1. Micromechanics Interfaces Interphases Mesomechanics Resin Rich Volumes Voids
John Summerscales

2. Outline of lecture
Micromechanics
Interface/interphase
Voids
Meso-mechanics
Resin Rich Volumes (RRV)

3. Micromechanics I composite material modelled using rules-of-mixtures
composite structure may be laminate with complex stacking sequence
[0/90]s four-ply laminate in-plane: Ex = (2E1+2E2)/4 Ey = (2E2+2E1)/4
for out-of-plane (bending) need to calculate effect of layer/ply/lamina properties w.r.t. neutral axis

4. Micromechanics II assumptions in classical laminate analysis
data integrity
linear elasticity
perfect fibre/matrix & layer/layer bonding
continuous strain throughout
plane stress
small deflections (beam theory)

5. Micromechanics III
FEA for orthotropic plates uses 2D shell elements with four (4) elastic constants: E1, E2, G12, 12
FEA for orthotropic solids uses 3D brick elements with nine (9) elastic constants
sandwich panels/beams have a specific option in Autodesk Helius

6. Interface or Interphase
Interface: where fibre meets matrix as a sharp change between phases or a monomolecular layer of “sizing agent”
Interphase: where fibre meets matrix with a graded transition between phases: usually metal- or ceramic-matrix composites at >10 nanometer scale

7. Meso-mechanics
bridges the microstructure-property relationship of materials with non-continuum mechanics
uniform distribution of fibres gives
highest strengths
low in-plane permeability
hence fibre clustering is
bad for strength
good for processing by LCM processes

8. Resin Rich Volumes (RRV)
… or … areas (RRA), regions (RRR) or zones (RRZ)
three-dimensional (3-D) features associated with
low fibre volume fraction
high areal weight fabrics
inhomogenous strain fields
… leading to early failure
permits larger voids

9. Voids Stone and Clarke (Non-Destructive Testing, 1975) suggested
at low void content (<1.5%), the voids tend to be spherical with diameter 5-20 μm
at higher void contents, the voids are cylindrical and the length can be an order of magnitude greater than the diameter
cylindrical voids are generally oriented parallel to the fibre

10. Voids
Judd and Wright (SAMPE J, 1978) reviewed 47 papers and concluded:
considerable scatter in results
the interlaminar shear strength of composites decreases by about 7% for each 1% voids up to at least the 4% void content level, beyond which the rate of decrease diminishes.
other mechanical properties may be affected to a similar extent.
true for all composites regardless of the resin, fibre or fibre surface treatment used in their fabrication". 

11. Voids Ghiorse (SAMPE Q, 1993) found that:
in the range zero to 5%, each 1% increase in void content decreased the interlaminar shear strength of carbon fibre epoxy composites by 10% and decreased the flexural modulus by 5%.

12. Vf + Vm + Vi +Vv = 1 where Vx is the volume fraction of x and ...
Volume fractions
Vf + Vm + Vi +Vv = 1 where Vx is the volume fraction of x and ...
x = f for fibre
x = m for matrix
x = i for interface (normally negligible)
x = v for voids (normally in the matrix)