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A combination of two distinct phases found in carbon steel. The two phases are ferrite (alpha iron) and cementite (the intermetallic Fe3C), they form a layered structure as in pearlite but in this case the layer separation is smaller than the wavelength of visible light and so the pearlescent appearance of pearlite is absent when viewed through a microscope.


This occurs when a sample of ductile material is compressed. Assuming that the material has a positive Poisson Ratio the width of the sample must increase so that the volume of the material can be accommodated. This results in a simple cylinder shape bulging in the middle thereby resembling a traditional wooden barrel.


Beach Marks

These are the characteristic sign of a failure due to fatigue. The failure starts with a small crack of defect. Each cycle of loading the piece results in the crack growing by a small amount and the production of another line in the beach marks. Eventually the crack reaches a critical length and the piece fails. Since the fatigue process can take some considerable time to reach the critical length the beach marks often look old and tarnished whist the final failure site looks fresh.

Beach Marks in a Fatigue Failure

Beach marks in a fatigue failure.

Bird Cannon

A device for testing aircraft jet engines against birdstrike. a dead (but not frozen) bird, usually a chicken is fired in to a running engine. In order to pass the test, the engine must not eject fragments such as blade parts that could damage other engines or the fuselage. The engine itself is not expected to continue working.


Birdstrike is distinct from foreign object damage in that it only relates to small animals that are sucked into the running engine. Usually birdstrike occurs close to either takeoff or landing as it is due to birds congregating near the runway. However other animals such as small to medium sized mammals can also be sucked in to the engine if they cross the runway and occasionally aircraft have hit birds whist flying at altitudes of several kilometres. It is standard practice for the bird species concerned to be identified either by feather fragments or by DNA analysis

Body Centred Cubic

One of the standard crystal lattices. It is formed by placing an atom, or more generally a lattice point, at each corner of a cube and one in the centre. The packing density of the structure is approximately 68%.

Body Centred Cubic Structure

Body centred cubic structure.

Boyle’s Law

One of the basic gas laws. It states that for an ideal gas, if the temperature is constant then the pressure is inversely proportional to the volume.

Bravais Lattices

Bravais lattices are the basic repeating units that can make up a crystal, these are rather like the three dimensional equivalents of space filling tiles. There are fourteen Bravais lattices in total and they are all based on either simple or distorted cubes or a hexagonal prism. Prior to university studies the most frequently encountered are: simple cubic, body centred cubic, face centred cubic and hexagonal.

Simple CubicBody Centred CubicFace Centred Cubic

Simple cubic, body centred cubic and face centred cubic (with corner and front face atoms removed)

Bridgeman Furnace

A Bridgeman furnace is a device to produce directional solidification. If a sample is cooled without particular care then at some point the initial columnar growth crystals (which all grow in the same direction) are replaced by equiaxed crystals that grow in all possible directions simultaneously. The Bridgeman furnace maintains columnar growth crystals by providing a very steep temperature gradient (so that only a very small area is at the correct temperature for solidification) and a very low rate of growth so that the solidifying material has the necessary time to grow on the existing solid front rather than creating new, isolated crystals). The Bridgeman furnace is used in both research and in industry, where one of its roles is to produce directionally solidified and mono-crystaline turbine blades that are resistant to creep at the high temperatures found in aircraft jet turbine engines.

Brinell Hardness

The measure of the hardness (resistance to permanent deformation) of a material as assessed by the Brinell test. A small, hard ball (usually either hardened steel or tungsten carbide) if pressed into the material with a known force for fixed period of time. The size of the indentation is then measured and hardness calculated from the from a standard formula.

Brinell Hardness Testing Equation

Brinell hardness testing procedure and equation

Bulk Modulus

One of the standard elastic moduli. The bulk modulus is a measure of how easily the volume of a sample changes in response to a change in external pressure.

Bulk Modulus MeasurementEquation

Bulk modulus testing and equation

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