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Kirchoff Assumptions
-when are they used |
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Definition
1-plane stress
2-no edge effects
these assumptions are used for off-axis loading situations |
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Definition
| used for transverse thermal expansivity |
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| what is the purpose of silane coupling agents? |
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Definition
| o glass has a high affinity for water so the silane reacts with the water to ultimately generate improved fiber-matrix bonding (covalent bonds) for glass fibers in polymer matrices. Failure in silane coupling agent situations is often due to the stiffness. Toughness will not be high because components are brittle. |
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Definition
o Tensile-Shear interactions equal zero and there is no distortion. Also, the young’s modulus is isotropic [0 60 120]
interaction ratios go to zero therefore the material has an isotropic young's modulus |
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Definition
| Contains a symmetry/mirror plane [0 90 90 | 90 90 0] |
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| curvature measurement methods |
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Definition
-Interferometry (ie Newton’s rings)
-Reflect a light beam from curved surface
-Deflection of specimen end
-Can monitor change while they’re occurring ie during coating formation |
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Definition
| this is the point about which a bi-material beam has a stress of zero. In a single material beam it is simply the center but it is more complicated when materials with different Young's moduli are laminated |
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Definition
| analytical focus on stress transfer via shear at cylindrical interfaces. Perfect bonding and no end effects are assumed |
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Aspect ratio
what does it mean for this value to be low/high |
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Definition
| [s=L/r] when s is low, stiffness is low, misalignment reduces stiffness and isotropic order makes stiffness the same as for s=1 (particulate), long fiber stiffness value when tanh(ns)/ns<<1 |
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| what is the significance of being below the critical aspect ratio |
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Definition
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| axial strength approximation |
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Definition
| to a good approximation, σ*=fσf as the fibers take on most of the load |
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| tensile shear interactions |
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Definition
| o C11 is a direct strain but C12, C13, etc. are poisson strains and correspond to having a normal strain from shear stresses and vice versa. Consider and interaction ratios: |
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Definition
-Thermal expansion
-Phase transformations (solidification, resins, curing, martensite transformation)
-Plastic deformation (shot peening) |
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| failure when σ1>=σ1* or we are therefore assuming there is no interaction between other failure modes |
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| Tsai Hill failure criterion |
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Definition
| Mixed-mode failure”: in practice some interaction often observed particularly between transverse and shear modes |
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Definition
| main objective of making composite is to toughen it, promote crack deflection, and generate frictional slidding by adding a tough metal constituent |
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| aim is sometimes to deliberately create weak interfaces to deflect cracks and increase toughness |
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| both fiber and matrix are usually very brittle and Gc for the fiber is approximately that of matrix |
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| What is the significance of transverse/shear properties of a material |
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Definition
| o much harder to predict from matrix and fiber properties as it is likely to depend on the nature of the interface. These values tend to be significantly less than the axial strength. Composites are often weaker in compression than tension related to their tendency to collapse laterally associated with shear properties |
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| what is the limiting factor for load transfer? |
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Definition
| Load transfer happens via shear stresses. Matrix has to adhere to fibers strongly enough to let this happen |
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| How does load transfer effect stress distribution |
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Definition
| Load transfer from the matrix to the fiber leads to a rise in the stress of the system at the midpoint of the wire |
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| in the slab model, what are the equivalent names for equal stress and equal strain? |
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Definition
stress: Reuss
strain: Voigt |
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| of the different assumptions to estimate E, what are the most accurate and why |
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Definition
| equal strain--equal stress--Halpin-Tsai |
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| are there any aspects of the fiber composite that are isotropic? |
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Definition
| the material is transversely isotropic |
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| Why does the S tensor only have 1, 2, 6 terms? |
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Definition
In the Kirchoff assumptions, we are discussing plane stress. Therefore the 3, 4, and 5 components are irrelevant.
S16 corresponds to tensile/shear interactions |
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| How can you get Poisson ratios from Sbar matrix values |
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Definition
vxy=-ExSbar12
vyx=-EySbar12 |
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| what are interaction ratios and what do they correspond to? |
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Definition
| They have to do with tensile-shear interaction behavior (normal stress from shear stress and vice versa) and are based on Young's modulus, Sbar, shear stress, and shear strain |
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| What is the shear lag model |
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Definition
it is used for short fiber composites to develop an understanding of stress transfer via shear
there is interest in teh effect of the fiber length and aspect ratio on stiffness |
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Definition
in fibers, the stress builds up to a plateau at the center of the fiber and the build up distance is symmetric and known as the stress transfer
this value is related to teh aspect ratio of the fiber |
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| what are some possible inelastic phenomena in composites? |
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Definition
cavitation
matrix cracking
fiber fracture
creep |
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| What tool is used to do single fiber pushout tests? |
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Definition
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| why are transverse and shear properties hard to estimate |
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Definition
you must also take into consideration the nature of the interface as well as poisson effects
additionally for transverse thermal expansivities, differential poisson contractions are relevent |
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what are some of the problems that exist with off axis tensile testing
what is a method of reducing these problems and why |
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Definition
tensile shear interaction effects
gripping can be difficult
edge effects
cant set the different stress components independently
tubular specimens however have no edge effects, minimal tensile shear distortions, better control over stress state (can independently apply transverse tension, shear, internal P |
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| what are some methods for curvature measurement? |
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Definition
interferometry
deflection of specimen end
coating crack formation |
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