R1 Classification and structure of materials

Posted: October 10, 2010 in Resistant Materials

Initial knowledge – 1

Current working knowledge – 2

Having previously studied design and technology at GCSE and A level, I have naturally come into contact with various materials when manufacturing products.  However I have never really thought about the classification and structures of the materials I was using.  OK, so I knew there were different hardwoods and softwoods but I have never really thought about why the different materials would be suitable for different applications.  It is an important factor of design that the appropriate materials are selected for the correct tasks.  That is why I feel it is important we were set the research tasks in the first week to look at the different materials we will come into contact with this year and in our teaching year and throughout our careers.

The presentations were informative and are available on the internet so for quick reference I have used them and some of my own research to have a look at the different types  of materials, their classification and structure.

Wood

Wood obviously comes from trees, so what is the structure of a tree?  Here is a cross section of the trunk:

  • The bark protects the tree against damage and extreme weather conditions
  • The phloem transports sugars round the tree.
  • The cambium is where all the cells are produced.
  • Sapwood is where the sap still flows.
  • Heartwood is sapwood that has gone through a chemical change and now supports the tree.
  • The annual rings basically tell you how old the tree is

I often thought that hardwood and softwood were the actual density of the individual woods however this is not the case, it means there botanical classification!

Hardwood is typically more expensive than softwood.

Density: Hardwood has a higher density and is therefore harder.  Softwood has a lower density, therefore most softwood varieties are softer than hardwood.

Found in regions: Trees supplying hardwood are found throughout the world from the Boreal and Taiga forests of the North to the tropics and down into the far South.  Softwood is found in the northern hemisphere.

Examples: Examples of hardwood are mahogany, teak, walnut, oak, ash, elm, aspen, poplar, birch, maple etc.  Examples of softwood trees are pine, spruce, cedar, fir, larch, douglas-fir etc.

Growth: Hardwood has a slower growth rate than softwood.

Type: Hardwood are mostly deciduous. Some European evergreen trees that yield hardwood are holly, boxwood and holm oak.  Softwoods are evergreen.

Properties: Broad leaves; enclosed nuts and a higher density.  Not all hardwood is actually hard e.g. poplar and basswood.  Softwood is less dense,  less durable, it has high calorific values and they are coniferous trees.

Shedding of leaves:Hardwoods shed their leaves over a period of time whereas softwoods don’t shed their leaves.

Definition: Hardwood comes from deciduous trees that drop their leaves every year. Softwoods have trees that are conifer and have needles, and normally do not lose needles.

Applications: Hardwood is used for trimmings and furniture but less frequently than softwood.  Softwood is widely used as woodware for building (homes/cabins) and furniture.

Read more: Hardwood vs Softwood – Difference and Comparison | Diffen http://www.diffen.com/difference/Hardwood_vs_Softwood#ixzz1IIFNKXvT

Wood is anisotropic which means its structures and properties vary in different directions its compressive and tensile strength acting parallel to the grain is greater than the strength perpendicular to the grain.  It is an elastic material and will bend when put under even small loads.  If the load is too big then the wood will of course snap.  Wood has an elastic or proportional limit.  For load values below the proportional limit values of load and deflection are proportional to each other.

Wood is a very good thermal conductor hence the reason it is used in fire making and the heat will travel faster going with the grain than across it.  Electric current will travel through wood due to impurities in the form of metallic ions.  There aren’t very many of these in dried wood therefore wood is a good insulator and hence why utility poles are made from it.  Wood also has interesting acoustic properties and they can have interesting implications.  Some wood have the properties of dampening sound while others will resonate.  For example some woods such as spruce are used in violins as it has excellent resonant qualities.

Metal

Metals are split into two classifications also, ferrous and non- ferrous.  Ferrous metals are commonly those metals which contain iron. They may have small amounts of other metals or other elements added, to give the required properties. All ferrous metals are magnetic and give little resistance to corrosion.  Compounds containing iron having a valance of +2 are ferrous, those compounds containing iron having a valence of +3 are ferric.  Most commonly used ferrous metals are Mild Steel, High Speed Steel, Stainless Steel, High Tensile Steel and Cast Iron.  Ferrous metals are known for their ability to allow for oxidation which is a property known as corrosion. Oxidation of ferrous metals can be seen in a reddish brown deposit on the surface which is an oxide of iron.

