14-08-2012, 10:12 AM
Defects in Composite Materials
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MANUFACTURING TECHNIQUES
Fibers and matrix are directly processed into a finished structure
Pultrusion
Filament Winding
Fibers incorporated into matrix to make ready-to-mold sheets: prepregs and SMCs
Autoclave Molding
Compression Molding
Void Content
During incorporation of fibers into matrix at the time of manufacturing, air or other volatiles may get trapped, known as voids
Caused due to inability of resin to displace air
It significantly affects mechanical properties
Higher void content over 2% leads to lower fatigue resistance, greater susceptibility to water diffusion
Factors contributing towards void formation
Rate of fiber pull
Resin viscosity
Solvents used, moisture, chemical contaminants in resin may remain dissolved, but volatilize during high temperature curing
Air entrapment during lamination
Shrinkage
Reduction in volume or linear dimensions
Caused by curing and thermal contraction
Rearrangement of polymer molecules into more compact mass
Thermal shrinkage during cooling period after curing reaction
Desirable for easy release of part from mold
Warpage is a distortion where the surfaces of the molded part do not follow the intended shape of the design and
Sink marks are caused by localized shrinkage of the material
Voids in Compression Molding
Voids and inter-laminar cracks due to premature gelation
For slow mold closing speeds, SMC flow pattern depends on charge thickness, higher thickness, the more the outer layers undergoes extensional deformation
Blisters are inter laminar cracks due to large pockets of entrapped air at high pressures
Weld lines in compression molding
Weld lines are linear domains of poor fiber orientation that are formed at the joining of the two divided flow fronts
Strength of the part in the direction normal to the weld line aka knit lines are reduced
Can be reduced by proper charge placement
Common area is behind core pins for molded-in holes, so better to drill for holes in high stress area
Warpage in compression molding
Caused by variations in cooling rates between sections of different thickness or different fiber orientations
It is critical in case of thin section molding
Differential cooling rates may also lead to complex residual stresses
Pultrusion
Rate of heat transfer from the cured material to the die walls is increased owing to lower die temperature near the exit, else interlaminar cracks may form
Control of pulling force and design of fiber guidance system influences the fiber alignment
Filament winding
Voids, delaminations and fiber wrinkles are the common defects here
Voids are caused due to poor fiber wet-out, air bubbles in resin bath, excessive resin squeeze out
Delaminations caused by excessive time lapse between two consecutive layers of windings
Wrinkles are caused due to improper winding tension and misaligned roving, unstable fiber paths that causes fibers to slip on mandrel causes misalignment
Radiography
Internal structure of molded part is examined by impinging a beam of radiation on one of its surfaces and recording the intensity of the beam as it emerges from the opposite surface
Uses X-rays conventionally
Records internal defects as a shadow image on photographic film
Gamma rays more useful in thicker parts due to shorter wavelengths and higher penetrating powers