Micromechanics Interfaces Interphases Mesomechanics Resin Rich Volumes Voids John Summerscales

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

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

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)

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

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

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

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

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

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". 

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%.

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)