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Functional Behaviour of Materials: Liquid Crystals

Description

This set of animations looks at birefringerence, linear and circular polarisation and LCD display. From TLP: Liquid Crystals

Subjects

liquid | crystal | nematic | birefringence | phase | transformation | LCD display | DoITPoMS | University of Cambridge | animation | corematerials | ukoer

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Strain induced birefringence in a thermoformed PET cup

Description

The base of the cup is the least strained part of the sheet. Nevertheless, residual strain, increasing with radius, is evident when the specimen is viewed between crossed polars. The polymer chains are more highly aligned where the strain is greatest and this leads to greater birefringence (rotation of polarised light). Hence a circumferential pattern of colours is observed. If heated above the glass-transition temperature of PET (70-80 °C), the cup will tend to retract towards its unstrained form of a sheet. The dark 'Maltese cross', spreading out horizontally and vertically from the centre, indicates the extinction directions or the orientations of the two polarising films, where the intensity of transmitted light is lowest. Note: most clear plastic cups are made from polystyrene (PS)

Subjects

alignment | birefringence | cup | drawing | polyester | polyethylene terephthalate (pet) | polymer | strain | thermoforming | doitpoms | university of cambridge | micrograph | corematerials | ukoer | Engineering | H000

License

Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales http://creativecommons.org/licenses/by-nc-sa/2.0/uk/ http://creativecommons.org/licenses/by-nc-sa/2.0/uk/

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Injection-moulded polystyrene ruler

Description

The colours in the image are the result of birefringence and relate to the residual strain in the polystyrene. The pattern of strain is indicative of the flow of material during the injection process and it highlights the injection point (known as a 'gate') at the end of the ruler; the molecular alignment is greatest near this point. Towards the edges of the ruler and along its length, the material becomes more relaxed and as the molecular alignment falls, the retardation of light is less. If heated above the glass transition temperature of polystyrene (about 100 °C), the material will tend to relax, particularly along the centreline, near the gate. This will result in a wrinkled form.

Subjects

alignment | birefringence | injection moulding | polymer | polystyrene (ps) | ruler | sprue | strain | doitpoms | university of cambridge | micrograph | corematerials | ukoer | Engineering | H000

License

Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales http://creativecommons.org/licenses/by-nc-sa/2.0/uk/ http://creativecommons.org/licenses/by-nc-sa/2.0/uk/

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Strain-induced birefringence in a notched polycarbonate bar

Description

The colours in the image are the result of birefringence and relate to the strain-induced alignment of the polycarbonate molecules. Because the specimen is below the glass transition temperature of PC (145 °C), the material will relax when the stress is removed and there will be no residual alignment. The colours reveal the pattern of stress concentration around the notch.

Subjects

bending | birefringence | polycarbonate (pc) | polymer | stress | doitpoms | university of cambridge | micrograph | corematerials | ukoer | Engineering | H000

License

Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales http://creativecommons.org/licenses/by-nc-sa/2.0/uk/ http://creativecommons.org/licenses/by-nc-sa/2.0/uk/

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Strain-induced birefringence in a polycarbonate bar

Description

The colours in the image are the result of birefringence and relate to the strain-induced alignment of the polycarbonate molecules. Because the specimen is below the glass transition temperature of PC (145 °C), the material will relax when the stress is removed and there will be no residual alignment. The colours reveal the pattern of stress concentration around the points of contact with the rig, with a neutral axis along the centre and opposite values of birefringence on either side.

