VTOL $800 * SST $1500 • Space Shuttle $45,000 Chapter 2 describes the basic principles (micromechanics) of fiber composite materials. As an example, to a good first approximation, the stiffness under loading in the fiber direction (unidirectional fibers) may be determined by the simple law of mixtures. This is simply a sum of the volume (or area) fraction of the fibers and the matrix multiplied by the elastic modulus. The strength estimation is similar (for a reasonably high fiber-volume fraction) but with each elastic modulus multiplied by the breaking strain of the first-failing component. In the case of carbon fiber/epoxy composites, this is generally the fiber-breaking strain. If, however, the lowest failure strain is that of the matrix, the first failure event may be the development of extensive matrix cracking, rather than total fracture. This damage may or may not be defined as failure of the composite. However, toughness is usually much more than the sum of the toughness of each of the components because it depends also on the properties of the fiber/ matrix interface. Therefore, brittle materials such as glass fibers and polyester resin, when combined, produce a tough, strong composite, most familiarly known as fiberglass, used in a wide range of structural applications. Control of the strength of the fiber/matrix interface is of paramount importance for toughness, particularly when both the fiber and the matrix are brittle. If the interface is too strong, a crack in the matrix can propagate directly through fibers in its path. Thus it is important that the interface is able to disbond Table 1.3 Summary of the Approach for Development of a High-Performance Fiber Composite • Fibers t Polymer Matrix • Composite - stiff/strong/brittle/low - low stiffness and strength - toughness through density ductile or brittle synergistic action - high temperature - can be polymer, metal, (woodlike) capability or ceramic - high strength and - able to carry major load - transmits load to and stiffness in fiber as reinforcement from fiber direction, weak at - usually continuous - forms shape and protects angles to fiber axis - oriented for principal fiber - tailor fiber directions to stresses optimize properties