Non ferrous metals are metals that do not contain iron or they are an alloy of metals which do not contain iron as a component. Most, but not all, ferrous metals are magnetic in nature but in magnetism, ferrous metals vary depending upon the amount of iron they contain. Stainless steel, though it contains iron is not magnetic in nature because of the process that makes it stainless. It is put in nitric acid to get rid of iron and what remains is a lot of nickel thus making it non magnetic though it still classifies as a ferrous metal.

Non ferrous metals have properties different from ferrous metals and are used for industrial applications.   They are mainly used because of reduced weight, higher strength, non magnetic properties, higher melting points and resistance to corrosion, whether chemical or atmospheric. These non ferrous metals are also ideal for electrical and electronic applications.

Metals structure and the arrangement of atoms

Metals are large structures of atoms that are held together by metallic bonds, large but also variable numbers of atoms are involved but it depends on the size of the piece of metal.

Most metals are close packed which means they fit as many atoms as possible into the available volume. Each atom in the structure has 12 touching neighbours.  This metal is known as 12-co-ordinated.  Each atom has 6 other atoms touching it in each layer.

There are also 3 atoms touching any particular atom in the layer above and another 3 in the layer underneath.

This second picture shows the layer immediately above the first layer. There will be a corresponding layer underneath.

Some metals ( those in Group 1 of the Periodic Table) are packed less efficiently, having only 8 touching neighbours. These are known as 8-co-ordinated.

The left hand diagram shows that no atoms are touching each other within a particular layer . They are only touched by the atoms in the layers above and below. The right hand diagram shows the 8 atoms (4 above and 4 below) touching the darker coloured one.

It is misleading to think that all the atoms in a piece of metal are arranged in a regular way.  Any piece of metal is made up of a large number of ‘crystal grains’, which are regions of regularity. At the grain boundaries atoms have become misaligned.

Plastics

 Plastics can be classified according to several criteria. Generally an initial rough classification can be made according to their chemical structure.  The initial differentiation is between cross-linked and non-cross-linked materials. Thermoplastics are not cross-linked, elastomers and thermosets are cross-linked materials.

Plastics are made of linear or branched molecules. In thermoplastic materials there is no chemical connection between individual macromolecules. Therefore, they can be reused several times.   As a disadvantage, they can be chemically dissolved.

With thermoplastics we further distinguish between those plastics in which the macromolecules are arranged at random and those materials with some areas arranged in a regular way. If the arrangement of the macromolecules is random, the materials are termed amorphous. They can be easily identified by their transparency if no color pigments are admixed. Materials with molecules arranged regularly in some areas are termed semicrystalline. They are not transparent even if pigments are not admixed.

Because the macromolecules are entangled, complete crystallization is impossible. This means that between crystalline areas there still amorphous regions. The proportion of crystalline sections in relation to complete crystallization is described by the degree of crystalline and can be influenced by the process conditions during processing. The degree of crystallinity depends strongly on the material itself. The simpler the chain structure, the higher is the degree of crystallinity.

There are also plastics that can be produced in either an amorphous or a semicrystalline state, depending on the processing parameters. Amorphous and semicrystalline thermoplastics have different properties with regard to processing. They also show different performance characteristics.

The other large group of plastics, besides the thermoplastics, is the cross-linked materials, which can be further divided into slightly and strongly cross-linked systems. Unlike thermoplastics, these materials cannot be refused and processed several times. Cross-linking means that chemical connections are created between individual macromolecules, in a chemical reaction. Slightly cross-linked materials are termed elastomers. They do not dissolve in solvents, but swell chemically.

As the number of transverse connections between molecules increase, the material becomes harder and more brittle, and is no longer swellable. Strongly cross-linked plastics are termed thermoses. The numerous macromolecules have become one single molecule having a very complex cross-linked structure.

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