Subjects

bending | birefringence | polycarbonate (pc) | polymer | stress | doitpoms | university of cambridge | micrograph | corematerials | ukoer | Engineering | H000

License

Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales http://creativecommons.org/licenses/by-nc-sa/2.0/uk/ http://creativecommons.org/licenses/by-nc-sa/2.0/uk/

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Birefringence in a film of polypropylene

Description

The colours in the image are the result of birefringence and relate to the residual stress in the film, following the biaxial stretching process. The uniformity of the colour (with contrast only where an additional thickness of film exists or where wrinkling has resulted in a different apparent thickness) is indicative of both a uniform film thickness and of the uniformity of the drawing process used to make the film. Note that unlike polyethylene, the film has not been permanently strained in the region of the tear. This is because the monomer is bulkier and the material is relatively close to its glass transition temperature (~8 °C for PP, ~-30 °C for PE). Chain alignment and subsequent sliding is therefore much more sluggish and does not occur readily under an applied stress.

Subjects

alignment | birefringence | extrusion | film | polymer | polypropylene (pp) | doitpoms | university of cambridge | micrograph | corematerials | ukoer | Engineering | H000

License

Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales http://creativecommons.org/licenses/by-nc-sa/2.0/uk/ http://creativecommons.org/licenses/by-nc-sa/2.0/uk/

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Birefringence in a film of polyethylene

Description

The colours in the image are the result of birefringence and relate to the residual stress in the film. The colour is generally uniform (with some contrast where an additional thickness of film exists or where wrinkling has resulted in a different apparent thickness). This is indicative of both a uniform film thickness and of the uniformity of the drawing process used to make the film. In the region of the tear, however, the colours indicate strain-induced orientation: because the material is above the glass transition temperature of PE (-30 °C) chain sliding occurs readily under and applied stress and the molecules become aligned along the tensile axis. The strain is permanent and does not relax when the stress is removed. However, if the material were annealed at ~100 °C, it would rela

Subjects

alignment | birefringence | extrusion | film | polyethylene (pe) | polymer | polythene | doitpoms | university of cambridge | micrograph | corematerials | ukoer | Engineering | H000

License

Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales http://creativecommons.org/licenses/by-nc-sa/2.0/uk/ http://creativecommons.org/licenses/by-nc-sa/2.0/uk/

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Injection-moulded polystyrene (PS) case

Description

The colours in the image are the result of birefringence and relate to the residual strain in the polystyrene. The pattern of strain is indicative of the flow of material during the injection process and it highlights the injection point (known as a 'sprue') which can also be identified by a small lump on the surface; the molecular alignment is greatest near this point. Towards the edges of the ruler and along its length, the material becomes more relaxed and as the molecular alignment falls, the retardation of light is less. If heated above the glass transition temperature of polystyrene (about 100 °C), the material will tend to relax, particularly near the sprue. This will result in a wrinkling of the component.

Subjects

alignment | birefringence | injection moulding | polymer | polystyrene (ps) | sprue | doitpoms | university of cambridge | micrograph | corematerials | ukoer | Engineering | H000

License

Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales http://creativecommons.org/licenses/by-nc-sa/2.0/uk/ http://creativecommons.org/licenses/by-nc-sa/2.0/uk/

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Injection-moulded protractor

Description

The colours in the image are the result of birefringence and relate to the residual strain in the polystyrene. The pattern of strain is indicative of the flow of material during the injection process and it highlights a 'weld line' where two distinct flows of material meet. The weld line is a common point of failure because there is limited intermixing of the two fronts during the time allowed.

Subjects

alignment | birefringence | injection moulding | polymer | polystyrene (ps) | protractor | sprue | weld | doitpoms | university of cambridge | micrograph | corematerials | ukoer | Engineering | H000

License

Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales http://creativecommons.org/licenses/by-nc-sa/2.0/uk/ http://creativecommons.org/licenses/by-nc-sa/2.0/uk/

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Injection-moulded protractor

Description

The colours in the image are the result of birefringence and relate to the residual strain in the polystyrene. The pattern of strain is indicative of the flow of material during the injection process and it highlights the injection point (at 152 degrees on the outer scale), and two 'weld lines' (at 338 and on the central bar) where two distinct flows of material meet. The weld line is a common point of failure because there is limited intermixing of the two fronts during the time allowed.

