  We develop a strain gradient plasticity formulation for composite materials
with spatially varying volume fractions to characterize size effects in
functionally graded materials (FGMs). The model is grounded on the
mechanism-based strain gradient plasticity theory and effective properties are
determined by means of a linear homogenization scheme. Several paradigmatic
boundary value problems are numerically investigated to gain insight into the
strengthening effects associated with plastic strain gradients and
geometrically necessary dislocations (GNDs). The analysis of bending in
micro-size functionally graded foils shows a notably stiffer response with
diminishing thickness. Micro-hardness measurements from indentation reveal a
significant increase with decreasing indenter size. And large dislocation
densities in the vicinity of the crack substantially elevate stresses in
cracked FGM components. We comprehensively assess the influence of the length
scale parameter and material gradation profile to accurately characterize the
micro-scale response and identify regimes of GNDs relevance in FGMs.
