Prediction of Stresses at Microscopic Level
The micromechanics method was extended to estimate the micro-stresses of the
graphite/epoxy composite laminate under combined thermal and external loads. The
procedure used to obtain the relation between macro- and micro- stresses is described in
the algorithm shown in Figure 2-8. Six independent sets of unit strains are applied to the
unit cell boundary as explained in the previous section and micro-stresses are calculated
in each element corresponding to the unit strain states. The temperature in the unit-cell
was also made equal to the temperature of the structure. The individual micro-stresses are
multiplied by laminate strains in each layer calculated using the laminate theory [26].
Macro Level Micro Level
Composite structure subjected Unit cells subjected to six independent set of
to external and thermal loads unit strain and temperature change
Compute micro-stresses in each element
Apply Laminated Plate
Theory
Use superposition to compute the micro-stresses
Obtain macro-strain results for in the composite structure from the macro-stains
each layers (EL, T, YLT)
r
Check failure of constituent phases based on the
strength properties of fiber and matrix
Figure 2-8. Flow chart of algorithm used to predict the failure due to micro-stresses.
The micro-stresses obtained by superposition are used to calculate the maximum
and minimum stresses in each finite element in the unit-cell model. The failure of
laminate can be predicted by using the maximum stress criterion for the fiber and matrix
phases. The maximum stress criterion is reasonable as the fiber and matrix are expected
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