Subjects

alignment | birefringence | injection moulding | polymer | polystyrene (ps) | protractor | sprue | weld | doitpoms | university of cambridge | micrograph | corematerials | ukoer | Engineering | H000

License

Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales http://creativecommons.org/licenses/by-nc-sa/2.0/uk/ http://creativecommons.org/licenses/by-nc-sa/2.0/uk/

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Injection-moulded polypropylene (PP) hinge

Description

The colours in the image are the result of birefringence and relate to the residual strain in the polypropylene. The pattern of strain is indicative of the flow of material during the injection process and reveals the molecular alignment in the constriction which constitutes a hinge. The aligned molecules provide a fatigue resistant, flexible joint. A sprue which has been cut off is also visible in the top left-hand corner of the image.

Subjects

alignment | birefringence | hinge | injection moulding | polymer | polypropylene (pp) | doitpoms | university of cambridge | micrograph | corematerials | ukoer | Engineering | H000

License

Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales http://creativecommons.org/licenses/by-nc-sa/2.0/uk/ http://creativecommons.org/licenses/by-nc-sa/2.0/uk/

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Sprue in a sample of polypropylene (PP)

Description

The colours in the image are the result of birefringence and relate to the residual strain in the polypropylene. The pattern of strain is indicative of the flow of material during the injection process and highlighting the injection point or 'sprue' from which all the material flows radially into the mould. The molecules are aligned parallel to the direction of flow.

Subjects

alignment | birefringence | injection moulding | polymer | polypropylene (pp) | sprue | doitpoms | university of cambridge | micrograph | corematerials | ukoer | Engineering | H000

License

Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales http://creativecommons.org/licenses/by-nc-sa/2.0/uk/ http://creativecommons.org/licenses/by-nc-sa/2.0/uk/

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Frozen stress photoelastic fringe pattern during fibre pushout

Description

The experiment indicates a more uniform of shear stress than that predicted by the shear lag model, suggesting that interfacial shear de-bonding strength values obtained from de-bonding loads on the basis of that model may be overestimates. (See 'The use of single fibre pushout testing to explore interfacial mechanisms in SiC monofilament-reinforced Ti-I. A photoelastic study of the test', Watson and Clyne, Acta metall. mater. Vol 40, No 1, pp 131-139, 1992 for more details.)

Subjects

araldite | birefringence | composite material | fibre | load distribution | photoelasticity | polymer | polymer composite | pushout | reinforcement | residual stress | shear | doitpoms | university of cambridge | micrograph | corematerials | ukoer | Engineering | H000

License

Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales http://creativecommons.org/licenses/by-nc-sa/2.0/uk/ http://creativecommons.org/licenses/by-nc-sa/2.0/uk/

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Frozen stress photoelastic assessment of load distribution around a cylinder

Description

The experiment indicates a more uniform of shear stress than that predicted by the shear lag model, suggesting that the model underestimates the magnitude of fibre loading, especially at low fibre aspect ratios. Eshelby type models, on the other hand, successfully predict the average phase stresses, but not the form of stress varying within the fibres.

Subjects

araldite | birefringence | composite material | eshelby | load distribution | photoelasticity | polymer | polymer composite | reinforcement | residual stress | shear | doitpoms | university of cambridge | micrograph | corematerials | ukoer | Engineering | H000

License

Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales http://creativecommons.org/licenses/by-nc-sa/2.0/uk/ http://creativecommons.org/licenses/by-nc-sa/2.0/uk/

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Frozen stress photoelastic assessment of load distribution around an ellipsoid

Description

The experiment indicates a more uniform of shear stress than that predicted by the shear lag model, suggesting that the model underestimates the magnitude of fibre loading, especially at low fibre aspect ratios. Eshelby type models, on the other hand, successfully predict the average phase stresses, but not the form of stress varying within the fibres.

