Designing Hybrid Materials

Transcription

1 MME445: Lecture 30 Designing Hybrid Materials Part 1: Composite and sandwich structures A. K. M. B. Rashid Professor, Department of MME BUET, Dhaka Learning Objectives Knowledge & Understanding Knowledge about structural and architectural materials Skills & Abilities Ability to fill holes in material property charts Values & Attitudes Realization of the potential for materials development Resources M F Ashby, Materials Selection in Mechanical Design, 4 th Ed., Ch. 11 1

2 Outline of today s lecture Introduction and synopsis Concept revisited holes in material-property space The method: A+B+Configuration+Scale Composites and sandwich structures Introduction & Synopsis The good and the bad about hybrids Hybrid corn Hybrid cars Improved yield, hardiness Low fuel consumption, emissions but Infertile but Expensive 2

3 Making hybrid materials Hybrid materials combine the properties of two or more monolithic materials (CFRP, GFRP) or, of one material and space (foams) or, of a single material in two different forms (dual phase steels, eutectic alloys, PSZ, ABS) Design variables: Choice of materials Volume fractions Configuration Connectivity Scale Holes in Material-Property Space Is it possible to create a material to fill this empty space? (A compliant-low thermal expansion material??) big empty area 3

4 What might we hope to achieve? The weaker link dominates particulate composites The best of both Zinc-coated steel Glazed porcelain The rule of mixtures unidirectional fibre composites (CFRP, GFRP) The least of both or weakest-link fire sprinkler systems (a wax-metal hybrid) Is hybrid a material? There is a certain duality in how hybrids are considered and discussed. Filled polymers, composites, or wood Galvanized steel (steel coated with zinc stinc???) Sandwich panels In case of comparison, we must think of the hybrid as a material in its own right, with its own set of effective properties If we are to design the hybrid itself, we must deconstruct it and think of it as a combination of materials (or of material and space) in a chosen configuration. 4

5 The method: A + B + Configuration + Scale A hybrid material is a combination of two or more materials in a predetermined configuration, relative proportion and scale (size and shape), optimised for a specific engineering purpose. A + B + Configuration + Scale lies in one area in the material property space lies in empty holes in the material property space 5

6 Assuming steel carries not current and copper example of a hybrid material filing a no load (the most pessimistic scenario), the hole in the material-property space performance of the cable will lie at the point shown on the figure. It has twice the resistivity of copper and half the strength of steel. It occupies a part of property space that was previously empty, offering performance that was not previously possible need strong, electrically conductive material for power line A + B + Conf + Scale Copper => min elect. resist. Steel => max YS Interleaving fine strands Hybrid materials: Four families of Configurations combine two solids, one (the reinforcement) in the other (the matrix) average properties outer faces material supported by a low-density core material greater flexural stiffness combination of material and space energy absorption properties or, Cellular materials sub-divided in 1, 2 or 3-dimensions lower stiffness and damage tolerance 6

7 Hybrid Type 1: Composites It is difficult to calculate/ predict the actual behaviour of the composite. Easier to find general bounds and limits that bracket the expectations/possibilities. Criteria of Excellence: Material Indices. Used to decide whether (or not) the hybrid outperforms existing materials. Fibre and particulate composites - the maths rule of mixtures for density (exact value) m matrix r reinforcing agent f fraction reinforcing agent rule of mixtures for stiffness (upper bound value along the fibre) (lower bound value across the fibre) same sort of equations available for strength, heat capacity, thermal and electrical conductivity 7

8 Composites for a stiff beam of minimum mass bounds for the elastic moduli of hybrids better Material indice, M Possibility of Al-alloy beam being replaced with Al-alloy Al 2 O 3 hybrid Al-alloy Be hybrid Be - fibres have a stronger effect due to their low density. Al 2 O 3 gives almost no gain. Commercial alloy: Al 62%Be M = 6.5 (compared to 3.1 for Al alone) filling property space with composites: example 1 (modulus-density chart) 8

