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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