Subjects

araldite | birefringence | composite material | eshelby | load distribution | photoelasticity | polymer | polymer composite | reinforcement | residual stress | shear | doitpoms | university of cambridge | micrograph | corematerials | ukoer | Engineering | H000

License

Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales http://creativecommons.org/licenses/by-nc-sa/2.0/uk/ http://creativecommons.org/licenses/by-nc-sa/2.0/uk/

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Nematic liquid crystalline polymer

Description

Liquid crystalline polymers (LCPs) exhibit a mesophase above their melting temperature. In this phase, they are fluid yet they retain long-range orientational order. The self-alignment of crystallites manifests itself as patterns of optical birefringence when the liquids are viewed between crossed polars. Defects in the patterns are analogous to dislocations in crystal systems. This specimen is K24 (4-cyano-4'-8-alkylbiphenyl), which is nematic above 35 degrees C. The general gradation of colour is simply due to a wedge plate placed over the specimen.

Subjects

alignment | birefringence | disclination | liquid crystalline polymer (lcp) | polymer | doitpoms | university of cambridge | micrograph | corematerials | ukoer | Engineering | H000

License

Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales http://creativecommons.org/licenses/by-nc-sa/2.0/uk/ http://creativecommons.org/licenses/by-nc-sa/2.0/uk/

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Functional Behaviour of Materials: Liquid Crystals

Description

This set of animations looks at birefringerence, linear and circular polarisation and LCD display. From TLP: Liquid Crystals

Subjects

liquid | crystal | nematic | birefringence | phase | transformation | lcd display | doitpoms | university of cambridge | animation | corematerials | ukoer | Engineering | H000

License

Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales http://creativecommons.org/licenses/by-nc-sa/2.0/uk/ http://creativecommons.org/licenses/by-nc-sa/2.0/uk/

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Strain induced birefringence in a thermoformed PET cup

Description

The base of the cup is the least strained part of the sheet. Nevertheless, residual strain, increasing with radius, is evident when the specimen is viewed between crossed polars. The polymer chains are more highly aligned where the strain is greatest and this leads to greater birefringence (rotation of polarised light). Hence a circumferential pattern of colours is observed. If heated above the glass-transition temperature of PET (70-80 C), the cup will tend to retract towards its unstrained form of a sheet. The dark 'Maltese cross', spreading out horizontally and vertically from the centre, indicates the extinction directions or the orientations of the two polarising films, where the intensity of transmitted light is lowest. Note: most clear plastic cups are made from polystyren

Subjects

alignment | birefringence | cup | drawing | polyester | polyethylene terephthalate (PET) | polymer | strain | thermoforming | DoITPoMS | University of Cambridge | micrograph | corematerials | ukoer

License

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Injection-moulded polystyrene ruler

Description

The colours in the image are the result of birefringence and relate to the residual strain in the polystyrene. The pattern of strain is indicative of the flow of material during the injection process and it highlights the injection point (known as a 'gate') at the end of the ruler; the molecular alignment is greatest near this point. Towards the edges of the ruler and along its length, the material becomes more relaxed and as the molecular alignment falls, the retardation of light is less. If heated above the glass transition temperature of polystyrene (about 100 C), the material will tend to relax, particularly along the centreline, near the gate. This will result in a wrinkled form.

Subjects

alignment | birefringence | injection moulding | polymer | polystyrene (PS) | ruler | sprue | strain | DoITPoMS | University of Cambridge | micrograph | corematerials | ukoer

License

http://creativecommons.org/licenses/by-nc-sa/2.0/uk/

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Strain-induced birefringence in a notched polycarbonate bar

Description

The colours in the image are the result of birefringence and relate to the strain-induced alignment of the polycarbonate molecules. Because the specimen is below the glass transition temperature of PC (145 C), the material will relax when the stress is removed and there will be no residual alignment. The colours reveal the pattern of stress concentration around the notch.

