1IntroCompositeApplns.PropertiesVoigt, Reuss,HillAnistrpy.CTECellularMatls.WoodL7: Microstructure-Properties: CompositesFall 200727-301, Microstructure & Properties IA. D. RollettMicrostructurePropertiesProcessingPerformance2IntroCompositeApplns.PropertiesVoigt, Reuss,HillAnistrpy.CTECellularMatls.WoodLecture Objectives: Composites• The main objective of this lecture is to introduce youto microstructure-property relationships in compositematerials.• Composite materials constitute a huge class ofmaterials. The objective of this lecture will thereforebe to provide some definitions and describe some ofthe basic relationships.• Cellular materials will be emphasized because oftheir connection to natural materials (biomaterials)and especially wood, which you will study in thesecond Lab.3IntroCompositeApplns.PropertiesVoigt, Reuss,HillAnistrpy.CTECellularMatls.WoodKey points• Composites are regarded as artificial (man-made)mixtures of phases.• Classification of composites by reinforcement type(dimensionality) - particles, fibers and laminated.• Application of the Rule of Mixtures.• Dependence of composite properties on the spatialarrangement of the phases.• Upper and lower bounds on properties - example ofelastic modulus, Voigt and Reuss approximations.• Anisotropy of composite properties, e.g. elasticmodulus.• Properties of wood as a cellular material.4IntroCompositeApplns.PropertiesVoigt, Reuss,HillAnistrpy.CTECellularMatls.WoodExamples5IntroCompositeApplns.PropertiesVoigt, Reuss,HillAnistrpy.CTECellularMatls.WoodWhat are Composites?• Composite materials contain more than one phase.• Almost all materials contain more than one phase, sowhat’s the difference?• The term composite is typically applied to a materialwhen the multi-phase structure is constructed bydirect intervention (external to the material).• Composite Material Examples: glass fiber reinforcedplastic (GRP), wood, clam shell, Mars bar.• Multi-phase Material Examples: precipitationstrengthened aluminum alloys, Ti-6Al-4V, dual-phasesteel, transformation toughened alumina (Al2O3-CeO2).• Caution! There is some overlap between thecategories!6IntroCompositeApplns.PropertiesVoigt, Reuss,HillAnistrpy.CTECellularMatls.WoodWhy Use Composites? [Biomaterials]• In nature, the basic materials tend to be weak and/or brittle.Evolution has resulted in structures that combine materialstogether for properties that far exceed those that could beobtained in the basic materials.• The basic inorganic constituent of bone, for example, is calciumphosphate in the form of crystalline Ca10(PO4)6(OH) andamorphous CaPO3. This ceramic is brittle and not particularlystiff. The matrix of fibrous collagen is tough but even less stiff.When embedded arranged in the form of a cellular material,however, remarkable values of stiffness:density andtoughness:density are achieved [and land-based multi-tonnecreatures are possible].• A similar situation exists in wood where the basic materials arequite compliant but arranged in the multi-level composite formsthat we know, high values of strength:density andtoughness:density result.7IntroCompositeApplns.PropertiesVoigt, Reuss,HillAnistrpy.CTECellularMatls.WoodWhy Use Composites? [Man-made]• The basic reason for the use of composites is alwaysthe same: some combination of properties can beachieved that is impossible in a monolithic material[for a given cost].• In SiC-reinforced aluminum for brake rotors, forexample, the combination of light weight, toughness(from the Al matrix at ~ 2.7 Mgm/m3), and stiffness(from the SiC additions) is not possible in eitherconstituent by itself.• In Cu-Nb for high strength electrical conductors, thecombination of >1GPa yield strength and highelectrical conductivity (in the Cu) could never beachieved in either constituent by itself. In this casethe high strength is a synergistic property of thecomposite.8IntroCompositeApplns.PropertiesVoigt, Reuss,HillAnistrpy.CTECellularMatls.WoodKey aspects of composites• Composites are expensive to make, ascompared to monolithic materials, especiallyif the shape and arrangement of the phasesmust be controlled.• Therefore there must be a strong motivationfor making a composite structure to offsetthe cost.• The simplest composites are particulatecomposites. Laminates are next, followedby fiber composites. Woven structures arethe most complex.9IntroCompositeApplns.PropertiesVoigt, Reuss,HillAnistrpy.CTECellularMatls.WoodTypical Microstructures• We show next some typical microstructures.• In biomaterials, many are cellular compositesat some length scale.• Man-made composites are more often fullydense. The three major [structural] materialtypes are all used so the abbreviations MMC[metal matrix composite], CMC [ceramicmatrix composite], and PMC [polymer matrixcomposite] are commonly used.10IntroCompositeApplns.PropertiesVoigt, Reuss,HillAnistrpy.CTECellularMatls.WoodCellularBiomaterialsGibson: CellularSolidsNote thevariation indensity; also thepresence ofdistinct layers ofcells in somewoods, and inbone.11IntroCompositeApplns.PropertiesVoigt, Reuss,HillAnistrpy.CTECellularMatls.WoodMan-made ExamplesSiC fibers in Ti3Al matrixSiC fibers in a CAS ceramic matrixDowling: Mech. Beh.Matls.Note the typical lengthscale of ~100µm, andthe use of fibers forreinforcement. Thisbasic type of fiber-reinforced compositeis strongly anisotropic.The toughness of suchcomposites and theneed for limitedadhesion betweenfiber and matrix wasdiscussed in thelecture on Fracture.12IntroCompositeApplns.PropertiesVoigt, Reuss,HillAnistrpy.CTECellularMatls.WoodFood!From left to right,top to bottom:a) Breadb) Meringuec) Chocolate bard) Chipe) Malteser(Candy)f) Jaffa cake(cookie)13IntroCompositeApplns.PropertiesVoigt, Reuss,HillAnistrpy.CTECellularMatls.WoodFood for Thought!• How does ice cream represent a material inwhich the thermal-mechanical history iscritical to its microstructure which, in turn,controls its properties? Homework!14IntroCompositeApplns.PropertiesVoigt, Reuss,HillAnistrpy.CTECellularMatls.WoodExamples of composites• The classical example of a composite is concrete.• It is more complex than it appears. There aretypically coarse and fine particles (rocks!) embeddedin a matrix of silicates and sulfates. There is a highfraction of pores of all sizes. This is an example of aparticulate composite.• Ordinary concrete (properly made) has excellentcompressive strength but poor tensile strength.