3: Intermediate Materials Flashcards
What is the purpose of the ‘intermediates’ process?
To convert raw fibres (and matrices if prepregs) into workable materials: mats, fabrics and prepregs
What affect does increasing the number of processing stages have on costs?
Costs increase
Bundle (multiple filaments) sizes are measured in “hex”. Define 1 hex
1 gram per kilometre (1g/1000m)
If aligned: Glass-roving, Carbon-tow
If twisted: Yarn
What are some advantages of using yarn fibre bundles compared to aligned bundles?
-Reduced risk of bundle falling apart and fibre breakage
-More circular cross-section
However, reduces axial properties and costs more
List some properties of fibre mats
-Flat sheets (usually glass fibres)
-Chopped or continuous fibres
-Polymer binded (to stabilise the mat structure)
-Random fibre orientation, resulting in isotropic properties (however directionality can be induced during design)
-Common matrices used are: Polyester or epoxy
-Fibre volume fraction <40%
-Low cost
-High industrial application
Describe the manufacture process of chopped strand mat (CSM)
-Fibre strands are chopped and deposited on a conveyer belt at a defined rate
-Polymer binder is sprayed onto the mat
-The binder is soluble in styrene
-Processed in hand laminating
Describe the manufacture process of continuous filament mat (CFM)
-Similar process to CSM, however molten glass is deposited instead of chopped matting
-Continuous fibres form loops
-Processed using hand laminating or liquid composite moulding
What are some advantages CFM offers compared to CSM?
-Better formability
-Washing resistance
List some properties of textile reinforcements
-Patterned fibre arrangement (architecture)
-Filaments arranged into tows (micro level)
-Tows arranged into fabric architecture (meso level)
-Homogenised effective properties of textile sheet (macro level)
-Fibre volume fractions >50%
-Anisotropic due to tow alignment (usually orthotropic)
Define a “unit cell”
-The smallest repetitive unit allowing a macroscopic textile patch to be generated through translation along weft and warp directions
-Analysis of a unit cell fabric is sufficient to determine material properties
Describe the “weaving process”
-A longitudinal warp yarn is raised/lowered to form gaps (in predefined patterns)
-A transverse weft yarn is drawn through the gaps and inserted over/under the warp yarns
-The weft yarn is pushed against the fell of the fabric by a reed
-The process repeats forming a woven fabric
List some common weave architectures
-Plain weave: 1x1 (high crimp, stable fabric)
-Twill weaves: 2x2, 3x1, ect…
-Satin weaves: 5x1, 4x1, 8x1 (low crimp, improved mechanical properties)
-Unidirectional weaves (unbalanced in yarn size and spacing)
What is crimp and what are some material by-products of it?
The “waviness” of the yarns (cohesion of yarn layers when high levels of interlacing)
-Through thickness properties increase
-In-plane properties decrease (compared to straight fibres)
-crimp proclivity varies with weave architecture
How do woven fabrics usually deform and what effect of material properties does this have?
Mainly in shear (fabric shear is known as “drape”)
-The yarn crossover points act as junctions, therefore both volume fraction and fibre orientation are affected (both depend on shear angle “alpha”)
Describe the purpose and properties of spread tow fabrics
-To improve mechanical properties due to a reduction in crimp at smaller thicknesses
-More expensive due to additional processing
Explain the manufacture process of Non-Crimp Fabrics (NFC)
Continuous process:
-Layers of aligned tows are laid up on a flat bed at defined orientations (eg. 45, 90, ect)
-Needle bars at the end of the bed stitch layers together with a sewing thread
-Layers are stitched together
What are the properties of knitted fabrics?
-Same knot pattern for fibre fabrics as knitting
-Better at fitting moulds
-Decreased mechanical properties
What is the test for how far a fabric can shear before wrinkling?
Picture frame shear test
What is the test for how easily fabric layers slide relative to each other or tool surface?
Fabric friction test
What is the test for what compaction pressure is required to achieve a specified fibre volume fraction?
Fabric compression test
Explain the picture frame shear test
-Shear frame mounted on a universal testing machine
-Cruciform specimen clamped in the frame
-Cross-head moves upwards at a constant speed
-Measure cross-head displacement and force at the load cell
-Works for bi-directional fabrics, yarns aligned with sides of frame (doesn’t work for multiaxial fabrics or random mats)
Explain the Fabric Friction Test
-Aluminium table attached to universal testing machine
-Brick-shaped steel sled with defined mass connected to a cross-head via fishing line and “no friction” pulley
-Fabric layer is bonded to the sled/table or both
-Pull sled at constant speed
-Record pulling force and displacement to calculate the COF
Explain the Fabric Compression Test
-Fabric specimen sandwiched between two flat steel compression plates
-top plate is adjusted to ensure the plates are parallel
-Compression force and Gap between the plates are measured
-Vf plotted against compaction pressure
Vf=mass/(densityareameasured height)
compaction pressure=Fc/A
Explain a typical Thermoset Prepreg material
-Unidirectional tape or fabric pre-impregnated with partially cured (B-stage) resin system
-Usually carbon fibre & epoxy resin
-Limited storage life, so stored at low temps to minimise cure (-18 degrees)
-Able to conform to highly geometrically complex parts (when laid by hand, then autoclave cured)
-Can produce large low geometrically complex parts (when manufactured with ATL)