Subjects

bending | birefringence | polycarbonate (PC) | polymer | stress | DoITPoMS | University of Cambridge | micrograph | corematerials | ukoer

License

http://creativecommons.org/licenses/by-nc-sa/2.0/uk/

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Strain-induced birefringence in a polycarbonate bar

Description

The colours in the image are the result of birefringence and relate to the strain-induced alignment of the polycarbonate molecules. Because the specimen is below the glass transition temperature of PC (145 C), the material will relax when the stress is removed and there will be no residual alignment. The colours reveal the pattern of stress concentration around the points of contact with the rig, with a neutral axis along the centre and opposite values of birefringence on either side.

Subjects

bending | birefringence | polycarbonate (PC) | polymer | stress | DoITPoMS | University of Cambridge | micrograph | corematerials | ukoer

License

http://creativecommons.org/licenses/by-nc-sa/2.0/uk/

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Birefringence in a film of polypropylene

Description

The colours in the image are the result of birefringence and relate to the residual stress in the film, following the biaxial stretching process. The uniformity of the colour (with contrast only where an additional thickness of film exists or where wrinkling has resulted in a different apparent thickness) is indicative of both a uniform film thickness and of the uniformity of the drawing process used to make the film. Note that unlike polyethylene, the film has not been permanently strained in the region of the tear. This is because the monomer is bulkier and the material is relatively close to its glass transition temperature (~8 C for PP, ~-30 C for PE). Chain alignment and subsequent sliding is therefore much more sluggish and does not occur readily under an applied stress.

Subjects

alignment | birefringence | extrusion | film | polymer | polypropylene (PP) | DoITPoMS | University of Cambridge | micrograph | corematerials | ukoer

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http://creativecommons.org/licenses/by-nc-sa/2.0/uk/

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Birefringence in a film of polyethylene

Description

The colours in the image are the result of birefringence and relate to the residual stress in the film. The colour is generally uniform (with some contrast where an additional thickness of film exists or where wrinkling has resulted in a different apparent thickness). This is indicative of both a uniform film thickness and of the uniformity of the drawing process used to make the film. In the region of the tear, however, the colours indicate strain-induced orientation: because the material is above the glass transition temperature of PE (-30 C) chain sliding occurs readily under and applied stress and the molecules become aligned along the tensile axis. The strain is permanent and does not relax when the stress is removed. However, if the material were annealed at ~100 C, it would

Subjects

alignment | birefringence | extrusion | film | polyethylene (PE) | polymer | polythene | DoITPoMS | University of Cambridge | micrograph | corematerials | ukoer

License

http://creativecommons.org/licenses/by-nc-sa/2.0/uk/

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Injection-moulded polystyrene (PS) case

Description

The colours in the image are the result of birefringence and relate to the residual strain in the polystyrene. The pattern of strain is indicative of the flow of material during the injection process and it highlights the injection point (known as a 'sprue') which can also be identified by a small lump on the surface; the molecular alignment is greatest near this point. Towards the edges of the ruler and along its length, the material becomes more relaxed and as the molecular alignment falls, the retardation of light is less. If heated above the glass transition temperature of polystyrene (about 100 C), the material will tend to relax, particularly near the sprue. This will result in a wrinkling of the component.

Subjects

alignment | birefringence | injection moulding | polymer | polystyrene (PS) | sprue | DoITPoMS | University of Cambridge | micrograph | corematerials | ukoer

License

http://creativecommons.org/licenses/by-nc-sa/2.0/uk/

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Injection-moulded protractor

Description

The colours in the image are the result of birefringence and relate to the residual strain in the polystyrene. The pattern of strain is indicative of the flow of material during the injection process and it highlights a 'weld line' where two distinct flows of material meet. The weld line is a common point of failure because there is limited intermixing of the two fronts during the time allowed.

Subjects

alignment | birefringence | injection moulding | polymer | polystyrene (PS) | protractor | sprue | weld | DoITPoMS | University of Cambridge | micrograph | corematerials | ukoer

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http://creativecommons.org/licenses/by-nc-sa/2.0/uk/

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