9 filling property space with composites: example 2 (strength-density chart) Hybrid Type 2: Sandwich Structure Faces: material A Core: material B high bending stiffness and strength at low density structures with increased I and Z that resist bending and buckling 9

10 Beams and Panels: Shaping increases efficiency (more GPa/kg) E 1/2 / E 1/3 / Low density materials are paramount for efficient panels => foamed cores Sandwich structure: Properties defined Face: Modulus E f, Thickness t Increases I, takes load Volume fraction of face material :- -f = 2t/d the ultimate light, stiff hybrid: the sandwich panel Core: Modulus E c, Thickness c Prevents shear!! Core fraction : 1- f = 1- (2t/d)= c/d 10

11 Sandwich panel as a monolithic material: The maths Sandwich properties:, E, I Equivalent monolithic material properties: ρ, E, I ρ = ρ = m a d I = bd3 12 S = E I = E I E = 12 E I bd 3 Z = bd2 4 Z σ flex = M f σ flex = 4 M f bd 2 Rule of mixtures for density Fibre composites Sandwich panels Rule of mixtures for modulus Fibre composites (in tension) Sandwich panels (in bending) the sandwich in flexure is approx. 3 times more efficient than the most efficient fibrous composite, even when the fibres are all aligned normal to the axis of bending 11

12 unidirectional composites compared with sandwich structures E 1/3 / face sandwich core U-D Composites the performance of a sandwich structure compared with that of a composite made from the same two material Sandwich Panel: 3 times more efficient (GPa/kg, in bending) than the unidirectional composite (in tension) f 0.04 sandwich panel is 2.8 times lighter than a solid CFRP panel of the same stiffness or, (2.8) 3 = 22 times stiffer for the same mass 12

13 Filling property space with sandwich structures Those with 0.01 < f < 0.2 extend into an area that was previously empty. Using the index E 1/3 /ρ for a light, stiff panel as a criterion of excellence, we find that sandwiches offer performance not attainable before. Percolation A box full of marbles is only 66% full (75% full if the marbles are in an FCC of HCP arrangement). Between 25 and 34% of the volume is empty, interconnected space. Percolation may happen along the interconnected interstices. You need at least about 25-30% volume fraction of liquid to have interconnection (continuity) from top to bottom, hence percolation of properties. Percolation: important design tool for hybrids 13

14 switching percolation on and off V = 0.05 Isolated particles V = 0.10 Small Isolated clusters particles dispersed in a continuum matrix V = 0.15 Long Isolated clusters V = 0.2 Long interconnected clusters percolation switches on Percolation relates to the existence of a continuous path trough the structure. Dispersed particles touch at V f > 0.2 Minimum volume fraction for percolation: about 20% Example: mixing metallic powders with polymers result in electrically conducting polymers. The property disappears (switches-off) at V f < 0.2. Percolation affects other properties as well: Thermal conductivity Ductility and fracture toughness of composites Percolation is affected by the shape of the particles (fibres tend to touch each other more often than round particles) 14

15 Example of percolation: Flexible ferromagnets Monolithic ferromagnetic materials are stiff, metallic or ceramic, solids. Elastometric ferromagnetic hybrids offer several properties that these monolithic solids do not. The hybrids are made by mixing up to 30% of sub-micron-sized iron particles into an elastomer resin before polymerising it. The result is a compliant ferromagnetic material that has the property that it is magnetostrictive, and that its stiffness increases when placed in the magnetic field because the magnetic dipoles that are induced in the particles attract one another. The material has fast (1 ms) response time, making it suitable for vibration damping. Magnetostriction ( or Joule effect) is a property of ferromagnetic materials that causes them to change their shape when subjected to a magnetic field. The reciprocal effect, the change of the susceptibility of a material when subjected to a mechanical stress, is called the Villari effect. (Wikipedia) Next Class MME445: Lecture 31 Designing Hybrid Materials Part 2: Lattice and segmented structures 15